Dc-excited AC Electric Motors (cont.) The
electric motor's reluctance torque results from the saliency (preferred
direction of magnetization) of the rotor pole pieces and pulsates
at speeds below synchronous. It also has an influence on electric
motor pull-in and pull-out torques because the unexcited salient-pole
rotor tends to align itself with the stator electric motor magnetic
field to maintain minimum magnetic reluctance. The electric motor's
reluctance torque may be sufficient to pull into synchronism a lightly
loaded, low-inertia system and to develop approximately a 30% pull-out
The electric motor's synchronous torque is torque developed after excitation is applied, and represents the total steady-state torque available to drive the load. It reaches maximum at approximately 70° lag of the rotor behind the rotating stator magnetic field. This maximum value is actually the pull-out torque.
Pull-out torque is the maximum sustained torque the electric motor develops at synchronous speed for one minute with rated frequency and normal excitation. Normal pull-out torque is usually 150% of full-load torque for unity-power-factor electric motors, and 175 to 200% for 0.8-leading-power-factor electric motors.
Pull-in torque of a synchronous electric motor is the torque that it develops when pulling its connected inertia load into synchronism upon application of excitation. Pull-in torque is developed during transition from slip speed to synchronous speed, as electric motors change from induction to synchronous operation. It is usually the most critical period in starting a synchronous electric motor. Torques developed by the amortisseur and field windings become zero at synchronous speed. At the pull-in point, therefore, only the reluctance torque and the synchronizing torque provided by exciting the field windings are effective.
Timing Electric Motors: Timing electric
motors are rated under 1/10 hp and are used as prime movers for
timing devices. Since the electric motor is being used as a timer,
it must run at a constant speed.
Electric Motor: Basics of AC Motor Design Engineering