Page 28 - Industrial Power Engineering and Applications Handbook

Theory, performance and constructional features of induction motors 119
Tabte 1.2 Recommended frame sizes assumptions. The parameters that may affect the per-
formance of a motor are
Frame size Shaft height for B3 Frame size Shaft height for B3
(Figure 1.17) (Figure 1.17) 1 Voltage unbalance
motors motors 2 System harmonics
(mm) Imm) 3 Voltage variation
4 Frequency variation
56 56 225L 225
63 63 250s 250 5 Ambient temperature and
71 71 250M 250 6 Altitude.
80 80 250L 250 They may influence the performance of the motor by its
90s 90 280s 280
90L 90 280M 280 1 Output and
IOOS I00 280L 280 2 Torque. IEC 60034-1 stipulates for a minimum in-
1 OOL 100 315s 315
112s 112 315M 315 built capacity of a machine (all ratings and voltages)
112M I12 315L 315 to sustain an excessive torque of 60% for a minimum
112L I12 355s 355 of 15 seconds, without stalling or an abrupt change in
132s 132 355M 355 its speed. This stipulation is to meet the need for an
I32M 132 355L 35s excessive torque of a transitory nature due to the above
132L 132 400s 400 parameters or for a momentary excessive load torque
160s I60 400M 400 itself during operation. This stipulation, however, would
160M 160 400L 400 not apply to motors designed and manufactured to
160L 160 450 450
180s I80 5 00 500 specific requirements.
180M 180 560 560 3 Efficiency
180L I80 630 630 4 Power factor
200s 200 I10 710 5 Speed
200M 200 800 800 6 Slip and
2001, 200 900 900 7 Current
2259 225 1000 1000
225M 225 - -
Note According to the stipulations of IEC 60034-1, any factor
According to IEC-60072-1 and 60072-2 noted above which may influence the performance and cause an
excessive current than rated, or because of an excessive load itself,
Note Designations S, M and L denote the variation of length in motors rated up to 3 15 kW and rated voltage up to 1 .O kV should
the motor housing for the same shaft height. be capable of withstanding a current equal to 150% of the rated
current for a minimum of two minutes. No such tolerance is, how-
ever, recommended for motors beyond 315 kW and all HT motors.
1.5 Preferred ratings at different This stipulation would not apply to motors that arc dcsigncd and
voltages manufactured to specific requirements.
The extent to which these performance data may be
The preferred ratings at different rated voltages according influenced, by the non-standard opcrating conditions is
to publication MG-I are given in Table 1.3. discussed briefly below.

1.6 Influence of service conditions 1.6.1 Voltage unbalance and system harmonics
on motor performance As standard practice, all motors are designed for a balanced
and virtually sinusoidal supply system, but it may not
The performance of an induction motor is influenced by be feasible to obtain the designed supply conditions
the service conditions, when these differ from the design in practice. Hence, a motor is designed with a certain
in-built capacity to sustain small amounts of voltage
unbalances and some degree of harmonic quantities, such
Table 1.3 Preferred horsepower rating for different voltages that the voltage waveform may still be regarded as
sinusoidal.
Supply system (kV) Preferred rating (h.p.)
Voltage unbalance
0.415* Up to 600
3.3 200-6000 The performance of a motor is greatly influenced by a
G.6 1000- 12 000 voltage unbalance in the supply system. It reduces its
11 3500-25 000
output and torque and results in a higher slip and rotor
loss. This subject is covered in more detail in Section
*This voltage is now revised to 0.4 kV as in IEC 60038. I2.2(v). For likely deratings, refer to Figure 12.1. A system
‘MG- Reference to a publication on Motors and Generators by with an unbalance of up to 1 % or so calls for no derating,
1
:
NEMA (National Electrical Manufacturers Association, USA) which whereas one having an unbalance of more than 5% is not
is adopted universally. recommended for an industrial application, because of a