Synchronous Motors and Generators

1
Synchronous Motors and Generators
2
Synchronous Motors

• Constant-speed machine
• Propulsion for SS Queen Elizabeth II
• 44 MW
• 10 kV
• 60 Hz
• 50 pole
• 144 r/min

3
Synchronous Motors (continued)

• Construction
• Stator identical to that of a three-phase
  induction motor now called the armature
• Energize from a three-phase supply and develop
  the rotating magnetic field
• Rotor has a DC voltage applied (excitation)
• Rotor could be a permanent-magnet type

4
Synchronous Motors (continued)

• Operation
• Magnetic field of the rotor locks with the
  rotating magnetic field rotor turns at
  synchronous speed

5
Salient-Pole Rotor
Excitation Windings
6
Salient-Pole Rotor with brushless excitation
7
Synchronous Motor Starting

• Get motor to maximum speed (usually with no load)
• Energize the rotor with a DC voltage

8
Salient-Pole Motor operating at both no-load and
loaded conditions
Angle d is the power angle, load angle, or torque
angle
9
Rotating Field Flux and Counter-emf

• Rotating field flux ?f due to magnetic field in
  the rotor. A speed voltage, counter-emf, or
  excitation voltage Ef is generated and acts in
  opposition to the applied voltage.
• Ef ns?fkf

10
Equivalent Circuit of a Synchronous Motor
Armature (One Phase)
11
Phasor Diagram for one phase of a Synchronous
Motor Armature
12
Synchronous Generators
13
Motor-to-Generator Transition
14
Motor-to-Generator Transition (cont)

• Begin with motor driven from the infinite bus and
  the turbine torque in the same direction as the
  motor torque.
• The motor operates normally, driving the water
  pump.

15
Motor-to-Generator Transition (cont)
Phasor Diagram
VT Ef IajXs
16
Allow the Turbine to take part load
Motor becomes a generator as d becomes gt or zero
Excitation voltage is not changed and the vector
traces an arc
The power angle decreases to zero and then
becomes positive
17
Motor Action
Power angle is negative
18
Motor to Generator Transition
Power angle is now 0
19
Generator Action
Power angle is positive
Note Iacos?i is reversed!
20
Generator Action (cont)

• In order for Ia to reverse direction, voltage Ef
  must become a source voltage
• Ef gt VT

Ia