Electric motors have many applications in residential, commercial, and industrial settings. To accomplish their intended functions, they require reliable control and protection. Motors face exposure to several hazards that can reduce their service life. As well as major electrical faults can damage them beyond repair.

Well-designed motor control is complemented with adequate protection devices. The following are some of the main electrical hazards that must be considered:

• Overload
• Short circuits and ground faults
• Stalling during start-up
• Voltage imbalance
• Phase loss and phase reversal
• Undervoltage and overvoltage

Given the range of hazards to which they are exposed, electric motors must use a combination of devices. This article provides an overview of the most common protective features.

Motor Circuit Protectors (MCP)

A motor circuit protector is a magnetic device that responds to the current drawn by a motor. This device offers protection against high-current hazards. For example, short circuits, ground faults, and stalling during start-up. The magnetic field caused by these currents triggers the MCP, which disconnects the power supply to protect the motor.

An MCP does not respond to minor overloads, voltage issues, and phase imbalance, since these events do not produce a current peak. Therefore, other protection devices are necessary for these electrical hazards.

Overload Relay

Overload occurs when an electric motor draws continuous current above its nameplate value. This causes overheating. Even if a motor is slightly above rated current, it is still considered an overload.

An overload relay is a protection device with a slight delay in its response. This allows high-current events with a short duration, such as the inrush current when starting. An overload relay disconnects a motor when a current peak lasts longer than expected in normal operation. Based on their operating principle, overload relays can be described as thermal or electronic:

• A thermal overload relay uses a bimetallic strip that expands with the heat released by high current. As the current magnitude increases, the thermal expansion and motor disconnection occur faster.
• An electronic overload relay accomplishes the same function. But it measures current directly instead of relying on thermal expansion.

Electronic overload relays are more accurate and unaffected by ambient temperature. This is because they do not rely on thermal expansion. Although, they are also more expensive than thermal relays.

Motor Protection Circuit Breakers (MPCB)

A motor protection circuit breaker is a combined device. It has both an instant response and a delayed response. An MPCB has a magnetic protection element similar to an MCP. The element disconnects the motor immediately in response to high currents. Such as those caused by short circuits and ground faults. However, an MPCB also includes a delayed response like an overload relay. Which can be thermal or electronic.

In a few words, an MPCB combines the functions of an MCP and an overload relay. Additional protections are still necessary for voltage and phase issues.

Voltage and Phase Monitors

Ideally, a three-phase motor should operate at rated voltage, and the phase voltages should be perfectly balanced. Yet, achieving this is unlikely in actual projects, and there is some tolerance for voltage variation and phase imbalance:

• According to the US Department of Energy, most motors are designed to operate within 10% of their rated voltage. For example, if the nameplate voltage is 220V, the motor can operate from 198V to 242V.
• A small phase imbalance is acceptable. But the difference between the lowest and highest voltages should not exceed 4%.
• Undervoltage, overvoltage, and phase imbalance reduce the service life of motors. They also cause performance issues and overheating.

Electric motors can also be affected by single-phasing. Which occurs when one phase is disconnected completely. A motor can continue running in this condition. But it becomes unstable and its windings overheat. Also, noise and vibration increase with single-phasing.

Motors can also be affected by phase reversal. Which happens when two-phase voltages are switched. A phase reversal causes the motor to rotate in reverse, and the consequences will depend on the load. For example, a fan will move air in the opposite direction. But heavy industrial motors with the wrong phase sequence can cause severe accidents.

A voltage monitoring relay disconnects a motor when its operating voltage falls outside the optimal range. Typically, within 10% of the rated voltage. On the other hand, a phase monitoring relay disconnects a motor in response to phase loss, reversal, and excessive imbalance.

Conclusion

Electric motors need adequate controls and protection devices to meet their performance and safety requirements. Many electrical hazards can affect motors. No single protection device cannot respond to all. For this reason, motor controls must combine various protective features. 

The specific protection requirements for electric motors depends on the application. But in general, they must be guarded against voltage variations and excessive current. In the case of three-phase motors, protection devices must also ensure that phases are balanced and sequenced correctly.