Normally, equipment is sized to handle maximum load under worst-case conditions. This kind of sizing is applied to, for example, pumps, fans and motors. However, maximum load is only needed occasionally. If equipment constantly runs at full power, it uses wasteful amounts of electricity.

An important thing to consider when choosing a drive is where it will be installed. This has an impact on the type of ingress protection required. For instance, if the drive is to be mounted outdoors or in a harsh environment, an IP66/UL Type 4X or better ingress protection rating will be needed. If it is going to be mounted in an enclosure, an IP20/UL Type Open rating may well be enough. A wide range of suitable protection classes is available for AC drives to meet all installations.

In typical cases, a drive can be quickly and simply started and the speed reference can be given by using the keypad, the I/O terminal or a Fieldbus. PC tools are available for fast commissioning of the drive. If training is needed training courses are available for operation and maintenance personnel.

When set point is used, the drive needs both the set point reference and the corresponding actual value. The reference and actual value typically represent flow, pressure or temperature. The set point is given to the drive, and the drive controls the speed based on the actual value. An internal PID regulator ensures that the set point is maintained.

Many customers buy a drive on the basis that it will act as a motor starter for their application – and, in many ways, it will. However, in some applications, a contactor is required before or after the drive to positively disconnect the motor when it is stopped.

The principle of a frequency inverter is very simple. It is not complicated to implement with modern electronics. Most frequency inverters are based on a double-conversion scheme which implies:

• rectification of mains voltage;
• stabilisation of the supply voltage;
• generation of the output voltage with a set frequency and amplitude, which is fed to the motor power contacts.

Apart from the conversion electronics, the unit is equipped with a control module, indication system, protection against overloads and other emergencies. Some models have remote control capability as well as one or more sensors for feedback control.

A variable frequency drive is selected on the basis of such key characteristics as:

• motor power;
• speed control range;
• number of phases;
• ability to connect external sensors;
• type of control system for the operation of the equipment.

If you do not want to know exactly how to choose a frequency inverter, it is best to inform the supplier of these parameters and of the system for which it is being purchased. For example, it may be a pumping system, a ventilation system, or a machine tool motor.

A detailed diagram of how to connect the motor to the variable frequency drive is included in the documentation of the device and is not too complicated for a specialist. If it is a single-phase VFD for domestic use, it can be connected by a home handyman with wiring experience. For single-phase, the connection involves two inputs and two motor outputs; for three-phase, it involves four power inputs and three or four motor outputs, depending on which circuit is used to power the motor. It may also be necessary to connect a remote control unit, if it is intended by the design, and sensors, if the frequency inverter is controlled by feedback. Please note that this equipment should only be connected to an industrial system by a qualified person with the necessary authorisation to carry out such electrical work.

Once the system is assembled and configured, there is no problem with how to turn the variable frequency drive on or off. As a rule, starting and stopping modes of such systems are preset during the initial setup of the equipment, and their activation in normal mode is no more complicated than starting an electric motor with an ordinary starter.