VFD Fundamentals


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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Serial Communications

Serial communications are all around us, from our USB memory sticks to the Internet. Most variable frequency drives include a serial interface, which can be used instead of hard wiring to control as well as set up the drive. Serial communications need some form of definition in order for equipment to be compatible. The hardware of the system is often defined using a standard such as RS485, which says what is a 1 and what is a 0 in terms of volts. It’s necessary to define how the data is managed to prevent ‘collisions’ between signals. Many systems use a master slave system so that the slave only sends a signal when the master has asked for one. A token system can also be used so that whichever ‘node’ has the token is the master – the token can be passed around. Many fieldbus systems use master/slave systems. A fieldbus is one where all the specifications of the system is published and freely available to encourage its use. Examples are Modbus, Canbus, Profibus etc.

Finally, the content of the signal or telegram must be defined. Modbus, for example, has a simple structure shown in Figure 2. Ethernet, the system used on the Internet is more complex, and is shown in Figure 3. Many existing Fieldbus systems now have Ethernet like derivatives, such as Modbus TCP or Profibus; these use hardware similar to IT networks. Serial communication use is growing rapidly and will be used for more and more installations in the future, Invertek variable frequency drives support many different Fieldbus systems.

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Motors, Motors, Motors

Induction motors have long been the simple, reliable solution for power in industry. Recently however, new technologies and the need for ever increased efficiency have encouraged the development of different types of motors that can be driven by variable frequency drives.

Permanent Magnet Motors (PMMs) are not new. By replacing the rotor windings with permanent magnets, efficiency is easily increased (and maybe the motor is smaller as well) as there are no longer any losses in the rotor. PMMs really need vector control to give smooth operation, and the motors are more expensive than a corresponding induction motor.

Synchronous Reluctance Motors (SRMs) have a rotor made up of steel laminations only; this is magnetised by the stator field just as a horseshoe magnet magnetises a nail. Again, a vector drive is needed for good performance, but reduced rotor losses result in improved efficiency.

Brushless DC motors (BLDCMs) come in many shapes and sizes, but are often designed ‘inside out’. That is, the stator is fixed and in the centre of the motor, and the rotor, including the permanent magnets, is on the outside. Small versions of BLDCMs (used in cooling fans for example) include a simple inverter and position sensor built into the motor.

All these motors need slightly different control systems. Invertek drives have been developed with these motors in mind, and are already used in a wide variety of applications from pumps to elevators. Set up using Vector Control is straightforward, and there are application notes to help. In the future, new motor technologies will be developed for better performance and efficiency; Inverter variable frequency drives will control them.

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Vector Control

‘Vector Control’ is used by drive salesmen in the same way ‘Turbo’ used to be by car salesmen, but just what is Vector control?

A simple variable frequency drive takes the speed demand (or setpoint) from the user input and selects an output frequency and voltage that theoretically suits the motor. Usually that means selecting a voltage proportional to the output frequency, with minor modifications to help at low speeds. That’s good enough for many applications, but with better microprocessors and improved current measurement, vector control is now available on all but the simplest drives.

Instead of supplying the motor with a voltage and frequency and hoping for the best, the drive calculates the torque and flux that is required to produce the desired result –usually the motor speed. Because the torque and flux are continually calculated, taking into account the precise rotor speed and position, the performance is greatly improved. This results in improved torque at low speed, and better response to step loads. Efficiency is also usually better. Vector control relies on the drive having a good ‘model’ of the motor; for a good model, the drive must be programmed with the motor parameters such as voltage, frequency, power factor etc. An autotune will allow the drive to measure additional parameters to improve the model. Vector control is needed when working with Permanent Magnet, Synchronous Reluctance and similar motors, described in the previous article.

Invertek P2 drives offer a reliable Vector control system that is easy to set up and suitable for many types of motor. The E3 drive includes a simple vector control to give improved performance, especially at low speed. There are instructions in the manual and application notes to help you use these drives to their best advantage.

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Torque Control and Master Slave Operation

Most applications of drives are not concerned with the actual speed of the motor, but with the result. In a fan application, the output frequency of the drive will be varied (either manually or as part of a control system) to control the flow rate, pressure or temperature.

However, in some applications it is useful to control the torque output. Torque is the rotational equivalent of force, and at a fixed speed the more torque, the more power.

To control torque, the drive measures the output current, separates the current into the magnetising (flux generating) and load components, and then varies the output frequency to maintain the load current constant (keeping the magnetising current constant).

So a torque control system is a closed loop control system similar to those described in an earlier article, and therefore has adjustable parameters such as gain and integral settings. Torque control is more easily operated using Vector Control, as Vector Control uses Torque and Flux control systems in any case.

Torque control is used in applications such as winding (to control tension) and similar applications. Torque control is very useful to share loading among motors which are mechanically linked, such as on a long conveyer. The first drive is operated in normal frequency control, subsequent drives run in Torque control, taking the Torque output of the master drive (via the Analogue output for example) as a reference. So all the drives run with the same torque.

An easy way to do this is to use Invertek’s master/slave system. This will ensure all drives run with either the same frequency or Torque, simply by connecting the master to the slaves via an Ethernet cable and setting a couple of parameters. The frequency or Torque can be scaled for each slave, and local trim controls can be set up using the analogue input (See application note AN-ODP-2-027).

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Power Factor, Form Factor and RMS

When a motor runs directly from the mains, it draws a load current which supplies the torque, and a magnetising, or reactive current that develops the magnetic field. This current flows back and forth in the motor and cables, but does no useful work, and in small installations is not metered or paid for. The ratio between the total and reactive current is known as the power factor. Many other loads also draw reactive currents, so it makes sense to minimise and correct the power factor if possible. Power factor correction equipment can be used in larger installations.

Power Factor values don’t always reflect the heating effect of a waveform. When rectifiers are used (in drives, computer power supplies, LED lights etc.) ‘peaky’ current waveforms occur, and these can cause unexpected losses - as the heating effect is proportional to the square of the current. RMS values of waveforms take account of this, and are usually used for AC waveform measurements. The ratio between the RMS, and the absolute average of an AC waveform is called the form factor, and gives an idea of the level of distortion.

When a motor is used with a variable speed drive, the drive supplies and handles the magnetising current, so the power factor into the drive is very good. But the rectifier waveform has a high form factor, so the RMS input current is higher than might be expected.

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Setting Setpoints

What’s a setpoint? It’s the frequency you want to be supplied by the drive to the motor. Remember you don’t usually control the speed of the motor, that’s determined by the applied frequency less the slip (the small speed reduction dependent on load) and of course the number of pole pairs. So a four pole motor, fully loaded, supplied with a 25Hz frequency will run at about 700rpm, while a two pole machine will run at 1400rpm.

Most of the time we aren’t interested in the actual speed of the motor, we just want the conveyer to deliver the package at the right speed, or the air to supply the amount of cooling we want. If we do need more accurate speed control, we can enable slip compensation by setting the motor speed parameter. Vector control does a better job by calculating exactly what the motor does and adjusting accordingly. For real speed control we can fit an encoder to the motor and feed the speed information back to the drive.

The setpoint can be supplied to the drive in various ways. The easiest is to supply a 0 to 10V signal to pin 6 (with respect to pin 7) and the drive will produce an output frequency (by default, when enabled) of 0 to 50Hz (60Hz in US). There’s a 10V output on pin 5, so connect a potentiometer across 5, 6 and 7 and you have control. The input at pin 6 can be recalibrated for offsets and scaling, and can also accept 0 – 20mA, 4 – 20mA, 10 – 0V etc.

If you want to control the frequency from the built in keypad that’s possible, and various options can be selected (different start and control settings, reverse button disabled etc.)

The drive also includes adjustable fixed frequencies which can be selected using digital inputs, so a machine can run at fast, medium or slow fixed speeds.

Control using serial communications allows the setpoint to be continually updated over the the serial port.

The Master Slave controller allows a drive to derive it’s setpoint from a master drive via a simple connection.

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