Synchronous or Asynchronous Motor in Hoist and Crane Applications

Typical squirrel cage asynchronous motors are capable of 200 to 225% locked rotor torque generation at start, and without the slip in this type of motor, the starting torque generation isn't there. Synchronous motors are not used in any hoist and crane applications I'm aware of because of this, for the 36 years I've been in the industry. Once in a while we run into a crane repaired by people not as experienced as they should have been, one recently in Texas where they had installed a synchronous motor as a replacement for the original center drive bridge motor. (Both 5 HP Rated) They called here for advice as the motor wouldn't operate properly with the existing ac drive. I ran it and identified the motor. The end result was they opted to repair the original, asynchronous motor and the problem was solved.


The Synchronous motor was drawing excessively high current at start, as it suffered with lower starting torque capabilities. The drives we use, and others on the market that are crane rated, are capable in many cases of +/- 0.5% accuracy of selected motor RPM without encoders, 0.1% with encoders, using asynchronous motors. This allows precision control more than acceptable for agencies such as NASA and Lockheed, as an example, in rocket construction, including what was used in building the space shuttles. NEMA Design D motors are often converted from wound rotor, for example, to squirrel cage type motors by grounding out the slip rings and winding in a 10% slip factor in the rotor. These motor types have a significantly higher starting torque capability than NEMA B/C motors, those being common squirrel cage types. The D motors, however, don't have the level plain across the total RPM specter, losing some at the approach of the maximum RPM rating. However, once the hoist load or the traversal load is in motion, the high torque at start isn't needed as the load is already past the inertial starting stage.