# Scalar control of motor

By Dmitry Levkin
Scalar control (frequency control) is a control technique of a brushless AC motor, which is to keep the voltage/frequency ratio (V/Hz) constant throughout the full operating speed range, with control only the magnitude and frequency of the supply voltage.

The V/Hz ratio is calculated based on the rated values ​​(voltage and frequency) of the controlled AC motor. By keeping the value of the V/Hz ratio constant, we can maintain a relatively constant magnetic flux in the motor gap. If the V/Hz ratio increases, then the motor becomes overexcited and vice versa if the ratio decreases the motor is in an unexcited state.

The dependence of the power supply frequency of the electric motor from time for scalar control
Voltage variation of the motor power during scalar control

At low speed, it is necessary to compensate the voltage drop across the stator resistance, therefore, the V/Hz ratio at low speed is set higher than the rated value. The scalar control technique is most widely used to control induction electric motors.

## As applied to induction motors

At the scalar control method, the induction motor speed is controlled by setting the voltage and frequency of the stator, so that the magnetic field in the gap is maintained at the desired value. To maintain a constant magnetic field in the gap, the V/Hz ratio must be constant at different speeds.

Scalar control without speed sensor

As the speed increases, the stator supply voltage must also increase proportionally. However, the synchronous speed (frequency) of the induction motor is not equal to the rotation speed (frequency) of the shaft, and the slip of the induction motor depends on the load. Thus, the controller with scalar control without feedback cannot accurately control the speed when there is a load. To solve this problem, speed feedback and, therefore, slip compensation can be added to the control system [2].

Scalar control with speed sensor

## When used

Scalar control of AC motors is a good alternative for applications where there is no variable load and no good dynamics (fans, pumps) are required. The scalar control does not require a rotor position sensor, and the rotor speed can be estimated from the frequency of the supply voltage. When scalar control is used, a high-performance digital signal processor is not required, as is the case with vector control.