In the delivery documentation for a machine, phase-loss and phase-sequence protection are often treated as a single electrical accessory and passed over in a sentence. These two faults are of entirely different types, one burns the motor out and the other turns the pump backwards, and the way each is investigated is different as well, so they are worth setting out on their own.
Phase loss: the motor runs on two phases
When a three-phase induction motor loses one phase, the remaining two phases still form a rotating field and the motor still turns, at a speed close to normal. The problem is that the torque falls, the current redistributes between the phases, and the current in the phase that has not been broken rises sharply. The most dangerous aspect of this state is that it does not trip: the motor does not stop, it does not trip, and the sound may only change slightly, so the operator often does not treat it as a fault. The result is that the windings run continuously overheated, the motor burns out after several hours or longer, and the damage is not confined to the windings: the contactor, the fuses and the cables in the same circuit may be affected as well.
Phase sequence error: the pump runs backwards
A phase sequence error means that the sequence of the three phases is connected wrongly, and the consequences are entirely different from phase loss: the motor turns the wrong way and the pump turns the wrong way with it. When the pump runs backwards there is no mechanical overload and the current is basically normal as well, so the machine looks perfectly healthy, but the direction of the oil circuit is completely reversed and the inlet and the outlet are swapped. In a system with a non-return arrangement this shows up as a pressure that will not build and a vacuum system that will not pull; in a system without one, the oil may be driven into the wrong line or even pushed back into the oil tank.
- Phase loss: unbalanced current and overheated windings, with the insulation ageing under sustained high temperature.
- Phase sequence error: the rotation is reversed, and pumps and valves that are sensitive to rotation act in the opposite direction.
- What the two have in common: neither is a fault of the machine itself, but a problem on the supply side or the wiring side.
How the protection logic recognises the two faults
Phase-loss protection works by monitoring the balance of the three phase currents: when the deviation between the three phases exceeds the set range, the condition is judged to be phase loss and the control circuit is cut. The common approach for phase-sequence protection is to check the sequence before the machine is energised and refuse to enter the running state if it is wrong, or to detect an incorrect direction of rotation during running and stop the machine. Both are wired into the start and stop circuit of the machine, and the result of the action is the same: no start, or a trip during running. The value of fitting these two protections is that they turn a fault that destroys the machine slowly into a clear refusal to start.
Three steps in the investigation on site
First, check the three-phase voltage and the phase sequence at the incoming end, taking one measurement at no load and one under load with an instrument; a normal reading at no load does not mean the machine is normal under load. Second, check the connection positions of the three phase lines against the equipment terminals, with particular attention to whether anything was disturbed during maintenance or after a modification to the wiring. Third, match the record of the protection operation against the operating conditions at the time, and establish whether the condition was judged to be phase loss at start-up or whether it was a current imbalance during running; the two lead in different directions.
| Fault type | Electrical indication | Mechanical indication | Where to start |
|---|---|---|---|
| Phase loss | The three phase currents are unbalanced, one phase rises markedly | The motor still turns, the windings overheat continuously | The three-phase voltage and the wiring at the incoming end |
| Phase sequence error | Current and voltage basically normal | The rotation of the motor and the pump is reversed | Check the phase sequence against the required direction of rotation of the pump |
| Operation of the protection | The start and stop circuit is cut | The machine does not start, or stops during running | Check the operating record and the conditions at the time |
| Accompanying checks | The state of the contactor and the fuses | The coupling and the pump seals | Establish whether there is already associated damage |
Phase loss burns the motor and a phase sequence error turns the pump; neither of the two faults exposes itself, and both can only be found through the protection and through regular checking.