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Alternate Current Machines
10.12.2012
Contents
Electric Motors
AC Motors
Synchronous AC Motors
Asynchronous AC Motors
Losses in Motors
Power Calculations for Motors
Alternate Current Machines
25.11.2011
2
Electric Motors
If an electric current flows through a conductor in a magnetic
field, a magnetic force effects the conductor. A simple electric
motor can be formed if this conductor has a point to rotate
around. Faraday electric motor and Barlow Wheel are the first
experimental representations of the electric motor.
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Electric Motors Faraday Electric Motor
There is a free rotating wire
which is inserted in a glass
full of mercury (or salt
water) in Faraday Electric
Motor. The glass full f
mercury has a permanent
magnet on center.
If a current flow through
the wire, it starts to rotate.
This motion is the
representation of the
magnetic field produced
because of the current
flows on a wire.
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Electric Motors
An electric current passes through the
hub of the wheel to a mercury contact
on the rim; this is contained in a small
trough through which the rim passes.
Due to health and safety
considerations brine (salt water) is
sometimes used today in place of
mercury. The interaction of the current
with the magnetic field of a U-magnet
causes the wheel to rotate. The
presence of serrations on the wheel is
unnecessary and the apparatus will
work with a round metal disk, usually
made of copper.
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What is an Electric Motor?
An electric motor is a machine that converts
electrical energy to mechanical energy.
• Used is compressors, pumps, air condition
fans, electric vehicles, robot mechanisms,
cranes, etc.
• More than the two thirds of the load in
industry are the load of electric motors.
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Electric Motors
It is the ‘Lorentz Force’
that effects the charge
of ‘q’ which has the
velocity of ‘V’ in
magnetic field ‘B’. The
directions of this force,
the current and the
magnetic field can be
seen in the figure.
|FL|=q V B Sin α
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Electric Motors
The rotating part of an electric motor is called as rotor whereas
the fixed part as stator. If the rotor is consist of windings,
brushes are used to transfer the current. The brush is a carbon
part which has a contact with the terminals of the coils on the
axle of the rotor. In some DC motors, permanent magnets are
used in rotors and these types are called as Brushless DC Motors
(BLDCM). The problem for these types is to sense the position of
the rotor. Information about the position of the rotor is needed
to be sent to the driver of the motor.
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Electric Motors
In some AC motors, Aluminum bars are used as the rotor of the
motor. These type of motors are called as Squirrel Cage type
electric motors. When the current is changing periodically, the
rotor follows the current.
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Electric Motors
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Electric Motors
Electric Motors
Alternate Current Motors
(ACM)
Direct Current Motors
(DCM)
Synchronous
Induction
(Asynchronous
Externally
Excited
Mono Phase
Three Phase
Series
Self-Exited
Compound
Schunt
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Alternatif Akım Motorları
Alternate Current Motors
(ACM)
Synchronous ACM
Induction (Asynchronous) ACM
Slip Ring ACM
Squirrel Cage ACM
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Alternate Current Motors
The ACM’s are simplier in structure
and more economic than DCM’s.
An ACM generates more power
comparing with a DC motor that has
the same weight. Maintenance of
ACM’s is easier. However, their speed
control is harder. They can be
connected to the AC source directly.
If accuracy in velocity or position
control is needed, DCM’s are used.
But, ACM’s are used more than
DCM’s in industry.
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ACM’s
Basic Definitions
Free Running Current (I0): It is the current consumed from the grid with
nominal voltae and frequency, but without any load on motor.
Maximum Starting Current (Ik): It is the maximum current on nominal
voltage and frequency when starting a motor.
Starting Torque (MA): It is the torque generated by the motor during
starting under nominal voltage and frequency.
Nominal Moment (MN): It is the toque generated by the motor under
nominal power and speed.
Stall Torque (Mk): It is the maximum torque generated by the motor with
nominal voltage and frequency.
Pull-up Torque (Ms): It is the minimum torque delivered by the motor
with nominal voltage and frequency, between zero velocity and the
velocity with the stall torque.
1 kgm = 9,81 Nm ~ 10 Nm,
MN = 9550 x Nominal Power [kW] /Nominal Velocity of Rotor [RPM]
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ACM’s
Basic Definitions
Torque
[Nm]
http://www.ebmpapst.se/sv/dat/media/informati
on/definitions_for_ec_motors.pdf
MA
MS
MK
MN
Revolution [RPM]
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Synchronous Machines
“ Synchronous Machine is a machine that runs at a constant speed which
is proportional to frequency and number of poles. It can be run as a
generator or a motor. However, because of the constant running speed
these machines are generally used as generators. They are the most
common machines used in power plants. They can be manufactured to
generate electricity up to 2000 [MVA]. Cost effectivity due to unit power
generated, higher efficiency in greater power generation, less
maintenance and control processes made them to be manufactured in
greater powers.
(*see references)
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Synchronous Machines
Stators of Synchronous Machines are
manufactured using laminated cores
which have slots to place the coils on
them.
Synchronous Machines are divided into
two groups according to the structure
of the rotor that has exiting coil on it.
If the airgap between the stator and
rotor is constant every where, then it is
a round rotor (turbo) machine. Unless,
it is a salient pole synchronous
machine.
22 [MW], 13.8 [kV], 3,600 [RPM]
* http://www.ips.us/industries/fossil-fuelpower/
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Synchronous Machines
Round rotor synchronous generators are manufactured in small
number poles and high synchronous revolution per minute. They are
used in high velocity steam turbines. The length of the rotor is long
and radius of the rotor is small in this type of turbines.
The salient pole synchronous machines are generally have more
poles and are designed for lower synchronous rotational velocity.
Length of the rotors are short and the radius of the rotors are long.
Salient pole synchronous machines are used in hydro elecric power
plants and for compensating the power factor of the grid.
ns : velocity of the synchronous
machine
ns = 120 f / p f : frequency of the source
p : number of poles
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Asynchronous Machine
• Mono Phase Induction Machine
• Has only one stator coil.
• Uses only one phase.
• Rotor of an asynchronous machine can be a squirrel
cage.
• Needs a unit to start to motor.
• Are used in applications needs 3 ~ 4 HP (Fans, washing
machines, household devices… etc.)
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Asynchronous Machines
• Three Phase Induction Machine
• Magnetic field is generated by three phases
• Rotor can be either squirrel cage or composed of coils
• Can be started easily
• Has great power capacities
• There are applications from 1/3 HP to hundreds of
HPs: Pumps, compressors, conveyor drums, grinding
machines and etc.
• More than 70 % of the motors in industry are three
phase induction machines.
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Three Phase Asynchronous Machine
Industrial loads or high
power loads are
needed to be
connected to three
phase grid whose
phases follow each
other in 120 degrees
instead of mono phase
grid. Result of this
usage is smaller
currents.
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Three Phase Asynchronous Machine
t1
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Three Phase Asynchronous Machine
Rotating Magnetic Field:
In a three phase AC motor, a rotating field might be achived using
the coils which are located geometrically around stator (see Figure
below).
Terminals for a three phase asynchronous
machine:
Phase R input terminal: U, output terminal X
Phase S input terminal: V, output terminal Y
Phase T input terminal: W, output terminal Z
t1
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Three Phase Asynchronous Machine
Rotating Magnetic Field in a Three Phase Machine
i,u
1200
R
1200
S
T
1200
1
t1
3
t2
t3
t4
t5
t6
2
t1
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Three Phase Asynchronous Machine
Rotating Magnetic Field in a Three Phase Machine
t2
t1
Time Interval
t1
t2
t3
t4
t5
t6
t3
t4
t5
t6
IR IS IT
+
+
0
0
0
+
+
0
-
0
0
+
+
t1
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Three Phase Asynchronous Machine
WYE Connection
t1
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Three Phase Asynchronous Machine
Delta Connection
t1
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Three Phase Asynchronous Machine
Three Phase Asynchronous Motor
If the frequency of the flowing current is f ,the number of
rotation (or synchronous number of rotation or nember of
rotation of rotating field) is n. Equation of the number of
rotation of magnetic field is given below in unit of RPM.
Ns = 60 f / p
f [Hz]: Frequency of the source
p [ ]: Number of pole pairs
Three phase asynchronous machines do not form sparks. Their
number of rotation do not change so much with changing
loads. Thus they are said to be constant speed motors. Thus,
they are called as constant speed machines. Their efficiencies
are high. If the three phase grid is not present then the
monophase motors are used.
t1
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Three Phase Asynchronous Machine
Speed and Slip
In an asynchronous motor, the speed of the magnetic field
generated by the stator coils and the rotation speed of the rotor is
not the same. The value of the rotational speed of the rotor is
always smaller than the speed of the stator’s magnetic field. The
reason of the word ‘asynchronous ’ is this. The difference of these
speed is called as the slip. If ‘s’ is negative (rotor’s speed is greater)
then the electric machine is running as a generator.
The slip s is defined as 'the
s = [(Ns – Nr)/ Ns ] x 100
difference between synchronous
s [ %]: Slip
Ns [RPM]: Speed of the magnetic field.
Nr [RPM]: Rotational speed of the rotor
t1
speed and operating speed, at
the same frequency, expressed
in rpm or in percent or ratio of
synchronous speed'.
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Asynchronous Machine
Efficiency – Speed – Torque Curves
t1
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Asynchronous Machine
Slip Ring Type
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Losses in Electric Motors
Losses
Notation
Losses of mechanical
Pks
frictions
Iron loss(hysteresis and
PkFe
eddy current losses)
Ohmic power loss of
Pka
armature
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Losses in Electric Motors
Losses in Asynchronous Motors
Pe
3
Pm
Losses
Friction and air flow
losses
Iron loss
Loss of conductor
(stator - copper)
Loss of conductor
(rotor - alluminium)
Additinal load loss
2
1
4
1
5
Pfw
PXL
PR
PS
Pfe
Notation
Pfw
Pfe
PS
PR
PXL
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Losses in Electric Motors
Losses in Asynchronous Motors
Pe
Friction and Air Flow
They are constant losses during motor run,
independent from load and occur in bearings and
cooling fan propellers.
Pm
Iron Loss
Total affects of losses in cores of coils (hysteresis and eddy
current losses). It can be neglected even the rotor composed of
coils since the frequency of the induced voltage is low. It might
be observed as heat in laminated cores when the motor is
running. It is dependent to the material, thickness and
dimensions of the laminated core, the frequency applied to the
motor and the square of the voltage applied to the motor. It is
constant if the frequency and the voltage that the motor is
connected do not change.
Pfw
PXL PR
PS
Pfe
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Losses in Electric Motors
Losses in Asynchronous Motors
Pe
Conductor Loss (Stator)
It is heat loss. The current flow through the stator
coils creats this loss (I2RS ).
Pm
Conductor Loss (Rotor)
It is heat loss. The current flow through the stator coils or
cage bars creats this loss (I2RR ).
Additional Load Loss
It is the loss occurs in metal parts of the motor except
the laminated cores in rotor and stator because of the
leakage because of the load.
Losses
Friction and Air Flow Losses
Iron loss
Conductor loss (stator)
Conductor Loss (rotor)
Additional load losses
% 0,5 ~ 1,5
% 1,5 ~ 2,5
% 2,5 ~ 4,0
% 1,5 ~ 2,5
% 0,5 ~ 2,5
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PS
Pfe
Pfw
PXL PR
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Power Calculations in Electric Motors
Colours of wires (TS 6429)
i,u
R
t1
t2
S
t3
t4
T
t5
Blue- Brown – Black – Gray –
Yellow+Green
t6
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Power Calculations in Electric Motors
The nominal power of a DC motor might be expressed as the
equation below. UDC [V] is the voltage applied to the motor, and IDC
[A] is the current flow. Pinput [W] is the electrical power, Poutput [W] is
the mechanical poweror the nominal power, ωm [RPM] is the
rotational speed of the axle of the motor, Tm [Nm] is the torque
generated by the motor. Ploss [W] is the power loss, η [%] is the
efficiency of the motor.
Pinput =UDC IDC
Poutput =ωm Tm
Ploss = Pinput − Poutput
η=( Poutput / Pinput )x100
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Power Calculations in Electric Motors
In AC motors, because of the changing current characteristics, there is
an important point that , there are three powers called as apperant,
true and reactive. In AC motors, current is lagging voltage with angle
φ. This divides the power into two vector parts.
I [Amper]
[VAC]
[Hz]
Lm
Rm
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Power Calculations in Electric Motors
Apparent (imaginary)
Power
S=IU
Reactive (blind)
Power
Q = U I sinφ
[VAR]
[VA]
ϕ=P/S
True (real) Power
P = U I cos φ
[Watt]
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Power Calculations in Electric Motors
Example: A mono phase asynchronous motor draws 12.3[A] from
grid and its power factor is measured as 0.94. What are the powers
consumed?
Apparent Power= S = U I = 220 x 12,3 = 2706 [kVA]
Active Power = P = U I Cos φ =220 x 12,3 x 0,94 = 2,833 [kW]
Reactive Power = Q = U I sin φ or
= 1,028 [kVAR]
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Power Calculations in Electric Motors
Power Calculations in Three Phase Electric Motors
In a balanced three phase circuit:
P = √ 3 x U x I x cos φ
Q = √ 3 x U x I x sin φ
S=√3xUxI
P : True Power [Watt] ;
Q : Reactive Power [VAR];
S : App. Power [VA]
U : 380 [V] phase to phase voltage: 380 [V].
I : Current drawn from one phase: [A]
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Power Calculations in Electric Motors
Power Calculations in Three Phase Electric Motors
The current drawn from the three phase grid by an asynchronous
alternate current motor is 7 [A] and the power factor of the motor
is measured as 0.85. What are the powers consumed from the
grid?
P : True Power [Watt] ; Q : Reactive Power [VAR]; S : App.Power [VA]
U : 380 [V] Voltage btw. Phases: 380 [V]
I : Current drawn from one phase: [A]
Power in a balanced three phase circuit:
P = √ 3 x U x I x cos φ = √ 3 x 380 x 7 x 0,85 = 3916 [W]
Q = √ 3 x U x I x sin φ = √ 3 x 380 x 7 x 0,5268 = 2427 [VAR]
S = √ 3 x U x I = √ 3 x 380 x 7 = 4607 [VA]
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References:
1. http://ocw.mit.edu
2. http://en.wikipedia.org
3. http://www.energyefficiencyasia.org
4. www.amidesign.ch
5. Asenkron Elektrik Motorları, Ali Taner, 2011.
6. http://avstop.com/ac/apgeneral/typesofacmotors.html
7. http://www.daviddarling.info/encyclopedia/E/electric_motor.html
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