Which type of drive can be used for Hoisting Machinery?
Right Answer is:
All of the above
AC slip Ring Motor, Ward Leonard Method, Cumulative Compound Motor drive can be used for Hoisting Machinery.
Cranes and Hoists for electric motor drive
The requirements of the drive are as follows:
The acceleration and retardation must be uniform. For exact positioning of loads creep speeds must be possible. The motion of the crane is in all three dimensions. The drive must have high speeds in both directions horizontal and vertical. The speed must be constant while lowering the loads. Mechanical braking must be available in an emergency. Due to the heavy inrush of current at starts, fluctuations in supply voltage are possible. The drive motor must be capable of withstanding such fluctuations.
Among the d.c. motors, the series motors, and cumulative compound motors are most preferred for crane operations. The motors have good starting torque, high torque capacity at light loads, the simple arrangement of braking, electric braking at low speeds is possible. The only disadvantage is that the motors are less stable while the regenerative braking. So additional stabilizing circuits are necessary. Advances made in the technology of solid-state devices have enabled the use of thyristor converters and choppers for driving the d.c. motors used in cranes and hoists with good accuracy, reliability, and efficiency.
⇒ The characteristics of cumulative compound motor lie between those of shunt and series motors. The series field provides a high starting torque and the shunt field prevents overrunning in the no-load condition.
These motors are used for drives where high starting torque is required with the probability of the load being totally removed such as punch, press, shears, planning machine, conveyors, crushers, bulldozers, lid haulage gears, mine hoist, power fans, rolling mills, stamping press, and the large printing press.
⇒ In 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.
Ward Leonard system is commonly employed for elevators, hoists, and main drive in steel mills, as this method can give unlimited speed control in either direction. Since the generator voltage can be varied gradually from zero, no extra starting equipment is required to start up the main motor smoothly.