What does inverter mean?

When it comes to the motor of the machine tool, in the case of lathes and milling machines, at Heller we use high-quality technology to achieve excellent results. One of the essential components of our machines is the drive or inverter. Do you know what inverter means and how it works?

What is a variable frequency drive (VFD)?

Variable frequency drives are known as VFDs from the initials of Variable Frequency Drive; they can also be called AFDs or Adjustable Frequency Drives. This is a device that allows control of the rotational speed of alternating current (AC) motors by controlling the frequency supplied to the motor. We should clarify that the variable frequency drive is a type of variable speed drive, which, as its name indicates, is responsible for choosing the speed at which a machine tool will run.

As the voltage can vary along with the frequency, they are sometimes called VVVF drives (variable voltage variable frequency drives). You may have heard the name drive in industry, but also microdriver or inverter. What does the latter mean? It is the same term, which in Spanish means «inversor de corriente» (current inverter).

With these devices, the operator can regulate the speed as well as the direction of rotation of the machine tool's electric motor. They also allow you to control the speed and the start-up time and, in addition, they are highly recommended components for protecting the motor. Thanks to their easy installation, low maintenance and high performance, you will find them in our lathes and milling machines.

Before fitting an inverter to our machines, we take into account the type of motor that will be connected to the drive. The choice depends on the motor's voltage and its power.

What inverter means: benefits of the variable frequency drive

Why is it advisable to install a drive or inverter in machine tools? In general terms, this type of device is recommended on all machines, because it allows three-phase 220V motors to be connected to single-phase supplies. They are also easy to use and make it easy to control the motor speed and the pressure flow. Thanks to these devices it is possible to correct the motor's power factor and they allow a smooth start-up of the motors.

In addition, they provide energy savings, reduce the temperature of the equipment and mean less maintenance for the appliances connected to them.

Operating principle

Why does an inverter bring great benefits to industry? This is due to the device's operating principle. All variable frequency drives follow the same guidelines. The synchronous speed of alternating current motors is determined by the alternating frequency supplied to the motor, as well as the number of poles in the stator.

Motor speed is measured in revolutions per minute (RPM) and is calculated using a formula. RPM equals 120 times the alternating current supply frequency, divided by the number of poles. In synchronous and asynchronous motors, these poles can be 2, 4, 6 and 8. As a result, it is possible to obtain RPM of 3000, 1500, 1000 and 750 (in synchronous motors at 50 Hz).

In asynchronous motors the RPM are slightly lower, because this is caused by the asynchronism itself. There is therefore a lag between the rotor RPM and the RPM of the magnetic field.

How is the frequency changed? It is very simple, because all that has to be changed is the duty cycle in ON time and OFF time of a periodic square wave. As a result, the average value of the voltage varies between the maximum and the minimum. To make these changes, the inverter is fitted with so-called IGBT electronic switching devices. These are insulated-gate bipolar transistors and they work as switches. When they open and close under the control of specific software, a sinusoidal signal can be obtained. Varying the duty cycle of the square waves produces sinusoidal waves of frequency f (measured in Hz).

Parts of the frequency inverter

Now that you know what inverter means, you should know the parts that make up these inverters or drives. They generally consist of a motor, a controller and an operator interface.

• The drive motor is usually three-phase, although some are single-phase.
• Controller. It consists of solid-state conversion devices. These convert the input power into DC. To do so they use a rectifier bridge. This is how the intermediate power is converted into a quasi-sinusoidal alternating current signal through the switched inverter circuit.
• Operator interface. This is the part of the inverter that establishes a functional connection within the system, that is, a connection between the inverter and the motor. Thanks to this part, information can be exchanged between the device and the motor.