The knowledge base for
variable frequency drives

Explore 'how to' videos and tutorials; search an extensive database of installation, commissioning and application advice; and check for our engineers' responses to FAQs about variable frequency drives.

What is a Variable Frequency Drive?

A variable frequency drive is used to control the speed of three phase AC Induction motors.

An Induction motor acts like a transformer (shown in Figure 1). When an AC voltage is applied to the primary of the transformer, a voltage is induced on the secondary. The same thing happens with and induction motor; the primary is now called the stator, and the secondary the rotor, which is free to rotate (Figures 2 and 3). Again, there is transformer action, but now there is a force on the rotor due to the current and the magnetic field, so the rotor turns. As the rotor speeds up, it catches up with the magnetic field, which is changing continuously - effectively rotating - at the supply frequency of 50(60)Hz. It won’t reach this speed because if there is no changing magnetic field there is no transformer action, so no current and no torque. So the motor always runs a bit slower than this frequency, as shown in Figure 4. This reduction in speed is dependent on load and is known as slip. The base speed of motors also depends on the number of windings or pole pairs, which are fixed. So if we want to control the speed we must control the applied frequency. We must also control the voltage at the same time to control the magnetic field in the motor. To do this we use a variable frequency drive, described in the next article.

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Variable Frequency Drives in Practice

This article describes how a variable frequency drive is put together. The IGBTs and rectifiers are usually in a single power module which is mounted on a heatsink, normally cooled by a fan which switches on when needed.

‘Power’ printed circuit boards (pcbs) will carry the gate drive circuits, a power supply, current measurement circuits and an inrush circuit. The power supply is complicated because it must supply the drive circuits at different, high voltages, as well as the control and interface circuits at ‘safe’ low voltages. Power terminals – supply and motor connections- are also mounted on the power boards. A separate control board often includes the control terminals as well as the display and push buttons to set up and control the variable frequency drive.

The complete assembly may be mounted in a plastic case or, for higher power variable frequency drives, in strong steel case. IP20 versions are suitable for mounting in a cubicle or machine, while IP55 or IP66 versions may be offered for more exposed applications.

In the next article we’ll look at drive selection, installation, and commissioning.

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Variable Speed Drive Selection and Installation

Travelators benefit from smooth stopping and starting when controlled by drives

In the last article we looked at the practical construction of variable frequency drives; now we’ll get down to the practical side of choosing and installing a VFD.

If you have a motor in mind to control, you can select the drive simply by looking at the power and voltage of the motor. Remember to check the actual supply you are using, as dual voltage motors (which allow for star or delta connections) will be suitable for two supplies, for example 230 or 400V. The variable frequency drive must be chosen for one voltage or the other.

It is always worth looking at the motor rated current and that of the drive. With high efficiency motors it is sometime possible to select a smaller drive than the nameplate power suggests. That is, a 15kW variable frequency drive may supply enough current for an 18.5kW motor; it’s the current that counts!

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