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05 Nov 2018, Drives & Motor Control

Tips for sizing Variable Frequency Drives (VFDs)

Start in the know: It sounds obvious, but simply understanding what a VFD actually does will help keep you on track. Basically, VFDs convert input AC current to DC current. Then they invert it right back into a modified three phase AC output.

Variable Frequency Drive sizing should be based on the motor’s voltage and current, not its horse power. It’s key to ensure the VFD can satisfy the maximum current requirements of the motor at peak torque demand. Other key sizing considerations include:

Full load amps

Full load amps (FLAs) are those amps on which the motor will draw power when it’s operating at its full speed and torque. When sizing a VFD, it’s crucial to account for the motor’s FLA plus any other additional overloads that it may require.

Altitude

Altitude also influences VFD sizing, because VFDs are air-cooled. Air thins at high altitudes, which decreases its cooling properties. Most VFDs are designed to operate at 100% capacity up to an altitude of 1,000 meters; beyond that, the drive must be de-rated or oversized.

High overload usage

For high overload usage of the VFD, it’s important to understand precisely how the motor will be used in order to ensure that it has been sized to accommodate the motor’s own overload protection. This is necessary for more challenging applications, including positive displacement pumps cranes, conveyers and blowers. 

Motor type

The type and nature of the motor being used are also important to the size of the VFD. For instance, one should be very careful while operating a submersible motor since it will draw more current per horse power than conventional motors.

Application

Your application may require that you oversize the VFD to meet peak torque demands from the load. Even if your VFD has been sized to appropriately for the motor’s FLA, it might not be sufficient for motors built specifically for short high torque/current applications.

Ambient operating temperature

Ambient operating temperature also influences VFD sizing. Most VFD’s are sized to operate to 40 degrees Celsius without de-rating the VFD. Switching frequency in the VFD can also de-rate a VFD.


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