The Difference Between Variable Speed Drives and Variable Frequency Drives

VSDs and VFDs

About one-quarter of the world’s electrical energy is consumed by industrial electrical motors. In many cases, these motors are set to a steady power or torque output. This means that they can’t be powered up or down to suit system requirements and at times may be consuming much more power than the process requires.

Variable frequency drives (VFD) or variable speed drives (VSD) are devices used to regulate the speed and output torque of electrical motors. By varying the frequency and voltage of the power supply, the motor control speed can be adjusted to better suit the specific requirements of the application.

These units can be used on electric motors across a range of commercial and industrial applications including fans, pumping systems, air compressors, HVAC systems, appliances and more.

Benefits of VSDs and VFDs

When applied to commercial or industrial electrical motors, VFDs and VSDs provide a range of benefits. By matching the power supply to the process requirements, you can ensure that the motor is only using the electricity needed to carry out the work. Running the motor at the optimal speed helps to reduce energy consumption and electricity costs.

Reducing power consumption also helps to cut down on industrial carbon emissions without affecting process efficiencies.

VFDs and VSDs can also help to reduce operational strain by ensuring the motor isn’t working harder than it needs to. A VFD can also be used to gradually speed the motor up and down prior to turning the motor on and off. This reduces shock damage and long-term excessive wear on the motor.

Additionally, a VFD or VSD can provide precise control over acceleration, flow, monitoring, pressure, speed, temperature, tension, and torque. This can allow for better process control and improved precision and process sensitivity.

VSDs and VFDs

There are two main types of motor drive units used for controlling electrical motors: variable speed drives and variable frequency drives. While they essentially do the same thing (i.e. controlling motor speed) they do it in slightly different ways.

  • Variable speed drivesvariable speed drives
    VSDs change the speed of a motor by varying the input voltage. Therefore they can be used with both AC motors and DC motors. VSDs control the amperage and voltage being supplied to the motor. For AC motors, VSDs use a rectifier circuit to convert the AC to DC. The voltage of the DC can then be altered to change the speed of the motor.The process is controlled by a microprocessor that monitors the incoming voltage supply, speed set-point, DC link voltage, output voltage and current to ensure the motor operates within established parameters.
  • Variable frequency drivesvariable frequency drives
    VFDs also control motor speed, but they do it by altering the voltage and frequency and so can only be used with AC motors. Adjusting the frequency of an AC motor will control its speed, while changing the voltage will alter the torque. VFDs are commonly used to control start-up and slow-down motor speeds to reduce shock damage, voltage sag and start and stop wear and tear.

Understanding which type of until your system requires will ensure you can get the most out of your speed control system.