View single post by David Hoatson
 Posted: Wed Sep 16th, 2020 01:59 am
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David Hoatson

Joined: Sat Oct 5th, 2013
Location: Chestertown, Maryland USA
The start windings are positioned at a different angle than the start windings. Maybe 15 degrees off. 
The AC voltage is a sine wave:

To get the fan spinning in the correct direction, the starting coils need to be fed a sine wave that is delayed or advanced 15 degrees relative to the run windings. Together, the two coils make a “rotating magnetic field”. 

It turns out that the current through a capacitor is a sine wave 90 degrees ahead of the voltage and the current through a coil (inductor) is 90 degrees behind the voltage. This is called “phase shift”:

But, the resistance of the coil changes the phase angle shift to less than 90 degrees. 

So, to drive the starting coil with a delayed or advanced sine wave, it just needs a different resistance/inductance combination or different resistance/capacitance combo. Combined, this is called “reactance”.  Instead of +90 or -90 phase shift, you can make whatever you want. In our case, we wanted 15 degrees. 

Some early 1900’s R&M ceiling fans used nichrome wire in the start coils. Nichrome has 10x higher resistance than copper and gives a nice phase shift. But, as it ages, it gets brittle and breaks.  

You can also get a phase shift by having a different inductance (# turns of wire) or different resistance (diameter of wire and length of wire).

Emersons and other fans have an extra coil in the speed coil to make phase-shifted current. 

Even with a centrifugal start switch, the start windings need a phase shift. 

In a shaded pole motor, copper bands wrapped around part of the magnetic poles delay the magnetic field. 

Last edited on Wed Sep 16th, 2020 02:02 am by David Hoatson