March 2018 | Dreisilker Electric Motors

Why are Overload Relays needed?

When a motor starts, it typically requires 6 times the full-load current rating. After the motor ramps up to operating speed, the current drops off. Motors are designed to handle this overload condition only for a short period of time. If a motor maintains this overload condition, the motor will overheat and potentially become damaged.

While fuses and circuit breakers can protect your system from short circuits, ground faults, or an overload, they are not the proper protection device for motors. As noted above, motors pull significantly more amps at startup than their full-load current rating. Any fuse used with a motor would need to be rated to handle this higher startup amp draw, therefore it would fail to protect the motor from overload conditions beyond normal startup. Overload relays are designed to allow temporary overloads for a specific period during startup. If the overload persists, the overload relay will trip and break the circuit to protect your motor. Overload relays can be easily reset after the overload is corrected.

Overload Relay Trip Classes

Overload relays have a trip class rating for different applications. The most common trip classes are Class 10, Class 20 and Class 30. The number in a trip class is simply the total number of seconds that the motor is allowed to overload before the circuit trips. For example, if you have an overload relay with a Class 10 rating, your system will allow an overload condition for 10 seconds before the overload relay trips to protect your motor.

Types of Overload Relays

A few different types of overload relays include Bimetal Overloads, Ambient-Compensated Overload Relay, and Electronic Overload Relays.

Bimetal Overloads use a bimetal strip that acts as a trip lever. When there is an overload condition, the bimetal strip becomes heated and will bend to close and trip the circuit.

Ambient Compensated Overload Relays are similar to Bimetal Overloads. The main difference is that the Ambient Compensated Relays allow for there to be an ambient temperature, such as the temperature of the surrounding environment. These relays can prevent false tripping by allowing the ambient temperature to be higher.

Electronic Overload Relays do not have heaters found in Bimetal and Ambient-Compensated Overload Relays. The Electronic Overload Relays also offer phase loss protection by detecting phase losses and disconnecting the motor from the power source. There are many types of Electronic Overload Relays to fit a lot of applications.

Installing overload relays in motor applications will prevent motors from running in overload conditions and can protect your motors from damaging heat. There are many types and settings for overload relays. If you need guidance finding the right overload relay for your application, call us today!

Motors and generators are often mounted in a location that makes them susceptible to contamination. Over time, these contaminants cause major problems that can lead to bearing or complete motor failure. In this article, we’re going to outline four common scenarios in which contamination impacts motors and generators. We’re also going to provide solutions to keep your bearings and motors clean and free from contamination.

Breakdown of bearing lubrication or damage to the bearing surface.

Contaminants can breakdown your bearing lubrication and damage the bearing surface. This will cause a buildup of heat from friction. If the problem isn’t corrected, the added heat can lead to a catastrophic failure of your bearings and motor or generator.

Blockage of airflow or liquid cooling passages.

Motors naturally create heat which is caused by wasted energy. Motors and generators are designed with cooling fans, external blowers, vents, heat sinks, and liquid cooling systems to allow for heat to dissipate. Contaminants can clog the vents, cooling pipes, and cooling surfaces and prevent the motor from cooling properly. This added heat will eventually break down the mechanical and electrical components of your motor and can lead to motor failure.

Creating an imbalance to rotating components.

When contaminants stick to your motor rotors, fan blades, or other rotating components, they add excess weight. This weight creates an imbalance which can lead to increased vibration. The vibration caused by an out-of-balance rotating component will decrease the life of your bearings and could cause motor or bearing failure.

Deteriorating electrical insulation:

Not only will contaminants not allow your winding not to cool, the electrical insulation will breakdown over time. Contaminants will allow currents to discharge from conductors in the windings. In form wound windings, insulation tapes and varnish/resin carbonizes and breaks down. In random wound windings “pinholes” in the enamel insulation cause an electrical short when stress caused by contaminants break down the insulation. The result of insulation breakdown is lowering insulation resistance which can eventually causes an electrical short. Also on brushed motors and generators, brush dust can build up and cause shorts as well.

What are some solutions to preventing contamination in motors?

Make sure your motor/generator is selected properly for the correct application

Example: This open vented motor filled up with corn dust. The motor has to be replaced with a totally enclosed fan cooled motor.

Install and maintain filters in your air or liquid cooling system to prevent blockages

Example: This printing press motor filled with paper dust because it was not filtered properly.

Electric Motor contaminated by metal chips

Control nearby sources of contamination

Example: This motor was splashed with coolant and metal chips from machine tools.

Ensure bearing, cooling systems, brakes, or other motor system components are sealed properly

Example: This motors seal wore out allowing liquid to enter the motor.

Ensure air patterns from cooling systems do not change allowing contaminates to travel in the wrong direction

Example: This motor is designed to run in one direction.

Motor cooling surface clogged with contaminants

Clean cooling vents and heat sink surfaces as needed without letting contaminates to ingress the motor/generator

Example: This motors cooling surfaces have clogged with contaminates.

Clean grease guns for lubricating motor bearings

Make sure bearing lubrication tools are clean and do not let contamination in

Example: These grease guns are kept in a clean storage area for lubricating motor bearings.

If you find your motor has been contaminated make a plan to send it in for minor reconditioning before it fails and becomes a more costly repair or replacement.

Example: A 1,600 kW generator being cleaned to remove contaminants.

Vacuum or blow out dust in brush applications per manufacturers recommendations

Example: Carbon/Graphite dust was not blown out of the motor on this brushed DC motor causing electrical problems.

Electric Motor Protection: Basics of Overload Relays