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    What Does a Variable Frequency Drive Do?

    April 5, 2019 | Energy Management, Facility Management

    A variable frequency drive, or VFD, is “a type of motor controller that drives an electric motor by varying the frequency and voltage supplied to the electric motor,” according to VFDs. It is usually used for running an air conditioning motor at variable speeds or ramping up their speed to give them a smooth startup. They work by adjusting the frequency of the motor to adjust the revolutions per minute.

    Why would you need a machine to adjust motor speeds?

    Many ACs run on three-phase power now. The opposite of it is direct-on-line (DOL) operation, where motor runs at full speed regardless of the demand. Obviously, DOL operation will waste some extra energy when the demand is lower. However, a main drawback of three-phase power is that it can only run at one speed. In order to compensate for this disadvantage, VFD was created in the 1980s to efficiently run the AC motor at different speeds.

    A VFD can convert the voltage to change the frequency of the AC motor, which is also known as the speed control. The reasons why we want to adjust the motor speed are, according to Danfoss:

    • Save energy and improve system efficiency
    • Convert power in hybridization applications
    • Match the speed of the drive to the process requirements
    • Match the torque or power of a drive to the process requirements
    • Improve the working environment
    • Lower noise levels from fans and pumps
    • Reduce mechanical stress on machines to extend their lifetime
    • Shave peak consumption to avoid peak-demand prices and reduce the motor size required

    Benefits of VFD

    Since the VFD was invented, many improvements have been made to it. For example, nowadays, VFDs have integrated networking and diagnostic capabilities to better control performance and increase productivity, making it easier to control and more efficient. According to Danfoss, VFD can commonly save about 40% of the energy, compared to the DOL operation.

    Thus, the advantages of VFD are:

    • Energy savings
    • Intelligent motor control
    • Reduction of peak-current drawn
    • Foster a comfortable working environment
    • Control starting current
    • Reduced power line disturbances
    • Lower power demand on start
    • Controlled acceleration
    • Adjustable operating speed
    • Adjustable torque limit
    • Controlled stopping
    • Reverse operation
    • Elimination of mechanical drive components

    Because of all of the benefits VFD brings, the applications of it account for 75% of all drives operating globally, according to Danfoss. The most common uses of VFD are in fans, pumps, and compressors. But it is also suitable to almost all motor controllers that required voltage change.

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