Page 60 - Industrial Power Engineering and Applications Handbook

Motor torque, load torque and selection of motors 2/41
2.4.1 NEMA recommendations onstartingcumnts 2.5 Load torque Or Opposing torque
With a view to achieve yet more standardization in motor
design, NEMA Standard MG- 1 has also recommended For smaller loads, say up to 20/30 kW, it may not be
the maximum locked rotor current of single-speed three- essential to pre-check the load curve with that of the
phase motors for the various rotor designs A, B, C, and motor. But one should ensure that working conditions or
D, for various recommended torque values. These have the load demand are not so stringent that they may cause
been derived for a 415 V a.c. system and are shown in a lock-up of rotor during pick-up due to a very low applied
Table 2.2. voltage or accelerating torque, or a prolonged starting
time as a consequence or due to a very large inertia of
Table 2.2 Recommended maximum locked rotor currents for rotating masses etc. For critical applications and for larger
various rotor designs motorb it is essential to check the speed-torque
requirement of the load with that of the motor. Loads
~
HP Approx. maximum Rotor design can generally be classified into four groups. Table 2.3
locked rotor current indicates the more common of these and their normal
torque requirements, during start-and variation with speed.
I 18 B.D. The corresponding curves are also drawn in Figures 2.10-
1 .5 25 B.D. 2.13. To ascertain the output requirement of a motor, for
2 31 B.D.
3 39 B.C.D. different applications a few useful formulae are given in
5 56 B.C.D. Appendix 1 at the end of Part I of this book.
7.5 71 B.C.D.
IO 98 B.C.D.
15 141 B.C.D. 2.6 Selection of motors
20 I78 B.C.D.
25 222 B.C.D.
3 0 265 B.C.D. The recommended practice would require that at each
40 3s4 B.C.D. point on the motor speed-torque curve there should be a
so 44 1 B.C.D. minimum 15-20% surplus torque available. over and above
60 529 B.C.D. the load torque, for a safe start (Figure 2.14). The torque
75 66 I B.C.D.
100 884 B.C.D. thus available is known as the accelerating torque.
125 1105 B.C.D.
150 1319 B.C.D.
200 1764 B.C. ~ 2.7 Time of start-up and its effect
Note For motors beyond 200 h.p., NEMA has not covered these on motor performance
data. It is, however, recommended that larger motors may be designed
to have even lower locked rotor current; than the above to reduce This depends upon the applied voltage, i.e. type of
the starting transient effects on the distribution system as well as switching, starting torque of the motor, counter-torque
on the motor windings.

Table 2.3 Types of loads and their characteristics
Seriai Load Characteristics of Sturting torque Opposing torque with Figurt. no.
110. load speed
Presses, punches, latches - Light duty 20-309 Torque remains constant 2.10
and drilling machines and at a very low value.
since the load is applied
when the motor has run to
speed
Fans. blowers, centrifugal The power is Medium duty 1040% Torque rises with square of 2.1 1
pumps and compressors proportional to the the speed (T Q N')
third power of the
speed (P = N3)
Rolling mills, ball mills, The power is Heavy duty 3040%. Near full-load torque 2.12
hammer mills, calendar proportional to the May be more and have to
drives and sugar square of the speed accelerate large masses
centrifuges (P = Nz) of heavy moment of
inertia, requiring a
prolonged time of start-up
Conveyors and hoists The power is Heavy duty lOO-llO% Torque remains constant 2.13
proportional to the throughout lhe speed range
speed (P - N) and at almost the full-load
torque