HomeFAQVoltage drop along with cable length

Voltage drop along with cable length

Voltage drop along the cable will reduce the actual starting current. If we for example choose 25mm2 cable, we would have around 9% voltage drop at nominal load, over the 400m cable length. (57A, at 1.6mV/A.m). This means that the current would be 57*1/0.09 A (or approximately 11*In) if the cable is shorted at the motor end, and if we assume that the locked rotor current for the motor is 7*In for full voltage on the motor terminals, the actual locked rotor current for motor + cable will be In*(7*11)/(7+11), or approximately 244A. This is also the largest current that we can experience if there is no actual fault on either cable or motor (we do not consider any possibility of the motor running backwards before start, or other abnormal situations).

The voltage at the motor will be around Un*11/18, or 61% Un; this corresponds closely to the voltage at a star/delta start. If only the motor can deliver sufficient torque to pull away under these conditions, we have arrived at just about the simplest and cheapest reliable installation possible.

For the mining installations we do in this country, there is a separate earth grid to which all metal housing is connected, so we ensure that the "earth" is raised to pretty much the same level as any motor housing, so earth fault considerations will not influence the cable sizing. We might choose to incorporate a simple Earth Leakage function in the motor protection relay, but that can very well depend on other local and project standards.

It goes without saying that for each project is carried out a step-and-touch-potential analysis, to ensure that no person will be harmed by touching any equipment during a fault.

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