Synchronous Motor Characteristics

Synchronous Motors are three-phase AC motors which run at synchronous speed, without slip.
(In an induction motor the rotor must have some “slip”. The rotor speed must be less than, or lag
behind, that of the rotating stator flux in order for current to be induced into the rotor. If an induction
motor rotor were to achieve synchronous speed, no lines of force would cut through the rotor,
so no current would be induced in the rotor and no torque would be developed.)
Synchronous motors have the following characteristics:
· A three-phase stator similar to that of an induction motor. Medium voltage stators are often used.
· A wound rotor (rotating field) which has the same number of poles as the stator, and is supplied by an external
source of direct current (DC). Both brush-type and brushless exciters are used to supply the DC field current to
the rotor. The rotor current establishes a north/south magnetic pole relationship in the rotor poles enabling the
rotor to “lock-in-step” with the rotating stator flux.
· Starts as an induction motor. The synchronous motor rotor also has a squirrel-cage winding, known as an
Amortisseur winding, which produces torque for motor starting.
· Synchronous motors will run at synchronous speed in accordance with the formula:

Synchronous RPM =(120 x Frequency)\(Number of Poles)

Example: the speed of a 24 -Pole Synchronous Motor operating at 60 Hz would be:
120 x 60 / 24 = 7200 / 24 = 300 RPM
Synchronous Motor Operation
· The squirrel-cage Amortisseur winding in the rotor produces Starting Torque and Accelerating Torque to
bring the synchronous motor up to speed.
· When the motor speed reaches approximately 97% of nameplate RPM, the DC field current is applied to the
rotor producing Pull-in Torque and the rotor will pull-in -step and “synchronize” with the rotating flux field in
the stator. The motor will run at synchronous speed and produce Synchronous Torque.
· After synchronization, the Pull-out Torque cannot be exceeded or the motor will pull out-of-step. Occasionally,
if the overload is momentary, the motor will “slip-a-pole” and resynchronize. Pull-out protection must be
provided otherwise the motor will run as an induction motor drawing high current with the possibility of
severe motor damage.
Advantages of Synchronous Motors
The initial cost of a synchronous motor is more than that of a conventional AC induction motor due to the
expense of the wound rotor and synchronizing circuitry. These initial costs are often off-set by:
· Precise speed regulation makes the synchronous motor an ideal choice for certain industrial processes and as a
prime mover for generators.
· Synchronous motors have speed / torque characteristics which are ideally suited for direct drive of large horsepower,
low-rpm loads such as reciprocating compressors.
· Synchronous motors operate at an improved power factor, thereby improving overall system power factor and
eliminating or reducing utility power factor penalties. An improved power factor also reduces the system voltage
drop and the voltage drop at the motor terminals.