Page 206 - Industrial Power Engineering and Applications Handbook

7/186 Industrial Power Engineering and Applications Handbook

7

Ud

C
inst.

3 4

1 1
(Off-delay) On Off
+ trip
inst. - instantaneous inst.
Figure 7.19 A typical scheme illustrating re-acceleration of a critical motor

7.19 Motors for thermal power authorities may sometimes stipulate norms and outline
station auxiliaries their basic minimum requirements for the benefit of users
and the manufacturers. In India, for instance, the Central
Electricity Authority (CEA) publishes manuals based on
These applications have considerably more stringent the actual feedback obtained from various thermal sites,
performance requirements than any other application. electricity companies, leading consultants and the NTPC
Circulating water pumps, boiler feed pumps, forced- (National Thermal Power Corporation), in addition to
draught (FD) and induced-draught (ID) fans, pulverizers motor manufacturers. Below are extracts from these
(ball mills) and condensate pumps are components in a manuals :
thermal power station that may require extra safety in a
standard motor to make it able to fulfil these requirements I Motors must be generally squirrel cage type and not
and withstand abnormal service conditions and system slip-ring.
disturbances. Abnormal operating conditions may be one 2 The preferred ratings for motors above 1 104000 kW,
or more of the following: must follow the ‘Renard Series’, R-20, according to
IS0 3, as shown in Table l.l(a).
3 The preferred synchronous speeds is as shown in Table
High ambient temperature
High humidity 7.7.
Dust-laden (coal dust and fly ash) environment 4 The enclosure and type of cooling is as shown in
Table 7.8.
Long continuous operation 5 Space heaters: motors above 30 kW must be provided
High fault levels
Overvoltages, caused by a fast bus transfer with anti-condensation space heaters suitable for
Re-acceleration in some drives 240 V 50 Hz (or 60 Hz), a.c. supply. The space heaters
Voltage surges, caused by system disturbances or must switch ON automatically when the motor is out
of service. Motors of 30 kW and below must be suitable
switching operations (for HT motors).
for connection to a 240 V, 50 Hz (or 60 Hz) a.c.
supply. Here also the windings must be connected
In a large power station, connected to a transmission automatically to the 240 V supply when the motor is
network through a power grid, system disturbances are a out of service.
common feature. For details see Section 17.5. 6 Terminals and terminal box
With a view to standardizing the basic requirements HT motors must be provided with phase-segregated
of an electric motor for essential services, electricity terminal boxes as illustrated in Figure 7.17.