Handbook of Electrical Installation Practice - BeKnowledge

426 Handbook of Electrical Installation Practice
facilities which provide early warning of advanced brush wear and reduce the possibility
of an unscheduled stoppage for maintenance.
Coolers
After a time any heat exchanger whether air/air or air/water will have developed
some dirt build-up which reduces the cooling effectiveness. Cleaning of the air passages,
tubes and water tubes is therefore necessary from time to time. Filters fitted
to force ventilating blowers need to be inspected and, if necessary, cleaned or
replaced depending on the type.
If the machine is to be shut down in freezing conditions any water circuits should
be drained. Condensation forms and accumulates in the air/water heat exchanger
and could be drawn into the machine through the air passages and enter the windings.
When a machine is to be shut down for any time, the cooling water flow should
also be stopped to limit the build-up of condensation. Even under normal operation
this condensation can be significant and justify raising the water temperature
in the cooler and reducing the ambient humidity to reduce the quantity of condensation
formed.
Windings and insulation
A machine which has been out of service for a long period could have too low an
insulation resistance to allow it to return to operation. If the resistance is less than
1MW on an l.v. machine, or equivalent value on h.v. machines, the windings need to
be dried out. The machine can be placed in an oven or dried using a hot air blower.
In either case the temperature should not exceed 90°C or damage could result to
rotating diodes for example. Initially, the insulation resistance will fall and then
slowly increase to a maximum level. The resistance readings should be monitored
as the drying out process proceeds and, to avoid misleading readings, the temperature
should be kept as constant as possible.
Before taking any insulation resistance measurements, any diodes or capacitors
should be disconnected or short-circuited. High-voltage insulation breakdown tests
should be avoided as they have a progressive effect in causing deterioration of the
winding insulation.