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Synchronous Motors and Generators
Synchronous Motors
• Constant-speed machine
• Propulsion for SS “Queen Elizabeth II”
– 44 MW
– 10 kV
– 60 Hz
– 50 pole
– 144 r/min
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
Synchronous Motors (continued)
• Operation
– Magnetic field of the rotor “locks” with the
rotating magnetic field – rotor turns at
synchronous speed
Salient-Pole Rotor
Excitation Windings
Salient-Pole Rotor with brushless excitation
Synchronous Motor Starting
• Get motor to
maximum speed
(usually with no load)
• Energize the rotor
with a DC voltage
Salient-Pole Motor operating at
both no-load and loaded conditions
Angle δ is the power angle, load angle, or torque angle
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
Equivalent Circuit of a Synchronous Motor
Armature (One Phase)
V  I R  I jX  I X  E
X X X
V  E  I (R  jX )
V E I Z
Phasor Diagram for one phase of a
Synchronous Motor Armature
Synchronous Generators
Motor-to-Generator Transition
Motor-to-Generator Transition (cont)
• Begin with motor
driven from the infinite
bus and the turbine
torque in the same
direction as the motor
• The motor operates
normally, driving the
water pump.
Motor-to-Generator Transition (cont)
Phasor Diagram
VT = Ef + IajXs
Allow the Turbine to take part load
Motor becomes a generator
as δ becomes > or = zero
The power angle decreases to
zero and then becomes positive
Excitation voltage is not changed
and the vector traces an arc
Motor Action
Power angle is negative
Motor to Generator Transition
Power angle is now = 0
Generator Action
Power angle is positive
Note: Iacosθi is reversed!
Generator Action (cont)
• In order for Ia to
reverse direction,
voltage Ef must
become a source
• Ef > VT