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225-275% of T, and a low starting current, up to a
maximum of five times the rated at the rated voltage as
well as frequent starts, stops and reversals. They are
normally short-time rated. To make these motors suitable
for frequent starts and stops, the rotor is designed so that
acceleration is quick and the heat generated during a
start is low. This now limits the temperature rise of the
rotor even after frequent starts, without sacrificing the
frame size. It is possible to achieve this by keeping the
GD2 of the rotor low. In such motors also, fan cooling
may be obstructed when a brake is mounted on the
extended shaft at the non-driving end (NDE). For such
installations also, sometimes a surface-cooled motor may
be preferred. Alternatively, to increase the cooling surface,
the housing may be designed with circular ribs, as shown
in Figure 7.l(b) and 7.2(b).

Lift motors
Generally same as the crane motors, but comparatively
silent in running and have a very low vibration level.
For general requirements of other types of lifting and
Figure 7.2(a) Surface-cooled loom motor without fins (cradle hoisting applications see Table 7.1.
mounted)
7.3 Determining the size of motor

For lifting/hoisting
The mechanical output of the motor for cranes and hoists
in lifting the hook load is the useful work done by it. The
losses produced in the crane or hoist mechanism are
taken into account by the mechanical efficiency of the
hoisting mechanism.
The output Ph of such motors is expressed by
kW
F.V
Ph = - (7.1)
102 ‘ q
where
F = useful load in kgf
V = lifting speed in m/s
q = efficiency of the mechanism
This output corresponds to a continuous duty of drive. It
must be suitably corrected for the duty cycle the motor
has to perform (see equation (3.1 l)), Le.
Figure 7.2(b) Loom motor with circular ribs (cradle mounted)
Pheq = tl + t* + t3 + ...
higher frames for such motors, compared to a standard
motor. The latest practice is to employ a variable-speed For traverse
drive. As for ‘Cop Bottom Build’ and ‘Nose formation’,
the frame must operate at a slower speed to minimize the Pt = 1.027 . T,,, . N, . kW (7.2)
end breakage, while for the remaining yam it may operate 1000 . q
at the higher speed. where
T,,, = maximum torque, consisting of torque, resulting
from weight load, friction and acceleration in
7.2 Crane motors kgf.m
q = efficiency of the whole mechanism for traversing.
Crane and hoist motors
Correct this output also depending upon the duty cycle
Such duties require a high starting torque, of the order of as noted in equation (3.12), i.e.