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


David Hoatson

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 phaseshifted 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 

