Page 209 - Industrial Power Engineering and Applications Handbook

Special-purpose motors 7/189

Table 7.9 Sources emitting sound and their likely loudness
level
Noise level (dB) Sound source
0 Threshold of hearing
20 Rustling sound, whisper, homes
(quiet places)
40 Motor cars
60 Normal conversation
80 Street traftic
100 Light engineering workshop
120 Thunderllightning

working day, without undue fatigue, is considered at
90 dB as noted in Section 7.9. For recommended
values of sound level for airborne noise, refer to the Speed -
IEC publication. See also Table 7.9 for sources in s= 1
day-to-day life that emit noise and their likely noise Slip at 80%
levels. voltage
17 Temperature detectors
HT motors must have a minimum six numbers of Figure 7.21 Determining the accelerating torque
resistance temperature detectors (RTDs) (See section
12.8).
All bearings of HT motors must be provided with minutes and can be assumed to occur once an hour. Also
embedded temperature detectors (ETDs). likely overloading by 20% for the same period in the same
18 Polarization index test duration. Only one contingency occurring at a time. The motor
Motors rated 7500 kW and less must be considered should also be capable of running, without stalling if the voltage
suitable for dielectric tests or operation only when drops to 70% momentarily, say, for 25 cycles.
the polarization index or the value of the insulation Ambient temperature = 50°C
resistance (at 40°C) is at least the minimum Permissible starting = DOL in a squirrel cage or rheostat in
recommended values. a slip-ring motor
Motors rated above 7500 kW must have both the Desired starts = six equally spread starts per hour and
polarization index and the insulation resistance above three consecutive hot starts not to
be repeated for one hour
the minimum recommended values. Service factor = 1.1, alternatively 1.15
The recommended minimum value of the
polarization index for motors having class B or F Solution
insulation must be 2 when determined according to Deratings
IEC 60034- 18- 1. (a) For +lo% voltage and frequency variation: 90%
(Section 1.6.2(C)).
Note The above recommendations are those of an electricity (b) For 50°C ambient temperature: 92% (Section 1.6.2).
authority of one country and may vary for other countries. (c) At 80% voltage, the full load torque of the motor will drop
to 0.73*, Le. 0.53 times the rated torque, as shown in
Table 1.5. Also the motor will tend to stall unless adequate
7.20 Selection of a special-purpose torque is available on the motor torque curve, i.e. Tpo
must not be less than 1/0.53 or 189%. The motor will thus
motor start to drop its speed until it reaches a point where a
motor torque of 189% of 450 kW is available. The motor
will now operate at a higher slip, causing higher slip losses.
Example 7.1 Assuming the mill torque at the reduced speed to be the
Select a suitable motor for a ball mill with the following load same as at 100% speed, then the kW requirement of the
details: mill
kW = 450 kW- N. T
r.p.m. = 400 through V-belts
-
.
Startina torque = 40% rising to 100% at full load This will also decrease in the same proportion as the
(Figure 7.51). increase in slip. For a rough estimate, we may also ignore
Gd of rotating masses = 10 000 kgm’ the higher slip losses for an equal reduction in the required
kW. However, due to the lower voltage the motor current
Supply system: will increase proportionately and will be IJ0.8 or 1.251,.
Voltage = 3.3 kV f 10% The motor will thus run overloaded by 25% for 20 minutes
Frequency = 50 Hz ? 3% which is likely, and not more than once an hour. The
frequency of occurrence must be known. A higher derating
Combined voltage and frequency variation not to exceed may be necessary if such a condition is frequent.
f 10%. Voltage may fall to 80% during running for about 20 (d) Overloading of 20% for 20 minutes per hour need not be