Can an ac motor work with just a single phase (not talking of two- phase machines)? ie if you spin the motor shaft can it work or must you have at least two phases? I cannot understand why a single phase does not produce a rotating field. if the current changed then the field changes with the current and the phasor must rotate if it is sinusoidal.
Hardy
the field is rotating and but theres no difference between cw and ccw so a motor would run in a random direction, inertia keeping it running at what ever direction it happens to start.
-Lasse
it does produce rotating field how ever, its balanced between the poles and the rotor will just sit there, you need to spin the shaft left of right to offset this balance. Then it'll start.. You won't have an much efficiency but because the there is a balance talking place in the field how ever, between the swing load of the rotor and back emf, it helps counter act this..
Single phase motors have a third leg and is used to offset this balance.
There are various ways to do this.
Cap to produce phase C at all times or just for starting which is what they call them RUN or START CAPS the RUN cap is much smaller than a START cap and usually found in motors with variable speeds. (split phase) etc..
Then you have the non CAP versions that simply has a start coil with a centrifugal switch that disconnects when rpm's reach a minimum speed..
etc...
A single phase machine produces a field that flips back and forth. This can be expressed mathematically as a pair of fields, rotating in opposite directions at the same speed.
Electrical machines that work in single phase do so because the rotor will generate more torque in response to a field whose speed is closer to the rotor's speed than not -- so once the rotor is spinning it gets pulled more by the 'part' of the field that's rotating its way.
The rotor will still respond to the other 'part' (or it'll act like its just one field, flip-flopping, depending on how you want to look at things). This generally results in a vibration at the line frequency plus the rotor's electrical frequency (i.e. if it's an induction machine then it'll spin at less than 60Hz, and the vibration will be that less-than-60Hz plus 60Hz).
There are other ways of looking at this that don't ask you to view the one flip-flopping field as a superposition of two different fields. They can all be made mathematically equivalent, but the "two superimposed fields" model is more consistent with three- and higher phase motors, so it's generally the one that's taught.
-- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" was written for you. See details at http://www.wescottdesign.com/actfes/actfes.html
A single phase exciting current goes through zero twice per cycle. A rotating field never is zero.. So, a single phase might create one Cartesian component of a rotating field, but not both.
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