What is the variable frequency drive (VFD)

- Oct 24, 2018-

Variable frequency drive (VFD) is a type, by varying the frequency supplied to the motor and the motor drive voltage of the motor controller. Other names for VFDs are variable speed drives, adjustable speed drives, frequency converters, AC drives, microdrives and inverters.

The frequency (or Hertz) is directly related to the motor speed (RPM). In other words, the faster the frequency, the faster the RPM. If the application does not require the motor to run at full speed, the VFD can be used to reduce the frequency and voltage to meet the motor load requirements. As the application motor speed requirements change, the VFD can simply increase or decrease the motor speed to meet speed requirements.


How does the VFD work?

The first stage of the variable frequency AC drive is the frequency converter. The converter consists of six diodes, similar to the check valves used in piping systems. They allow current to flow in only one direction; the direction of the arrow in the diode symbol. For example, whenever the Phase A voltage (voltage is similar to the pressure in the piping system) is more positive than the Phase B or Phase C voltage, then the diode will open and allow current to flow. When Phase B becomes more positive than Phase A, the Phase B diode will open and the Phase A diode will close. The same is true for the three diodes of the bus negative. Therefore, when each diode is turned on and off, we get six current "pulses". This is called “six-pulse VFD”, which is the standard configuration of the current inverter.

We can eliminate the AC ripple on the DC bus by adding a capacitor. The capacitor operates in a similar manner to a reservoir or accumulator in a piping system. This capacitor absorbs AC ripple and provides a smooth DC voltage. The AC ripple on the DC bus is typically less than 3 volts. Therefore, the voltage on the DC bus becomes "about" 650 VDC. The actual voltage depends on the voltage level at which the drive feeds into the AC line, the level of voltage imbalance on the power system, the motor load, the impedance of the power system, and any reactors or harmonic filters on the drive.

A diode bridge converter that converts alternating current to direct current is sometimes referred to simply as a converter. A converter that converts direct current back to alternating current is also a converter, but in order to distinguish it from a diode converter, it is often referred to as an "inverter." In the industry, it has become common to refer to any DC-AC converter as an inverter.

When we turn off a top switch in the inverter, the phase of the motor is connected to the positive DC bus and the voltage on that phase becomes positive. When we turn off a bottom switch in the converter, this phase is connected to the negative DC bus and becomes negative. Therefore, we can make any phase of the motor arbitrarily positive or negative, and thus can produce any frequency we want. So we can make any stage positive, negative or zero.

If we want to reduce the motor frequency to 30 Hz, then we only need to switch the inverter output transistor more slowly. However, if we reduce the frequency to 30Hz, then we must also reduce the voltage to 240V to maintain the V / Hz ratio (for more information on this, see the VFD motor theory presentation). If our only voltage is 650VDC, how do we reduce the voltage?

This is called pulse width modulation or PWM. Imagine that we can control the pressure in the water pipe by switching the valve at high speed. Although this is not practical for piping systems, it works well for VFDs. Note that during the first half of the cycle, the voltage is ON for half of the time and OFF for half of the time. Therefore, the average voltage is half of 480V or 240V. With the pulse output, we can get any average voltage at the output of the VFD.

What is the VFD

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