Under normal operating conditions, phase displacement is a function of generating equipment and stays relatively fixed, but the line voltages can vary depending on the individual loads connected at that time. For balanced loads, such as three-phase motors, unbalanced phase voltages lead to unbalanced currents flowing in the motor windings. Consequently, circulating currents are set up, heating is increased and uneven, and torque is reduced.

Condition of all windings in the motor

The stator windings connected to the supply are prominent and obvious areas of concern. Noisy operation and reduced torque of a three-phase motor can mean that the bars of the rotor might need attention. Many cages consist of the aluminum cast into a shape in the laminations, and little can be done in the way of maintenance; many of the larger motors, however, have prefabricated bars and rings of copper, which are welded into place. It is possible to repair or replace damaged items, whether broken or simply loose in the rotor.

Phase reversal

The induction motor has three identical windings, and three equal currents flow in them, all spaced at 120° E to each other. If one phase is reversed, however, these conditions no longer hold true. Two of the three currents that flow are at 60° E to each other, and the load system is unbalanced. The same condition applies to delta-connected loads.

(If phase currents become unbalanced, windings generate increased heat, and torque is greatly reduced.)

Reversal of one phase for star-connected motor. Source Ahmed Faizan Ahmed

As a result of this incorrect connection, the motor loses most of its torque and is often unable to start against even a light load. If able to start at all, it usually rotates very slowly and has unequal values of current in the phase windings. The values of the current approach those drawn during normal starting, but remain high. The motor usually emits a ‘growling’ noise and has an associated vibration due to the sustained high current values.

Single phasing

Single phasing is a condition that occurs when one line of a three-phase supply is open-circuited and is not able to supply current to a three-phase load. The term is also used when one of the three-phase windings in a load is open-circuited.

The condition for single phasing in a star-connected load and a break in either the line or the phase winding reduces the circuit to a single current path.

There is one main current path from L1 to L3 through phase A and another path from L1 to L3 through phases B and C in series. Both currents are in parallel with each other, although not necessarily in phase with each other. A delta-connected induction motor is shown with phase C open-circuited. There are two current paths — L1 through phase A to L3 and L2 through phase B to L3.

Single-phasing of a three-phase induction motor due to circuit condition. Source: Ahmed Faizan Ahmed

The rotating magnetic field is either destroyed or unbalanced and causes unsatisfactory operation of the motor. The motor rotates at slower speeds if it can start at all because of a much-reduced starting torque. It usually draws higher than normal currents in the parts of the circuit still operating, with values approaching starting current values in some circumstances. It can also emit a low-pitched growling noise like that which occurs during a phase reversal.

(If single phasing occurs while the motor is operating at normal speeds, the normal humming sound often changes to a higher-pitched whine.)

Overloading

Standards are laid down for the operation and performance of induction motors. In particular, standards are set for the starting and running torques of these motors. There are also stated limits for torque a motor can exert when it is unable to start or is stalled by the connected load. The limits are, of course, variable, depending on the size or rating of the motor.

These requirements are built into the motor design and beyond the control of anyone attending to the maintenance or installation of a motor. Overloading is a condition normally brought to the attention of a technician only when a motor is behaving in an abnormal manner.

The motor may run at a slower speed and higher temperature than normal. The varnish used on the windings might start to smell, while in extreme cases, smoke might start to issue from the windings.

Once a motor is unable to meet the requirements of the applied load, it will come to a rapid stop, and locked rotor conditions then apply. Current more than normal full load will then be drawn, and the installed motor protection must rapidly disconnect the motor from the supply to prevent damage.