Thermal Overload Protection

Overview

The thermal overload protection is used to protect asynchronous motors against thermal overload conforming to the IEC 60947-4-1 standard.

The thermal overload protection can be used to protect:

• Three-phase IE1, IE2, IE3, and IE4 asynchronous motors.

• Single-phase asynchronous motors.

The thermal overload condition causes the overheating of the motor. Thermal overload conditions occur:

• During the starting phase when the starting time is too long, or if there is stalling conditions.

• During operation if there is jam condition or any condition that results to increase abnormally the currents flowing in the motor.

For more information on protection of single-phase motors, see Single-Phase Motor Application.

Operation

The overload relay calculates the thermal state and thermal capacity of the motor continuously.

• When the motor thermal capacity used exceeds 90%: the Alarm LED on the overload relay turns on steady orange to signal an alarm, indicating that the motor is approaching a thermal overload. The motor will stop shortly, if no action is performed to solve the thermal overload. The thermal overload alarm is cleared by the overload relay when the thermal capacity used falls below 80%.

• When the motor thermal capacity used exceeds 100%: the thermal overload protection triggers a trip and the state of the auxiliary contacts changes.

Settings

The settings of the thermal overload protection are Ir/FLA threshold and trip class.

• Ir/FLA threshold

  The Ir/FLA threshold corresponds to the rated current of the motor or full load amp and can be adjusted with a 64-position rotary switch.

  

  The table below provides the setting range of the Ir/FLA threshold:

  Commercial reference	Ir/FLA setting range
  LR9G115C	28–115 A
  LR9G225C	57–225 A
  LR9G500C	125–500 A
  LR9G630C	160–630 A

• Trip class

  The trip class allows to adapt the tripping time of the thermal overload protection to the application. Trip classes are defined by the IEC 60947-4-1 standard. The trip class can be selected with a rotary switch.

  

  The following table indicates the tripping time (Tp) depending on the trip class selected:

  Current in the load	Class 5E	Class 10E	Class 20E	Class 30E
  7.2 x Ir	3 s < Tp ≤ 5 s	5 s < Tp ≤ 10 s	10 s < Tp ≤ 20 s	20 s < Tp ≤ 30 s

Tripping Curves

The tripping curves of the thermal overload protection indicates the tripping time (Tp) for each trip class depending on the current load and the thermal state of the overload relay . The two states are the cold state and the hot state:

• Cold state: Thermal state of the overload relay when it was not loaded previously and the thermal capacity used is 0%.

• Hot state: Thermal state of the overload relay when it was loaded at setting current and the thermal capacity used is 75%.

Class 5E 1 Hot State 2 Cold State	Class 10E 1 Hot State 2 Cold State
Class 20E 1 Hot State 2 Cold State	Class 30E 1 Hot State 2 Cold State

Manual Reset

The thermal overload protection can be reset manually. For more information, refer to Trip Reset.

Automatic Reset

WARNING
UNINTENDED EQUIPMENT OPERATION Before setting to AUTO R, ensure that automatic restart of the motor will not result in unsafe conditions. Failure to follow these instructions can result in death, serious injury, or equipment damage.

The automatic reset of the thermal overload protection can be enabled or disabled by using a rotary switch.

When the automatic reset is enabled, the thermal overload trip is reset automatically in 2 minutes maximum.