Motor Overload Setting Table: A Complete Guide
Setting up a motor correctly ensures safety and long-term performance. One of the most critical parts of this setup is understanding the motor overload setting table. Whether you’re an engineer, technician, or someone working in facility management, you must get these values right. Overload protection is not just a regulation—it’s a necessity.
The purpose of the motor overload setting table is to match the protection device with the motor’s full-load current (FLC). This ensures that the motor operates within its thermal limit. Let’s explore how this table works and why it matters.
What is a Motor Overload Setting Table?
A motor overload setting table is a reference chart. It shows how to set overload relays or motor protection devices based on the motor’s rated current. These values are derived from standards such as IEC or NEMA.
Most overload relays are adjustable. But if they’re not set properly, motors can overheat, fail prematurely, or trigger unnecessary shutdowns.
In a practical sense, this table helps engineers and electricians decide the correct setting on thermal overload relays. It often includes details like:
- Motor rating in kW or HP
- Voltage level
- Full-load current (FLC)
- Overload relay setting range
These parameters work together to define safe operating limits for electric motors.
Why is Overload Protection Necessary?
Motors draw a large amount of current during startup. But this is temporary. If a motor draws more than 110–125% of its rated current for an extended time, it overheats. This is where Overload Setting for Motor plays a crucial role.
Overload protection detects these high-current conditions. It then disconnects the motor before it suffers damage.
For example, a 5.5 kW motor running on 400V may draw a full-load current of 11.2 A. If it’s overloaded and draws 14 A continuously, the overload relay should trip to protect the windings.
Basic Principle of Overload Relay Settings
The overload relay is often set between 115% and 125% of the motor’s full-load current. The exact value depends on:
- Duty cycle of the motor
- Ambient temperature
- Type of overload relay (thermal, electronic, etc.)
Let’s look at a sample motor overload setting table to understand how to apply these settings.
Motor Overload Setting Table (Three-Phase Motors)
Here’s a sample table for standard 3-phase induction motors running at 400V, 50 Hz.
| Motor Power (kW) | Voltage (V) | Full Load Current (A) | Overload Setting Range (A) |
|---|---|---|---|
| 0.37 | 400 | 1.0 | 1.1 – 1.3 |
| 0.75 | 400 | 1.8 | 2.0 – 2.3 |
| 1.5 | 400 | 3.2 | 3.5 – 4.0 |
| 2.2 | 400 | 4.6 | 5.0 – 5.6 |
| 4.0 | 400 | 8.0 | 9.0 – 10.0 |
| 5.5 | 400 | 11.2 | 12.0 – 13.5 |
| 7.5 | 400 | 15.0 | 16.5 – 18.0 |
| 11.0 | 400 | 21.0 | 23.0 – 25.0 |
| 15.0 | 400 | 28.0 | 30.5 – 33.0 |
| 18.5 | 400 | 34.0 | 36.5 – 39.0 |
These values may slightly vary depending on the manufacturer and efficiency class of the motor.
How to Use the Motor Overload Setting Table
To use this table effectively:
- Identify your motor’s rated power and voltage.
- Find the full-load current (FLC) from the nameplate or catalog.
- Use the motor overload setting table to find the recommended setting range.
- Set the overload relay within the given range, preferably closer to the FLC but not below it.
Always verify the settings with actual measurements if possible. Mismatched settings may result in nuisance tripping or overheating.
Tips for Accurate Overload Settings
- Always match the relay setting to the actual current, not just the nameplate.
- Use Motor Circuit Protection Tables to verify compatibility between cable size, breaker size, and relay setting.
- If your motor is running in a high-temperature environment, derate the overload relay setting.
- Motors operating with frequent starts may need a lower setting.
Thermal vs Electronic Overload Relays
Thermal overload relays use bimetallic strips that bend when heated. They are simple, low-cost, and suitable for most motors. But they react slower and less accurately.
Electronic overload relays are more accurate. They can detect phase imbalance, ground faults, and have adjustable trip classes. These are ideal for precision control applications and large motors.
In either case, the motor overload setting table helps to ensure you’re applying the right settings.
Common Motor Overload Mistakes to Avoid
- Ignoring Full Load Current (FLC)
Always refer to FLC, not just HP or kW. - Setting Too Low
This causes nuisance trips. - Setting Too High
This exposes the motor to overheating. - Using the Wrong Relay Type
Some relays are not meant for heavy-duty motors or variable frequency drives. - Not Considering Ambient Temperature
Temperature derating is often ignored but is critical for accuracy.
Role of Motor Protection Circuit Breaker (MPCB)
A Motor Protection Circuit Breaker integrates short-circuit and overload protection in one unit. These breakers also use motor overload setting tables internally. They are adjustable, compact, and easy to reset after a trip.
MPCBs are ideal for industrial motor panels. They reduce wiring, panel space, and setup time.
Here’s an example of MPCB settings compared to traditional thermal overload relays:
| Motor Size (kW) | FLC (A) | Thermal Relay Setting (A) | MPCB Setting Range (A) |
|---|---|---|---|
| 1.5 | 3.2 | 3.5 – 4.0 | 3.2 – 4.5 |
| 4.0 | 8.0 | 9.0 – 10.0 | 8.0 – 11.0 |
| 11.0 | 21.0 | 23.0 – 25.0 | 21.0 – 27.0 |
This makes the MPCB highly versatile and user-friendly.
Special Considerations for Overload Settings
- Star-Delta Starters: The relay should be placed in the line side, not delta side.
- Soft Starters: Allow for extended acceleration time. Set the relay at the higher end of the range.
- Variable Frequency Drives (VFDs): Use the VFD’s built-in protection settings, not external relays.
In each case, reference the motor overload setting table to determine the most suitable base values.
Summary and Best Practices
Understanding and using the motor overload setting table properly is essential for efficient motor protection. These settings protect your investment, reduce downtime, and prevent hazards.
Always:
- Start with the motor’s full-load current.
- Use the manufacturer’s recommendations.
- Factor in environment and duty cycle.
- Match the relay or MPCB setting accordingly.
- Double-check with Overload Setting for Motor guides or Motor Circuit Protection Tables.
Proper overload setting is a small step, but one with major consequences. An accurately set relay is the difference between years of motor operation and premature failure.
Follow Us on Social:
Subscribe our Newsletter on Electrical Insights to get the latest updates in Electrical Engineering.
#MotorProtection, #OverloadRelay, #MotorOverloadSetting, #ElectricalEngineering, #MotorStarter, #OverloadProtection, #ElectricalSafety, #IndustrialAutomation, #ControlPanelDesign, #MotorCurrent, #OverloadRelaySettings, #MotorControl, #ElectricalMaintenance, #IECStandards, #ThermalOverloadRelay