AC motors are often paired with variable frequency drives (VFDs), which control the speed of the motor by adjusting the frequency of the supply voltage. Depending on the application and the level of speed control required, VFDs can be controlled using either scalar or vector methods. The most common type of VFD control is the scalar method, called volts/hertz (V/Hz) or volts/frequency (V/ph).
The terms variable frequency drive (VFD) and variable speed drive (VSD) are often used interchangeably, but there is a difference between the two.
A variable speed drive (VSD) is any drive that can control the speed of a device, including AC and DC motors. VSDs can be mechanically, hydraulically, or electrically driven.
Variable frequency drives (VFDs) are used to control the speed of AC motors by varying the frequency of the voltage supplying the motor.
AC motors are designed for a magnetic field (flux) of constant strength. The strength of the magnetic field is proportional to the ratio of voltage (V) to frequency (Hz), or V/Hz. But a frequency converter controls the speed of the motor by changing the frequency of the applied voltage. According to the synchronous speed equation:
Changing the voltage frequency affects the motor speed and the magnetic field strength. When the frequency decreases (the motor speed decreases), the magnetic field increases and generates excess heat. When the frequency increases (the motor speed increases), the magnetic field decreases and the generated torque decreases. In order to maintain a constant magnetic flux, the V/Hz ratio must remain constant. This allows the torque to be maintained constant regardless of the frequency.
V/Hz control maintains a constant relationship between voltage (V) and frequency (Hz). Image source: Square D
Voltage/frequency control of VFDs avoids variations in magnetic field strength by varying the voltage and frequency to maintain a constant voltage/frequency ratio. The appropriate V/Hz ratio is determined by the rated voltage and frequency of the motor. For example, a motor rated for 230 V and 60 Hz will operate best with a V/Hz ratio of 3.83 (230/60 = 3.83).
Traditional V/F control does not use feedback and changes the motor voltage and frequency based only on an external speed command. For closed-loop V/F control, feedback from an encoder can be added to measure the actual speed of the motor. Based on the difference between the actual speed and the reference speed, an error signal is generated, and the controller generates a new frequency command to compensate for the error. Although closed-loop V/F control can improve speed regulation, it is rare due to the additional cost and complexity of encoders and feedback equipment.
Voltage/frequency (V/Hz) control is a simple and inexpensive method of controlling variable frequency drives and is generally considered the most common VFD control scheme. It is suitable for constant and variable torque applications and can provide up to 150% of rated torque at zero speed during starting and peak loads. The speed control range is 2% to 3% of the maximum rated frequency, so this method is not suitable for applications where precise speed control is critical. The most common application of voltage/frequency control is the control of industrial equipment such as fans and blowers.
A unique advantage of voltage/frequency control over other methods is that it allows one VFD to control multiple motors. All motors will start and stop at the same time, and they will all run at the same speed, which is very useful in certain process applications such as heating and cooling.
The V/Hz control method allows one variable frequency drive to control four motors in a cooling tower. Image source: variablefrequencydrive.org
As we have just described in detail, the scalar V/Hz or V/f control method varies the voltage (V) and frequency (f) of the motor supply to maintain a fixed constant ratio between them, so that the magnetic field strength remains constant regardless of the motor speed. The corresponding V/Hz ratio is equal to the rated voltage of the motor divided by its rated frequency. V/f control is usually implemented without feedback (i.e., open-loop), although closed-loop V/f control (including motor feedback) can be implemented. Scalar control provides speed control within only 2–3% of the rated motor frequency, so these methods are not suitable for applications requiring precise speed control. Open-loop V/f control is unique in that it allows a single VFD to control multiple motors and is perhaps the most commonly used VFD control method.
In contrast, vector control, also known as field-oriented control (FOC), controls the speed or torque of an AC motor by controlling the spatial vector of the stator current in a manner similar (but more complex) to DC control methods. Field-oriented control uses complex mathematical techniques to transform a three-phase, time- and speed-dependent system into a two-axis (d and q), time-invariant system. For more information on comparing scalar and vector VFD control methods, see the Motion Control Tips article: What are the main VFD control methods for AC motors?
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Post time: Feb-08-2025