Speed control of wound rotor induction motors thru slip ring resistances variation

Thread Starter


Joined Nov 20, 2011
In the late 1960's, active in R&D @ Philips Power Electronics, I developed a working prototype for a speed variable wound rotor induction motor, where:

- the classical variable resistances (rheostats) were replaced by a chopper with variable duty cycle (the thyristor was just there, my job was to find applications for it...)

- the chopper was built into a cylindrical shape and was made solidary of the rotor

- the speed setting was transmitted magnetically from the stator to the rotor

- the slip energy was dissipated, which was deemed acceptable for a fractional horse power motor (less than 1 HP) : the market segment objective

- I found a way to collect and dissipate the slip energy. An air transformer was buit with a single winding secondary, shorted and finned. This aluminum secondary had a lateral 'U' shaped crossection. The chopper coil, the primary of the air transformer, was slid in this U shaped cavity, the shorted cooled secondary. Both were potted into one solid component.

Philips wrote 4 patent applications to cover this.

A good team was assembled with SUBEM, a Belgian induction motor producer (the second in revenue). The project was stopped by Philips for political reasons. I left the company, disappointed, and went on.

I very much would appreciate your comments !
Did you come across a scheme like this ?

Very cordially,


Joined Mar 24, 2008
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Joined Oct 3, 2010
Well that sounds more clever than something I could come up with, but ultimately turned out to not be what the market was looking for apparently. If the market didn't bite on the idea in the 60's, they sure won't now. Everything is going to VFDs. Your design sounds bulky, and material-intensive. For fractional HP motors you can get a VFD that is the size of a soda can for cheap, and is more efficient in that it doesn't burn of excess energy in a finned shorted coil; it only uses what it needs. I hope this doesn't sound like I'm talking smack about your idea; like I said, it's better than I could come up with, but you asked for input so there's mine; just shootin' you straight.
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