Synchronous Motor
AC Motor
An alternator run in reverse: the DC-excited rotor locks step with the rotating stator field and turns at exactly synchronous speed regardless of load — and, over-excited, it behaves as a power-factor-correcting 'synchronous condenser'.
Schematic symbol
Motor
How synchronous motor appears in a circuit diagram.
Types & variants
Key specs
Speed
RPMExactly Ns = 120f/P — zero slip. Load changes swing the torque angle δ, not the speed.
Excitation
V/A (DC)Rotor field supply. Under-excited it draws lagging current; over-excited, leading — the basis of the V-curves.
Power rating
kW / MVAUsed at large ratings (hundreds of kW to tens of MW) where its efficiency and PF control pay off.
Torque angle δ
degreesAngle between rotor and stator fields; torque ∝ sin δ, and beyond ~90° the motor pulls out of synchronism.
Pull-out torque
% of ratedMaximum torque before losing sync — typically 150–250% of full load.
Markings
Nameplate adds excitation voltage/current to the usual kW, V, A, RPM, PF data. PMSM/servo types instead specify torque constant and back-EMF constant.
Standard values
Line-start industrial machines: 500 kW–20 MW, 3.3/6.6/11 kV. PMSMs dominate at small sizes: washing machines, EV traction, servo drives, all inverter-fed.
How to choose
1) Choose it for very large constant-speed loads (compressors, mills) where high efficiency and unity/leading PF cut the power bill. 2) As a synchronous condenser purely for PF/voltage support. 3) For variable speed, a PMSM with a drive beats induction on efficiency and size. 4) Remember it's not self-starting — plan damper-winding (induction) starting, a pony motor, or an inverter ramp.
Pinout & package
Three-phase stator like an induction motor plus rotor field via slip rings or a brushless exciter. Damper (amortisseur) bars in the pole faces provide starting and stability. PMSMs look like BLDC motors: magnets on the rotor, three leads plus encoder/resolver.
Example circuits
- V-curve experiment: armature current vs field current at constant load
- Over-excited synchronous condenser compensating a factory's lagging power factor
- Started as an induction motor on damper bars, then field switched in to lock into sync
- PMSM + field-oriented-control inverter as an EV traction drive
Common failures
Loss of synchronism (pole slipping) from overload or excitation failure — violent current and torque pulsations; exciter or slip-ring problems; damper bar cracking from repeated starts; PMSM magnet demagnetisation from overheating.
How to test
Megger stator and field windings; verify field circuit continuity and exciter output; run the V-curve test to confirm excitation behaviour; for PMSM, spin the shaft and scope the balanced sinusoidal back-EMF on the three leads.
Substitutes
Induction motor + VFD for most drives below a megawatt; capacitor banks replace synchronous condensers at small scale (but can't adjust continuously). PMSM and BLDC are near-interchangeable terms at small sizes — sinusoidal vs trapezoidal drive is the practical difference.