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