by Daniel Peters, Drives Application Engineer, Yaskawa America

Input current imbalances happen when a variable frequency drive (VFD) with a full-bridge rectifier operates under lightly loaded conditions. Here we explain what to do about it.

One phenomenon that electricians often see after installing a VFD is measurable current imbalance on the input to the VFD when that VFD operates under light load. In across-the-line (ACL) starters in which a three-phase induction motor runs at full speed and voltage, imbalances are problematic and need further diagnosis. However, when a VFD provides variable speed in response to variable loads, the two systems are really as comparable as apples and oranges.

In fact, imbalance at the inverter input under lightly loaded conditions is a normal phenomenon of the electrical circuit. Imbalance arises from the connection of the electrical supply to the VFD, not to the motor itself. More specifically, the issue is the VFD’s full-bridge rectifier.

First some background: A common VFD rectifier section is composed of six diodes. These semiconductors conduct current in one direction to act like a check valve in water-supply systems. (Check valves only let fluid flow when the pressure on the supply side is greater than the pressure on the discharge side.) Diodes work in roughly the same way, but the pressure equivalent is the electromotive force or voltage.

Diodes perform myriad functions in electronic circuits; in the case of a bridge rectifier, diodes convert ac current to dc current. The dc current then connects to a circuit of specialized transistors called insulated gate bipolar transistors (IGBT) with the motor coil between them. A transistor is simply a semiconductor switch that can turn on and off at high speeds, typically 2 to 15 kHz. It is the IGBTs that directly control voltage and frequency sent to the motor coil.

So, current imbalance on a lightly loaded VFD is normal in the presence of any voltage imbalance phase to phase. In addition:

• The dc bus voltage of the VFD is unregulated and varies depending on line-supply voltage levels and motor load.
• As long as current draw on all the input legs is at or below the drive input current rating and the voltage imbalance of the three-phase supply is within NEMA recommendations, there should be no excessive wear to VFD input components. The VFD separates the connection of the line supply to the motor, so the VFD protects the motor from unbalanced line voltage.
• Input line reactors and dc bus chokes are simple, cost-effective devices that add impedance to the VFD input and mitigate voltage imbalance. Another benefit to their addition is a reduction of reflected harmonics from non-linear current draw.

More specifics on VFD line
For what follows, refer to the figure showing a common VFD circuit on a three-phase supply. As mentioned, the way the bridge rectifier comes between the line and the motor makes comparisons of VFD and ACL setups invalid. Current imbalance on ACL motors commonly arises from voltage imbalance phase-to-phase on the line supply and should be maintained to NEMA limits of 1% (NEMA MG1, 12.45). If not, motor performance suffers and motor-coil damage can occur.

Because the VFD severs the direct connection of the line supply to the motor, it acts as a buffer and actually protects the motor from phase-to-phase voltage imbalance. A VFD can even operate a three-phase motor on a single-phase line. This phase conversion is a common application for VFDs where only single-phase current is available. Sizing VFDs for actual single-phase input should follow VFD manufacturer guidelines to ensure that the VFD can supply rated power to the motor without excessive loading to VFD components.

A caveat: Our discussion here concerns lightly loaded VFDs on three-phase power with small voltage imbalances of 1 to 3%. Voltage imbalances on the electrical grid are common in most areas, but plant engineers should find ways to correct them if they’re outside NEMA specifications.

Troubleshooting tip: To verify whether the current imbalance arises from the supply or upstream components, rotate the three input phases to the VFD and compare the current measurements before and after. If the current imbalance tracks the rotated phases, the supply or upstream components are likely responsible. If the imbalance stays with the drive input phases, the VFD may be responsible.

To be clear, any presence of impedance-producing devices is known and their introduction isn’t a troubleshooting technique. The purpose of rotating the lines is to isolate where the imbalance begins in the supply line and to isolate the device responsible for the imbalance.

In other words, tests on the line side of each device isolate the imbalance.

Light loading is a likely operational condition where VFDs vary the speed of fans and pumps (which follow the laws of affinity). This is a common VFD application because speed control is an efficient way to regulate flow and reduce power consumption. In this condition, even voltage imbalances within NEMA specifications can cause unbalanced current draw phase-to-phase with the highest line voltage supplying most of the current. This is due to the design of a rectifier that passes current like a check valve and the fact that the supply power is alternating from positive to negative (ac) voltage.

Because voltage is a measure of electromotive force, the diode only conducts when line-supply voltage is greater than the dc bus voltage on the diode’s other side. This means that diodes only conduct current at the peaks (positive or negative) of the voltage waveform. This means that the dc bus voltage will measure at the peak of the ac sine wave.

When engineers measure the ac line-supply voltage with common multimeters at a fixed value, actually displayed on the meter is a voltage expressed in units of root mean squared (RMS) voltage. Voltage measured is alternating from positive to negative, so the average value is zero. The RMS value is the area contained in the sine wave between zero and peak. This measure represents the amount of electromotive force supplied by the ac line. So, the peak of the voltage waveform is the RMS value times √2 (or 1.41). For example, 460 volts RMS x 1.41 equals 648.6 volts peak. If an engineer measures the dc bus voltage of a VFD supplied with 460 Vac, he or she will measure 648 Vdc. As the motor is loaded down, the dc bus voltage naturally drops.

A diode in which supply voltage exceeds the reduced dc bus voltage will turn on and replenish the dc bus. Because the three phases of the line supply are offset by 120 electrical degrees, only one pair of diodes (one positive, one negative) out of the six conduct at any time. Once the line voltage drops below the dc bus voltage, diodes turn off to let a pulse of current through to the dc bus circuit. This is why this type of bridge is also called a six-pulse bridge and why it draws power in a non-linear manner (so the current waveform doesn’t match the voltage waveform).

Current only conducts to the dc bus when there’s a voltage difference between the dc bus and line supply. So when the VFD is lightly loaded and one of the supply lines has a higher line voltage, it will resupply the dc bus first, and conduct longer than the other phases.

Once the dc bus is pulled down further by the motor load, the other diodes begin to turn on. But imbalance in current remains, depending on the level of phase-to-phase voltage imbalance.

In short, current imbalance on the input of the VFD is normal with any level of voltage imbalance but is greatest under light load. It is not an excessive load to VFD input components on a three-phase supply.

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