Which of the following motors is preferred for boom hoist of a traveling crane?
Right Answer is:
AC Slip Ring Motor
SOLUTION
A hoist having a spar projecting from the mast to support and guide the load is called Boom Hoist.
For crane travel, trolley travel and boom hoist of traveling crane require high starting torque and constant speed in operation. Hence slip ring induction motor is preferred.
In the SLIP RING induction motor the ends of the rotor windings are externally connected by a variable rheostat (resistance is varied in order to give it proper starting and running current). So more the resistance, more the torque. When we add resistance to the rotor the torque is high, the slip is high and the current is reduced.
With the correct value of (usually) resistance inserted in the rotor circuit, a near-unity relationship between torque and supply current at the start can be achieved, such as 100%full load torque (FLT), with 100% full load current (FLC) and 200% FLT with 200 percent FLC. This is comparable with the starting capability of the dc machine.
Not only high starting efficiency but also smoothly controlled acceleration historically gave the slip ring motor great popularity for lift, hoist, and crane applications.