AC motor help needed

mctrivia

I am trying to find out if the rotor on a split-phase motor uses an electro magnet or a permanent magnet. The reason is I am studying for a test and came arose this question:

The rotor of a split-phased motor is:
a) not electrically connected to the winding
b) electrically connected to the centrifuge switch
c) electrically connected in series with the winding
d) electrically connected in parallel with the winding.

b&c are both obviously wrong. And I am pretty sure the answer is a but I can not find any diagrams that accurately depict the rotor of a split-phase motor or any document saying how the rotor gets it magnetic field.

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mctrivia

found the answer: the key is to find that the rotor is often called a squirrel cage rotor:
www.magnetonlymotors.com/related%20topics

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Lots of propeller based products in stock at affordable prices.

yarisboy

The question appears to relate to small single phase AC motors that are self starting. The term "split phase" indicates that the incoming single phase is "split in-two" by differing time domain (torque)responses of two components in the stator. The split phase description would rule out small permanent magnet rotor synchronous with a single stator time domain pole set. They used to be in AC alarm clocks before your time. To the typical civil service job interviewer split-phase means one thing and one thing only. (good luck with the interview). I'm not that smart so here is what I've observed. In my fridge the fan motors have shaded pole motors. When I'm modifying the Grainger (shaded pole)motor to fit in a Kenmore fridge I notice two things. 1) I saved 50% on cost. 2) the second winding is a single loop of 1/8" copper wire that responds differently in time than does the single AC line coil that magnetically excites the rotor. This motor is cheap and just accepts the thermal loss associated with leaving the second pole set engaged. To the civil service "expert" this is probably not a split phase motor. When I do the annual inspection on my heavy bench grinder it has two windings. One is hooked to the on/off switch. The second has a second speed cut-out switch that chops power to it when the grinder has made 77% (torque pull out speed) of its fully loaded slip RPM. This is a very high torque start motor so stand away from the wheels when starting. Some of these have a capacitor on the start coil that further boosts starting torque. If the grinder cost a lot more it may also have a second run cap across the run coil to increase efficiency(power factor). I also have a cheap polishing grinder that is low torque/slow start. It has an on/off switch and a capacitor hooked to only one of it's windings. This run-cap alters the time domain response of one of the windings enough that the motor will self start in a no load condition. When I get into my home 5-ton HVAC system the design and control of the single phase motors gets more complicated with several definitions. As a retired and experienced person I'd not pass the civil service exam because I'm over-qualified. My dumb brother-in-law would because he'd just go find out the caned definitions and answers on the civil service exam. Typically the interview panel members wouldn't know the difference between a potential start relay and a thermal start relay but by comparing your answer with what is on their interview directive they've earned there keep and decided your fate. Good luck.

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MOORE'S LAW: The capabilities of electronics shall double every 18 months.
cloyd's corollary: Hardware is easy, software is hard.

erco

XLNT reply, yarisboy! I only recently delved into AC motors (my neighbor's ancient garage door opener, no schematic) and had to sort out all sorts of switches and caps that controlled it. It used a split-phase motor with an internal centrifugal switch on the rotor that opened up at speed. THAT was the problem. Too much crud buildup inside this open frame motor was preventing the switch from working properly. Removing the crud buildup and adding some lube solved the problem. VERY high starting torque when it all works properly.

Thanks for your description of the time domain response of these motors. Very insightful stuff!

http://en.wikipedia.org/wiki/AC_motor#Split-phase_induction_motor· mo' good info

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·"If you build it, they will come."

Post Edited (erco) : 5/2/2010 5:50:27 AM GMT