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Frequency Inverter (VFD) Selection Guide

  • 2018/11/29

 AC drives are also known by various other names such as adjustable speed drives, adjustable frequency drives, variable frequency drives, variable speed drives, frequency converters,  frequency inverters. This article is about the basic guide of how to select a Frequency Inverter.

About universal AC drive selection, it is a topic concerned by lots of people, here we share some basic guidelines about how to specify an AC drive.

The most critical selection factor: working current. Actually, choose the frequency inverter according to the working current is the final step in the entire selection process. Here is to emphasize. Sizing the ac drive according to the motor's actual operating current (not the nameplate current) rather than the nameplate power.

The basic principle, while working for a long time: ac drive output current > Motor current

 In an application, the first step is selecting the motor, then the ac drive, which means the ac drive is selected based on the existed motor. The operating current of the ac motor is related to the actual working conditions. Estimate the motor operating current changeable relationship under certain working conditions, in order to determine the appropriate ac drive models.

 Generally, for driving a constant torque motor, selecting the ac drive based on the rated current. For example, a 10 hp motor, 15A rated current, can choose a 10 hp ac drive, 17A output current.

In general, driving the loads like fans and pumps also can select the ac drive based on the rated current.

For frequent short-time overload running motors, it is necessary to calculate the overload cycle, it requires the ac drive maximum output current Imax is bigger than the peak current of the motor, and the VFD's I2t is within its allowable range. It may select a higher-level ac drive. For example 10hp motor, 15A rated current. Intermittent duty, one second overload two times (ie current is 30A), then stop running 29 seconds. In this case, specifying the ac drive according to its overload curves.

In this aspect,  m-driver ac drive is doing well, generally 2 times short-time overload capacity.

For large motor large, if the workload is light, we can select a smaller ac drive according to the actual situation.

Other factors for selecting ac drive


Ambient temperature, transport and storage temperature, enclosure.

Input line voltage level, frequency, ac drive output frequency range.

The ac drive efficiency, overload capacity, cooling methods.

Size, structure, installation way.


The most important is specifying the ac drive according to practical application, not just the guide on paper.