All motors have a “torque vs speed” characteristic.

DC machines are very simple: constant torque from zero speed to some “base speed”, and then a “constant power” ranging from base speed to top speed. In the constant torque range, acceleration is dependent on applied voltage, with the field under constant full current excitation. In the constant power range, voltage is held constant and the field current is reduced, thereby achieving an increase in speed (hence the term “field weakening”).

AC machines are somewhat more complex, since the curves are nowhere near as linear. The key points are:

– “starting torque”, which is the torque achieved at the locked rotor (zero speed) condition

– “pull in torque”, which is the available machine torque at the point where the machine pulls into synchronism (synchronous machines only)

– “pull out or breakdown torque”, which is the peak torque the machine can sustain momentarily before stalling

– “load torque”, which is the amount of torque actually required by the process at any operating point

– “accelerating torque”, which is the difference between what the machine is capable of producing and the load torque

A machine is rated for the “full load torque” condition which is the rated torque performance of the machine. In imperial (lb.ft) units, that would be 5252 * HP / RPM. It can produce this torque continuously, provided it has the rated conditions of applied terminal voltage and applied terminal current (for both rotor and stator, as applicable).

The time required to start a motor is dependent primarily on the accelerating torque available and the combined inertia (motor + remainder of drive train).

Note that available starting and pull-in torque during the transient operation of starting is proportional to the square of the applied voltage – if the voltage dips below 1.0 per unit, the available torque will be significantly reduced.

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