Motor Protection Device (Simocode)

A system that is perfectly balanced all the time is hard to find. If you don't set a threshold for the allowable imbalance then your relay will treat any imbalance, no matter how small, as a reason to trip. Take the time to do the calculations and set the relay properly; the aggravation you save will be your own.

A 1% voltage unbalance will cause a 6-8% unbalance in current in induction motors. There are many papers on the web which will help you decide how much unbalance to permit. The motor manufacturer may also tell you what is the figure.

A core balance CT will not sense unbalance, It will sum all currents vectorially, which will be zero. It will act only when there is an earth leakage.

The Simocode is detecting actual current imbalance, not voltage imbalance. You are not required to set it, but it's a good idea. Current imbalance in an AC motor leads to heating in the rotor due to negative sequence current. This current circulates inside of the motor but does not show up in the external current sensors as excess until it is too late and the damage is done. But is is predictable based on current imbalance, so that is how you protect against it.

There is no set value or calculation to use, it is load dependent. If for example your motor is only 50% loaded, it can survive a lot more current imbalance than a motor that is running near full load. I generally would set a Simocode at 30% current imbalance for tripping, then set a 20% Alarm value to get a pre-warning. Also, make sure you click the box that says to "bias the thermal model". What that does it automatically lower the thermal model trip threshold based on the heating effects of a current imbalance, based on the amount. That is the feature that will truly protect the motor.