Variable frequency drives (VFDs) have been commonly used in industrial and commercial applications for decades. The basic design topology of the VFD with a diode bridge, DC bus capacitors and insulated gate bipolar transistor (IGBT) output has proven to be reliable and beneficial from application and financial perspectives. In general, these applications have been installed on a balanced three-phase power supply, meaning that the impedance line to line is balanced. Therefore, under normal conditions, the voltage and current are balanced phase to phase. While there can be differences relative to ground—which may be done intentionally for single-phase loads such as lighting—the phase-to-phase relationship remains balanced.
In many rural areas, the balanced, three-phase supply is commonly replaced by a grounded, open-wye to center-tapped, grounded, an open-delta configuration. This is done mainly to save on installation costs since the three-phase supply only requires two transformers and two current-carrying conductors. The open-delta configuration is generally suitable for locations at which the majority of the load is single-phase and a relatively small amount of three-phase load exists. End users and installers of rural pumping systems need to be aware that an open-delta supply is feeding their application because unbalanced supplies can cause the overheating of three-phase motors that are run across the line. Installing a VFD in front of the motor will isolate the motor from the imbalance. However, the imbalance is transferred to the VFD. Therefore, when installing VFDs or other three-phase loads on open-delta supplies, proper steps must be followed.
Topology of most common VFDs
If proper precautions are not taken when installing a VFD on an open-delta supply, premature failure of the drive can be expected. Types of VFD failures seen are one or more shorted diodes and/or DC bus capacitor failures. When or if these failures will occur depends on the actual load placed on the VFD and the quality of the power being delivered to the drive. Open-delta systems often exhibit increased voltage fluctuation as single-phase loads are applied and removed from the sometimes unevenly sized transformers. Furthermore, open-delta configurations are often used at the end of the line where high line impedance may further exacerbate voltage fluctuation.
Understanding that the time to premature failure may be quite long, possibly years, is important. A properly installed and sized VFD should be able to run trouble free for several years, and some systems have been running more than a decade. Understanding installation and properly accounting for any variables, such as an open-delta supply, is the best way to maximize a VFD’s run life.
This article examines the results of one field test on an actual open-delta supply. Two key areas will be affected by the installation of a VFD on an open-delta supply:
- The load imbalance’s effect on the VFD’s components
- Mitigation of harmonics produced by the VFD
Identifying an Open-Delta Feed
How do users identify if their power is from an open-delta feed? Most rural pump installations are at the end of a utility wire run and have dedicated transformers feeding them. First, look at the utility pole that holds the transformers supplying the service and identify the current-carrying lines that run to the transformers. Simply count the transformer cans hung on the pole. Generally, a balanced, three-phase system will have three transformer cans, although there are single cans containing all three windings, so noting the current-carrying lines is important.
|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.