When a motor stops after starting i.e. it fails to carry load, the causes may be because of :
1. hot bearings, which increase the load by excessive friction.
2. excessive tension on belt, which causes the bearings to heat.
3. failure of short cut-out switch.
4. single-phasing on the running position of the starter.
The usual cause of blow-outs in induction motors is single phasing.
By single-phasing is meant the opening of one wire (or leg) of a three-phase circuit whereupon the remaining leg at once becomes single-phase. When a three-phase circuit functions normally, there are three distinct currents flowing in the circuit. As is known, any two of these currents use the third wire as the return path i.e. one of the three phases acts as a return path for the other two. Obviously, an open circuit in one leg kills two of the phases and there will be only one current or phase working, even though two wires are left intact. The remaining phase attempts to carry all the load. The usual cause of single-phasing is, what is generally referred to as running fuse, which is a fuse whose current-carrying capacity is equal to the full-load current of the motor connected in the circuit. This fuse will blow-out whenever there is overload (either momentary or sustained) on the motor.
If already running and carrying half load or less, the motor will continue running as a single-phase motor on the remaining single-phase supply, without damage because half loads do not blow normal fuses.
If motor is very heavily loaded, then it will stop under single-phasing and since it can neither restart nor blow out the remaining fuses, the burn-out is very prompt.
A stationary motor will not start with one line broken. In fact, due to heavy standstill current, it is likely to burn-out quickly unless immediately disconnected.
The Y -phase connected across the live or operative lines carries nearly three times its normal current and is the one most likely to burn-out.The other two phases R and B, which are in series across L2 and L3 carry more than their full-load currents.
When currents flow in single-phasing star-connected motor, with L1 disabled, the currents flowing in L2 and L3 and through phases Y and B in series will be of the order of 250 per cent of the normal full-load current, 160 per cent on 3/4 load and 100 per cent on 1/2 load.
Protection against single phasing:
The motors be protected against single-phasing by:
(i) By incorporating a combined overload and single-phasing relay in the control gear.
(ii) By incorporating a phase-failure relay in the control gear. The relay may be either voltage or current-operated.