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#1
12-17-2008, 11:25 AM
 bonkers Junior Member Join Date: Dec 2008 Posts: 14
motor start and run windings

When doing a voltage test cross a run or start winding to common should the meter red o volts if the winding is closed and 120 if the winding is open or 120 in either case. Would it act the same as a switch or would the resistance of the windings make a difference.
#2
12-17-2008, 12:56 PM
 mik3 Senior Member Join Date: Feb 2008 Location: Cyprus, but now in UK (GMT+0) Posts: 4,846 Blog Entries: 9

It will be 120V either case. It is not the same a switch because windings have resistance and reactance (they are inductors). Even switches have a small resistance.
#3
12-17-2008, 02:52 PM
 bonkers Junior Member Join Date: Dec 2008 Posts: 14
spectrum analizer

Funny, I got a big laugh out of that one. Exactly what is a spectrum analizer, and thanks for answer on motor windings, but what if motor winding is open, wouldn't the reading on the meter read 120 volts instead of 0 volts. This is assuming that the motor is not running, and only current is going through the windings with one open and one closed. In another words, motor is only getting current through one winding and trying to start, while the other winding is open. Or is one winding having an effect on the other winding since they are both in paralel, would this be the inductance reactance your talking about.
#4
12-18-2008, 02:28 AM
 c_h_a_r_l_i_e_ New Member Join Date: Dec 2008 Posts: 1

in all 3-phase motors all of the windings should have the same resistance and receive the same voltage. in some cases motors motors will have a cyntrifigul switch that kicks out at about 75% of it top rpm. so when the motor is running the start winding is kicked out thus it will not read voltage because the circuit is broken by the switch. if you need to figure out which winding is what on the termenal the run windings have the highest resistance, the run windings will have less. hope this helps

intellitec college student
#5
12-18-2008, 08:01 PM
 bonkers Junior Member Join Date: Dec 2008 Posts: 14
compressor motor

Actually, I have to be honest, i am studying electrical troubleshooting for refrigeration and appliances, my question pertains to compressor motors in ac units and heat pumps. The information I have states that in a compressor motor there are two resistances each one is different. Start to common is higher, run to common is lower, and run to start is the highest, I believe, I don't have my material with me at the moment, it may be common to start is lower than common to run, but i do know in a compressor motor the run and start windings will give the highest reading in ohms. My question pertains to a motor, compressor, that is not starting, but has 120 volts to the windings and one is good and one is bad, if you read 120 across one winding and o across the other, in volts, then according to my material one winding has to be open, and one closed. I know the windings are hooked together in paralel, and I assume if your taking a reading in volts across one winding in series, then that must isolate the other winding in paralel, so i assume you would get a reading in volts depending on an open or closed winding. There is the law of inductance and reactance, which I am not to familiar with, I do know it has to do with opposing a current in a resistance circuit. So, in my hypothetical situation, since in a open switch in a series circuit would read 120 volts on the meter, and there is no inductance or reatance across the switch you will obtain a volt reading. Now, a voltmeter across one winding in a compressor motor that is not running will obtain a reading open or closed, but in a different enviroment than a switch in series in a circuit. I assume a reading of 120 on each winding open or closed, my material says one winding will read 120 if good, one if open will read o volts. Is this correct, or wrong. Thanks for help.

 Tags motor, run, start, windings

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