Handbook of Electrical Installation Practice - BeKnowledge

indicated by an arrow on the fan casing. Providing that the fan motor has been connected
with the correct phase rotation as specified in the connection diagrams, this
test should be just a formality but nevertheless an important formality because
wrong fan rotation will almost certainly significantly reduce its effectiveness with
consequent overheating of the main machine. This overheating would not be
detected by any overcurrent relay as already stated and damage to the main
machine windings could result.
Direction of rotation should similarly be checked on the main machine itself.
Again, the correct direction is indicated or specified on the nameplate or elsewhere.
Often machines, particularly the larger ones, are designed for rotation in only
one direction. Reversing the direction of rotation therefore may require some
mechanical alteration, albeit only a change to the shaft driven fan. Unidirectional
fans are more common on larger motors because they are both more efficient and
less noisy.
Alternators which have been in storage for some time may have insufficient
residual magnetism to allow proper excitation to build up. Larger alternators having
a permanent magnet pilot exciter should not suffer this problem. Some manufacturers
provide the normal exciter with pole face permanent magnets embedded to
eliminate the possibility of failure to excite.
When failure to excite is experienced and a maximum voltage regulator setting
is not successful and any rotating diodes have been examined for correct functioning,
flashing of the exciter field winding is necessary.This can be achieved by running
the alternator up to speed and by touching the leads from a 6 or 12V battery across
the exciter field terminals in the correct polarity.
Checking of a rotating rectifier system requires each diode in turn being removed
from its mounting plate and its forward and reverse resistances being measured.
The forward resistance should be low and its reverse resistance high. If both resistances
are approximately equal a failed diode is indicated and this should be replaced
by an identical unit.
When refitting a diode a silicon heat sink compound should be lightly smeared
on the diode seating to improve heat transfer and inhibit corrosion. A torque
spanner to the correct setting must always be used when tightening semiconductors
to their mounting plates or heat sinks.
Insulation resistance tests can be made on a brushless alternator using a 500V or
1000V instrument but not on the rectifiers themselves. Similarly, high voltage flash
tests should not be attempted unless the rectifiers have first been short-circuited and
any auxiliary equipment such as the voltage regulator disconnected.
Once the preliminary tests have been completed and the machine is running and
loaded it is desirable to check for vibration and to monitor and log all meter readings
and speed.
MAINTENANCE
Rotating Machines 423
Rotating electrical machines are inherently robust and reliable and require little
maintenance. Generally, maintenance involves maintaining standards of cleanliness
and regular inspection. A machine which is running within its design parameters is