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Motor Protection

Overload Trip Class - What does it really mean?
Posted: 1 Jul 03

Quite often we here of applications in which the motor protection/overload relay 'trips' during normal motor starting. The remedial action taken by many when faced with this problem is to select and install a replacement overload relay, generally one that provides a higher 'Trip Class' setting, for example, Trip Class 20 in lieu of the standard Trip Class 10.

Note: There appears to be a general misconception in industry that Overload Trip Class relates directly to the starting time of a machine. This is not the case!

All due care must be taken when selecting a protection relay offering higher Trip Class, as this can result in under protection and subsequent failure of the motor.

To select an overload relay with a more suitable trip class, you must in the first instance obtain data relating to the motor’s thermal withstand capabilities. For example, you need to know how many seconds (from cold condition) the motor can the sustain Locked Rotor Current before it is compromised.

This information is readily available from most leading manufacturers of motors and is generally provided in one of two formats.

a) Specific values for Locked Rotor Current and maximum Locked Rotor Time (from 'cold condition' ) are given.

b) A Motor Thermal Withstand Curve is provided.

With this information available to you, you can refer to the tables given in IEC 60947 to identify the most appropriate Overload Trip Class. This is defined as the one that provides a trip curve as close as possible to but below the overload curve of the motor. Adopting this process will ensure nuisance tripping is minimised and that the motor is adequately protected at all times.

Note: If the above processes are adopted but the trip conditions continue, there are 4 possible causes.

1. The motor (and overload relay) are not given sufficient time to cool between starts.

2. Assuming reduced voltage start (star/delta, auto-transformer, primary resistance, soft start etc., the motor is not delivering torque sufficient to accelerate the connected load to speed. That is the starting current and starting time under RVS conditions exceeds that permissible by the overload curve.

3. A more advanced protection strategy such as motor thermal modelling may be required. Motor thermal modelling allows the user to 'match' the curves of the protection device to the connected motor. This is of particular advantage when the motors thermal withstand capabilities and the start condition (starting current and starting time) fall between two curves defined by standard overload trip classes.

4. The motor is simply too small for the application.

Put simply, the most appropriate protection strategy is the one that allows the motor to be fully utilised without nuisance tripping or fear of motor burn-out.

Regards,
GGOSS

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