Page 62 - Industrial Power Engineering and Applications Handbook

Motor torque, load torque and selection of motors 2/43
Heat generated;
H = Is: . R ' t, watt. s. (W.S.)
also H=W.6.8

_. or 8=- "C
6
~ ~~ W.
f 15-2070 Of T, where
Minimum
W = weight of heated portion in kg
6 = specific heat of the material of windings, in watt . s./
kgm ("C)
8 = temperature rise in "C (Table 1 1. I )
A possible way to restrict the temperature rise is the
use of a material having a high specific heat. An increase
in the weight would be futile, as it would require more
material and prove to be a costly proposition. A motor's
constructional features should be such as to provide good
Speed 4 N, heat dissipation through its body.
Figure 2.14 Accelerating torque (Ta)
Sharing of heat
The rotor and stator heats, during start-up and run, are
interrelated and vary in the same proportion as their
GD2 of the load, as referred to the motor speed, will be respective resistances. (See circle diagram Figure 1.16
different. Equating the work done at the two speeds: in Section 1.10.)

GO; . N,' = GO,' . Nf If H, = the heat of rotor in W.S.
and H, = the heat of stator in W. s.
or GD; = GD; ($)?
(2.9)
where
GD; = weight moment of inertia of load at a speed N,*. While the total heat generated in the stator and the
rotor may be comparable, there may be a significant
difference in the temperature rise of the respective parts
Example 2.1 as a result of the bulk of their active parts and area of
A 100 kW, 750 r.p.m. motor drives a coal mill, having GD: as heat dissipation. For the same material, the rotor will
600 kg.m2 through belts, at a speed of 500 r.p.m. Then its have a much higher temperature rise compared to the
effective GD: at motor speed will be stator, in view of its weight, which may be several times
less than the stator. During start-up, therefore, the rotor
500
GDf = 600 x ( 75(1) will become heated quickly and much more than the
stator. Repeated start-ups may even be disastrous. During
a run, however, when the temperature has stabilized, an
= 600 x 0.445
overload will render the stator more vulnerable to damage
= 267 Kg m2 than the rotor. The rotors, as standard practice, are designed
for much higher temperature rises (200-300°C) and may
Note For simplicity, the synchronous speed of the motor is be suitable to withstand such marginal overloads.
considered, which will make only a marginal difference in
calculations.
Corollary
2.7.1 Motor heating during start-up During start-up thc rotor, duc to its lighter weight compared
Irrespective of the type of switching adopted or the load to the stator, and during a run, the stator, due to overload
driven by the motor, each time it is switched it generates are more vulnerable to damage through excessive heat.
heat. in both the rotor and the stator components. The
magnitude of the start-up heat will depend upon the inertia Example 2.2
of the rotating masses, the type of switching, the torque A rotor fails during start-up, possibly due to a lower supply
developed by the motor and the opposing (load) torque voltage than desired or a smaller accelerating torque than
required or reasons leading to similar conditions. In such
etc., as can be inferred from equation (2.5). The higher cases the rotor fails first, due to higher rotor currents and a
the time of start-up, the higher will be the heat generated. prolonged acceleration time or a locked rotor. At this instant,
The corresponding temperature rise of the stator or the unless the motor control gear trips, the stator may also fail
rotor windings can be measured as below: due to excessive heat. Instances can be cited where even