Page 211 - Industrial Power Engineering and Applications Handbook

Special-purpose motors 711 91

Coupling speed
Figure 7.22 Accelerating torque with the use of fluid coupling

engaged with load, up to Nr, Tr2 z 1 6ST, which will now be possible with a standard squirrel cage
motor. The motor starting torque requirement can also
(b) calculating average ordinate\ at points I to 6
be relaxed with the use of flexible couplings. if this can
= i[XO + 135 + 185 + 215 + 215 +;(215 + 1001 economize on the cost of the motor.
= 165%
Checking the suitability of a slip-ring motor
Lo'id aberage torque E 0.68Tr
With the same pull-out torque as above, if we select a
.. TCi2 = 1.65T, - 0.68T, slip-ring motor and arrange the starter to develop dn
= 0.97Tr = 450 x 974 x 0.97 averdge pick-up torque of 200%, the avdilable accelerating
torque will be (200 - 68) = 132%
980
= 433.33 mkg 450 x 974 ,.32
i.e. 980
and GD; = GD<4 + GD: + GD?
1866 x 980
= 200 + 200 + 1666 or 590 mkg and starting time 1, = 375 x 590
= 2066 kgm' or 8.26 seconds.
N,? = 980 - 800 Assuming a safe start time of8.26 x 1. 15. i.e. 9.5 seconds,
a motor with a safe hot stall time of 30 seconds will be
= 180 r.p.m.
adequate to withstand three consecutive hot starts. since
... I,? from 0.8Nr to N, = 2066 x 180 3 x 9.5 < 30 seconds.
375 x 433.83
(The actual speed will be slightly less due to coupling Nore In a slip-ring motor I,, is much less compared to a squirrel
slip) cage motor, say 200-300% I,. and therefore a hot thermal with\tand
time of only 6 to 9 \econds ~hould be quite safe for such a duty. e.?.
= 2.28 seconds
Heating, H = If,. K t, lr: . R t2
:. Total accelerating time = 2.01 + 2.28
= 4.29 seconds
With permissible tolerance = 1. 15 x 4.29
Since the thermal withstand time refers to locked rotor conditions
= 4.93 seconds and normally corresponds to 600 to 700% I,.
:. at 300% I,,. the equivalent thermal withstand time, considering
For three consecutive starts, the motor must possess a the hot thermal withstand time at 600% starting current as 8
minimum thermal withstand time of about 14.8 seconds wconds will be