How do Crane Motors Work?

Answer 1

I'm not 100% familiar with this but this should be pretty close to the answer. Simple/standard crane motors work by applying power first through a rotating steel shaft into a gearbox. The gearbox allows the gearing ratio to be changed (to trade rotation speed for torque or torque for rotation speed). Another steel shaft carries the power out of the gearbox, again through rotation, to a spindle (think big thimble) around which steel cable is wrapped. So the motor rotates this spindle to wind in the cable or unwind the cable to lift things or lower things.

Here is a good page on cranes:
http://en.wikipedia.org/wiki/Crane_%28machine%29

Here is a good page on motors:
http://en.wikipedia.org/wiki/Electric_motor

Answer 2

Crane motors, for the most part, fall under the "variable speed" category of machines. This means there must be some criteria used to determine/measure the rotational speed of the rotor, and (usually) some means of determining absolute position. Also, there must be a means of varying the speed of the machine while providing sufficient torque for the application.

DIRECT CURRENT:
Most commonly used variable speed devices in the world (historically). These are usually listed as having two speeds; a "base" speed and a "top" speed. The range from zero to base speed is achieved by energizing the main stator field winding at full current, and varying the armature voltage. Effectively, this means a constant volts-per-hertz operation envelope, corresponding to variable power/constant torque. The higher speeds - base to top range - are achieved by what is referred to as field weakening. In this case, the armature voltage is held constant at the maximum, while the stator field strength is reduced by lessening the current applied to the winding. Effectively, this means an inconstant volts-per-hertz, corresponding to constant power/variable torque operation.

INDUCTION:
More or less, these operate the same as a DC machine, except that the higher range (base to top) is achieved by varying the frequency applied to the system ... something that is now achievable through the use of PWM drives, for instance.

SYNCHRONOUS:
These are not often used in crane applications because of the range of speeds involved, the rapid response requirements, and the difficulty of maintaining adequate speed/position control.

Typical load profiles for cranes are a combination of two different curves.

First, there is a "loaded" condition. During this phase, the load is lifted and then lowered. The machine must produce torque, because there is the "hook load" plus the actual material load. Depending on how big a lift is being done (versus the torque production of the machine), it is pretty common to find the machine working at or below base speed.

The second phase corresponds to the "unloaded" condition. Here, there is just the hook load - thus the machine can be operated much more quickly (field weakened) to return it to where another load of material can be obtained.

For sizing a machine, both portions must be understood and torque requirements adequately defined - as must the time alloted to each portion of the cycle.