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)
Synchronous Motors (continued)
• Operation
– Magnetic field of the rotor “locks” with the
rotating magnetic field – rotor turns at
synchronous speed
Cylindrical (Round) Rotor
Constructed from solid steel forging to withstand large
centrifugal stresses inherent in high-speed operation
Used for high speed, low inertia loads (low starting torque)
Salient-Pole Rotor
Excitation Windings
Salient-Pole Rotor
with shaft-mounted DC exciter
Need carbon brushes to make
contact with the commutator
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
The VARISTOR or resistor in shunt with the field winding prevents high
voltage from being induced during locked-rotor and acceleration.
The induced current helps to accelerate the rotor by providing additional
torque.
Brushless Excitation
How it works
• Frequency-sensitive Control circuit
–
–
–
–
–
Looks at emf induced in the field
fr = sfs
At locked-rotor, s=1, fr = fs
Open SCR1 – block current from field
Close SCR2 – connect discharge resistor across the
field
How it works (continued)
• As the speed approaches ns, fr gets small,
fr = sfs approaches 0
– Open SCR2 – disconnects the discharge resistor
– Close SCR1 – allows field current to flow
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 DC current 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
Armature-Reaction Voltage
• Rotating armature flux, ar is caused by the
three-phase stator currents. The induced speed
voltage caused by the flux ar cutting the stator
conductors.
• Ear = nsarka
Armature-Reaction Voltage (continued)
• Ear = nsarka
• ar proportional to armature current Ia
• Ear = (Ia)(jXar)
– where Xar = armature reactance (Ω/phase)
Equivalent Circuit of a Synchronous Motor
Armature (One Phase)
V  I R  I jX  I X  E
T
a
a
a
X X X
s
l
l
a
ar
V  E  I (R  jX )
T
f
a
V E I Z
T
f
a
a
s
s
ar
f
Phasor Diagram for one phase of a
Synchronous Motor Armature
Synchronous-Motor Power Equation
• In most cases, the armature resistance is
much smaller than the synchronous
reactance, so the synchronous impedance
Zs is approximately equal to jXs .
The Equivalent-Circuit and Phasor Diagram
IaXscosθi = -Efsinδ
The Synchronous-Motor Power Equation
• VTIacosθi = -(VTEf/Xs)sinδ
• VTIacosθi = power input per phase = Pin,1Φ
• -(VTEf/Xs)sinδ = magnet power per phase
developed by a cylindrical-rotor motor
(a function of Ef and δ)
• Pin,1Φ = -(VTEf/Xs)sinδ is the synchronousmachine power equation
• For three phases,
– Pin = 3(VTIacosθi)  proportional to Iacos θi
– Pin = 3(-VTEf/Xs)sinδ  proportional to Efsinδ