Power Quality: Single Phasing in Equipment

Single phasing of a motor occurs when one of the phases of the motor fails, which causes voltage imbalance due to the remaining phases trying to deliver the full horsepower needed to function. The result of single phasing in a power system is most pronounced with three-phase induction motors. Most three-phase motors will continue to operate with power from only two phases, however, this will result in overheating and failure if not protected.

Most of the commonly used protective devices, such as conventional circuit breakers or thermal overload relays for motors, are not designed for detecting single phasing conditions. For lightly loaded three-phase motors, say 70% of normal full-load amperes, the phase current will increase by the square root of three under secondary single-phase conditions. This will result in a current draw of approximately 20% more than the nameplate full load current. If the overloads are sized at 125% of the motor nameplate, circulating currents can still damage the motor. Therefore, it is recommended that motor overload protection be based upon the actual running current of the motor under its given load, rather than the nameplate current rating. The most common fault on the utility system is a single phase-to-ground fault. It is over twice as likely to be a single-phase outage as a three-phase outage. Normal events are outages caused by weather conditions, third party contacts, utility equipment failure, etc.

In accordance with SWEPCO’s Terms and Conditions, it is a customer’s responsibility to protect their three-phase motors from single phasing.

Strategies to use to help protect your equipment

1. Install a circuit breaker with a built-in shunt trip overload relays that are adjustable to 125% of full load current with a time delay to allow for startup. This type of breaker is more expensive than a conventional overcurrent relay breaker. The most important application issue to keep in mind is the type of sensing used with the relay as both voltage and current sensing is offered. While voltage sensing is less expensive it is not as accurate because motor back-EMF (reverse current) can mask the loss of phase.
2. Install a phase failure relay with a motor controller or tie it in with the undervoltage trip of the breaker if the breaker has this extra cost option. This type of relay looks at phase sequence, unbalance and undervoltage. It is also the fastest trip but can cause nuisance tripping.
3. Use dual element fuses, which is the least expensive. The key element is proper sizing and two elements that allow for a time delay for start-up. The only disadvantage to this method is that there must be spare fuses. The built-in time delay allows for momentary sags in voltage while not causing nuisance tripping.
4. Specify a higher standard motor that has a much higher heat resistance and better insulation since a major cause of motor failure is overheating the insulation. Therefore, improving the insulation value lengthens the time before failure. The simplest improvement is to specify a service factor of 1.15 instead of 1.0. Instead of the typical class B insulation, use a higher rating. These are extra cost options as well.

Some customers use an inexpensive solution, commonly referred generically as a “motor saver.” Typically, this inexpensive solution leads to nuisance tripping caused by normal utility events, not just lost of phase conditions. So, as you look for ways to help reduce single phasing with equipment, consider some of the strategies mentioned above for your operations.