Sigma currents are essentially stray currents that circulate in an electrical system. The sigma currents are created as a result of the signal frequency, voltage level, capacitance and inductance in conductors. These circulating currents can find their way through protective earth systems causing mysterious nuisance tripping or in some cases fluting of a bearing race or excess heat in windings.
Sigma current can be found in the motor cabling and is the sum of the current of the three phases at any one point in time. In a perfect situation, the sum of the three currents would equal zero. In other words, the return current from the drive would be equal to the current to the drive.
Sigma current can also be understood as asymmetrical signals in multiple conductors that can capacitively couple currents into the ground conductor.
The Fluke 190-204 ScopeMeter™ isolated 4 channel portable oscilloscope with a wide bandwidth (10kHz) current clamp (Fluke i400S or similar) can identify sigma currents in order to help avoid damage or downtime from this stray current.