Variable Frequency Drive vs. Variable Speed Drive: Electric Motor Speed Control

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Across nearly every sector, electric motors are playing a vital role in modern technology. In an effort to decrease electric motor energy consumption in these energy-hungry devices―and optimize their efficiency―manufacturers have employed various drive-technologies to modify the motors' speed. These technologies, which include variable frequency drives (VFD) and variable speed drives (VSD), control motors in variable conditions.

Electric Motor Types and Applications

Electric motors are used in manufacturing processes and system controls in industrial and agricultural settings; if it moves, chances are it uses an electric motor. In the transportation sector, electric motors are used in a variety of applications, such as:

  • •  Vehicle powertrains
  • •  Hybrid motor systems
  • •  Subsystem controls like windshield wipers
  • In the residential sector, electric motors are used in everything from kitchen appliances to toys, with motor sizes ranging from a few hundred milliwatts to a few thousand watts. Even the commercial sector utilizes electric motors in items such as:

  • •  Vacuums in hotels
  • •  Forklifts in wholesalers
  • •  Mixing bowls in restaurants
  • •  Building facilities systems like HVAC and elevators

Regardless of their applications, electric motors use a startling amount of energy. They consume 66% of energy in the industrial sector, which constitutes nearly 40% of global energy use. Commercial ventilation and cooling units use over 30% of a building's energy, of nearly all of which is consumed by electric motors.

Electric Motor Speed Control: VFD vs. Variable Speed Drives

There are two primary methodologies for controlling electrical motors: variable speed drives (VSDs) and variable frequency drives (VFDs).

What are VSDs?

These drives change the speed of a motor by changing the input voltage and can be used with both AC motors and DC motors.

Variable speed drives supply specific amperage and voltage to a motor. If the supply power is in AC form, VSDs utilize a rectifier circuit to convert the AC to DC at a specified voltage and amperage that the drive can adjust. Changing the voltage of the DC changes the speed of the motor. A DC motor VSD can consist of a very simple circuit, which makes them generally less expensive than VSDs.

What are VFDs?

Variable frequency drives also control the speed of a motor, but they do so by changing the voltage and frequency and can thus only be used with AC motors. 

VFDs control the frequency and voltage of the AC signal. Adjusting the frequency of an AC motor controls its speed, while changing the voltage will change the torque provided by the motor. VFDs are also used to control the start-up and slow-down speeds of electric motors and to prevent overcurrent in the motor. These devices are extremely effective at increasing the motor's efficiency in a variety of AC motor applications where speed control is imperative, and may even be required in certain applications where energy conservation is a priority.

CThe Difference Between VSDs and V

Both VSDs and VFDs seek to decrease an electric motor's unnecessary power consumption and optimize the efficiency and operation of systems that utilize electric motors. While both technologies rely on manipulating the power that an electric motor receives, the two drives have critical differences when it comes to application. Check out more information on setting up electric motors and some popular products.

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