5 ways to choose a variable frequency drive

When choosing a variable frequency drive (VFD), six factors you should take into consideration to ensure that you specify the correct AC drive for your application.

Full Load Amperage
the first decision to make when choosing a VFD is making sure the drive can handle the motors current demands. Check the motor nameplate for the Full Load Current requirement, then find a drive that’s rated for at least that much current.  If you are feeding the drive with single-phase power, be sure to use the drive ratings for single-phase.  Variable frequency drives are significantly derated for single-phase operation.

Overload
The size of the VFD should be chosen based on maximum motor current at peak demand and not chosen based upon motor horsepower. Constant starting, stopping and dynamic loads affects the electronics inside the VFD far more than the effect they have upon the local power bus and a full voltage motor starter. Therefore, peak demand current should be used.

Application Type
Choose between variable torque (VT) and constant torque (CT) and separate ratings for each. Use VT ratings for fans and pumps or consult the CT ratings for conveyors and general machine control. It is important to know the application type because the drive specifications are organized accordingly. If you aren’t sure which one to use it’s recommended to go with CT.

Altitude
The altitude at which you’re using your VFD also has an effect on cooling. As the altitude increases, the air becomes less dense. This decrease in air density decreases the cooling properties of the air. Most VFDs are designed to operate at 100% capacity at altitudes of up to 1000m. If you’re at a higher altitude, the drive must be oversized to compensate for the decrease in cooling.

Carrier Frequency
You want the lowest carrier frequency your motor can handle. Most of the time the default carrier frequency will work fine, but if you need to reduce the audible noise, the heat dissipation or the power consumption, then make sure you are able to modify the carrier frequency for the drive.

If the design requires greater reliability and longevity, it would be wise to pick a product that has a history of reliability and good support.