Page 208 - Industrial Power Engineering and Applications Handbook

7/188 Industrial Power Engineering and Applications Handbook
In HT motors the terminal box for space heaters impedance, because of the saturation effect, than at a reduced
and embedded temperature detectors must be voltage. The Zst at full voltage therefore would be slightly
separate from the main terminal box. higher than that calculated from the starting current, measured
The terminal box must be capable of withstanding at a reduced voltage. Hence the allowance for this. See also
the system fault current for at least 0.25 second. Figure 27.2(b).
The terminal box must be suitable for being turned
through 180”, for bottom or top cable entry. 10 Frequency of start: The HT motors must be suitable
The terminal box must have the same degree of for two starts in quick succession when the motor is
protection as the motor. hot or three equally spread starts in an hour (not to be
For large motors, 1000 kW and above, if they are repeated in the second successive hour).
provided with star windings the three neutral end 11 Stresses during a fast bus transfer: A fast bus transfer
leads must be connected to a separate terminal box between the unit and station supplies is an essential
to enable mounting of CTs for differential protection. feature in a power station to maintain an uninterrupted
The neutral terminal box need not be phase power supply to the unit and station auxiliary services
segregated. (Figure 13.21). The changeover may be as fast as
7 Performance three or four cycles which is adequate to break before
Motors must be designed for an ambient temperature make in the modern interrupting devices and hence is
of 50°C. safe to adopt. The motors should, however, be suitable
HT motors must be generally wound with class F of withstanding excessive voltages on account of this
insulation and the temperature rise should not exceed up to 150% of the rated voltage for at least one second
the prescribed limits of class B insulation. LT motors during such bus transfers.
up to 250 kW, however, can be wound with class B
insulation. Nure The time gap between a fast bus transfer is so small that
The performance of the motors must conform to it will not permit the various drives to slow down sufficiently to
IEC 60034- 12. affect their performance or the system if they are made to re-
8 Voltage and frequency variations accelerate (reenergize) on restoration of power.
During start-up, the motors must be suitable for
accelerating to 80% of the rated voltage. In a fast bus transfer scheme, the phase angle is
Motors must run at full h.p. under the following monitored through special relays (Section 16.1 1) which
initiate the changeover, so that the self-induced e.m.f.
conditions:
Variation in frequency: f5% of the motor does not slip too far in the phase angle,
A@, between the two voltages and the motor is not
Variation in voltage: flO%
Combined variation in frequency and voltage: f10% exposed to excessive overvoltages. At higher A6 the
Motors must not stall due to a momentary dip in relays will block the transfer and protect the system
voltage up to 70% of the rated voltage, i.e. from excessive voltages. If the fast bus transfer fails
to change over automatically, slow changeover takes
7’ to be more than T,./(0.625)’, i.e. 256% for a place and the motor-induced e.m.f. is allowed to decay
1200 r.p.m. motor and TJ(O.605)’, i.e. 273% for a by up to 40-20% before the bus transfer takes place.
1000 r.p.m. motor and less, as shown in Table 1.5.
Motors must run satisfactorily for 5 minutes at a For insulating and bracing the windings, such bus
transfers may occur up to 500 times in the total lifespan
supply voltage of 75% of the rated value.
of the motor.
12 Starting time and locked rotor withstand time
Nore This operating condition is, however, not specific for For motors with a starting time of up to 20 seconds,
it does not stipulate the frequency of occurrence of such a with the minimum permissible applied voltage
contingency. It may be assumed that this condition will not
occur more than once before thermal stability is reached. (80%), the locked rotor withstand time under hot
See also Section 3.7. In normal practice a motor meeting the conditions at rated voltage must be at least 2.5
other operating conditions noted above in all likelihood will seconds more than the starting time.
satisfy this requirement also without needing yet another For motors with a starting time of more than 20
derating. See also Example 7.1, seconds but within 45 seconds, at the minimum
permissible applied voltage (SO%), the locked rotor
9 Starting current withstand time, under hot conditions at rated voltage,
On DOL the I,, must not exceed 600% of I, but for must be at least 5 seconds more than the starting time.
boiler feed pumps it must be limited to 450% of I,, 13 Bearings: The bearings will preferably have an arrange-
subject to the tolerance stipulated in IEC 60034-1. ment for self-lubrication.
To determine the starting current, when the test is 14 Bearings must be insulated wherever necessary, to
conducted at a reduced voltage (1/& Vr ), prevent them from shaft currents (Section 10.4.5).
allowance must be made for the saturation effect 15 Over-speed: Motors will be designed to withstand
while estimating the value of I,, at the rated voltage. 120% of ff, for at least 2 minutes without any
See also Section 1.6.2(A) also (the important note mechanical damage.
on starting torque). 16 Noise level: Motors must conform to the requirements
of IEC 60034-9. A safe sound pressure level for a
Note At full voltage, the stator core will offer a lower human body to perform better, during an 8-hour