voltage troubles


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Old 08-09-14, 07:10 PM
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voltage troubles

well today i went to my shop to do some wire brussing of some rusty stuff and got my air tools all set up and went to turn on the air and nothing happened ....so i checked to make sure that the switch was on ..on the air compressor ...and it was....so i went back and looked at the breaker ....it was fine ...so i go the multi meter out and started testing ....well my breaker is good ....my switches are good and at my switch on the inside of the shop i got 220v and outside at the compressor at the line going to it i only get around 40v and as im holding the meter there the voltage drops .....why am i getting low voltage ...i have not found a split in the line and it was all working the other day
 
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Old 08-09-14, 07:49 PM
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What voltage do you get at the disconnect? Is this a single phase compressor installed outside? I assume what you really need is 240 volts?
 
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Old 08-09-14, 07:51 PM
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Welcome to the forums.

It sounds like dirty, loose corroded terminals on the inside switch.
You checked the voltage at the switch terminals inside.... not the actual wiring inside.
 
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Old 08-09-14, 08:00 PM
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i checked all switches ...and i checked all the wires ...and connections of those said wires and everything is good ....its got me puzzled
 
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Old 08-09-14, 08:08 PM
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If you've eliminated everything else then you have a broken wire.
 
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Old 08-09-14, 08:38 PM
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ok so i just went out and did some testing ....the breaker is good ....changed it out with one that was working ....turned the breaker on ...at the inside switch i was getting 15 volts with the switch on the pump off.....turned the switch ON at the pump and rechecked the voltage on the inside switch and was getting 240v ....and with the inside switch on went and checked the voltage at the pump and was only getting 20v ....then when i turned the inside switch off and checked the voltage at the pump it was 0v.....and the switch on the pump is good as well....
 
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Old 08-10-14, 07:20 AM
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Somewhere along the line there is a loose connection or broken wire.

Perhaps underground outside your workshop where a woodchuck chewed the direct burial or PVC conduit encased cable and only today the wire finally corroded through (faster with aluminum exposed to moisture) and the wire ends dipped in the wet soil mixed with the corrosion resdues and the decomposing remains of electrocuted woodchuck let 220 of the 240 volts at a couple of milliamperes show up at your meter with the compressor off but once the compressor kicked on, all except 15 or 20 or 40 volts were dropped at the higher resistance spot where the aformentioned mess was, the exact voltage the compressor or pump got depended also on its internal resistance.

At all times in an electrical circuit, the voltage between any two points is equal to the current flowing between those points times the resistance of that portion of the circuit between the two points including any device or appliance there. For motorized equipment and some electronics, your voltmeter is not sufficient to give you suitable data to compute things; you also need to use "complex numbers" which are typically taught in second year high school algebra and which can also be explained using plane (two dimensional; not plain) geometry.

Caution: When measuring voltage at a switch, measure between switch terminal and neutral or, with a grain of salt, between the terminal and a grounded object. Measuring between the two switch terminals does not give results that you can make good use of.
 

Last edited by AllanJ; 08-10-14 at 07:50 AM.
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Old 08-10-14, 10:45 AM
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Due to the characteristics of a 240v circuit..... measuring from one terminal to ground is only effective if there is no load connected. Otherwise the AC flows thru the load and back and will not allow for useful testing.

You have what appears to be a basic circuit there. Am I correct ?

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Old 08-10-14, 11:53 AM
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Addition: When measuring voltage in 240 volt circuits, you need measurements between the two hot lines (under differing circumstances including with the appliance turned on or turned off). You can make this measurement even if the two hot lines where you are measuring are connected to (the two poles of) the same double pole switch. You want to see the 240 volts (except when measuring on the far side (the load side) of a switch that is flipped off).
 
 

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