sunclipa At any given moment, any motor operates under one of two conditions - constant torque, or constant (output) power.
DC motors achieve their constant torque range by varying the applied voltage (constant ratio between volt and rpm). Stationary field magnetic strength (i.e., field current) remains constant throughout. For the constant power range, the applied voltage is fixed (at the rated condition) and the field strength is reduced by lowering the field current.
AC motors work similarly. For constant torque, they generally vary the applied line voltage with frequency (a constant ratio). For variable torque, they hold the applied voltage constant and weaken the magnetic strength of the "field".
So
- DC motor. Reduce speed, maintain torque. = > Lower the voltage (linearly).
- AC motor. Reduce speed, maintain torque. = > Lower the voltage AND the frequency (linearly).
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