Thermal Overload Relay: Working Principle, Types and Important Settings Calculation
A thermal overload relay is one of the most reliable protection devices used with motors. It protects the motor from overheating caused by excess current. This simple device has been used for decades because it reacts to heat in almost the same way a motor winding would. That is why electricians and engineers still prefer it in many control panels. A thermal overload relay gives dependable protection at a low cost, and its operation matches the heating pattern of induction motors very closely.
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Modern industries use motors in pumps, compressors, conveyors and fans. In all these applications, any rise in current can damage insulation. This is where a thermal overload relay plays its role. It monitors the load current continuously. When the current goes above the safe limit, the relay trips the motor and avoids further heating. This prevents costly breakdowns and improves the life of the motor.
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Understanding how a thermal overload relay works helps in selecting the right model and setting it correctly. A wrong setting can cause nuisance trips or fail to protect the motor on time. This article explains the principle, the major types and how to calculate the settings with practical examples.
Working Principle of Thermal Overload Relay
A thermal overload relay works on the heating effect of current. When current flows through the heater elements of the relay, heat is produced. This heat warms a bimetal strip inside the relay. The strip bends when it gets hot. When the bending reaches a certain point, it activates a tripping mechanism.
This operation is slow and matches the motor’s heating curve. Motors can handle short overloads during starting or sudden load variations. The relay avoids tripping during these short bursts. But when overload continues for a longer time, the bimetal gets hotter and finally causes the relay to trip. This is why a thermal overload relay provides time-delay protection without using any electronics.
The relay usually has a reset mechanism. After tripping, the bimetal needs time to cool. Once it cools, the operator can reset the relay manually. Some designs also allow automatic reset, but this is used carefully to avoid unsafe motor restarts.
Key Components
- Heater elements
- Bimetal strip
- Trip latch
- Reset button
- Auxiliary contacts
The NO and NC contacts of the relay are used to cut control power to the contactor coil. This stops the motor supply. Additional contacts can signal alarms or shutdowns in automated systems.
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Types of Thermal Overload Relays
Different industries use different types depending on accuracy and environmental needs. All these types follow the same basic principle but differ in construction and features.
Bimetallic Thermal Overload Relay
This is the most common type. It uses one or more bimetal strips heated directly or indirectly by current. Indirect heating uses separate heater coils, while direct heating passes motor current through the strip. This type of thermal overload relay offers simple operation and long service life.
Electronic Thermal Overload Relay
This type uses electronic sensors to measure current, but it still follows the thermal trip characteristics. It imitates the heating pattern electronically. These relays are more accurate and provide extra functions such as unbalance protection and earth fault indication.
Single-Phasing Protection Relays
Some modern thermal relays include phase-loss protection. When one phase fails, the motor draws higher current in the remaining phases. The relay senses this unbalance quickly and trips. Although technically improved, it is still categorized under the thermal family because the basic tripping behavior copies bimetal heating.
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Thermal Overload Relay Settings
Correct settings ensure the motor runs safely. The main setting on a thermal overload relay is the full load current (FLC). The relay dial is adjusted to match the motor nameplate current. This allows the relay to carry the normal load without nuisance trips.
A common guideline is to set the dial between 100% and 115% of the motor rated current. Small motors generally use 100%. Larger motors use up to 115% because they can tolerate slightly higher temperature rises.
Other adjustable features include trip class. Trip class defines how fast the relay trips during overload. Most applications use Class 10. Heavy-duty loads sometimes use Class 20.
Table 1: Recommended Trip Classes
| Motor Load Type | Recommended Trip Class | Explanation |
|---|---|---|
| Standard motors | Class 10 | Trips within 10 seconds at 600% current |
| Heavy-duty loads | Class 20 | Allows longer starting times |
| Special machinery | Class 30 | Used rarely for very long start periods |
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How to Calculate Thermal Overload Relay Settings
Setting the relay correctly is important. The process is simple when motor data is available. You only need the motor full load current and application type.
Step-by-Step Method
- Check the motor nameplate.
- Note the full load current (FLC).
- Set the relay current dial to the same value or slightly higher based on the application.
- Select the trip class required for the load.
- Perform a test run and observe motor temperature.
- Fine-tune the setting if nuisance trips occur.
Example Calculation
Consider a 3-phase induction motor with these values:
| Motor Parameter | Value |
|---|---|
| Rated Power | 11 kW |
| Full Load Current | 21 A |
| Starting Load | Standard |
| Ambient Temperature | Normal |
To set the relay:
- Dial setting = 21 A
- Trip class = Class 10
This ensures the relay trips when the load exceeds safe limits for a longer duration. If the motor starts frequently or drives a hard load, the setting may be increased slightly but should not exceed 115% unless recommended by the manufacturer.
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Advantages of Using a Thermal Overload Relay
Many plants still prefer installing a thermal overload relay because of its simplicity. It does not need external power. It works even when sensor electronics fail. Maintenance is minimal. The relay also matches the thermal behavior of motors closely, giving dependable long-term protection.
Key advantages include:
- No external power required
- Low cost
- Simple wiring
- High reliability
- Accurate thermal mimicry of motor windings
The device is also easy to replace. Many contactor manufacturers design relays that snap directly under the contactor frame. This saves space in the panel and reduces wiring time.
Common Applications
A thermal overload relay is used wherever induction motors run. It protects motors used in industries, agriculture, HVAC systems and commercial buildings. Common applications include pumps, blowers, machine tools, compressors and small conveyor systems.
It is also used in motor control centers since it responds well to long-time overloads. Many electricians prefer it in rural installations because it can withstand voltage fluctuations better than sensitive electronic relays.
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Troubleshooting Tips
When the relay trips frequently, check these points:
- Loose connections
- Undersized cables
- High ambient temperature
- Mechanical load issues
- Incorrect relay setting
If the relay does not trip during overload, it may have a jammed mechanism or wrong dial adjustment. Regular inspection helps maintain accuracy.
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Conclusion
A thermal overload relay remains one of the most trusted devices for motor protection. It operates on a simple heat-based principle and offers dependable performance. With the right selection, correct settings and proper installation, it provides long-life protection for motors of all sizes.
Understanding its working principle, available types and calculation method ensures reliable operation in any industrial setup. The simplicity, accuracy and rugged nature of a thermal overload relay make it a preferred choice in control panels across the world.
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