Misconceptions about Variable Frequency Drives

VFDs are important and they have numerous benefits. However, there are some myths and misconception about the VFDs. The sad thing is that the myths interfere with the solutions provided by VFDs to various OEMs.

VFD components

As a fact, VFD panels share some common components such as output inverter section, bridge rectifier, DS bus capacitor bank, and soft-charging bank. The VFDs vary in their reliability of components, the switching of the inverter sections and the efficiency of thermal dissipation scheme. Some variable frequency drive panels achieve lower voltage intensification at the motor. This is because of the three level output as it makes it possible for the output pulse to change between full-bus level pulses, voltage-level pulses and half-bus. Other types of VFDs have matrix-style inverters which do not have a bridge rectifier or DC use. The VFDs use bidirectional switches which are capable of connecting to any one of the three output phases. This makes it possible to have a four-quadrant operation where power flows freely from motor to line or line to motor.

Control and Program

Even though VFDs have many parameter settings, it does not mean you have to use all of them. Every application requires specific parameters. Therefore, you do not have to worry about other parameters that you may not use to set up your VFD.

Output is Sinusoidal

When you use a motor starter to start an induction motor, it requires connecting the 3 phase leads to a 3 phase power. Each of the three phases has a frequency of 60 HZ and a 460 V, 230 V and 575 Volts of voltage amplitude. The voltage creates an equal sine wave current waveform which is equal to what is provided at the motor leads. VFDs output work differently. They change the 3 phase input into a dc voltage using dc bus capacitors to filter and store it. The dc voltage undergoes a transformation process where each output phases uses an insulated bipolar transistor. The process produces variable frequency output and variable voltage.

The user of the VFD panels has control over the period of time the IGBT remains ON or OFF. This means they have the power to regulate the root mean square (RMS) level of output voltage. There is a linear relationship between output voltage and output frequency which leads to a constant torque except in pumps and fans which do not need constant torque. The rate of increase in output voltage and output frequency should be at the same rate. The resulting waveform is therefore not sinusoidal.

Higher Input than Output Current

Logically speaking, a VFD should have a high input current than the current. This is because it loses current through its thermal component. In real sense, when you measure the output and input power, the input power is lower.

Resolves PF (power factor) issues

VFDs internal capacity buses are essential to the motor as they supply the reactive current. This prevents AC line from providing the reactive current and it brings down the displacement PF. When you install a VFD, the input displacement PF improves but it does not describe the PF calculation. For accurate calculations, you need to consider the reactive power required when AC is rectified to DC.  The current produced when AC changes to DC is discontinuous. This creates a ripple-like voltage waveform. The only way to calculate PF is by adding total harmonic distortion to the displacement PF.

Enables Motor to Run Slower or Faster

Variable frequency drive panels change their output frequency and output voltage. This gives motor the ability to operate slower or faster than their rated speed. The process presents three limitations. The motors run out of toque when they reach close to their maximum operating speed. In such a case, the VFD runs out of voltage. When totally enclosed fan cooled motors operate at slow speeds, it reduces the amount of cooling air that enters them. For maximum efficiency, they should operate below 15HZ. Since most motors have a recommended speed, it is not good to go above the limit.


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