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Thread: Someone feel like explaining backfeed to me? lol

  1. #1
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    Someone feel like explaining backfeed to me? lol

    Howdy!

    Have been working on a walk in cooler that utilizes a 208v condenser.
    Had 208v going to the start winding after the potential relay switched over to the run capacitor/run winding.
    It was explained to me that this was backfed voltage.

    This was never taught to me in school and I can't really find anything online pertaining to HVAC electrical back feed.

    Does anyone have any material or can you explain to me how it works?

    What voltages do you get back feed on?


    Thanks!

  2. #2
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    Back feed is easy, sorta, and I am going to tell you more than I know till someone smarter comes along. I am not going to say this is 100% correct but the way it was explained to me, and seems like an easy way to think about it is this.

    You have a start and run winding in a PSC motor. The start winding gets it running then the run wire takes over. The start winding then becomes somewhat like a generator as the rotor spins in the winding. The electricity generated is added to that being supplied and you get Back EMF. It is this back EMF that causes the potential relay to open when a compressor starts.

  3. #3
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    Thread Starter
    Quote Originally Posted by BNME8EZ View Post
    Back feed is easy, sorta, and I am going to tell you more than I know till someone smarter comes along. I am not going to say this is 100% correct but the way it was explained to me, and seems like an easy way to think about it is this.

    You have a start and run winding in a PSC motor. The start winding gets it running then the run wire takes over. The start winding then becomes somewhat like a generator as the rotor spins in the winding. The electricity generated is added to that being supplied and you get Back EMF. It is this back EMF that causes the potential relay to open when a compressor starts.
    Thanks for taking time to respond.

    So can you get backfeed on 120v? All voltages?

    Does the potential relay not sense the predetermined voltage (at minimum running speed say 75%) and cut the start capacitor/start winding out of the circuit?

    Thanks!

  4. #4
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    First there is a difference between backfeed and Back EMF. Backfeed is when you energize one circuit but the power feeds back and energizes something you didn't want. Example: If G is jumpered at the board on some thermostats it will backfeed and turn on the condenser. Back EMF is when a nonenergized winding is putting out a voltage greater than what is energizing the other winding. You can find back EMF on most multispeed motors that are not ECM. In time you may find a transformer for a humidifier that is bad with no apparent reason. If you check where it is tied into the circuit you may find it is getting feedback on the heating fan speed from the cooling fan speed.

  5. #5
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    Not really backfeed. But the thing to remember with any equipment in this trade is that every pin is always hot.
    If you're working on 240 volt single phase equipment, then typically only one leg gets interrupted to shut the operation down. So you'll have 120 volts backfeeding through the equipment.
    Same rule applies on 3 phase equipment. On most 3 phase motors you only need to interrupt one or 2 legs
    You don't squat with your spurs on.
    And you NEVER put the torches away before pressure testing.

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  7. #6
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    When you were seeing 250 volts across your Capacitor ... some of us were calling it Backfeed , but thats not exactly the correct term. The correct word is EMF

    But still ... if you were testing a 220V system and took a reading across the RUN Capacitor going from Com to Herm , you might see 420V

    Thats normal

    I dont think I've ever tested the Run Cap on a 120v system , but it could easily read 150v while running

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  9. #7
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    I would avoid just interrupting one leg as a control logic if I were you. <g>

    PHM
    -------

    Quote Originally Posted by Restaurant mech View Post
    Not really backfeed. But the thing to remember with any equipment in this trade is that every pin is always hot.
    If you're working on 240 volt single phase equipment, then typically only one leg gets interrupted to shut the operation down. So you'll have 120 volts backfeeding through the equipment.
    Same rule applies on 3 phase equipment.

    "On most 3 phase motors you only need to interrupt one or 2 legs"
    PHM
    --------
    The conventional view serves to protect us from the painful job of Thinking

  10. #8
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    Thread Starter
    Thanks every for your replies. I will look into Back EMF!!

    Thanks!

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