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Old 10th Oct 2010, 1:46 am   #18
kalee20
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Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,059
Default Re: Motor type - synchronous or not?

I'm with you on both counts, Kat - feeling the torque, and the method of speed control. It wouldn't work with a synchronous motor. It might not stall, but it would give very uneven running as the rotor gets further and further behind the rotating field and then suddenly slips a cycle giving a jump to the rotor movement - only for this to repeat.

Yur motor can't be a synchronous type.

In fact to me a friction-brake is a bit of an inelegant method anyway to control speed. I've seen medium-price B & O turntables with an eddy-current brake to control speed of an induction motor - the gearing and pulley sizes chosen such that 'free' speed (ie nearly synchronous) gives a turntable speed slightly too fast. Then the braking brings it down to the correct speed. The eddy current brake is just a thick copper or aluminium disc on the rotor, with a coil and pole-piecss (having variable DC shoved through it) close by.
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