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Thread: Spec-ing Relays

  1. #1
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    Spec-ing Relays

    I have 3 compressors, 460/60/3 with Rated Load Amperage of 27.3 and a Locked Rotor Amperage of 178. I want to add some current monitoring relays for control feedback. Do I need a relay that can handle the 178 Locked Rotor Amperage? I am assuming I will need to oversize the relays somewhat due to the in rush current being higher? I can not find in the manual whether the compressors are across the line start or not.

    Any rules of thumb with spec-ing relays would be helpful.
    Thanks

  2. #2
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    If I wanted to monitor compressor current I would put CT's on the legs on the load side of the existing contactors.
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  3. #3
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    Is that good practice or not? Monitoring compressor current that is. I've been told by maintenance that they like being able to monitor amperage pulled as it's an easy way to monitor the status of the coils. As coils dirty, current pulled increases.

    Also, in terms of how much oversized should a relay be for a given load, is there a rule of thumb?

  4. #4
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    I can think of a dozens other reasons why amp draws increase on compressors. I think this is a poor way to predict when to clean a coil.

    Contactor ratings may state incadescent load and motor loads are different. Reason is an incandescent load does not have a starting current. Rating is already "built in" to the relay capacity

  5. #5
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    If you could elaborate on the other reasons, I would appreciate that.

  6. #6
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    Quote Originally Posted by Klob View Post
    Is that good practice or not? Monitoring compressor current that is. I've been told by maintenance that they like being able to monitor amperage pulled as it's an easy way to monitor the status of the coils. As coils dirty, current pulled increases.

    Also, in terms of how much oversized should a relay be for a given load, is there a rule of thumb?
    Condenser or Evaporator?

    The increase or decrease in amperage is minimal at best!

    Short of a start up and commissioning I see no advantage in monitoring that # without several other data.

    Chillers we record 15 - 20 pieces of information every hr 24 / 7. That's for trouble shooting purposes. On smaller units No!

  7. #7
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    Evaporator. No one has expressed to me the need to monitor the condenser fans.

    Our units range on the low end from 5 tons up to 50 tons, maybe a couple units are a little large but I haven't seen anything over 75. We are not running a chilled water system.

  8. #8
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    Quote Originally Posted by Klob View Post
    Evaporator. No one has expressed to me the need to monitor the condenser fans.

    Our units range on the low end from 5 tons up to 50 tons, maybe a couple units are a little large but I haven't seen anything over 75. We are not running a chilled water system.
    You can monitor it but I see no advantage.

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