AC motors are often paired with a frequency converter (VFD), which adjusts the frequency of the supply voltage to control the speed of the motor. Depending on the application and the level of speed control required, the VFD can be controlled using scalar or vector methods. The most common type of VFD control is a scalar method called volts/hertz (V/Hz) or volts/frequency (V/f).
The terms variable frequency drive (VFD) and variable speed drive (VSD) are often used interchangeably, but there are differences between them.
A variable speed drive (VSD) is any drive that can control the speed of a device, including AC and DC motors. Frequency converters can be controlled mechanically, hydraulically or electrically.
A variable frequency drive (VFD) is used to control the speed of an AC motor by varying the frequency of the motor supply voltage.
AC motors are designed for a magnetic field (flux) of constant strength. The magnetic field strength is proportional to the ratio of voltage (V) to frequency (Hz) or V/Hz. However, according to the synchronous speed equation, the VFD controls the speed of the motor by varying the frequency of the applied voltage:
Changing the voltage frequency affects the speed of the motor and the strength of the magnetic field. As the frequency decreases (motor speed decreases), the magnetic field increases and excess heat is generated. As the frequency increases (for higher motor speeds), the magnetic field decreases and less torque is produced. To maintain a constant magnetic flux, the V/Hz ratio must remain constant. This ensures torque stability regardless of frequency.
V/Hz control maintains a constant relationship between voltage (V) and frequency (Hz). Image source: Square D
V/Hz control of the VFD drive avoids changes in magnetic field strength by varying the voltage and frequency to maintain a constant V/Hz ratio. The corresponding V/Hz ratio is determined by the rated voltage and frequency of the motor. For example, a motor with a rated voltage of 230 V and a frequency of 60 Hz works best with a V/Hz ratio of always 3.83 (230/60 = 3.83).
Traditional V/Hz control does not use feedback and only changes the motor voltage and frequency in response to external speed commands. For closed-loop V/Hz control, you can add encoder feedback to measure the actual motor speed. An error signal is generated based on the difference between the actual speed and the set speed, and the controller generates a new frequency command to compensate for the error. Although this improves speed control, closed-loop V/Hz control is less common due to the increased cost and complexity of the encoder and feedback hardware.
Voltage/Hz control is a simple and inexpensive method of controlling a variable frequency drive and is generally considered the most common VFD control scheme. It is suitable for both constant and variable torque applications, delivering up to 150% of rated torque at zero speed for 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 requiring precise speed control. The most common application of V/Hz control is to control industrial equipment such as fans and blowers.
The unique advantage of V/Hz control over other methods is that it allows one VFD to control multiple motors. All motors will start and stop simultaneously and run at the same speed, which is beneficial for certain processes such as heating and cooling.
The V/Hz control method allows one VFD to control four cooling tower motors. Image source: variablefrequencydrive.org.
As we just detailed, the scalar V/H or V/f control method changes the voltage (V) and frequency (f) of the motor to maintain a fixed, constant ratio between the two, so no matter the speed of the motor, the magnetic field is all constant. The correct V/Hz ratio is equal to the motor’s rated voltage divided by its rated frequency. V/Hz control is typically implemented without feedback (i.e., open-loop), although closed-loop V/Hz control (including motor feedback) is also possible. The speed control range with scalar control is only 2% to 3% of the motor’s rated frequency, so these methods are not suitable for applications requiring precise speed control. Open-loop V/Hz 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 to (but more complex than) DC control methods. Field-based control uses complex mathematics to transform a three-phase time- and speed-dependent system into a two-axis (d and q) time-independent system. Read the Motion Control Tips article to learn more about how scalar and vector VFD control methods compare: What are the basic methods for controlling VFD AC motors?
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Post time: Dec-26-2023