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Ac Motor Controller


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I came cross a special device consisting of an electronic card firing a series of transistors which themselves are connected to the stator windings. The rotor of the motor is connected to a series of resitors through a series of contactors. I am told that when powered, the motor ramped slowly to rated speed similar to a soft starter. This application is used on a hoist unit.This is a japanese invention according to the description I can read on the box.

This equipment is now faulty and literature is unavailable.


Any guess on this technology?



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Are they transistors or SCRs going to the stator? If SCRs, it might just be a soft starter on a wound rotor induction motor. if it is transistors, it may be a VFD of some sort, but there would need to be more than just transistors. A VFD would have a rectifier (diodes or SCRs) and then a converter (transistors) as well as caps.


Either way, it sounds as though the rotor resistors denote it to ne a wound rotor. Nothing terribly unique about that, especially in the hoist industry.

"He's not dead, he's just pinin' for the fjords!"
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Hello Bob


I agree with jraef, the "transistors" will almost certainly be SCRs that are used to control the voltage on the stator. The speed torque curve of the motor is modified by the rotor resistors and the torque output is fine tuned by the variation of the voltage applied to the stator. There is often a tacho attached to the shaft as well.

This enables the motor to provide a controlled positive torque while operating in reverse.


This type of technology has been manufactured by a number of manufacturers in the past. I would suggest that a search on websites of Siemens, ABB etc will probably reveal some information on the basic technology.


Best regards,

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