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variable frequency drives

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What is a Variable Frequency Drive? (Part 1)

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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What is a Variable Frequency Drive? (Part 2)

We saw in the last article that an AC induction motor runs at a speed dependent on the applied frequency. Therefore to control the speed we need to control this frequency. We also need to control the applied voltage as well to keep a constant magnetic field.

A variable frequency drive does this by taking the AC supply, converting it to DC, and then converting it back again to AC. The VFD consists of three main sections. The rectifier is made up of six (or four for single phase supplies) diodes that conduct one way only. In the arrangement shown in figure 1, a DC voltage is built up on the capacitor (which stores energy a little like a battery) and supplies the load resistor.

Figure 2 shows the rectifier and capacitor connected to an inverter. The inverter uses six IGBTs to convert the DC back to AC. The IGBTs are fast, electronic switches; by switching them on and off very rapidly, and adjusting the time that they are on and off, the current on the motor can be controlled as needed. Varying the on and off times is known as Pulse width Modulation (PWM). Because the motor current changes relatively slowly, the pulses of current are averaged out, and a nice sine wave current of any practical frequency can be built up, as shown in figure 4. The average voltage can also be controlled, but the motor voltage will still consist of a series of square waves.

The diodes in the inverter allow current to continue to flow in the motor as the IGBTs switch on and off.

The next article will look at the practical design of a variable frequency drive.

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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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