342 HANDBOOK OF ELECTRICAL ENGINEERING
may not present a problem to the motor but the driven machine could be damaged if it is allowed to
operate continuously in this state.
Undercurrent protection is often specified as a back up to the process control systems. It has a
typical setting range of 30% to 80% of the nominal relay current. A time delay is incorporated into
the relay and its range is typically 2 to 120 seconds.
12.7.9 High winding temperature
Resistance temperature detectors e.g. 100 ohm platinum elements, or thermocouples are usually
embedded in the three-stator phase windings to detect overheating in the vicinity of the conduc-
tors. A set of three is normally used, and a second set of three specified as spare detectors. The active
elements are wired to a simple threshold relay that gives an alarm when the temperature is exceeded.
12.7.10 High bearing temperature
Similar detectors and relays as those in sub-section 12.7.9 are used to detect excessive temperature
in the bearings of the motor. The relay gives an alarm when the temperature is exceeded.
12.7.11 Excessive vibration
Excessive vibration in the shaft of a motor can be caused by several functions:
- Damaged rotor conductors.
- Damaged bearings, especially rolling element bearings.
- Low oil pressure in the bearings.
- Unbalance in the driven machine e.g. vane damage in a pump, blade damage in a compressor.
- Loose coupling or gearbox components.
The measurement of vibration should be made by a non-contacting transducer, i.e. it should not make
direct contact with the rotating shaft.
12.8 Low Voltage Induction Motor Protection
In general a large amount of the theoretical aspects of the protection of high voltage motors applies to
low voltage motors. However, some functions are not normally required, in particular, core balance
earth faults (50 N and 51 N) see the note below:-
- Differential stator current (87).
- Undercurrent (37).
- High winding temperature.
- High bearing temperature.
- Excessive vibration.