What is Single-Phasing in a 3-phase system?

This engineering brief introduces the concept of a single-phase fault as applied to a three-phase system. It emphasizes the potentially destructive impact on motors. It also extends the concept to include phase imbalance which is equally detrimental to the three-phase induction motor.

What is a 3-phase system?

Our national power grid is an example of a three-phase system. The large generators, switchgear, and distribution systems are all built to supply three-phase alternating current. In a three-phase system, each waveform is offset from the others by 120 electrical degrees.

There is a rich history describing how this system was developed in the early 20th century. The distilled narrative revolves around the three-phase motor which was, and remains, the largest consumer of electrical power. The three-phase system remains prevalent because it is optimized in terms of engineering economics, optimization, and reliability.

Figure 1: Picture of a three-phase motor next to a reversing motor starter with thermal overload block.

What is a single-phase fault?

A single-phase event occurs when one leg of a three-phase system is disconnected or otherwise lost. At first, this may appear to be a poor definition, as 2 of 3 phases are still present. However, that’s not how things appear inside the three-phase motor. From the motor’s perspective, the remaining two phases appear as a single phase:

  • With all three phases: a rotating magnetic field is present in the motor’s stator. Recall that this rotating magnetic field is one of the crowning achievements of 19th century electrical engineering with widespread adoption in the early 20th century. This rotating field is the enabling technology upon which all advantages of the three-phase motor rest. This includes small size, low weight, and high efficiency.

  • With a missing phase: the stator’s magnetic field is no longer “rotating.” Instead, the magnetic field is expanding and contracting in a single physical plane. Consequently, the three-phase motor operates as a single-phase motor.

Tech Tip: The rotating magnetic field concept also applies to the single-phase motor. For example, one of the most common failure modes for a single-phase motor is damage to the phase shift capacitor. A single-phase motor with this type of failure will not self-start. This is such a common failure that many motors are built so that the user can quickly replace the capacitor as shown in Figure 2. Recall that this capacitor is a critical component in many single-phase motors as it creates a phase shift. From the motor’s perspective, this phase shift appears as a rotating magnetic field. Another way to visualize this situation is to recognize that the capacitor turns a single-phase motor into a two-phase (quadrature current) motor.

Figure 2: Picture of a used single-phase motor with the cover removed exposing the user-replaceable phase shift (starting) capacitor.

Motor symptoms in a single-phase fault condition

Assuming the motor is hard connected to the AC feeder with no protection, there are three likely results all of which are bad:

Fail to Start

A three-phase motor will not start unless all three phases are present to produce the rotating magnetic field. All three phases are required to produce the rotating magnetic field. In a single-phase situation the motor winding will be damaged.

Already running with heavy load

If the motor is already running when the fault occurs, it will likely stall when a phase is lost. Without protection the motor winding will once again be damaged.

Already running with a light load

In this situation the motor may continue to run. However, extended operation in this condition could damage a winding.

Tech Tip: Note that a phase does not need to be fully disconnected. An unbalanced 3-phase system can also damage the motor. If you suspect a problem, be sure to safely measure the 3-phase supply voltage as well as the motor current. It’s a three-step calculation. Step one is to calculate the average e.g., average voltage. Step 2 is to determine the deviation of each phase from the average. The last step is to divide the largest deviation by the average. The result for voltage should be less than 1 %. Note that a slight voltage imbalance can cause a large current imbalance.

As an example, consider a 208 VAC system with phase phase-to-phase voltage of 203, 202, and 210 VAC as measured at the motor. The average is 205 VAC. The greatest deviation is 5 VAC. The percent deviation is calculated as 100 * (5/205) = 2.4%. This system is considered unbalanced and in need of repair at the next available opportunity. Any motor operating at 100% mechanical load will likely have a reduced lifespan.

How can the motor be protected?

The first line of defense is to incorporate a motor starter. Recall that a motor starter is composed of two parts including the contactor and a thermal overload block. For long motor life, it is vital that the overload block be matched and adjusted to the associated motor. This is the central part of this article and bears repeating:

For long motor life, it is vital that the overload block be selected and adjusted to match the associated motor.

Think of the motor starter overload block as an empathic device that reflects the state of the motor’s windings. Recall that the traditional motor starter is a thermal device with heaters and bimetallic elements that respond to the motor’s current. In a perfect system there is a 1 to 1 correspondence between the motor’s winding temperature and heat in the thermal overload block. In an overload condition both the motor coils and the thermal overload block will be hot. If this condition continues for too long, the thermal overload block will trip thereby depriving the motor starter’s coil of power. Loss of coil energy then open the contactor disabling the motor.

This business with the motor and motor start is subtle. Initially, you may have thought that a 1% voltage deviation wasn’t a big problem. However, as pointed out, a small voltage imbalance can lead to a large current imbalance. Which in turn leads to overheating of a single or pair of motor windings. This situation is further complicated by the variable loads that are placed on a motor. The complexity increases when we have a heavily loaded motor operating in an unbalanced system. Without delving into the specifics, recognize that a motor must be derated when operating in an unbalanced system. The derated mechanical output horsepower can be calculated. However, you may be better off improving the hygiene of the feeder to maintain a better phase balance free of harmonics.


In all cases, a properly selected and configured motor starter will provide a good first line of protection. Motor overload conditions as well as excessive current deviation caused by single phase faults will be detected by the motor starter.

Naturally, the sophistication of the system should be related to the cost of the motor. A 1 hp motor fed and protected by a rudimentary motor starter is a good combination. A 100 hp motor should have state of the art monitoring and protection that reflect the significant capital investment of the motor. Routine monitoring and maintenance of your 3-phase feeder is a necessary step to protect your 3-phase equipment investment.

Best Wishes,


About the author

Aaron Dahlen, LCDR USCG (Ret.), serves as an application engineer at DigiKey. He has a unique electronics and automation foundation built over a 27-year military career as a technician and engineer which was further enhanced by 12 years of teaching (interwoven). With an MSEE degree from Minnesota State University, Mankato, Dahlen has taught in an ABET accredited EE program, served as the program coordinator for an EET program, and taught component-level repair to military electronics technicians. Dahlen has returned to his Northern Minnesota home and thoroughly enjoys researching and writing articles such as this. LinkedIn | Aaron Dahlen - Application Engineer - DigiKey

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Hey APDahlen,

Thank you for sharing this information, this is very insightful. Now my all confusion about three phase system has gone.


Thank you jamesbolt,

Welcome to the DigiKey forum.

Three phase is such a rich topic. I was once in a commercial building that lost one leg. The service technicians were not happy as several of the HVAC motors did not survive.

In another application, I worked on a 7.5 hp two-speed fan that was constantly throwing belts. As we troubleshot, we found that the motor was spinning CW for low speed and CCW for high speed. That certainly explains the squeal and belt issues. A two-wire swap of the high-speed motor connections and we never had a problem again.