AC motors are often paired with a Variable Frequency Drive (VFD) that regulates the speed of the motor by adjusting the frequency of the supply voltage. Depending on the application and the required level of speed control, variable frequency drives can be controlled by scalar or vector methods. The most common type of VFD control is the scalar method, called Volt/Hertz (V/Hz) or Volt/Frequency (V/f).
The terms Variable Frequency Drive (VFD) and Variable Frequency 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 equipment, including AC and DC motors. Frequency converters can be operated mechanically, hydraulically or electrically.
A frequency converter (VFD) is used to control the speed of an AC motor by changing the frequency of the motor supply voltage.
AC motors are designed for a constant magnetic field (flux). The strength of the magnetic field 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 changing the frequency of the applied voltage:
Changing the voltage frequency affects the motor speed and field strength. When the frequency decreases (lower motor speed), the magnetic field increases and excess heat is generated. As frequency increases (for higher motor speeds), the magnetic field decreases and less torque is generated. In order for the magnetic flux to remain constant, the V/Hz ratio must be kept constant. This maintains stable torque regardless of frequency.
The V/Hz control maintains a constant relationship between voltage (V) and frequency (Hz). Image credit: Square D
The V/Hz drive control of the VFD avoids changes in magnetic field strength by varying the voltage and frequency to maintain a constant V/Hz ratio. The correct V/Hz ratio is determined by the rated voltage and frequency of the motor. For example, a motor rated at 230V and 60Hz will perform best when the V/Hz ratio is always 3.83 (230/60 = 3.83).
Conventional V/Hz control does not use feedback, but only changes the voltage and frequency of the motor according to an external speed command. For closed loop V/Hz control, encoder feedback can be added to measure the actual speed of the motor. 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. Despite improvements in speed control, closed loop V/Hz control is rare due to the additional cost and complexity of the encoder and feedback hardware.
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 both constant and variable torque applications and can provide 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 where accurate speed control is critical. Most often, V/Hz control is used 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 at the same time, and they will all run at the same speed, which is beneficial for certain processing operations such as heating and cooling.
The V/Hz control method allows one VFD to control four motors in a cooling tower. Image source: variableFrequencydrive.org
As we just detailed, the scalar V/Hz or V/f control method changes the voltage (V) and frequency (f) of the motor to maintain a fixed, constant relationship between the two so that the magnetic field is constant. The proper V/Hz ratio is equal to the rated voltage of the motor divided by its rated frequency. V/Hz control is typically open-loop (i.e., open-loop), although closed-loop V/Hz control (including motor feedback) is possible. The speed control range of scalar control is only 2% to 3% of the motor rated frequency, so these methods are not suitable for applications requiring precise speed control. Open loop V/Hz control is unique in its ability to allow one 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 space vector of the stator current in a manner similar to (but more complex than) DC control methods. Field-oriented control uses complex mathematics to convert a three-phase, time- and speed-dependent system into a two-coordinate (d and q) time-invariant system. For a detailed comparison of scalar and vector VFD control methods, see the article Motion Control Tips: What are the main VFD control methods for AC motors?
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Post time: Jul-31-2023