How to Troubleshoot Faulty Thermal Overload Relays

What are Thermal Overload Relays?

Thermal overload relays are devices used to protect electric motors from overheating and causing damage due to excessive current. They work by monitoring the current flow through the motor and tripping open if the current exceeds the relay's preset value for a period of time.

Thermal overload relays contain a heating element that gets hotter as more current flows through it. If the current exceeds the relay's threshold, the heating element will activate the relay to open its contacts and disconnect power to the motor. This protects the motor windings from overheating and potential fire hazard.

Some key features of thermal overload relays:

• Protect motors from overcurrent damage
• Contain a heat-sensitive element that trips open on overload
• Allow motors to handle temporary overcurrent spikes
• Have adjustable trip current settings
• Provide automatic reset or manual reset options
• Offer overload signal contacts for external monitoring

So in summary, thermal overload relays are invaluable protective devices that preserve the lifespan and safe operation of electric motors.

Common Causes of Faulty Thermal Overloads

Thermal overloads can malfunction or become faulty for a variety of reasons:

Incorrect Sizing

• If the thermal overload relay is undersized for the motor, it may trip prematurely at normal operating currents.

• Oversized relays may fail to trip even on overload conditions.

• Use the motor nameplate full load amps (FLA) to select the right thermal unit.

Loose Connections

• Loose wiring connections can alter current readings at the thermal unit.

• Inspect terminals for discoloration or overheating signs and tighten connections.

Defective Heater Element

• The internal overload heater may open-circuit due to age or overheating damage.

• This renders the overload relay inoperative.

• Test heater element continuity or replace the entire unit.

Environmental Contamination

• Dirt, dust, moisture, or oil can contaminate the thermal unit over time.

• This alters the heat transfer and makes the unit sluggish.

• Clean any contamination and ensure motor environment is clean.

Excessive Voltage Unbalance

• Voltage unbalance on a 3-phase supply can produce unstable heating at the overload relay.

• It may trip randomly or fail to protect the motor.

• Check supply voltage balance and use a 3-phase motor protection relay if needed.

Wrong Relay for Application

• Standard thermal units may be unsuitable for high inertia loads that need time delay overload protection.

• Special relay classes exist for various applications.

• Consult technical guides to select the right class of overload for the specific load type.

Step-by-Step Troubleshooting Procedure

Follow these key steps to methodically troubleshoot and diagnose faulty thermal overload relays:

1. Verify Power Supply

• Check 3-phase power to motor for any blown fuses.

• Test voltage and frequency on all 3 phases.

• Inspect for faults, grounds, or open circuits.

• Restore healthy power supply before further testing.

2. Review Motor Current

• Clamp ammeter to each phase and read motor current draw under loaded conditions.

• Compare to motor nameplate FLA rating.

• If current is higher than FLA, there may be mechanical or electrical issues with the motor or driven load.

3. Test Overload Unit Continuity

• Shut off power supply and disconnect overload relay.

• Use ohmmeter to check continuity across main contactor terminals.

• Heater element should show some resistance. No or infinite reading indicates defective heater.

4. Check Trip Settings

• Verify if thermal unit trip current and class settings match the motor FLA and load characteristics.

• Adjust settings according to motor datasheet if needed.

5. Monitor Operation

• Reinstall overload relay and motor starters.

• Power up the motor and monitor current draw and relay operation.

• Does the relay trip? If yes, when does it happen during starting or running?

• This helps narrow down the fault to components.

6. Thermal Unit Testing

If relay malfunction is still unclear, conduct thorough testing:

• Use a thermal overload tester to simulate current and test trip times.

• Compare to published time-current curves to detect defective units.

• Consider replacing the unit if malfunction persists.

Preventing Premature Thermal Overload Tripping

Here are some tips to prevent nuisance tripping of healthy thermal overload relays:

• Allow sufficient cooling time between motor starts.

• Avoid restarting motors quickly after overload trip.

• Select overload size based on motor characteristics and load duty.

• Use ambient temperature compensated overload relays where needed.

• Limit motor starts to less than 6 times per hour to avoid overheating.

• Verify voltages are balanced on 3-phase motors.

• Ensure cabling is properly sized to avoid voltage drops.

• Keep motor environment and vents free of dirt buildup.

Proper sizing, installation, and maintenance are key to maximizing thermal overload relay performance and lifecycle.

When to Replace Faulty Units

• If heater element shows open circuit or wrong resistance

• Units fail testing against time-current curves

• Units trip prematurely or fail to trip when expected

• Evidence of internal damage like discoloration or cracking

• Age exceeds manufacturer recommended replacement timeline

• Units lack required certifications and testing standards

• Parts availability for repair or calibration is limited

Do not take chances with damaged thermal overload protection. Replace faulty units immediately to avoid catastrophic motor failures.

Summary

• Thermal overloads protect motors from overcurrent damage

• Faulty units can result from incorrect sizing, loose wiring, contaminated contacts, voltage unbalance and more

• Methodically verifying power, testing continuity, and monitoring operation helps troubleshoot issues

• Replace damaged thermal overload relays to maintain proper protection

• Proper installation, settings, and preventive maintenance prevent premature tripping

Following structured troubleshooting and adopting best practices ensures thermal overload relays guard motors reliably for the long run. Let me know if you need any clarification or have additional questions!