Download Purpose of the Exercise Sessions

Purpose of the Exercise Sessions
What is the topic about ? 5 Exercise sessions
Got an overview of the topic
& practical examples
Theory sessions
Understands the topic in depth
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Choose a Student Delegate
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Agenda
5 sessions :
1.
2.
3.
4.
Three Phase Electric Power
Magnetic Circuits
Transformer
Brushed DC Machine
& Brushless DC Motor FYI
5. Sync. AC Machine
& Linear Motor FYI
6. Async. AC Machine 3-Phase
7. Async. AC Machine 1-Phase FYI
+ extra session to refresh the
basics
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Lab Schedule
Please carefully fill in the following doodle :
http://doodle.com/poll/zhuemd6saw9kbdc6
before the next lecture. Feel free to ask your student delegate for
help in order to be consistent with other lectures.
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Basics of Electricity for Chemistry Students :)
Plan extra lecture for you
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Advertisement - Reference Book
This book is recommended for this lecture,
further lectures and as a good reference for the
rest of your life :
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1. Three Phase Electric Power
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Three Phase Electric Power – Some History
Thomas Edison vs. Nikola Tesla
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Three Phase Electric Power –
Y and Delta Configurations
There is a « line voltage » and a
« load voltage ». Total current in
the Y config. is zero
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Exercises
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Three Phase Electric Power – Exercises
1. What are the advantages of three-phase electric power
transmission over 1, 2 phases or DC ?
2. Calculate the voltages and current magnitudes and phases in the Y
(« Wye ») and Delta configurations
3. (pg 339 Wildi 3rd edition) : What is the problem with connecting the
motor on the right of the following picture as it is ? Explain. How could
the connection be improved ?
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2. Magnetic Circuits
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Magnetic Circuits – Microscopic View
More and more magnetic domains orientate in the same
direction as H increases. For steel all domains are oriented
in the same direction above 1 to 2 T. Beyond this point the
material saturates (it « acts like a non-magnetic material »)
Power is lost to reorient the
magnetic domains. We call these
losses « hysteresis losses », they
increase linearly with the
frequency
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Magnetic Circuits – The Reluctance Model
Similar to resistance. The magnetic flux will
tend to flow where the reluctance is low
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Exercises
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Magnetic Circuits – Exercises
1. For the 2 magnetic circuits that will be
drawn on the board, calculate the
reluctances and fluxes in various sections
of the circuit
2. Compare permanent magnets and
electromagnets i.a. in terms of the
magnetic field they can generate
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3. Transformer
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Transformer
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Transformer – Single Phase Working Principle
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Transformer – 3 Phase Working Principle
One could add an extra ferromagnetic bloc shared by
and geometrically similar for all three phases but no
flux would go through if the three phases are
balanced. The left picture is not geometrically
symmetrical for all 3 phases, the center winding will
thus not work exactly like the 2 others
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Transformer - Imperfections
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Saturation
Hysteresis losses in the magnetic material
Eddy current losses in the conductive magnetic
material (reduced with lamination)
Winding resistance
Leakage fluxes
Please note that the leakage
fluxes by themselves do not
create losses. They indirectly
generate losses by increasing
the reactive power transfer.
This increases the overall
currents and those currents do
create losses in the wire
resistance
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Transformer – Equivalent Electric Circuit
Considering the previous slide one can easily understand :
To maximize the efficiency one has to reduce the
resistances as well as the reactive power required to
magnetize the transformer. Reactive power will be
reduced if the leakage flux decreases and the
magnetizing inductor mu increases : i.e. if the magnetic
coupling is improved
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Transformer – Consequence of Saturation
A saturated transformer will see its magnetizing
inductor (x mu) drop. If the supply voltage U1 is high
enough for the magnetic core to saturate, then the
« excess » of supply voltage will cause a big current to
flow through an inductor that seems to be surrounded
by a non magnetic material and thus can not oppose
itself to the high current.
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Transformer – Output U-I Graph
1. Case : The load is an inductor (phi = +pi/2) :
No active power transfer to the load but reactive power is sent to the inductor :
this demagnetizes the transformer and thus decreases the transformer flux
which leads to a lower output voltage U, linearly
2. Case : The load is a capacitor (phi = -pi/2) :
Here reactive power is given TO the transformer and thus the transformer flux
and the output voltage increase with I, again linearly until saturation
3. Case : The load is a resistor (phi = 0) :
No reactive power transfer. Voltage decreases due to the imperfections.
Optimal would be to get a horizontal line
Output voltage of the transformer
versus output current for loads
with different power factors.
Perfect transformer gives a
horizontal line.
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Exercises
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Transformer – Exercises
1. Current and voltage were measured at both
sides of a transformer with firstly a shortcircuited other side and secondly the other
side as an open-circuit. Deduce the parameters
of page 6's equivalent transformer circuit
2. Watch an example of U-I plot for an average
transformer connected to a resistor
Any question before moving to the
motor/generator sessions ?
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Lab
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Transformer Lab - Warning
Warning :
This and the following labs will involve high currents and
voltages, think before acting !
For this lab :
●
When switching off the transformer please always bring
back the autotransformer voltage to 0 to avoid problems
when switching the transformer on again later
For the short circuit measurements please keep the
autotransformer voltage as low as possible. It should not
exceed 10% in practice for this test. Do not do the shortcircuiting by yourself
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Never exceed rated current or voltage
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Transformer Lab – Components – The Transformer
Our transformer...
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… and how we start it
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Transformer Lab – Components – Supply Voltage
Autotransformer
Gives an output voltage whose
value depends on the angular
position of the black indicator on
top. This output voltage is the
input voltage of the primary
winding of our transformer.
Working principle :
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Transformer Lab – Components – The Load
Load connected to
the transformer's
secondary winding :
On the right side of the table.
When on « 0 » the secondary
winding is an open circuit.
When on « 1 » it is 27 Ohm, 2
is 27/2 Ohm, 3 is 27/3 Ohm,...
You can not short circuit the
secondary winding with this.
You will need to manually
short-circuit it on the back side
of the table as explained later.
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Transformer Lab – Components – Power Measurement
Fluke
After the tricky part of setting the
right parameters it will give you all
measurements you might need,
including active and reactive power.
Note that Fluke will need to know if
the 3 phases are connected in a Y or
Delta configuration to provide the
correct measure.
How to connect the clamps :
All with the arrow in the same
direction.
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Transformer Lab – Question 1
Question 1
Locate all components that will be
used in the lab and understand how
they are connected.
What are the current, voltage and
power ratings of the transformer ?
Why are there 2 U/I ratings but only 1
power rating ?
You should not exceed those ratings
for the rest of the lab.
Hint : You will find all info written on
top of the transformer as on the
picture. All ratings as well as the
voltage measurements shown on the
board are indicated for the line voltage
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Transformer Lab – Question 2
Question 2
What is the transformer ratio ? How
does it change when the primary or
secondary winding is in a Y or Delta
configuration ?
Hint : Switching configuration is
easily done by pressing on the
corresponding buttons (« H » is not
used here) when the transformer is
off. The picture shows the selection
for the primary winding. All voltages
shown on the board are line
voltages
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Transformer Lab – Question 3
Question 3
Perform 2 measurements to deduce the impedances of the
transformer's electric model. Do your approximations make
sense? Explain the electric model, the two tests and the
underlying approximations/assumptions to your supervisor
before measuring.
Hints : Refer to « page 7 » in the lecture book for help. Ask
for the « Fluke » for this question. When the output load is
on « 0 » you have an open circuit. To short circuit the
secondary winding you will have to manually do it on the
back side of the table as on the picture. When shortcircuited the board will stop showing the measurements on
the secondary side but you can still measure P and Q on the
primary side.
Do not forget to start the short-circuit test with 0%
at the autotransformer and do not forget to take into
account the Y and delta voltage ratio to guarantee to
keep I out lower than the rating
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Transformer Lab – Question 4
Question 4
Measure the U-I output characteristic for
the connected resistive load. What would
be the optimal curve ? Are we close to it ?
Why don't we have the optimal curve ?
Link the voltage drop to the measured
impedances in the elecrical model.
What would you get for a capacitive and
inductive load ? Why ?
Be prepared to explain it to your
supervisor.
Hints : Put the autotransformer to 100 %
and choose the right configuration (y or
delta) for primary and secondary to
maximize the output voltage. You do not
have to perform the short circuit test, just
interpolate graphically your results.
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Transformer Lab – Question 5
Question 5
What are two different ways to measure the efficiency of
the transformer ?
Put the tranformer in the question 4 configuration that
you set to maximize the output voltage.
Evaluate then the transformer efficiency by the output
over input power ratio. Is it good ? How does it compare
to a combustion engine ? What does a high efficiency
mean for the size of a motor?
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Electrical Motors Classification
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4a. Brushed DC Machine
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Brushed DC Machine - BDC
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The brushed DC motors
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Recall Previous Lectures
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A Brushed DC Motor
Only 2 connections
for DC power
supply
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Not hermetically
sealed : brushes
need aeration
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Brushed DC Motor – Cut View
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Brushed DC Motor Components – The Stator
The stator generates a stationary magnetic field that surrounds
the rotor. This field is generated by either permanent magnets or
electromagnets
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Brushed DC Motor Components – The Rotor
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The rotor windings produce a magnetic field when energized
The magnetic poles will be attracted to the opposite poles (on
the stator)
As the motor turns, the windings are constantly being
energized in a different sequence. This switching of the rotor
windings is called commutation
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Brushed DC Motor Components – The Commutator
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BDC motors do not require a controller to switch current in the
motor windings. The commutation is done mechanically with
the commutator
The brushes and commutator are the parts that are most prone
to wear
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Brushed DC Motor – Pro's & Con's
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Readily available in all sizes and shapes
Easy to drive
Brush wear
Sparks from the brushes may cause explosion
RF noise from the brushes may interfere
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Brushed DC Motor – Applications
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Toys
Low cost consumer goods
...
Good option for everything cheap driven by a battery
Not used as power generators since AC power
transmission is used
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Brushed DC Motor – Types
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Permanent Magnet BDC
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Brushed DC Motor – Permanent Magnet
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Brushed DC Motor – Permanent Magnet
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No stator circuit copper losses
Simple construction and less space required
Weaker flux then shunt fields thus lower induced
torque
(Risk of demagnetization from extensive heating)
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Shunt Wound BDC
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Brushed DC Motor – Shunt Wound
Shunt-wound Brushed DC (SHWDC)
motors have the field coil in parallel with
the armature
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Shunt Wound – Induced Voltage E
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Generated in the rotor windings : loops that see a changing
induction field generate an induced voltage E
E depends on the rotation speed and flux only
E increases linearly with the rotation speed
E increases linearly with the flux flowing throught the loops
E opposes the input voltage and limits the current that can
flow through the rotor windings
The electric circuit of a SHWDC is the following :
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Shunt Wound Load - Speed Characteristic
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Speed quite indep. of load : Indeed, no matter how much the torque
increases, I rotor will not stop increasing until the previous opposing voltage
E is reached... and previous E can only be reached with previous rotating
speed
Slow decrease of speed though because R rotor causes I rotor to drop a
little
End of graph speed increase causes instability (think about a fan in water).
Speed incrases because I rotor is big and causes the global flux to decrease
(left image) because it is deviated to the corners and thus the reluctance
increases, thus causing the rotor's induced voltage E to decrease and so I
rotor to increase a lot ...but efficiency drops 01/03/2016
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Shunt Wound – How to Vary the Speed – Adding R Rotor
It works but since I rotor is big the efficiency drops
sharply. This method is only used to limit starting current
when switching on the motor.
For a better efficiency one could change U using power
electronics.
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Shunt Wound – How to Vary the Speed – Adding R Stator
Ie' < Ie
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If Ie decreases then E decreases and I rotor
increases a lot thus the speed increases
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Low power dissipation since Ie is low
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Limited to speeds not too slow (else Ie is so big that
the magnetic material saturates)
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Shunt Wound – How to Vary the Speed – Supply Voltage
U increase causes I rotor and I stator (and thus B)
to increase.
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Brushed DC Motor – Shunt Wound
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Speed roughly independent of load, thus :
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Excellent speed control
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Suitable for varying load applications (fans,
centrifugal pumps,...)
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Series Wound BDC
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Brushed DC Motor – Series Wound
Stator and rotor currents here are the same.
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Brushed DC Motor – Series Wound
If the load increases, then the rotor current (= stator current)
must increase. However, if the stator current increases then E
increases, which tends to decrease the rotor current.
Thus, in order to still be able to get a higher rotor current, the
motor must spin slower to decrease E to a level where the
needed rotor current can be drawn.
In conclusion, the higher the torque, the slower the spinning.
Please note that :
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The load has a maximum
limit in order not to
saturate
The load has a lower limit
below which the speed
might be fast enough to
destroy the motor due to
centrifugal forces
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Series Wound – How to Vary the Speed
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Resistor in parallel with the stator windings : if R
decreases then it speeds up (the rotor current speeds up
the motor while the stator current slows it less down by
decreasing E)
Varying the supply voltage U with a resistor in series with
the motor
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Brushed DC Motor – Series Wound
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One of the highest torque for BDC motors
Suitable for high torque applications (trains,
elevators, electric cars) & high starting torque
Difficult speed control
Spinning speed must be limited
Not suitable for low loads
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DC Motors – Where does the Energy Transfer Happen ?
Where in the DC Motor does the energy transfer
from electrical energy to mechanical energy
happen ?
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The brushed DC generators
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Brushed DC Generator vs. BDC Motor
It is the same machine, with some minor differences, e.g.
the brush position slightly moves to take into account the
magnetic field rotation in opposite directions for motor
and generator :
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Brushed DC Generator – Unloaded Curve
The graph shows the generated voltage without load
when varying the excitation current Ie. One can clearly
see the hysteresis cycle
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Brushed DC Generator – Separately Excited U – I Graph
I causes an induction field deflection which increases the reluctance and
thus decreases the flux and U. The winding resistance also decreases U.
This however only starts being visible for high I so that up to a certain I
limit the generator acts like an excellent voltage source.
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Brushed DC Generator – Series Wound U – I Graph
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I causes an induction field deflection which increases the
reluctance and thus decreases the flux and U. The winding
resistance also decreases U.
For low I, U increases linearly since Ie and thus the flux
increase linearly
For high I the output voltage U decreases faster than for a
separately excited configuration since the stator winding
resistance adds up to the voltage drops
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Brushed DC Generator – Shunt Wound U – I Graph
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I drawn from the load + winding resistance + induction
field deflection = sharp output voltage decrease
If Ie gets too low the output voltage and current
collapse
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Exercises
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Brushed DC Motor – Exercises
1. Discuss possible systems that could be used for emergency braking of a
brushed DC motor (mechanical braking, dynamic braking, braking by voltage
inversion and no active braking) and compare the time required to slow down
(Wildi pages 407 through 409)
2. Example 28-6 (Wildi page 409) about the braking system of a brushed DC
motor. Calculate the time to slow down the motor to 80 RPM in case of a
dynamic braking system and in case of no braking system for the given motor
characteristics
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Lab
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Brushed DC Machine Lab - Warning
Warning :
This and the following labs will involve high current
and voltages, think before acting !
Never exceed rated current or voltage
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Brushed DC Machine Lab – Components
DC motor
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DC generator
(« dynamo »)
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Brushed DC Machine Lab – How to Start it
Start the motor.
Make sure you
do it with I
excitation at
maximum or it
might not start.
Wait a bit
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After motor is started
start the generator by
pressing the « génératrice
ON » button and connect
the excitation circuit by
pressing the button on the
picture
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Brushed DC Machine Lab – Components – The Load
The Load is a resistor
and is connected to
the DC generator's
(« dynamo ») output :
On the right side of the table. As
shown on the resistors top and
on the bottom picture one can
see that the resistor can be
adjusted from a short-circuit to
an open circuit when rotating
the wheel
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Brushed DC Machine Lab – Question 1
Question 1
Make sure you are aware of the current and
voltage ratings for both the DC motor and
generator. You will find this information on
the machines similarly to the picture.
Additionaly to that:
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Why does the motor start in 3 audible
steps ? How is the motor started ?
In which fixed configuration is the motor
wired (look at the board drawings for that)?
What is the (probably) only difference in the
construction of the DC generator in front of
you and the DC motor ? Why ? Hint : Think
about the position of the brushes
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Brushed DC Machine Lab – Question 2
Question 2
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What is the effect of increasing and
decreasing the shunt motor's excitation
current ? Why ?
What is the effect of increasing and
decreasing the generator's excitation
current when separately excited ? Why ?
What is the effect of decreasing the
generator's load ? How does it change
the rotating speed ?
Hint: You can easily increase the
excitation currents by rotating the little
buttons called « Ie » clockwise
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Brushed DC Machine Lab – Question 3
Question 3
Measure the speed versus torque behavior of
the DC shunt wound motor. Compare your
results with the theory shown on the figure
on the right. Explain the physical reasons
behind the curve to your supervisor.
Hints : You do not have a torquemeter as you
will have in the asynchronous lab. There is
however a simple way to increase the torque
applied to the motor by using the generator.
Do not forget to keep all currents and voltages
below the values found in question 1.
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Brushed DC Machine Lab – Question 4
Question 4
Measure the unloaded U-I
excitation graph for the
separately excited DC
generator. You should get
something similar to the figure
on the right. Increase Ie
generator and then decrease
it again. What do you
observe ? Why ?
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Brushed DC Machine Lab – Question 4
Question 5
Measure the U – I graph of the shunt
wound DC generator at 1500 RPM. For
this test to provide good results please
set the generator's excitation current at
a value that leads to 100 V output
voltage when unloaded at 1500 RPM.
Explain the physical reasons for this
curve to your supervisor.
Hints : This test is to be done at constant
speed. Use the motor side to keep the
rotating speed constant.
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Brushed DC Machine Lab – Exercise
Exercise
A shunt wound DC motor is specified as
followed :
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Rated input voltage U = 110 V
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Rated input current I = 110 A
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Rated speed = 3000 RPM
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Rotor winding resistance = 0,1 Ohm
1. We want to start the motor with a 110 V
voltage and limit the in-rush current at twice
the rated current. What resistance should
the resistance in series with the rotor have
during starting ?
2. If we neglect the rotor series resistance :
what torque will the motor produce when at
rated speed with a 110V input voltage and a
85 A input current ?
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4b. Brushless DC Motor
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Brushless DC Motors (BLDC) - FYI
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Brushless DC Motors (BLDC)
Additional
cables for
controlling
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Can be fully hermetically
sealed and is thus more
robust since there are no
brushed
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Brushless DC Motors – Electronical Control
The goal is to spin the motor by creating a
rotating magnetic flux in a given direction
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Brushless DC Motors – What it is Made of
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A stator with windings
A rotor with permanent
magnets (external or
internal)
An angular position
sensor for control
purposes
An electronic controlling
chip
4 poles, 2 phases BLDC
(external rotor removed)
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Brushless DC Motors – What it is Made of
To spin the motor we need to
electronically control the switching of
the stator windings. For that we need
to know the angular position of the
rotor. We get it :
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With Hall sensors
With an angular position encoder
By measuring the back
electromotive force
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BLDC Motor – Which Elements do you Recognize ?
B
A
D
E
C
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Brushless DC Motors – Electronic Control
The goal is to spin the motor by creating a
rotating magnetic flux in a given direction
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Brushless DC Motors – Field Oriented Control
FOC needs the rotor flux to be 90° ahead of the
resultant stator flux. FOC control has the maximum
torque per amp.
Implementation can be easily done e.g. using an FPGA
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BLDC – Pro's & Con's
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Increased efficiency vs BDC Motors
Increased reliability vs BDC
Longer lifetime (no brush)
Reduced/more controllable EMI
Windings in the stator cooled down by conduction thus no
need for airflow as in BDC : can be fully enclosed and
protected from dirt
Electronically programmable behaviour thus very flexible
and greater capabilities
More expensive than BDC because of the electronics
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BLDC Motor – Applications
Brushless motors fulfill many functions originally
performed by brushed DC motors thanks to their
robustness and flexibility, but cost and control
complexity prevents brushless motors from
replacing brushed motors completely in the lowestcost areas. Example of applications are :
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HDD, CD players
Cooling fans
Humanoid robots joints
Power toothbrushes
...
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Stepper Motor
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Stepper Motor- Working Principle
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Stepper Motor- Working Principle
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5a. Synchronous AC Machine
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Synchronous AC Motors
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Synchronous AC Machine – Working Principle
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RPM = supply frequency /
number of poles
The rotor is made up of
permanent magnets or DC
electromagnets
The stator generates a
rotating magnetic field
rotating at f/p RPM
The rotor follows the stator
rotating magnetic field,
lagging behind it or ahead
of it by the mechanical
angle
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Synchronous AC Machine – Laminated Stator
Rotor sees no flux change : not laminated
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Synchronous AC Machine –
The Mechanical Angle
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When the machine is unloaded the opposite
poles follow each other : this stable
equilibrium position neither generates a
torque, nor generates electric power
When a mechanical motor torque is applied
to the rotor it is pushed ahead of its unloaded
position. This brings the rotor poles closer to
the opposed poles of the stator and thus
generate a braking torque so that the rotor
stabilizes at a given positive mechanical
angle. This braking force applied to the rotor
is transformed into electric power: the
machine acts like a generator
When a mechanical load is applied to the
rotor then it lags behind the unloaded
position. This brings the poles to repel each
other and thus produces a motor torque on
the rotor which keeps the rotor in
synchronicity : for this to happen the network
will give electric power : the machine acts
like a motor
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Synchronous AC Machine –
Machine Instability : « Stalling »
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Torque is zero when the poles exactly
face each other (be it N-N or N-S)
Torque is maximum on half way
(remember FOC?)
If the mechanical load applied to the
motor increases too much and brings
the mechanical angle beyond half way
then the torque produced by the motor
will start decrease, increasing even
further the mechanical angle : this
instability is called « stalling »
Stalling means a loss of synchronicity
Stalling for big machines causes
powerfull harmonics to be generated
on the network and might harm it
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Synchronous AC Machine Behn-Eschenburg Model – Physical Interpretation
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Simpler model than the Potier model
Valid if the machine is not saturated
Enables to define the «electric angle » : angle between the loaded and
unloaded output voltage
Ev and U have a phase shift when I is not zero
When the mechanic angle is negative the rotor lags behind the stator,
thus E will peak AFTER U => negative electric angle
When the mechanic angle is positive the rotor is ahead of the stator,
thus E will peak BEFORE U => positive electric angle
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Synchronous AC Machine –
The Electric Angle
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When the mechanical angle is zero
the stator windings see the induced
voltage appearing at the same time
as the network thus Ev and U have
the same phase and the electric angle
is zero
When the mechanical angle is positive
the electric angle is positive (Ev's
phase is in ahead of U)
(refer to the Behn-Escheburg electric
circuit for that)
Electric angle : angle between the
resultant stator and rotor magn. fields
The relation between the two angles
is :
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108
Synchronous AC Machine –
The Electric Angle
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109
Synchronous AC Motor –
Link with the BLDC Motor & FOC Control
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Sync. AC Motor – How to Start it ?
When not at sync. speed the average torque = 0.
We can thus add e.g. a squirell cage in the rotor to
start the sync motor as an async motor that has a
non zero starting torque (cfr. Next lecture)
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Sync. AC Generator – External U-I Curve Explained
1. Case : The load is an inductor (phi = +pi/2) :
No active power transfer to the load but reactive power is sent to the
inductor : this demagnetizes the motor and thus decreases the motor flux
which leads to a lower output voltage U, linearly
2. Case : The load is a capacitor (phi = -pi/2) :
Here reactive power is given TO the motor and thus the motor flux and the
output voltage increase with I, again linearly until saturation
3. Case : The load is an resistor (phi = 0) :
No reactive power transfer. We get something in between, similar to the
transformer
Note that I does not change its
sign for reactive power to be
given or taken but the reactive
power does because of the
impedance sign change
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V Curves
A possible Explanation to Understand them
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Consider a synchronous generator (with electromagnets on the rotor) that is
connected to a power network of constant voltage U and that spins at a constant
speed given by the network frequency:
If we neglect the (tiny) imperfections of the motor then the induced voltage E in the
stator must be close to U and thus be constant
Since E depends only on the flux and the rotation speed and we also know that E and
the rotation speed are constant, we can deduce that the flux is constant in the
generator
We also know that only the reactive power in the machine will create flux
The reactive power can either be generated via Iexcitationrotor in the rotor or via the
reactive component of Istator in the stator
Since the flux is constant, the total reactive power (the sum of what the rotor and the
stator create together) must be constant
THUS : on next slide's V curves one can see that for a low I rotor (i.e. An
« underexcited motor ») the rotor does not generate enough reactive power to get
the required flux and thus the stator must absorb reactive power from the network to
reach the constant sum of reactive power in the generator: in this case the motor
acts like an inductor on the network
For an appropriate I rotor the rotor generates exactly enough reactive power and
the motor thus does not take or give reactive power to the network : it has a unity
power factor (and its efficiency is maximized)
For an even bigger Ie the rotor generates too much reactive (power compared to the
constant reactive power sum) which is thus given away to the network : the motor
acts like a capacitor on the network
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Synchronous AC Machine – V Curves Application
V curves are measured at constant active power W !
If no mechanical load is plugged to the rotor then one can use the
synchronous motor to stabilize the network's power factor to 1 by varying Ie
(rotor) such that it absorbs the excess of reactive power on the network or
generates what is missing. Such machines are used in practice and can
produce as much as hundreds of MVAR
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Exercises
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Synchronous AC Motor – Exercises
1. Re-explain by yourself how an external inductive or
capacitive load can affect the sync. generator
magnetization and the output voltage
2. Adapted from Wildi Example 37,1 page 628
3. Explain the link between the BLDC and
synchronous motors. What is field oriented control ?
What are its advantages ? How could it be
implemented in practice ?
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Lab
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Synchronous AC Machine Lab - Warning
Warning :
This and the following labs will involve high current
and voltages, think before acting !
Never exceed rated current or voltage
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Synchronous AC Machine Lab – Components
Separately
excited DC
motor
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Synchronous
Machine
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Synchronous AC Machine Lab – How to Start it
Press the 2 « marche » buttons
Start the DC motor. It will
start in 3 audible steps by
progressively increasing
the motor's input voltage.
Please set the excitation
current Ie to the maximum
before starting to limit the
in-rush current
Press on « charge », this
will connect the load to
the sync. generator's
output, then press « on ».
Do not press « reseau » as
the generator's output is
not close to the networks
voltage and frequency yet
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Sync. AC Machine Lab – Components –
Network Comparison
Network voltage & frequency
versus generator output
comparator :
This will be used to bring the generator output
smoothly to the power network so that the
generator can be connected to the network.
V line has an arrow on the outer side of the circle
which indicates the network voltage (divided by 2
via a transformer). It also has an inner arrow which
shows the generator's output voltage.
The two boxes below compare the network
frequency with the generator frequency as well as
the phase difference. To connect the generator to
the network the middle box should optimally have
its arrow pointing to the top without moving : this
means a frequency match as well as a phase
match. This in turn guarantees a smooth
connexion to the network without vibration or
voltage spikes.
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Synchronous AC Machine Lab – Components –
The Load
Load is connected to sync.
generator's output :
The load is a resistor (top picture) in series
with a variable inductor (bottom picture). The
three loads for the three phases are
balanced and connected in a delta
configuration.
For the resistor : when on « 0 » the
resistance is infinity. When on « 1 » it is 36
Ohm, 2 is 36/2 Ohm, 3 is 36/3 Ohm,...
If you want to connect a « purely » inductive
load to the generator's output you will have
to short circuit the resistors as explained in
the next slide.
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Sync. AC Machine –
Connecting a Purely Inductive Load to the Output
To connect a « purely »
inductive load to the
generator's output :
You have to short circuit the resistive load
with cables as shown on the picture. The
current in the cables should stay below 10
Amps.
To vary the inductance you just have to
turn the wheel on the bottom picture.
Please note that only one every two
contacts is connected to the inductive
load so that as you turn the wheel you will
go through an open circuit connection
before being again connected to an
inductive load with a different value than
before.
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Synchronous AC Machine Lab – Question 1
Question 1
Make sure you are aware of the current
and voltage ratings so you know what
you should not exceed.
Additionnaly to that :
With the power switched off, open the
part containing the brushes of the
synchronous generator (bottom picture).
The brushes are there to power the DC
electromagnets of the rotor. However,
there are three brushes : how do you
explain that ?
Hints : You will find all information
written on the back side of the
machines, similarly to the top picture.
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Synchronous AC Machine Lab – Question 2
Question 2
Measure the U-I output curve of the synchronous
generator for a purely inductive charge as well as for a
resistive charge. U unloaded should be chosen to 70 V.
We do not ask to measure I short-circuit. Do your
measurements follow theory as displayed on the picture ?
Explain to your supervisor why for a purely inductive
charge U decreases faster than for a resistive charge and
why the decrease is linear. What is the U-I curve for a
resistive load with a perfect generator ? Make a link to
the U-I curves of the tranformer.
For a purely inductive charge, how many watts are
transferred to the load ? Explain in your case how
reactive power transfer leads to power losses.
Hint : This test requires a constant rotor speed. You can
adjust the rotor speed by changing the excitation current
of the DC motor as explained in the DC machines lab. To
get a resistive or inductive load and change the
resistance and inductance please refer to the dedicated
slide in the lab introduction.
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Synchronous AC Machine Lab – Question 3
Question 3
We would like to connect the synchronous generator to the network :
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Which 3 conditions do we have to fullfill to have a smooth connexion to the
network ?
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What would happen if the generator's output voltage was lower than the
network voltage ? Explain it
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What would happen if the generator's output frequency was lower than the
network frequency ?
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Connect the generator to the network after making sure the 3 requirements
are met ! Attention : To turn on the network comparator press the
« reseau » button. Press the « on » button below it ONLY when the 3
requirements are met
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Now that the generator is connected, see what happens when you change
the dc motor's and the generator's excitation current. Explain why
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Increase the DC motor's Ie until the sync. generator's output power gets
negative. How comes it is negative ? How does the sync. machine now act
like ? And the DC motor ?
Attention : Ask your supervisor to check all parameters before connecting
the generator to the network
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Synchronous AC Machine Lab – Question 4
Question 4
Make sure you understand what the V curves are. Explain to your
supervisor what it represents, in which conditions it is to be measured, why
it looks like a V. Since all points in a same V curve have the same active
power transfer under the same network voltage, why does the stator
current still change ? What consequence does this have on the efficiency ?
Deduce where the efficiency is maximized. Based on the V curves, explain
how one could stabilize the network using an unloaded synchronous motor.
Why do we want the synchronous motor to be unloaded ?
Measure the « 0 Watt » V curve. Do you
get the theoretic curve ?
Hint : You will have to go through
question 3 again to connect the
generator to the network if you
powered off the machine in the
meantime !
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5b. Linear Motors
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Linear Motors - FYI
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Linear Motor – Just to Mention it
A synchronous linear motor is equivalent to an
unfolded synchronous AC motor.
The two coils are energized in "quadrature". If the
bottom coil leads in phase, then the motor will
move downward, and vice versa.
Linear motors are not only used in maglev trains
but is also suited e.g. for electric hair trimmers or
anything else needing a linear motion
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6. Three Phase Async. Machine
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3-Phase Asynchronous AC Motors
(aka. Induction Motors)
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Asynchronous Rotor vs. Synchronous Rotor
Construction Difference
Asynchronous
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vs. synchronous
133
Asynchronous AC Motor – Construction
The squirrel cage is a little twisted to
smoothen the torque over time by
averaging it all over the bar
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Asynchronous AC Motor – Equivalent Electric Circuit
The asynchronous motor's electrical behavior is similar to a
transformer except that the secondary winding is not
connected to a load but is short circuited in the rotor. Indeed
the motor will not output any current but will transform the
input current into mechanical power. An additional difference
with the transformer is that the secondary side rotates, thus
the transformation ratio E2/E1 is not n but n x g : thus here
R2' = R2 / (n² g) and X2' = X2 / (n² g).
Note : R2' does not give only losses, R2/n² however does.
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Asynchronous AC Motor – Working Principle
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Since a short-circuited loop does not want to see its flux change it will
create a magnetic induction via eddy currents in order to try to keep the
flux constant. By doing so the rotor will create poles that are attracted
by the rotating stator resultant flux : a torque appears
Torque is zero at sync speed thus sync speed is not reachable due to at
least a friction torque
Slip is the difference between synchronous speed and operating speed
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Asynchronous AC Motor
Rotor as a pure inductor vs. as a pure resistor
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Look at a B, U and I phasor diagram
Compare the angle between the B rotor opposition due to eddy currents
and the B stator-to-rotor in case of a pure inductor in the rotor and a
pure resistor
If the rotor is a pure inductor then B opposed lags 180° behind B statorto-rotor, thus the rotor poles face exactly the opposite stator poles : this
is a stable equilibrium and the torque is zero
If the rotor is a pure resistor then B opposed lags 90° behind B-stator-to
rotor, thus the rotor poles are 90° behind the stator poles (for 1 pair of
poles) : this creates a non zero torque
THUS : the contribution of the inductive part of the rotor is a zero torque
while the resistive part of the rotor creates a non-zero torque
We thus want to have a « 1 » power factor in the rotor electric circuit
We want to have the « 1 » power factor with the smallest R rotor since R
creates losses
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Asynchronous AC Motor – C versus g diagram
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When starting to spin, the rotor sees a high frequency magnetic field.
The rotor's leakage inductance's impedance dominates over the rotor's
resistance which leads to a low power factor and a low torque
As the speed increases the frequency of the magnetic field seen by the
rotor decreases. Thus its leakage inductor's impedance decreases and
the power factor increases : the torque increases
At synchronous speed the rotor sees a DC magnetic field and thus there
is no torque : thus the rotor spins a bit slower than synchronous speed
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Asynchronous AC Motor – How to Vary the Speed –
R Rotor
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Add a resistor in series with the rotor's conductors :
At g=1 (zero speed) : The self inductance of the rotor (windings) >> R rotor
because everything is surrounded by magnetic material. Thus the active power
transferred to the rotor is very low since U and I are almost in quadrature
(phase = 90°). When increasing R rotor the angle moves away from 90° and
the torque increases
At g close to 0 (sync speed) : The frequency seen by the rotor is low and thus
the impedance is dominated by its resistance : the power factor is close to 1
and increasing R rotor just leads to lowering I rotor and thus the torque
decreases
THUS : Increasing R rotor slows down the motor when is g close to 1 but harms
the efficiency (R rotor dissipation with a big I rotor)
Useful to increase starting torque
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Asynchronous AC Motor – How to Vary the Speed –
Supply Voltage
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Increasing U increases the starting torque, the speed
and the efficiency (since g gets closer to 0),
decreasing U decreases the speed and the efficiency
Reachable speed range is very limited
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Asynchronous AC Motor – How to Vary the Speed –
Frequency
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Increasing the supply frequency increases the speed but
increases g and decreases the efficiency
To keep a high efficiency we need to increase U as we
increase the frequency
Very large reachable speed range
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Asynchronous AC Motor – How to Start it
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For a squirrel cage : R rotor is very low and the
self-inductance L rotor is high since it is
surrounded by magnetic material : thus the power
factor is close to 0 (low active power transferred
to the rotor) and the torque is low. Make sure
however to decrease the supply voltage U to
guarantee a low enough rotor current
With windings in the rotor : One can add a
series resistor to those windings to increase the
starting torque by increasing the power factor.
Moreover this decreases the starting current.
While starting the resistance is progressively
reduced and removed
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Asynchronous AC Motor – Construction Variant –
Double Cage
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The self inductance of the internal cage is high since it is sunk in
a magnetic material. Its resistance is low
The self inductance of the external cage is low since it is
surrounded by an air gap. Its resistance is high
At low speed the high internal cage's inductance keeps I rotor
(internal) low : low produced torque. The low external
inductance causes a big current to flow in the external cage. This
leads to a high overall torque
At sync. speed the biggest contribution to the torque comes from
the lowest resistance cage : the internal one
The sum gives an enhanced torque profile
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Exercises
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Asynchronous AC Motor – Exercises
1. Write an electric equivalent circuit for a squirrel cage
asynchronous motor and show how to measure its parameters in
practice. Based on the electric equivalent, show what power is
mainly transferred to the rotor (P or Q) when g = 1 and when g = 0.
Deduce from this where the efficiency is highest
2. Is an asynchronous generator best suited to generate electric
power in a power plant ?
3. Example 33-8 page 537 in Wildi
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Lab
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Asynchronous AC Motor Lab - Warning
Warning :
This and the following labs will involve high current and
voltages, think before acting !
For this lab :
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The cables that you will use to measure the input
current will be at a high voltage : do not touch them !
Watch out for your fingers when touching the rotor
Never ever unplug the ammeter from the current
transformer when current is flowing : this WILL
lead to electric arcs, damage the current transformer
and possibly cause injuries
Never exceed rated current or voltage
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Async. Motor Lab – Components
DC generator (« dynamo »)
which we use as a variable
mechanical load for the
asynchronous motor
3-phased
asynchronous motor
which we want to
understand
Speed & torque sensors
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Async. Motor Lab – How to Start it
Always put the two woodenconductive parts in as on the
picture to deviate the « inrush » current from the current
transformer. Remove the two
parts when started !
Press those two buttons
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Make sure those two switches are up
Start the async. motor,
remove the 2 wooden
bridges and you're done !
149
Async. Motor Lab – Components – Supply Voltage
Autotransformer
Gives an output voltage whose
value depends on the angular
position of the black indicator
on top. This output voltage is
the input voltage of the
asynchronous motor. Working
principle :
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Async. Motor Lab – Components – The Load
Load is connected to the
DC generator's
(« dynamo ») output. It
enables to electrically
change the async.
motor's load torque :
On the right side of the table. When
on « 0 » the DC motor output is an
open circuit. When on « 1 » it is 36
Ohm, 2 is 36/2 Ohm, 3 is 36/3
Ohm,...
When the load decreases the DC
generator (« dynamo ») generates
more electric power and thus the
torque applied on the asynchronous
motor by the DC generator increases
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Async. Motor Lab – Components –
DC Motor Excitation Current
Changing the excitation
current of the DC
generator's (« dynamo »)
stator is the second way to
change the torque applied to
the async. motor:
Increasing the excitation current (i.e.
the DC generator's stator current)
increases the flux in the rotor and
thus increases the DC generator's
output voltage and thus the output
electric power... thus increasing the
torque applied on the async. motor's
shaft !
Note that moving the wheel
clockwise increases the excitation
current.
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Async. Motor Lab – Components – Current Transformer
Current transformer converts
here a current in the 50 Amp
range to a current in the 5 Amp
range :
We use this system to bring the too high
current in the standard 5 Amp range which
can be measured by our wattmeter. The
ammeter part of the wattmeter should be
plugged as indicated on the picture.
Working principle :
It works exactly like a transformer and will
not saturate as long as the secondary
winding is in short-circuit. If a current is
flowing through your connected ammeter do
not disconnect the ammeter as the flux
would brutally increase in the transformer
and create very high voltages !
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Async. Motor Lab – Components – Power Measurement
Request 2 wattmeters !
You will have to use the 2 wattmeters
method, make sure you master it
before starting the lab !
Use the current transformer as
explained before to reduce the current
range. Plug the ammeter connections
in the current transformer and the
voltage connections at the right place
in the table. Do not forget to
multiply what you read by 10, the
current reduction factor !
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Async. Motor Lab – Question 1
Question 1
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Why does the async. Motor have 2
current ratings but only one power
rating ?
Show that the motor indeed is an
asynchronous motor !
What is the motor's synchronous speed ?
What is the slip value at the rated
speed ? What about the torque ?
Why is the motor input connected in a Y
configuration when starting ?
Hints : You will find all information written
on the back side of the async. motor as on
the picture. The motor is a motor with 2
pole pairs!
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Async. Motor Lab – Question 2
Question 2
Explain the asynchronous motor's
electric model and the link to the
transformer model. Measure its
impedances. Which two tests do you
need to perform for that ? Explain it to
your supervisor.
Why is the magnetic coupling worse
than for the transformer ?
Attention : Before doing the « shortcircuit » test with the rotor blocked
make sure to put the autotransformer
to 0% and increase it slowly during the
test while making sure you stay within
the current limits set in question 1.
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Async. Motor Lab – Question 3
Question 3
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Measure C with respect to g to capture the
stable zone of the theoretic graph on the picture.
Based on the motor power factor, explain why
the torque increases as the rotor starts to spin.
What is the effect of putting a series resistor on
the rotor at g = 1 and g= 0 ? Explain it to your
supervisor
Show that the async motor's efficiency increases
when g gets closer to 0. In our case what is the
best efficiency you can measure ?
What is the effect of increasing the
autotransformer output voltage ?
Hints : You can easily measure the efficiency of
the async. motor by dividing the mechanical power
by the input electric power. To change g you just
have to play around with the DC motor
(« dynamo ») load, with its excitation current
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7. Single Phase Async. Machine
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Single Phase Asynchronous AC Motors FYI
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Single Phase Asynchronous AC Motor –
Interactive Lecture
1. In which applications is it used ?
2. Draw a one pole single phase AC motor
3. What problem appears which was not present with 3
phases ?
4. How is this problem solved ?
5. What other problem is inherent to the usage of a single
phase ?
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Additional Motor Types
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Additional Motor Types
There are many more types of motors/generators that
are best suited for specific applications. We can not
discuss them all in this lecture, please read the Wildi
book for more.
Variable reluctance
motor : acts like an
async. motor but is able
to catch the
synchronous speed and
to stick to it for low
torques. It is the
cheapest synchronous
motor to produce
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