Page 56 - Industrial Power Engineering and Applications Handbook

Motor torque, load torque and selection of motors 2/37
2.1 Motor speed-torque curve

Refer to Figure 2.1 where
T,, = starting torque or breakaway torque.
T, = minimum, pull-in or pull-up torque.
Tpo = pull-out, breakdown or maximum torque, obtainable
over the entire speed range. In a good design this
should occur as close to the rated slip as possible
to ensure that the motor runs safely, even during
momentary overloads, load fluctuations exceeding
the load torque, or abrupt voltage fluctuations,
without harmful slip losses (equation (1.9)). In
some specially designed rotors, however, to achieve
a high starting torque sometimes the pull-out torque
Tpo may not be available on the speed-torque curve.
It is possible that in such cases the T,, may be the
highest torque developed by the motor in the entire
speed range (Figure 2.2).
T, = rated or the full-load torque and should occur as
near to the synchronous speed as possible to reduce Speed -
slip losses. Nr
S = rated slip at which occur the rated torque and
current. Figure 2.2 T,, too high to have Tpo on the speed-torque
curve
2.2 NEMA rotor designs

As a further step towards standardization and to achieve
more harmony in motor sizes and designs, for better NEMA,* in its publication MG-I for Induction Motors,
interchangeability in the motors produced by different has prescribed four rotor designs, A, B, C, and D, covering
manufacturers, in the same country or by other countries, almost all sizes of LT motors, to possess a prescribed
minimum T,,, Tpo and pull-up torques. These torques are
generally as drawn in Figure 2.3 to meet all normal
industrial, agricultural or domestic needs. (Refer to the
said publication or IEC 60034-12 for values of these
torques. IEC 60034-12 has also provided similar
stipulations.)
However, motor manufacturers may adopt more flexible
designs with more reserve capacity and better speed-
torque characteristics to suit the requirements of a
particular sector. These are particularly for installations
where the distribution system may have wider voltage
fluctuations or the load itself may have varying load
demands. It is possible that the same motor may have to
drive more than one type of loads at different times. An
agricultural pump motor may be one such application
where it may also have to drive a thrasher or a winnower
at different times. A motor with higher flexibility would
be more desirable for such applications.
Tr Manufacturers, depending upon market needs, may
adopt all or a few such designs or even have their own
designs, still conforming to such stipulations. Special
applications may, however, call for a custom-built motor
as noted later. As a standard practice all HT motors are
custom-built for each application and no rotor dcsigns
Speed --C are prescribed for these.
T,, : Starting torque
T, : Pull-in or DUII UD toraue
Ti : Pull-out 0; breakdown torque (maximum torque)
Tr : Rated torque
Figure 2.1 Defining a motor torque *NEMA - National Electrical Manufacturers Association, USA.