Page 100 - Handbook of Electrical Engineering

80 HANDBOOK OF ELECTRICAL ENGINEERING

3.9.4 Terminal Boxes

Motors and generators should be provided with properly designed terminal boxes. They should
be capable of withstanding a full three-phase fault without destruction and with the minimum of
subsequent repair work and materials being needed. The duration of the fault would be typical of
the relay or fuse protection provided in the switchboard. With a generator the limitation of damage
by the corrective action of the switchgear protective devices is not as effective as for a motor. The
switchgear in feed to the terminal box fault can be isolated by the relays in the circuit breaker.
However, unless the exciter can be shut down or the machine brought to rest there is a possibility
that the generator will feed its own fault. These events are rare but possible, and when they do occur
they are very disruptive to the production from the plant.

Large high voltage machines are usually protected by differential stator current (87) relays
and earth fault (51G) and (64) relays. These systems require current transformers to be fitted close to
the winding terminals. It is very desirable to mount these transformers inside the main terminal box.
Frequently it is necessary to have two main terminal boxes, one for the high-tension transformers
and cable connections, and one for the star point transformers and NER cable connection. It is also
preferable to fit the transformers in the star point ends of the windings because these are at almost zero
potential for the majority of the life-time of the machine. This minimizes the problems in designing
adequate space at the high-tension ends of the windings to locate these items. The usual alternative
is to fit them at the switchgear end of the feeder cable or bus-ducting which also has the advantage
of including these in the zone of protection.

3.9.5 Cooling Methods

The majority of motors are cooled by a simple shaft mounted fan which is attached to the non-drive
end and blows air across ribs or channels in the outer surface of the enclosure. This method is
satisfactory with machines rated up to about 1000 kW, thereafter a more elaborate system of air-to-
air (CACA) or air-to-water (CACW) heat exchangers is necessary. In all cases the main enclosure
should be totally enclosed and sealed from the surrounding atmosphere by machined faces and shaft
seals. This concept is also called ‘totally enclosed fan cooled or TEFC’, where the fan referred to is
generally the internal fan which circulates the enclosed air along the air gap and amongst the windings.
IEC60034 Part 6 and the NEMA standard MG1 give details of motor and generator enclosures.
Externally mounted fans on the shaft or in the heat exchangers should be made of a material
that cannot produce a spark if the blades happen to touch their surrounding metalwork. See also
sub-section 5.1.8 for further comments on the construction of induction motors.

3.9.6 Bearings

Machines rated up to about 150 kW generally use rolling element bearings, one of which usually
acts as the centralising and thrust-carrying element for the shaft. The lubricating medium is grease.
Some driven machines impart a longitudinal thrust on to the shaft of motors (150 kW is also near the
limit for the use of low voltage machines when direct-on-line starting is to be used). The above limit
may be extended to 500 kW for high voltage machines. Above 500 kW the practice is to use sleeve
bearings with or without forced lubrication. As the ratings increase the use of forced lubrication
becomes necessary, and with it the need for a cooling system for the lubricant.