This engineering brief introduces the 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. It concludes with a recommendation to protect your expensive motor by incorporating a motor starter with a properly tuned thermal or solid-state overload relay.
Key Takeaways
- Loss of a phase or poor phase balance can damage (overheat) a three-phase induction motor.
- A motor starter with a properly matched overload relay can automatically disconnect the motor before winding damage occurs.
- A motor that is already running when a phase is lost may continue to run depending on the load torque. There is a high risk of the motor running until it is burned out.
- Loss of a starting capacitor in a single-phase motor is similar to the loss of a phase in a three-phase. In both situations the rotating magnetic field is lost.
This article is part of the DigiKey Field Guide for Industrial Automation
Location: Understand It → Motors
Difficulty: 
Student — difficulty levels explained
Author: Aaron Dahlen | MSEE | Senior Applications Engineer, DigiKey
Last update: 10 Mar 2026
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 cost-effectiveness, efficiency, and reliability.
Figure 1: Picture of a three-phase motor next to a reversing motor starter with thermal overload block.
Author’s reflections (experience): Losing a phase in a commercial building is a not a good situation. If left unchecked, some of the building’s loads will still work while others will not. The most visible symptom is clusters of overhead lights that no longer work.
For these lights it’s an inconvenience (provided they aren’t phase-to-phase connected). However, it’s very bad for the three-phase motors. I’ve seen this situation frustrate electricians, as the often need to replace the resulting burned-out motors.
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:
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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.
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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.
What happens to a motor when an input phase is lost?
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. Without this rotation the motor will not turn and will quickly burn out.
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 a three-phase motor be protected against a missing phase?
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:
Tech Tip: 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 opens the contactor disabling the motor.
The relationship between 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.
Conclusion
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,
APDahlen
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About This Author
Aaron Dahlen, LCDR USCG (Ret.), is a Senior Applications Engineer at DigiKey in Thief River Falls. His background in electronics and industrial automation was shaped by a 27-year military career as both technician and engineer, followed by over a decade of teaching.
Dahlen holds an MSEE from Minnesota State University, Mankato. He has taught in an ABET-accredited electrical engineering program, served as coordinator of an electronic engineering technology program, and instructed military technicians in component-level repair.
Today, he has returned to his home in northern Minnesota, completing a decades-long journey that began with a search for capacitors. Read his story here.