|Balanced, three-phase system with three tranformer cans||Balanced, three-phase system with single transformer can||Unbalanced, three-phase system with two transformer cans, note the two different size transformers (50 and 25 kVA)|
Depending on the customer service of the utility, end users may also be able to call and find out what type system is installed at the service address. Users may need to speak to someone in the engineering department. This information should be available to the end user or any contractor installing equipment at the site. Generally, if only two transformer cans are located on the pole, it will be an open-delta (unbalanced) system.
Open-Delta Systems & VFDs
To understand the effects that this configuration will have on the VFD, end users should begin by looking at the basic schematic diagrams of the two systems. Figure 1 shows a balanced and unbalanced system. This is not the only possible configuration of either a balanced or unbalanced system. Figure 1 simply shows two common configurations found in general service. The open delta configuration shown in Figure 1 also has two different size transformers, with the large 50-kilovolt-ampere (kVA) unit being center-tapped to ground so that it can be used for 240-volt, single-phase loads.
The voltage measurement to ground being unequal is not an indication of an unbalanced system because a center-tapped, closed delta would also generate these levels and still represent balanced impedance phase to phase. Unbalanced line-to-line systems will create unbalanced voltage and current phase to phase that is capable of producing excessive heating in standard three-phase motors run across the line. However, when a VFD is placed on this supply, the effects of the imbalance will cause the diode bridge to draw current unevenly from the three legs.
Figure 1. Balanced (top) and unbalanced (bottom) systems
Because the VFD rectifies the incoming supply and inverts it to the motor, it will effectively isolate the motor from the imbalance. The same imbalance that can cause heating in the motor is now transferred to the VFD. This can cause excessive heating of the conductors and any switch gear in front of the drive. It can also cause excessive heating in the diode bridge, as the diodes on the high leg will switch on first and stay on longer than the other legs.
The diode bridge of the VFD will draw current from the utility in pulses and not follow the incoming voltage sine wave, which is the cause of harmonic distortion that gets reflected back to the utility. This is the normal function of the diode bridge. However, on an unbalanced, three-phase input, not only will the diode bridge draw current in pulses, the pulses will not be balanced line to line. This will cause uneven heating of the diode bridge as well as all conductors carrying current to the VFD.
Figure 2 displays the actual current traces of an open-delta system such as the one shown in Figure 1. The current shown in Figure 2 supplies a VFD running a submersible pump for irrigation water. In the top part, all the current traces are on the same zero reference line, and seeing the imbalance of the current pulses is easy. In the bottom part, the same traces are separated so that each one can be seen separately, which allows users to see the imbalance not only phase to phase but also pulse to pulse within the same phase. While the “rabbit ear” $1 is common for a six-pulse diode bridge, the imbalance in current draw is not.
Figure 2. Current traces of an open-delta system running a submersible pump for irrigation