Three-Phase Power and Power Quality Considerations on Farms

For decades, agriculture and farming have been vital to North Carolina’s economy, and our state is a leader in tobacco, soybean, poultry and hog production, and more. Critical to the success of agricultural farmers, producers and supporters is stable and reliable incoming power. If disruptions occur that affect proper air circulation and ventilation, chickens will suffocate; if watering is impacted, plants will die. The pumps and fans used by farmers are driven by motors that require electric power — power supplied by the grid.

One consideration for farms is that it can be difficult for those served by single-phase power (Figure 1) to take advantage of pumps and fans that rely on polyphase or three-phase motors, which are typically more efficient and come in larger output ratings than single-phase motors. Tasks such as ventilating, irrigating, grain handling, feather plucking and many others utilize three-phase motors. There are, however, two ways for these farms to use three-phase equipment: 1) getting three-phase power added by the electric utility provider, and 2) installing equipment that converts single-phase to three-phase power between the grid and end-use equipment.

Figure 1: Single-Phase Voltage Waveform — Grid Power

In the first scenario, an agricultural producer would approach their electric utility with a request to install three-phase power at their farm. If the farm is the only electric user on that utility transformer, the upgrade could be costly — potentially tens to hundreds of thousands of dollars depending on the distance between the farm and the nearest substation or existing three-phase power infrastructure — and the utility would likely charge the customer for it. (A farm far from the nearest substation may also be more likely to encounter power quality issues than a farm closer by.)

The second option is less costly but still may be expensive given the constraints of a producer. Equipment that converts single-phase to three-phase power is available on the market and includes variable frequency drives (VFDs), static phase converters and rotary phase converters.

A VFD is typically used in the motor industry to regulate the speed of an electric motor. It can slow a motor feeding a variable-flow process (like a pump or fan) to properly serve the application while saving energy. This occurs thanks to the affinity laws, which state that if you can reduce the speed of a system, you can reduce the input power of that system by the cube of that speed reduction. As an example, if a fan that spins at 1,800 revolutions per minute (RPM) and draws 10 kilowatts (kW) of power could be reduced to 900 RPM, the power could be reduced by 2³ (or 8). So, the 10 kW of power draw would drop to 1.25 kW (in an ideal scenario; these numbers are likely overestimated for an actual application).

A VFD can further be valuable on farms as some models can take in single-phase power and output three-phase power. The three-phase out is not an AC signal but rather a pulse-width modulated waveform (more similar to a square wave than a sine wave as supplied by the grid, see Figure 2). This output can serve a motor but would not be appropriate for non-motor loads.

Figure 2: Pulse-Width Modulated Voltage

A VFD, however, may not be ideal for long-term motor reliability if the motor it is supporting was not originally designed for use with a VFD (i.e., is not an inverter-duty motor). In fact, a VFD can shorten motor life if not properly wired and appropriately grounded (adding shaft grounding rings can deal with bearing currents).

VFDs are also susceptible to transient voltage events, such as voltage sags and swells, which can be good for certain applications but detrimental to others. VFDs often serve as a protection device and isolate incoming grid power from impacting the motor leads. If a VFD senses a transient voltage event or momentary power outage on its input terminal, it will likely trip the output and shut off power at the three-phase output terminals. This would hinder an application that requires constant air movement, like ventilation in a poultry house or water flow through an irrigation pump. If a VFD is used in these types of applications, it can be programmed for ride-through if momentary outages are common.

Phase converters represent another approach. A static phase converter takes in single-phase power and outputs three-phase power using a capacitor bank to assist with starting a three-phase load. Static converters are useful with applications that start and run for short periods. A rotary phase converter takes in single-phase power and outputs three-phase power using a rotational inductive transformer in addition to a capacitor bank. Rotary converters are useful with applications that run continuously. Phase converters broadly are also suited to serve other loads if a farm has non-motor electrical equipment.

One downside to phase converters is the severe voltage unbalance they supply at loads below current rating. There are ways to mitigate this with different auto-transformer tap settings during converter installation, but if the load is not matched closely to the size of the phase converter, voltage unbalance will be present at the motor or other equipment terminals. For a three-phase motor, voltage unbalance leads to current unbalance, and current unbalance leads to hot spots in the windings on the phase with the highest current (see Figure 3). Hot spots, in turn, lead to premature motor failure.

Figure 3: Hot Spots in an Electric Motor

Phase converters handle transient voltage events and momentary outages well based on testing in our lab, but they do include capacitors. Rotary converters specifically utilize a centrifugal switch for the spinning transformer. These electronics can be sensitive to transient voltage events if there are many over the life of the equipment. If you have consistent light flicker or other noticeable power quality events at your farm, those should be communicated to the vendor before moving ahead with a phase converter.

We’ve completed extensive testing on all of the above options, and each offers a unique solution to obtaining three-phase power on a farm and for navigating power quality issues. There is no one-size-fits-all approach, though, so your particular application must be considered. If three-phase equipment is needed long term, it’s best to start with your electric utility. If your utility is unable to install three-phase service or it is cost-prohibitive, you’ll want to weigh the features and benefits of VFDs and phase converters.

If you need help identifying the best path forward, we’d be happy to work with you.