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3 Pin Relay Wiring?

bob_v

Feb 7, 2024
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Believe this came out of a dishwasher. Searched all the identifiers but can't find a valid part #. Measured coil resistance and it's 75 ohms. W/o power 3rd pin isn't connected to anything. Tried connecting 120v across the coil and tripped circuit breaker. I'm a newbie and it's frustrating me that I can't understand this simple device. Any help would be greatly appreciated. Thx in advance , Bob
 

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Minder

Apr 24, 2015
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I think you will find that is a induction motor start relay, goes in series with the start capacitor and drops out when the motor has started and the start winding current has dropped.
It only has a few heavy turns, so the current would be very high if connected across 120v !
 

bob_v

Feb 7, 2024
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I think you will find that is a induction motor start relay, goes in series with the start capacitor and drops out when the motor has started and the start winding current has dropped.
It only has a few heavy turns, so the current would be very high if connected across 120v !
Thank you! I understand basically what you’re saying but not the specific details. Doesn’t sound like anything I’ll need and a bit advanced for my current knowledge. At least I don’t feel as stupid now. I’ll just throw it away before I get in any more trouble.
 

Minder

Apr 24, 2015
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Thank you! I understand basically what you’re saying but not the specific details. Doesn’t sound like anything I’ll need and a bit advanced for my current knowledge. At least I don’t feel as stupid now. I’ll just throw it away before I get in any more trouble.
You never throw anything away, it may come in handy some day !! :p
 

bob_v

Feb 7, 2024
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Ok Minder, I'll keep it ... b/c you said so. I'll label it "Caution - induction coil relay?". Maybe someday I'll understand it's technical use, and maybe even have a need for it. I'll think of you every time I look at it. I'm currently more interested in dc circuits ... but eventually 120v things. One of my goals is to use micro SBC w/wifi and log when and how long my sump pump comes on. Since I can't get at the float easily think I can measure current surges on the plug using the ACS712 current sensor. Got a way to go to get there but the SBC and programming is a no brainer (been there done that), electronic interfacing, esp to 120v is the challenge. Thanks again for caring and fast response!
 

Martaine2005

May 12, 2015
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Ok Minder, I'll keep it ... b/c you said so. I'll label it "Caution - induction coil relay?". Maybe someday I'll understand it's technical use, and maybe even have a need for it. I'll think of you every time I look at it. I'm currently more interested in dc circuits ... but eventually 120v things. One of my goals is to use micro SBC w/wifi and log when and how long my sump pump comes on. Since I can't get at the float easily think I can measure current surges on the plug using the ACS712 current sensor. Got a way to go to get there but the SBC and programming is a no brainer (been there done that), electronic interfacing, esp to 120v is the challenge. Thanks again for caring and fast response!
Please don’t play with mains power until you understand it and RESPECT it.
 

Minder

Apr 24, 2015
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Maybe someday I'll understand it's technical use, and maybe even have a need for it.
It is used to allow current to flow into the series relay winding, the start winding and the start capacitor when the motor is started.
The initial high current picks up the relay and switches both cap and winding across the supply, after the motor comes up to run, the current in the two series components drop and the relay drops out.
 

bob_v

Feb 7, 2024
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Please don’t play with mains power until you understand it and RESPECT it.
Yes sir Martaine! As I said I have a long way to go and thank God there's people like Minder to help me get there rather than criticize me.
 

Bluejets

Oct 5, 2014
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Actually the "coil" section of the relay goes in series with the run winding and monitors the current there during start.
A common (one terminal of the relay coil) then switches the start circuit either in or out depending on the aforementioned current via the remaining terminal.
Determining which is the common terminal is difficult though as one has the low resistance coil to contend with during any measurement.
Frig relays are easier to decipher given their "standard" pinout arrangement.
 

bob_v

Feb 7, 2024
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Thx Bluejets. Most definitions of relays simply say they bring contacts together or apart. This has been a real eye-opener. You folks should be really proud of this forum, got some really class acts here! Most Appreciative, Bob :)
 
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