Page 189 - Industrial Power Engineering and Applications Handbook

Special-purpose motors 711 69
Table 7.1 Electrical requirements for lift and crane motors

Type of dllh Storfing torquP (TJ Pull-uut torque (T,,J Over .\perding If’freqiient
uccrlercition,
braking and
reverscils are
re y uIred
I Portal and semi-portal Standard as in t 2.25 of T, Up to 2.5 Nr or Yes
wharf cranes manufacturer’s design 2000 r.p.m.
whichever is les\
2 Oberhead travelling > 2.25 of T, Up to 2.75 of T, A, above Yes
cranes
3 I.iits (a) For squirrel cage ~ As above -
motors 2 2.25 5 2.75
of T, (at any speed)
(b) For 4ip-ring motors, ~ As above -
t 2.25 of T, (with
suitable resistances)
4 Power-driven winches E 2.0 of T, 2 2.25 of T, for squirrel cage and Yes
tor lifting and hauling 2 2.75 of T, for slip-ring motors

i T2.t, iT;.t2iT;Z.t,+... Manufacturers Association (IEEMA) has issued a standard
on crane duty motors in which an attempt is made to list
Teq (r.m.s.) = t, +I? i i the outputs against the IEC frames for S3-6, S,- I SO and
...
tz
S5-300 startdhow duty types.
Duty cycle
Static drives
Duty types Sj, S, and Ss as discussed in Chapter 3 are
normally applicable to crane and hoist motors. For duty With the availability of Vlf drives and other advanced
types S4 and Ss, the duty cycle per unit time is greater technologies through static controls, as discussed in
than S3. The most important factor is the number of Sections 6.2 to 6.4, the use of standard squirrel cage
switching operations per hour. A temperature rise in the motors for such applications is a preferred choice.
motor occurs during acceleration, braking and reversing.
Many crane manufacturers specify that the motor should
be suitable for half an hour or one hour duration according 7.4 Sugar centrifuge motors
to the British practice still followed in some countries.
In fact, it is not possible to correlate precisely these In sugar mills a rapid separation of sugar crystals from
ratings with any of the duty factors. Hence the motors molasses is achieved through the use of massecuite* and
are designed for any of the duty factors of IS%, 2S%, centrifugal force. The motor drives a basket full of molasses
40% and 60%. In fact the duty factors for different types which undergoes repeated cycles of operation, i.e.
of cranes have been standardized, depending upon their
operation. after several years of experience. For example,
the cranes operated in steel industries have different types Charging of massecuite: at a low speed, to prevent
of duty factors as follows: spillage, normally by a 24-28 pole motor.
Intermediate spinning: at half the maximum speed of
spinning, i.e. at 8 or 12 pole.
Hoisting 60% Spinning: at a very high speed compared to the above,
Traversing 40% generally at 4 or 6 pole.
Tral elling 60% Regenerative braking: When the process is complete
Slewing 40% and the residue molasses are purged, an oversynchro-
nous braking is applied by changing the motor from
For steel mill auxiliary drives or for material handling spinning (4 or 6 poles) to ploughing (48 or 56 poles).
equipment, the duty factor normally chosen for slip-ring This brake energy is then fed back to the mains (see
motors is either 40% or 60%. Section 6.20.1(B)).
Ploughing of sugar crystals: at very low speeds of 50
Standardization r.p.m. or so. This is achieved by a 48- or a 56-pole
motor. A further reduction in speed is obtained by
The fixing dimensions of the motors are standardized at conventional electrodynamic or d.c. electric braking.
national and international levels. However, the output-
frame relationship is not yet established for duty type *Massecuite is used to form and then remove sugar crystals from
rated motors. The Indian Electrical and Electronics molasses by a centrifugal technique.