Motor Overload Relay Maintenance and Testing with Megger Test Equipment

Megger_MS-2Megger_MS-2AWhen it comes to motor overload relays, the primary function of these vital systems is to prevent continued motor operation during overload conditions. Maintaining this equipment requires that proper analysis be given when nuisance trips occur to understand the root cause of the tripping before changing the overload relay rating. When a motor is tripped, rating the relay without knowing the cause could ultimately result in insulation deterioration and reduction in the life of the motor itself.

Through a regular maintenance schedule, motor overload relays may be inspected and tripping time tested in order to keep the motor performing optimally. Most companies perform regular interval testing every one to two years, but of course, testing intervals will vary based on the particular motor’s role in production.

Essentially, three motor overload relays are used today: thermal with melting alloy, thermal with bimetallic strip and electromagnetic. Maintenance programs for these types of relays should consist of the following:

  •        Cleaning – periodic cleaning will ensure that the relay continues to operate reliably. Dirt and dust from surrounding machinery can cause parts to stop moving in the relay and prevent proper heat dissipation, particularly in thermal relays.
  •        Tightening – when electrical connections become loose, additional heat may cause nuisance operation, and this condition will especially affect thermal relays.
  •        Inspecting – heater size is of particular importance for optimal performance in thermal overload relays. While oversized heaters are often installed to prevent unexplained tripping, the required heat needed may seem to require a larger heater when, in fact, the original heater is oxidized and providing a small current than the original design intended. For magnetic relays, setting should be adjusted for minimum current and time delay.
  •        Testing – after the initial cleaning, tightening of connections and inspection, the motor overload should be tested using simulated overload so that the trip time can be measured. Results should then be analyzed based on the manufacturer specifications and relay time curve so make sure that the overload relay is operating correctly.

Testing of motor overload relays may be accomplished using Megger test equipment such as the Megger MS-2 Circuit Breaker and Overload Relay Test Set and the Megger MS-2A Circuit Breaker and Overcurrent Relay Tester.

While both units can be used for verification of all types of processes including thermal, magnetic and solid state overload relays, these Megger test sets are self-contained and offers the ability to perform testing for circuit breakers, ground/fault trip devices and overcurrent relays. Their compact and lightweight design also allows for easy use in the field or plant.

Related electrical test equipment available for rent from Protec:

Megger MIT525 5kV Insulation Resistance Tester

Megger DET4TD Earth Tester

Megger BM15 Analog 5kV Insulation Resistance Tester

This electrical test equipment is available for rent, lease, or purchase from Protec Equipment Resources. We are here to help you with your testing equipment needs.

Protec’s Calibration Lab offers accurate and affordable calibration services for all makes and models of electrical test and measurement equipment. Contact us today for a quote.


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Editors Note: This blog was orginally posted on June 6, 2012