Q6Advanced Electric Drives

Stator Voltage Control for Speed Control of Induction Motor

Stator voltage control achieves induction motor speed control by varying the magnitude of the applied stator voltage (typically using a thyristor-based AC voltage controller, i.e., back-to-back thyristor pairs in each phase with controllable firing angle) while keeping the supply frequency fixed at its rated value. Since the induction motor's developed torque at any given slip is proportional to the square of the applied stator voltage (T proportional to V^2, following directly from the induction motor torque equation, since torque depends on the square of the rotor current, which itself is proportional to the applied stator voltage), reducing the stator voltage shifts the entire torque-slip characteristic curve downward (scaled by V^2) while the synchronous speed (which depends only on frequency, not voltage) remains unchanged.

For a given constant-torque load, reducing the stator voltage therefore forces the motor to operate at a new equilibrium point with higher slip (lower speed) where the reduced torque-slip curve intersects the load torque line, achieving speed reduction below the voltage-unreduced operating speed. However, because torque falls off as the square of voltage while slip must increase substantially to restore torque balance with a constant-torque load, stator voltage control can only provide a relatively narrow useful speed control range before the required voltage reduction becomes so severe that the motor's torque-slip curve can no longer intersect the load line at all (motor stalls), or before the substantially increased slip causes excessive rotor heating (since rotor copper loss is proportional to slip power, itself proportional to slip times air-gap power).

For this reason, stator voltage control is generally best suited to applications with a fan- or pump-type load (where load torque itself falls off rapidly at reduced speed, roughly following a square-law torque-speed characteristic, meaning the reduced torque-slip curve and the reduced load-torque curve can intersect at a stable, acceptable operating point over a reasonable speed range without excessive rotor heating), rather than constant-torque loads, where the required voltage reduction and resulting slip/rotor-heating penalty would be much more severe - this application-specific suitability, combined with the fundamentally limited speed range and reduced efficiency (due to increased rotor copper loss at higher slip) compared to variable-frequency drive methods, is why stator voltage control, while simple and low-cost to implement, has been largely superseded by variable-frequency (V/f or vector-controlled) drives for applications requiring a wide, efficient speed control range.