Variable frequency drives may be industry-standard technology, but they aren’t immune to myths and misconceptions. What’s worse is how they disrupt the development of VFD solutions for OEMs looking to implement them.
It’s time to dispel these myths once and for all, starting with the most common ones engineers will encounter.
VFD Myth #1: VFD Output is Sinusoidal
Starting an ac induction motor using a motor starter involves connecting its three phase leads to three-phase power. Each phase contains a frequency of 60 Hz and a voltage amplitude of 230 V, 460 V, or 575 V. This voltage creates a sine wave current waveform that is equal to the frequency when checked at the motor leads.
This is very different from what occurs at the output of a VFD.
VFDs rectify a three-phase input to a fixed dc voltage that is filtered and stored using dc bus capacitors. The dc bus voltage then goes through an inversion process that uses three insulated gate bipolar transistor (IGBT) pairs, one for each output phase. This inversion process yields a variable voltage, variable frequency output.
Essentially, the dc voltage is converted into ac.
In VFDs, users can control how long an IGBT switch in any given pair is turned ON or held OFF, which allows them to determine the root mean square (RMS) value of the output voltage. There is a fixed, linear relationship between output frequency and output voltage, allowing for constant torque (with the exception of applications like fans, pumps, etc., which do not require constant torque). The ratio of output RMS voltage to output frequency also determines the flux developed in the ac motor. Output frequency increases and output voltage should increase at the same rate, which keeps the motor flux constant.
The resulting voltage waveform that is applied to the motor winding is not sinusoidal.
VFD Myth #2: All VFD are the Same
Most VFDs contain the same components:
- Bridge rectifier
- Soft-charging circuit
- Dc bus capacitor bank
- Output inverter section
While the basic components are the same, VFDs differ in many ways:
- How the inverter section switches
- The reliability of the components
- The efficiency of the thermal dissipation scheme
Some VFDs also have a three-level-output section that allows the output pulses to switch between half-bus, voltage-level pulses and full-bus level pulses. Three-level output allows for a reduction in voltage amplification at the motor.
Finally, there are VFDs that have a matrix-style inverter that don’t have a dc bus or bridge rectifier. These types of VFDs use bidirectional switches that have the ability to connect any of the incoming phase voltages to any of the three output phases. This arrangement allows power to flow freely from line-to-motor or motor-to-line, allowing for fully regenerative four-quadrant operation.
VFD Myth #3: VFDs cure power factor (PF) issues
VFDs use their internal capacitor buses to supply the reactive current that motors require. This lowers the displacement PF and protects the ac line from being the source of the reactive current. While input displacement PF does improve when a VFD is installed, displacement PF does not fully describe PF calculation.
In order to properly calculate PF, it’s essential to include the reactive power demanded by the harmonics created when ac voltage is rectified to dc.
It’s also important to note that the current conducted by the diode bridge from the ac line to the dc bus is discontinuous. Since diodes only conduct when the voltage on the anode side is higher than that on the cathode side, diodes are only ON at the peak of each phase of the positive and negative portions of the sine wave. This causes the input current to become distorted due to a ripple-like voltage waveform.
You can calculate true PF by adding the displacement PF and the total harmonic distortion (THD), as in the following equation:
True PF ‘ Displacment PF1 + THD2
VFD Myth #4: VFDs Allow Motors to Run at Any Speed
Because VFDs can change their output voltage and output frequency, they give motors the ability to run faster or slower than their rated operating speeds. But there are many limitations to this process:
- Cooling: when totally enclosed fan cooled (TEFC) motors run at very slow speeds, less cooling air gets to them. It is not recommended to operate these motors at a full load below 15 Hz.
- Speed: most motors come with a maximum safe operating speed. It is not recommended to exceed this speed even with a VFD.
- Power: motors that approach their maximum operating speeds may run out of torque. More specifically, the VFD will run out of voltage.
VFD Myth #5: The Input Current of a VFD Should be Higher Than its Output Current
This is more a misunderstanding than a myth. Logic dictates that, because a VFD should lose current due to its own thermal component losses, a VFD’s input current should be slightly higher than its output current. But, if you measure both currents, you will often see that the input current is much lower. This is because the primary consideration should be power, not current.
Consider the following power equation:
The voltage is straightforward; the input voltage is always at the ac line voltage, and the output voltage varies with the speed based on the V/f pattern. It’s the current portions of the equation that bring about the misconception that input current should be higher than output current.
Generally, induction motors have two current components:
- Current that produces the magnetic field (which rotates the motor)
- Current that produces torque
The input current consumed by VFDs is proportional to the active torque demand or load of the motor. Current that produces magnetic fields generally do not vary with speed. Output current may seem higher than input current during low-torque conditions. This is because input current mirrors the current that produces torque (plus harmonics), but ignores current that produces the magnetic field. This is true even at full load conditions.
It’s all about balancing input and output power, not current.
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