Variable Frequency Drive (VFD) vs Variable Speed Drive (VSD)

What is the difference between Variable Frequency Drive’s (VFDs) and Variable Speed Drive’s (VSDs)?

There are two primary methodologies for controlling electrical motors with subtle differences:

– Variable Speed Drives (VSDs): Controls the speed of a motor by changing the input voltage and can be used with both AC and DC motors.

– Variable Frequency Drives (VFDs): Controls the speed of a motor by changing both the voltage and frequency and can only be used with AC motors.

Both VSDs and VFDs are used to decrease an electric motor’s unnecessary power consumption and optimise the efficiency and operation of electric motor systems, but VFDs refers to AC drives only and VSDs refers to either AC or DC drives.

VFDs control the frequency (control speed) and voltage (control torque) of the AC signal.

VSDs supply specific amperage and voltage to a motor. If the power supply is in AC form, VSDs utilise a rectifier circuit to convert the AC to DC at a specified voltage and amperage – changing the voltage of the DC changes the speed of the motor.

Which one should I use, VSD or VFD?

In the case of a DC system, the correct term is VSD, but most common industrial control systems are AC powered, in which case the acronyms are interchangeable (refer to the same thing). There are however also various other terms used in different industries – the most common terms are thus as follows:

• Variable Speed Drive (VSD)
• Variable Frequency Drive (VFD)
• Adjustable Speed Drive (ASD)
• Adjustable Frequency Drive (AFD)
• AC Drive
• Micro Drive
• Frequency Inverter
• Inverter
• Variable Voltage Variable Frequency (VVVF) Drive

For more information regarding VSDs, please refer to the following related blog posts:

• What is a Variable Speed Drive (VSD)?
• How do Variable Speed Drives (VSDs) Work?
• Advantages of Variable Speed Drives
• VSD Industry Applications
• Single-Phase To 3-Phase Converters – VSD – EMHEATER
• Choosing between a VSD and a Soft Starter
• Variable Speed Drive vs Solar Drive (for Water Pumps)
• Variable Speed Drive – EMHEATER – Intro Video
• EMHEATER VSD and Solar Drive Fault Diagnosis – Errors
• VSD – Horticulture – Reduce Costs- ESKOM Fact Sheet
• VSD – Oil/Gas/Petrochemical – Reduce Costs- ESKOM Fact Sheet
• VSD – HVAC – Reduce Costs- ESKOM Fact Sheet
• VSD – Farm Irrigation – Reduce Costs – ESKOM Fact Sheet