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 = sf s –At locked-rotor, s=1, f r = f s –Close SCR1 – block current from field –Open SCR2 – connect discharge resistor across the field

How it works (continued) As the speed approaches n s, f r gets small, f r = sf s 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 E f is generated and acts in opposition to the applied voltage. E f = n s  f k f

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. E ar = n s  ar k a

Armature-Reaction Voltage (continued) E ar = n s  ar k a  ar proportional to armature current I a E ar = (I a )(jX ar ) –where X ar = armature reactance (Ω/phase)

Equivalent Circuit of a Synchronous Motor Armature (One Phase)

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 Z s is approximately equal to jX s.

The Equivalent-Circuit and Phasor Diagram I a X s cosθ i = -E f sinδ

The Synchronous-Motor Power Equation V T I a cosθ i = -(V T E f /X s )sinδ V T I a cosθ i = power input per phase = P in,1Φ -(V T E f /X s )sinδ = magnet power per phase developed by a cylindrical-rotor motor (a function of E f and δ) P in,1Φ = -(V T E f /X s )sinδ is the synchronous- machine power equation For three phases, –P in = 3(V T I a cosθ i )  proportional to I a cos θ i –P in = 3(-V T E f /X s )sinδ  proportional to E f sinδ