A Look At Torque in Direct-Current-Excited Electric Motors

Pull-Up Torque

An electric motor's pull-up torque is defined as the minimum torque created from a standstill to the pull-in point. This torque must exceed the load torque enough to maintain a satisfactory acceleration rate under normal voltage conditions.

 

Reluctance Torque

A motor's reluctance torque is the result of the rotor pole pieces' saliency, which is the preferred direction of magnetization. It pulsates at speeds below synchronous.

 

Reluctance torque influences the motor's pull-in and pull-out torques as the unexcited salient-pole rotor tends to align with the stator electric motor's magnetic field to maintain minimum magnetic reluctance. An electric motor's reluctance may be enough to pull a lightly loaded, low-inertia system into synchronism and develop a pull-out torque of approximately 30 percent.

 

Synchronous Torque

An electric motor's synchronous torque is the torque created after the application of excitation. It represents the total steady-state toque available to drive a load. The torque maxes out at about 70 lag of the rotor behind the rotating stator magnetic field. The maximum value, however, is the pull-out torque.

 

Pull-Out Torque

Pull-out torque is the maximum sustained torque an electric motor develops at a synchronous speed for one minute with rated frequency and normal excitation. Normal pull-out torque is typically 150 percent of full-load torque for unity-power-factor electric motors. It's 175 to 200 percent for 0.8-leading-power-factor electric motors.

 

Pull-In Torque

A synchronous motor's pull-in torque is the torque developed when pulling the connected inertia load into synchronism upon the application of excitation. It's developed during the transition from slip speed to synchronous speed, as electric motors change from induction to synchronous operation. This tends to be the most critical period when starting a synchronous motor. At synchronous speed, the torque developed by the amortisseur and field windings becomes zero. As a result, only the reluctance and synchronizing torque provided by exciting the field windings are effective at the pull-in point.