And a winding INSIDE that single-phase powered motor generates another phase. The rotor, typically, has conductive elements (short-circuited ones, with zero voltage applied). Calling the motor 'single-phase' ignores the internal structure, just as most circuit diagrams omit that detail.
So 3-phase motors are actually 6-phase motors.
Cheap switching power supplies used to inject a lot of higher harmonics of the grid frequency (50Hz or 60Hz) back into the grid. This creates problems and all but the smallest switching power supplies incorporate electronic power factor correction, which is designed to make the current flowing into the rectifying stage look very like a sine wave. What comes out is usually a floating high voltage direct current which is dumped into a capacitor which feeds a fast-switching transformer-based DC-to-DC converter. Because the switching frequencies are high, the transformers can be small, light and fairly cheap and they do isolate the output from from the mains, even though you do have hook up at least one rail to earth to make everything safe.
Stopping the noise from the fast switches from coming showing up at the mains socket feeding the device and on it's output leads does require attention, and some cheap crap doesn't do it well.
Anthony William Sloman snipped-for-privacy@ieee.org wrote in news: snipped-for-privacy@googlegroups.com:
Good reference as to how a switcher operates with smaller transformers. PF correction is required in any CSA cert unit. Any gov contracted unit as well.
Not quite related, we made a gov contracted supply for a computer that auto-switched between a set of 48V battery terminals to 85-265 VAC. We had maximum ripple requirements at both "high line" and "low line" (of course the DC was zero). If the AC switcher dropped out, the DC circuit would take over, so there had to be a monitor/switchover watchdog circuit in it that decided the source but not so much that it switched between them uneccessarily.
The physics of motor movement make it impossible to start clockwise with only one phase of excitation, but 3-phase motors can work without any trickery (large-ish synchronous motors, for instance) internally. Or, they can be induction motors with internal extra phases in the rotor. The only apparent difference is that the synchronous motor motion locks exactly to the input frequency.
All capacitor-start or capacitor-run motors also form extra phases even if the input power is single phase.
The original quote was "To produce a rotating field in a motor, at least
3 phases are needed."My reply was that a second phase is created in a start winding to start a single phase motor, same as what you probably just said. Phase shift to a start winding in the stator is done with capacitors or inductance. "At least 3 phases" are not needed, single phase motors start on 2 phases.
But more to the point, as I said, a single phase motor will run (but not start) on only single phase. There is a "rotating field" in the stator (but it can be considered clockwise or counterclockwise). When the motor has started, induction works from the rotating field. "At least 3 phases" are not needed. Single phase induction motors run on a single phase.
Capacitor start induction motors use the capacitor with a start winding only for starting. Capacitor run motors are also capacitor start (likely a much larger capacitor), and are 2 phase.
Synchronous motors are another subject. Three phase "large-ish synchronous motors" won't simply start across the line. They require "trickery". Three phase induction motors don't have a problem.
bud-- snipped-for-privacy@void.com wrote in news:cGaSI.40954$ snipped-for-privacy@fx46.iad:
There is a grinding wheel at work that does not start on its own and will run in either direction, depending on the (mechanical) boost direction one gives it at the time of energization.
Usually, that's a bit of dirt in the centrifugal switch. On a (woodwork) table saw, a bit of sawdust. Chopstick and compressed air fixed that without dismounting the motor, last time mine had the issue.
whit3rd snipped-for-privacy@gmail.com wrote in news: snipped-for-privacy@googlegroups.com:
Except in this case. This is an old antique pedistal grinder with two wheels, and it has no centrifugal assembly in it. Better watch which way you plug the two wire cord in as well.
Probably could have freed it with a heat gun too. Tends to loosen things like that up. Toss in a squirt or two of good ol' WD-40.
But not with this grinder.
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