Standard general-purpose motors have several advantages of importance to the user of electric motor drives. These arise due to the high degree of standardisation in terms of rating, duty, dimensions, electrical and thermal characteristics, and construction and also because they are manufactured in large numbers.
The main advantages are their relatively low cost, they are easy to replace directly with another motor without any modifications, they are easy to repair, and are readily available. However, there are special requirements for particular applications 'special' motors are inevitable. In some cases standard frame sizes can be used with special shafts, finishes or mounting arrangements. In others, either special performance characteristics or space constraints demand a completely new design.
The term 'crane motors' can usually be applied to motors, which carry out the three primary crane functions:
Load lifting and lowering (hoist motors);
Motion along the factory;
Motion across the factory.
Generally one motor is employed for each of these functions, except in the case of bucket or grapple cranes where two motors work together to provide the opening and closing motion of the bucket and grapple and the hoisting motion.
Because of the precise control requirements (variable torque loads, acceleration, stopping and reversal) crane motors are either d.c. or a.c. wound-motor induction motors, although cage motors are used for small hoists and cranes where precise control is not necessary. Heavy-duty steelworks cranes which have wide load variations are equipped with d.c. series motors supplied from a constant voltage d.c. power supply.
The basic speed control is inherent in the motor speed regulation. Series-connected tapped resistance banks are switched to provide current limiting on starting and low-speed operation.
In cases where fine speed control is essential, Ward-Leonard or thyristor controlled d.c. motor drives are usually the first choice. Wound-rotor a.c.