What Factors Should Be Considered When Making a VFD Selection

With the continuous improvement of industrial automation, variable frequency drives (VFDs) have been widely used. It may be tempting to determine the size of the VFD based solely on horsepower. Do you know what other factors we should consider when choosing a VFD to ensure that it is specified correctly for your application?

1. Pay attention to full-load current and overload when making a vfd selection.

When making a vfd selection, pay attention to full-load current: the first step in this process is to ensure that the drive can handle the motor's current demand. Check the full-load current requirements on the motor nameplate and find a drive with a rated current of at least that large. If you are using a single-phase power supply to power the drive, be sure to use the rated value of a single-phase drive. For single-phase operation, the rated value of the VFD is significantly reduced.

When making a vfd selection, pay attention to overload: ensure that the drive can handle any overload that may be expected during start-up or intermittent additional load. You may need to upgrade the drive until you find one that can handle it. Due to starting requirements or shock loads, many applications encounter temporary overload conditions. Most AC variable frequency drives are designed to operate at 150% overload for 60 seconds. If the application requires overload beyond 150% or longer than 60 seconds, the AC VFD must be oversized. Note that applications that require replacing existing motor starters with AC drives may require overloads up to 600%, necessitating the use of medium voltage variable frequency drives.

2. Pay attention to height and temperature when making a vfd selection.

When making a vfd selection, pay attention to the height at which you are using it, which will also affect cooling. As altitude increases, the air becomes less dense. This reduction in air density reduces the cooling properties of air. Most VFDs are designed to operate at 100% capacity up to a height of 1000 meters. If you are at a higher altitude, the drive must be oversized to compensate for the reduced cooling.

When making a vfd selection, pay attention to temperature: AC drives generate a lot of heat and may cause the internal temperature of the enclosure to exceed the rated temperature of the drive. Ventilation or cooling of the housing may be necessary. Measure/calculate the maximum expected ambient temperature. If you have any questions about it, contact the VFD manufacturer who can advise on the correct cooling or ventilation measures to adopt, based on the expected ambient temperature and the requirements of the application.

3. Pay attention to carrier frequency when making a vfd selection.

When making a vfd selection, pay attention to carrier frequency: generally, you want to find the lowest carrier frequency that the motor can handle. In most cases, the default carrier frequency will work fine, but if you need to reduce audible noise, heat dissipation, or power consumption, make sure you can modify the carrier frequency of the drive.

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