Fan Controller wirering 240V need help

[mapj1]

The desk fans are brushed motors, or at least the ones I have liberated are.... and either have resistors swithced, or in one case a thrid brush, ofset to the two "normal' ones by about 60 degrees. Most odd.
Interested to know if any do use the induction motor plus cap trick..
When you've next got one apart let me know..
regards M

 

[felix]

I dredged the brain sludge today and unearthed some old stuff on induction motors. Yes, the standard shaded pole squirrel cage induction motor has a rather odd torque-speed curve. Torque increases with rising slip speed then falls off rapidly. Here's why - approximately.

The induced emf is maximum in those cage bars lying between poles and is proportional to slip speed. This should set up a magnetic field in the rotor at right angles to that in the stator, exactly what you want for maximum torque, but there's a snag.

Those cage bars are inductive. As the slip speed rises, the induced CURRENT begins to lag the emf and so the rotor field moves round until it's almost equal and opposite to the stator field. That satisfies Lenz's law but gives little in the way of torque.

The way out of this is to include plenty of resistance in the cage bars. Resistance of course means heat but that's not a major problem when the rotor is on the outside and in a good blast of cooling air as is the case in our ceiling fan.

Many axial flow cooling fans also have the rotor on the outside. This in itself won't make them good at low speed operation but one can always hope. What was the original question again ---

 

[mapj1]

Cheers Felix. I dont care what the original Q was - I like new info. I knew the curves were funny, but not why - Odd really as from time to time I design electronics to wrap round stuff like this..
Happy to be a bit wrong if I'm always learning.
M.