Download Foundations of Motors and Generators

Motors and Generators
©Dr. B. C. Paul 2001
1
More Fun with Flux
Mechanically Rotated Shaft
Slip Rings
Wires with brush
contacts to slip
rings
Electromagnetic Flux
The coil rotates through the flux enclosing
a changing amount of magnetic flux
Output Voltage
is
Sinusoidal AC
This induces a changing voltage in the coil.
Brushes contact the slip rings and put wires at the same
potential as the coil
2
This Machine Converts
Mechanical Energy to Electrical
This is a Generator
Note why mechanical to electrical conversion tends to produce an alternating current
3
More Practical Machine
By rotating the rotor
mechanically we can
create a rotating
magnetic flux
Rotor a big rotating
electromagnet
(Slip rings connect to a direct current that
magnetizes the rotor)
Stator - a ring
shaped iron shell
Current transfers onto
the rotor by means of
slip joints
The wire wraps around the rotor
creating magnetic flux that carries
through the rotor - across an air gap
4
and through the stator
Now What Will You Do?
We put three separate coils
through the stator
#1 Phase
#2 Phase
#3 Phase
The rotating flux is seen
as a changing flux in each coil
generating 3 voltage phases
This is the secret to 3 phase AC current production
5
This is Called a Synchronous Motor
• Three Phase Electric Current is run through the
stator
• The AC current produces a rotating magnetic flux
• We now Apply a DC current to the windings
around the rotor (via slip rings and brushes)
making it an electromagnet
• It follows the rotating magnetic field at
synchronous speed.
6
The Stator Current
• An AC voltage is applied to the Stator
• The Stator has an impedance Zs
• We also have an electromagnet sweeping around
the stator and inducing a voltage opposing the
applied voltage
Zs
Vt
Vg + Ia Zs = Vt
Generated back voltage
7
How Synchronous Motors Know
How Much Power to Draw
• The Rotor actually lags just behind the terminal
voltage (due to friction and load resistance)
• This means the induce voltage is slightly out of phase
with the terminal voltage
• The bigger the load the further the rotor is behind and
the more the induced voltage drops back out of phase
8
The Voltage Phase Effect
• When the induced voltage is way out of phase
with the applied voltage there is little cancellation
• Vg + Ia Zs = Vt
Constantof voltage and current
• Power is a product
• Voltage stayed the same
• Current went up - it draws more current under load
9
The AC Generator
(Called an Alternator)
• Coal is burned in a boiler
• The boiler makes high pressure super-heated steam
• The high pressure super-heated steam is cooled and depressurized through a steam turbine
• A governor on the turbine regulates rotation speed
• The turbine shaft goes through a gear reducer to
produce 3600 rpm rotation
• The Shaft connects to a rotor in an alternator that goes
through 60 rotations per second
• The rate of rotation controls the current frequency
10
The Alternator
• There are variations that have 4 poles on an X
shaped rotor instead of 2 poles on a bar shaped
rotor
• The advantage to 4 poles is that rotation need only be
1800 rpm
• Reduces wear on the brush contacts
• Can also build 6 poles and rotate at 1200 rpm
• Beyond this increasing poles makes a very big
impractical rotor
11
Alternators Can Work in Reverse
• Suppose a 3 phase current is passed through the
stator
• The rotor will act as a compass and try to align
with the magnetic field
• The rotor is now generating torque that can turn a
load
• An alternator running in reverse is called a
synchronous motor
12
Inductors
• Inductors allow conversion of electrical energy
back and forth from magnetic flux
• This ability allows us to build transformers that
are essential to moving power over electric lines
• The magnetic flux to electricity correlation allows
us to mechanically turn a magnetic flux and
produce electricity
• This is the foundation of motors and generators
13
Mining Applications
• In many mining applications the conversion of
electricity to kinetic energy is the primary
objective
• most of our equipment is built of inductors
14
Comments on Generation and
Transmission Voltage
• We have seen that motors and generators all
involve electric wiring in close proximity.
• The types of insulators needed to control real high voltages in
limited space are not always available
• This is the reason that power must be generated and used at limited
voltage
• With long range transmission we can build big
poles and space wires to use air insulating
• We need high voltage to control transmission loss and we can
get it.
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