The truth about five common VFD myths
Knowing the truth about VFD operation can simplify the selection process for choose a variable frequency drive.
No matter how commonplace variable frequency drives (VFDs) have become to you, somewhere someone is using one or considering using one for the first time (see Figure 1). Think back to when you first thought about applying one of today's pulse-width modulation (PWM)-based VFDs to an ac motor. Chances are you probably had a few misconceptions about their abilities and designs. This article addresses five common VFD myths and corrects misconceptions about their proper usage.
Myth No. 1: The output of a VFD is sinusoidal
People tend to be more familiar with running their ac induction motors using motor starters. With a starter, starting the motor involves connecting the 3-phase leads of the motor to 3-phase power. Each phase is a sine wave with a frequency of 60 Hz and usually has a voltage amplitude of 230 V, 460 V, or 575 V in the U.S. This applied voltage creates a sine wave current waveform with the same frequency if checked at the motor leads. So far, running a motor is quite simple.
What happens at the output of a VFD is an entirely different story. A VFD typically rectifies the 3-phase input to a fixed dc voltage, which is filtered and stored using large dc bus capacitors. The dc bus voltage is then inverted to yield a variable voltage, variable frequency output. The inversion process is carried out using three insulated gate bipolar transistor (IGBT) pairs-one pair per output phase (see Figure 2a). Because the dc voltage is inverted into ac, the output stage is also called "the inverter." The duration for which each IGBT switch in a given pair is turned ON or held OFF can be controlled, which determines the RMS value of the output voltage. The ratio of the output RMS voltage to output frequency determines the flux developed in the ac motor. In general, there is a fixed relationship between the two. When the output frequency increases, the output voltage should also increase at the same rate to keep the ratio constant and thus the motor flux constant. Normally, the relationship between voltage and frequency is kept linear so that a constant torque can be produced. The resulting voltage waveform applied to the motor winding is not sinusoidal (see Figure 2b). Note that sometimes the voltage by frequency (V/f) ratio can be quite nonlinear, which is typical for fans, pumps, or centrifugal loads that do not require constant torque but instead favor energy savings.
What makes this work is that, as the name implies, an induction motor is a big inductor of sorts. A characteristic of induction is its resistance to current changes. Whether a current is increasing or decreasing, an inductor will oppose the change. What does this have to do with the PWM voltage waveform in Figure 2b? Instead of letting the current pulse rise on the same order as the applied voltage pulse, the current will start to rise slowly. When the voltage pulse has ended, the current doesn't disappear immediately, it slowly starts to ebb. In general terms, before the current has fallen back to zero, the next voltage pulse comes along, and the current starts to slowly rise again-even higher than before because the pulses are getting wider. Eventually, the current waveform becomes sinusoidal, albeit with some jagged up-and-down transitions as the voltage pulses start and end (see Figure 3).
However, don't think that you can power your solenoid from a single-phase output of a PWM VFD. It's not that kind of ac voltage.
Myth No. 2: All VFDs are the same
The common ac VFD of today is a fairly mature product. Most commercially available drives contain the same basic components: a bridge rectifier, a soft-charging circuit, a dc bus capacitor bank, and an output inverter section. Granted, there are differences in how the inverter section does its switching, the reliability of the components, and the efficiency of the thermal dissipation scheme. But the basic components remain the same.
There are exceptions to this "all-the-same" thinking. For example, some VFDs offer a three-level-output section. This output section allows the output pulses to vary from half-bus, voltage-level pulses and full-bus level pulses (see Figure 4).
To achieve the three-level output, the output section must have twice the number of output switches, plus clamping diodes (see Figure 5). The benefit gained by using a three-level output is reduction in voltage amplification at the motor due to reflected wave, lower common-mode voltage, shaft voltage, and bearing current.
The matrix-style inverter is an even more atypical type of VFD. VFDs with matrix-style inverters do not have a dc bus or a bridge rectifier. Instead, they use bidirectional switches that can connect any of the incoming phase voltages to any of the three output phases (see Figure 6). The benefit of this arrangement is that power is allowed to flow freely from line-to-motor or motor-to-line for fully regenerative four-quadrant operation. The drawback is that filtering is required on the input to the drive because extra inductance is necessary to filter the PWM waveform so that it does not affect the input ac lines.
In addition to VFDs with three-level outputs and matrix-style inverters, there are more examples that prove not all ac VFDs are the same.
Learn about additional VFD myths including power factor (PF) issues and what a VFD input current should be.