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Slipring Motor

12/25/2017 1:01 PM

is it possible to run a slipring motor as an induction motor by shorting the rotor winding leads together and applying power to the stator

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#1

Re: slipring motor

12/25/2017 1:52 PM

You are bypassing the starting mechanism by doing this and reducing the starting torque considerably, perhaps by as much as 50% or more...A slip ring motor is an induction run motor, and once the motor is up to speed the rotor windings are in a shorted condition....resistance is applied between the windings of the rotor during start procedure to increase the torque available, and reduce energy usage....

http://www.brighthubengineering.com/diy-electronics-devices/43726-how-does-a-slip-ring-motor-start/#

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#2

Re: Slipring Motor

12/25/2017 9:54 PM

It is more than possible, this happens in standard operation of a slip ring motor...(with the minor note that 'applying power' doesn't actually change anything because power has already been applied since before rotor was shorted).

Slip ring motor rotors are shorted once up to speed. The resistance is added to the rotor windings during start up to reduce the out of phase characteristic of the otherwise mostly inductive reactance.

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#3

Re: Slipring Motor

12/26/2017 7:22 AM

It can be done but it's not recommended

http://www.lmphotonics.com/slipring.htm

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#4

Re: Slipring Motor

12/26/2017 1:34 PM

A "slipring" motor with wound rotor is an induction motor with provision for adding external resistance to the rotor.

To add to Solar Eagle's comment, starting energy loss in rotor circuit = rotational energy put into rotor and load inertia.

Overheating of the motor rotor [and stator] would limit the load inertia which could be started. By dumping the loss in the external rotor resistance, much higher inertia loads can be started and very high stator currents at low power factor from supply are avoided.

In theory, a slipring rotor could be run from rest with zero external rotor resistance, but practically this would likely exceed permitted rotor and slipring currents and be outside stator current and power factor limits set by supply.

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#5

Re: Slipring Motor

12/27/2017 4:22 PM

Only for as long as it takes the upstream over-current protection equipment to disconnect the fault!

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#6

Re: Slipring Motor

12/27/2017 6:42 PM

m.kumar,

In our area this was called a "wound-rotor" motor, and was frequently used with a controller that varied the external resistance to change the speed of the motor. With appropriate auxiliary equipment a very quiet motor was therefore used with high power factor over a fairly wide speed range to pump sewage from a wet well to the treatment system and maintain a constant level in the wet well.

During a 2-year period around 18-20 years ago I converted at least 10 of these to induction motors by the shorting of the slip rings (done by welding pieces of bus bar across the three slip rings), and then powered them from a variable speed drive to maintain the same performance of the motor. We had no difficulties with any of the motors, but did pay careful attention to the use of load reactors to minimize the presence of overvoltage spikes on the stator windings (due to the impedance mismatch between the supply conductors and the windings, and the high-frequency components of the VFD's output power). Today, the VFD's have very good filters to provide the same or better performance than the use of external load reactors.

--John M.

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#7
In reply to #6

Re: Slipring Motor

12/27/2017 7:55 PM

The key point to this is "... and then powered them from a variable speed drive to maintain the same performance of the motor."

The VFD was not only maintaining the performance, but was also allowing control of the rotor current / torque / speed relationship so as to not burn up the rotor. If you just short the rotor bars and try to start it across-the-line (DOL) or even with a soft starter, you create a very high slip motor using a rotor that is NOT designed to handle the high current in the rotor windings, because it was made to have an external resistance applied to it until the rotation acts to impede current flow.

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