As the purpose of the crane is to lift heavy load through rope or chains, so it demands a kind of motor, which has a very high starting torque to lift the load at low current. Squirrel cage induction motor has the problem of low starting torque due to low resistance. Therefore, they are not suitable for applications like crane or lifts.
On the other hand, a slip ring induction motor is also similar to the normal Squirrel cage induction motor. The only difference is in the construction of the rotor winding. The rotor winding is been connected with the slip rings with the help of carbon brushes. The other end of the slip rings is been attached to the external variable resistances. This variable resistance is at its maximum value when induction motor starts, resulting in maximum resistance, low current flow and more torque. This resistance drops slowly with the increase in the speed of the motor and is been completely eliminated when the motor is at its full speed. Due to this extra added resistance provided by slip rings, slip ring induction motor has a high starting torque. Which makes it perfectly suitable for applications demanding high starting torque such as cranes and lifts.
So the use of a slip ring induction motor is been based on the application requirement. Crane has to lift or pull heavy loads, so the motor used for this task needs to provide a very high starting torque. Normal squirrel cage induction motor is not capable of providing very high starting torque. In-fact from the discussion above, squirrel cage induction motor has poor starting torque. However, the modified version known as slip ring induction motor provides very high starting torque with the help of slip rings and added resistance. Which is the main reason for the use of slip ring induction motor in heavy lifting applications through ropes such as in cranes and in lifts.