What Should We Do If the Thermal Overload Relay Tripped?

How to Fix a Thermal Overload Relay That Keeps Tripping: Step-by-Step Instructions

If the feeder or blender of an auger filling machine suddenly stops working, one common reason is that the thermal overload relay tripped. This motor protection device is designed to prevent damage caused by excessive current or overheating.

So how can we determine whether the thermal overload relay has tripped, and what should we do next? This guide explains the causes, troubleshooting steps, and safe reset procedures.

How to Tell If a Thermal Overload Relay Has Tripped

The easiest way to identify whether a thermal overload relay has tripped is by checking the trip indicator.

• If the indicator pops up (as shown in A), the relay has tripped.
• If the indicator remains inside (as shown in B), the relay has not tripped.

If the thermal overload relay keeps tripping repeatedly, further inspection of the motor and load system is required.

What Causes a Thermal Overload Relay to Trip?

When a thermal overload relay has tripped, it indicates that the motor current has exceeded its preset protection threshold for a certain period of time. Unlike a circuit breaker that reacts instantaneously to short circuits, a thermal overload relay operates based on an inverse time-current characteristic (I²t principle). This allows it to tolerate short startup inrush currents while protecting the motor from sustained overload.

1. Short Circuit in Wiring

A short circuit causes an instantaneous surge of current far above rated values. In most cases, upstream protection such as an MCCB or fuse will trip first. However, if the fault is partial or prolonged, the overload relay may also operate.

2. Motor Mechanical Failure

Mechanical faults such as bearing seizure, shaft misalignment, or gearbox jam increase mechanical resistance. As load torque increases, motor current rises proportionally, eventually exceeding the overload setting.

3. Continuous Motor Overload

When a motor operates beyond its rated capacity for an extended period, it draws excessive current. Thermal overload relays are designed to tolerate short startup inrush currents but will trip under sustained overload conditions.

4. Phase Loss or Voltage Imbalance

Loss of one phase in a three-phase system causes the remaining phases to carry higher current, leading to rapid overheating and overload relay tripping.

5. Incorrect Relay Setting

If the adjustment dial is set below the motor’s rated Full Load Amperage (FLA), nuisance tripping may occur. Proper calibration according to the motor nameplate is essential.

How Should We Reset the Trip?

Understanding the Four Key Components
The four main functional parts of the thermal overload relay are shown below:

• 1). 整流盘：黑色标线所指的电流安培值表示该热过载保护器所设定最大承受电流值。
  Adjustment Dial: The current ampere value indicated by black line mark indicates the maximum withstand current value set by the thermal overload relay. An adjustment dial located on the unit allows the ampere trip setting.
• 2). 测试按钮(红色)：按下这个按钮，脱扣指示将弹出。
  Test Button (red one): Press the Test Button, then the trip indicator will pop up.
• 3). 复位按钮(蓝色)：按下复位按钮，脱扣指示将复位，电机将会接通电源接续工作。
  Reset Button (blue one): Press the Reset button, the trip indicator will be reset, and the motor will be connected to the power supply to continue working.
• 4). 脱扣指示(绿色)：如果弹出来则表示在脱扣状态。
  Trip Indicator (green one): If it pops up, it means it is in a tripped state.

How to Solve the Thermal Overload Relay Trip?

• 1) Disconnect the main power supply and verify zero voltage before inspection.
• 2) Check whether any foreign material is blocking the feeder screw or blender shaft.
• 3) Verify the adjustment dial setting matches the motor rated current.
• 4) After removing the fault, press the reset button.