Overload relay setting and direction of rotation

The overload relay: where it sits, why 35.2 A

MC carries the full line current, because DC takes its supply from MC’s outputs. The overload relay sits after that split, in the leads to U1, V1 and W1. Those leads always carry the winding current. In star that equals the line current; in delta it is the line current divided by √3.

At full load in delta each winding carries 61 ÷ √3 = 35.2 A. So the relay is set to 35.2 A, which is 0.58 × FLC.

Set it to the 61 A on the nameplate and the motor could run 73 % overloaded without a trip. Set it well below 35.2 A and it trips at normal full load.

Trip class 10

A class 10 relay must trip within 10 s at 7.2 times its setting: long enough for a normal start, short enough to save a stalled motor.

Phase order sets the direction

The direction is set by the phase order at U1, V1 and W1. By IEC 60034-8, L1-L2-L3 on U1-V1-W1 turns the motor clockwise, seen from the shaft end.

A correct delta always turns the same way as star: each winding gets its own phase minus a neighbouring one, which only shifts it by 30°. A wrong DC connection does not reverse the motor. It leaves a winding with both ends on one phase. After the changeover the motor growls, the current climbs towards starting current and the overload relay trips within seconds.

A fire pump turning backwards

What reverses a fire pump is the phase order swapped on both motor cables, or upstream: after a motor overhaul, a cable repair or work on the switchboard. It then runs backwards in star and in delta. A centrifugal pump turning backwards still delivers some water, roughly a third of its head, and the ammeter reads low. It is easy to miss.

How to reverse it

To reverse a star–delta motor, never swap only two leads of one cable: a winding would end up with both ends on one phase in delta. Swap the same two phases on both cables, U1↔V1 and U2↔V2, or swap two incoming phases at the starter. Isolate and lock off first, then bump the motor and watch the rotation arrow before you trust the pump.

After a motor overhaul, test the insulation resistance before the first start and check the rotation with the coupling off where you can: on some pumps a screwed-on impeller or impeller nut can work loose when it turns backwards.

Check yourself