Overcurrent relay setting and position

[ewandingwall]

Good evening,

I have a few questions requiring your expertise:

1)

For a motor with a nameplate FLC of 44A:

The overload relay trip setting should be set to 44A when connected in Delta and set to 44/(root 3)=25.4A when connected in Star.

Is this statement correct?

2)

Does the position of where the overcurrent relay is connected in the power circuit have any effect what the trip setting is set to?

For example, if the OCR is connected in series AFTER the main contactor, would it have the same trip setting is it was connected before the contactor? In this case, my understanding is the OCR is monitoring the line current as it is connected to the supply line in series with the contactor and the motor which is why the OCR is different as per my 1st statement.

There are cases where there is no individua OCR in some motor starter circuits but instead a motor circuit breaker GV-2 with both thermal overload and short circuit protection, so in essence, it acts as a combined circuit breaker and OCR.

Would the GV2 have a different thermal overload setting than an individual OCR because it is connected before the contactor?

Thanks very much,

Ewan

 

[westward10]

The GV2 would act no different to the overload relay.

 

[Lucien Nunes]

The FLC(s) stated on the motor are for the stated voltages. So a motor that will operate in delta at 400V and star at 690V will show one FLC for 400V and another for 690V, which would be in the ratio you describe. A 6-wire motor for star-delta starting intended only to operate at 400V will probably only have one voltage and FLC stated.

The overload relay setting applies to the current that passes through the relay. For a motor that is permanently in star, it does not matter where the relay is connected, line current = coil current = OLR current = FLC.

For a motor in delta, coil current = line current / sqrt(3). Therefore if the OLR is in the supply conductors, OLR setting = FLC, but if it is connected in series with the individual coils downstream of a star-delta starter, OLR setting = FLC / sqrt(3).

 

[ewandingwall]

The FLC(s) stated on the motor are for the stated voltages. So a motor that will operate in delta at 400V and star at 690V will show one FLC for 400V and another for 690V, which would be in the ratio you describe. A 6-wire motor for star-delta starting intended only to operate at 400V will probably only have one voltage and FLC stated.

The overload relay setting applies to the current that passes through the relay. For a motor that is permanently in star, it does not matter where the relay is connected, line current = coil current = OLR current = FLC.

For a motor in delta, coil current = line current / sqrt(3). Therefore if the OLR is in the supply conductors, OLR setting = FLC, but if it is connected in series with the individual coils downstream of a star-delta starter, OLR setting

= FLC / sqrt(3).

Thank you for the reply,

Would you set the OCR to exactly the motor FLC?

And is the full load current the maximum current that the motor insulation can draw without causing damage?

Thanks, Ewan