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 RotorExcitation 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 – Close SCR1 – block current from field– Open 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 = nsfkf

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 = nsarka

Armature-Reaction Voltage (continued)

• Ear = nsarka

ar proportional to armature current Ia

• Ear = (Ia)(jXar)

– where Xar = armature reactance (Ω/phase)

Equivalent Circuit of a Synchronous Motor Armature (One Phase)

( )

T a a a l a ar f

s l ar

T f a a s

T f a s

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-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 synchronous-

machine power equation• For three phases,

– Pin = 3(VTIacosθi) proportional to Iacos θi – Pin = 3(-VTEf/Xs)sinδ proportional to Efsinδ