VHzsäätö

VHzsäätö, commonly written V/Hz-säätö, is a basic control strategy used in variable-frequency drives (VFDs) to regulate alternating current (AC) motors, particularly induction motors. The goal is to keep the motor’s magnetic flux approximately constant as speed changes, by maintaining a constant ratio between the applied voltage and frequency (V/f).

Principle: In V/Hz-säätö, the stator voltage U is adjusted in proportion to the applied frequency f, so that U/f stays near its rated value. At low speeds both voltage and frequency are reduced to keep flux constant. At the rated speed, the voltage reaches its maximum, and beyond that speed the controller increases frequency while holding voltage at its rated level. This causes the V/f ratio to fall, flux to drop slightly, and torque to diminish if demands rise; the motor can still run but with limited acceleration and torque.

Implementation: The method is implemented in VFDs using power electronics that generate a PWM-modulated AC waveform from DC supply. The control loop can be open-loop (no speed sensors) or sensorless/with sensors. V/Hz control is valued for its simplicity, robustness and low cost, making it suitable for a wide range of applications.

Characteristics and limitations: V/Hz-säätö provides smooth torque at low to moderate speeds and is well suited for fans, pumps, conveyors and similar loads with relatively simple dynamic requirements. Its dynamic response and speed regulation are modest compared to more advanced methods. Above base speed, torque capability decreases due to voltage limitations, and precise speed control under varying load is more challenging.

Compared approaches: Field-oriented control (FOC) and direct torque control (DTC) offer higher dynamic performance and better torque regulation, but at higher cost and complexity. V/Hz-säätö remains a common baseline technique for cost-effective motor control.