How to Size a Variable Frequency Drive: Current, Load, Conditions and Options

1. Current matters more than power

A drive is sized by its rated output current: it must be no less than the rated current on the motor nameplate. Power in kilowatts is a guide, but motors of the same power can draw different current depending on pole count, efficiency and power factor.

Example: a 55 kW motor at 400 V draws about 100 A. A drive with a rated current of 100 A or more in the required duty is suitable.

Induction motor nameplate showing rated current — The motor rated current is taken from its nameplate — and the drive is sized by it.

2. Load type: normal or heavy duty

Most drives have two ratings — for light and heavy loads.

Duty	Load	Overload, typically
Normal (ND)	Pumps, fans — torque rises with speed	about 110–120% for 60 s
Heavy (HD)	Conveyors, compressors, mixers, crushers — high torque right at start	about 150% for 60 s

For heavy loads, a drive one frame size larger is often chosen. Exact overload figures are in the manufacturer catalogue.

3. Supply voltage

3 × 380–480 V — the main option for industry;
1 × 220 V input — for small motors when there is no three-phase supply;
690 V — for large motors when the design provides for it.

4. Installation conditions

The rated current of a drive is specified for certain conditions. In harsher conditions it is derated according to manufacturer tables:

temperature — the rating is usually given up to 40 °C; a hot room needs a margin or cabinet ventilation;
altitude — above 1,000 m air cools less effectively, and current is derated;
enclosure rating — IP20–IP21 for cabinet mounting, IP54–IP55 for local mounting in dust and moisture;
cold — at unheated sites in the North, the cabinet needs heating: in severe frost the drive will not start.

5. Options

Input reactor or harmonic filter — reduces mains interference and protects the drive from voltage surges;
Output reactor, dU/dt or sine filter — for long motor cables, to protect the winding insulation; the allowable cable length without a filter is stated by the manufacturer;
Braking resistor — for fast stopping of high-inertia machines and for hoists;
EMC filter — if sensitive electronics are nearby.

6. Hazardous areas

The drive is installed outside the hazardous area, and an explosion-proof motor must be approved for operation from a variable frequency drive — this is stated in its documentation.

What to send for selection

Motor nameplate: power, current, voltage, speed.
The machine: pump, fan, conveyor, compressor.
Duty: how often it starts and stops, whether fast stopping is needed.
Cable length from the cabinet to the motor.
Installation: cabinet or local, temperature, whether the room is heated.
Control: push buttons, pressure sensor, automation system.

Send us the motor nameplate and a description of the machine — we will select the drive and options.

Variable frequency drives catalogue

FAQ

Can a VFD be selected by motor power alone?

As a guide — yes, but the final check is the rated current of the motor and the drive. Motors of the same power can draw different current, and a drive sized exactly by power will trip on overload.

What happens if the drive is undersized?

It will trip on protection at start-up or under load, and running constantly at its limit it will fail sooner.

Can one drive control several motors?

Yes, if the motors run synchronously on one load. The drive is then sized by the sum of currents, and each motor is protected by its own thermal relay.

Does a pump need a braking resistor?

Usually not: a pump stops quickly due to fluid resistance. A resistor is needed for high-inertia machines and hoists.

Will a drive work in an unheated room in winter?

Only if its passport allows it or the cabinet has heating. For northern sites, cabinets are fitted with a heater and thermostat.