How Does a Motor Overload Work?

Electric motors convert electricity into mechanical energy and are a key to powering a variety of modern devices. All sorts of equipment and devices use electric motors, including vehicles, fans, blowers, cranes, toys and other electronic mechanisms. An overload happens when an excessive amount of electricity passes through the system to the motor, which can lead to motor failure. To prevent electric motor breakdowns, motor overload relays are used to protect motors from too much current, inadequate torque and general overheating, which is a prime cause of motor failure. Today, we dive into the question: “How does a motor overload work to prevent failures?”

How Does a Motor Overload Work?

An overload occurs when a motor draws too much current. An overload triggers a relay that causes the motor to shut down, which prevents damage due to overheating. Overload relays are a component within a motor starter, which monitors the current flowing through the circuit in order to protect the motor. When current goes above a predetermined limit for a specific amount of time, it causes the overload relay to trip.

This results from an auxiliary contact that interrupts the motor’s control circuit, which in turn stops electrical energy from flowing to the contactor. When power stops flowing to the motor, it cannot overheat, saving the motor from catastrophically breaking down or otherwise being damaged. These overload relays have the capability to be reset manually, though some reset automatically, allowing the motor to restart.

What a Motor Overload Relay Does

The basic principle on which overload relays work is simple. Most depend on a component called a bimetallic strip, which is actually two strips made from distinctive metals that, when heated, expand at different rates. This arrangement within the motor’s circuit uses this differentiation in temperature changes to bend the bimetallic strip when it’s heated.

Once the bimetallic strip heats up, it allows the contact to activate, breaking the power supply and stopping electricity from moving toward the contactor coil. This stops the flow of current toward the motor, deactivating it. An overload relay allows electricity to flow through it during normal operation, only tripping if excess current flows through it. Tripping gives an operator time to determine the cause of the overload without jeopardizing the motor or possibly causing injury.

Basic Parts of an Overload Relay

Overload relays are made up of more than contacts and a bimetallic strip.

Most also feature the following:

  • Terminals in the relay diagram are labeled as L1, L2 and L3, which mount directly towards the contactor, while motor supply connects to T1, T2 and T3 terminals.
  • Ampere range setting via a rotating knob controls the rate of current flow going toward the motor, and can be adjusted to higher or lower limits as needed.
  • Reset buttons are used to reset the relay once faults and trips have been cleared; these manual or auto-reset selection buttons allow operators to either manually or automatically reset after the relay trips, with auto-reset buttons capable of remote resetting.\
  • Auxiliary contacts interrupt the motor control circuit, cutting off current to the contactor.
  • Test buttons check the relay’s control wiring.

Electrical Knowhow: Does Motor Overload Workability Affect Relay Applications?

Electric motors are not all the same, and motor overload relays differ too in how they work, depending on the application for which they’re designed. Once these differences are understood, the question “How does a motor overload relay work?” can then be answered more definitively. Yet all overload relays have this in common: they protect the motor by deactivating it when too much electricity passes through the system.

Types of motor overload relays, along with their applications, include:

Thermal Overload Relay

These devices primarily just cut off power after exposure to prolonged and excessive electrical current, similar to how circuit breakers operate. The difference in how thermal overload relays work involves their secondary purpose, which is to measure the motor’s heating profile over time. When too much electricity passes through the motor’s circuit over a specified period, heating the motor excessively, it shuts the motor off by breaking the circuit.

Magnetic Overload Relay

These relays work by detecting the strength of the magnetic field that’s generated by electricity flowing to the motor, cutting off power not in response to high temperatures but rather because of changes to the motor’s core. It’s often designed with a coil that surrounds a variable magnetic core and which holds current going to the motor. The coil drags the core upwards and, once it expands far enough, it trips connections at the relay’s summit. Magnetic overload relays work well in places where there are extreme changes in ambient temperature.

Bimetallic Thermal Overload Relay

These overload relays rely primarily on the heating properties of the bimetallic strip, relying either on direct or indirect heating. Directly heated, these overload relays supply all the electrical current that flows to the motor, and it relies on the heat from the current’s flow to trip. Indirectly heated relays use a conductor within the relay that comes in close contact with the strip. Electrical flow heats both the conductor and bimetallic strip as the current flows towards the motor. When the current heats too much, it bends the bimetallic strip and activates a mechanism that halts the flow of electricity to the motor.

Electronic Overload Relay

Also called solid-state overload relays, these don’t have a bimetallic strip, resulting in less heat loss through the relay. Instead, they feature current transformers that measure the current continuously passing on to the motor during each phase. These current flows pass through the transformers and a circuit that evaluates them to identify an overload, tripping the circuit when overloads occur. Electronic overload relays are controlled by a microprocessor within a device designed to protect three-phase motors when they experience phase failure or when temperatures rise to a certain level, which can affect single phase motors as well. Some manufacturers include design features to protect against the motor stalling or providing an earth fault. Electronic overload relays work well in situations when motors must stop and start frequently.

Eutectic Overload Relay

This relay features a tube that surrounds an eutectic alloy, a winding heater and a mechanism that activates the tripping device. The term “eutectic” means “easily melted”, so an eutectic alloy refers to a blend of materials that goes directly from a solid to liquid state without passing through an intermediate stage. This alloy dissolves due to heat, enabling a spring-loaded ratchet wheel to activate the tripping device when an overload occurs. Once tripped, the relay can be manually reset with a reset button on the relay cover.

Refrigerator Overload Relay

In refrigerators, the overload relay is positioned within the compressor circuit. Current passes through the overload relay to the compressor motor windings. The relay brings the start winding into the circuit, but only includes it until the compressor begins to run at full speed. If the overload relay senses overheating, it shuts the compressor down temporarily to let it cool.

Contact Springer Controls

Springer Controls has been a trusted electrical controls industry leader since 1996, supplying motor starters, power-switching devices, 22 mm & 30 mm pilot devices, hoist controls, signaling devices, and other control devices across a variety of industries. If you have questions about motor overload and associated products, contact the experts at Springer Controls today!