- Synchronous Motor Definition: A synchronous motor is defined as a constant speed motor that runs at the synchronous speed of the supply, often used for constant speed operations and power factor improvement.
- Speed Control Factors: The speed of a synchronous motor depends on the supply frequency and the number of poles, with frequency adjustment being the practical method for speed control.
- Open Loop Control: Inverter fed open loop synchronous motor drive uses variable frequency without feedback, suitable for less precise speed control needs.
- Closed Loop Operation: Self-synchronous (closed-loop) operation provides accurate speed control by adjusting frequency based on rotor speed feedback, avoiding oscillations.
- Speed Control of Synchronous Motor: Speed control of synchronous motor is achieved through varying the supply frequency using solid-state devices, rectifiers, and inverters.
Speed control of a synchronous motor means changing the lock-in speed. Synchronous motors run at that speed for a given supply frequency and pole count. They are used where shaft speed must stay fixed. Some machines also run light to improve power factor. Large units can have lower loss than induction motors of the same rating, but that is a design result, not a rule for every size.
The speed of a synchronous motor is given by 
where f = supply frequency and p = number of poles.
Synchronous speed depends on supply frequency and the number of poles on the rotor. Changing the pole count on a finished machine is rare. With solid-state converters the stator current can be supplied at a chosen frequency, and that sets the shaft speed.
A combination of rectifiers and inverters is the usual industrial method. Large machines also use cycloconverters or load-commutated inverters. Two textbook arrangements are:
Inverter Fed Open Loop Synchronous Motor Drive
Here the synchronous motor is supplied by a variable frequency inverter in open loop. Open loop means the inverter has no rotor-position feedback. This method suits drives that do not need tight speed regulation. The mains feed a rectifier-inverter set at the chosen frequency, and synchronous speed follows that frequency.
In the figure above, the three-phase mains are converted to DC by rectifiers. An LC filter smooths the DC. That DC feeds inverters. The inverter may be a voltage source inverter or a current source inverter. Changing inverter frequency gives speed control of synchronous motor. Open loop is useful when several motors must run at the same set speed. The drawback is hunting: the rotor can oscillate about synchronous speed if the frequency command is independent of rotor angle.
Self Synchronous (Closed – Loop) Operation
Self-synchronous (closed-loop) operation locks inverter firing to rotor position. Speed is then set by the DC link or current reference, not by an independent frequency command. Rotor speed is compared with a preset, and the rectifier/inverter pair is adjusted so the stator field stays in step. If load rises, the drive changes voltage and current while the firing angle still follows the rotor, so the machine cannot pull out of step the way an open-loop V/f set can. Hunting from an independent frequency command is avoided.
