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  • Basic relay questions

    I've got an old Delta drill press form a closed down mill here, and it has developed an electrical issue. Someone at the mill added a foot switch that runs to a box off the motor than has hole in the top for the chuck key. When you put the chuck key in, you can use the foot switch. If the chuck key isn't in the hole, nothing works. There is a relay in the box, as well as a switch for the chuck key.

    It has arced now and then since I got it, and has now stopped working unless I use the wall outlet (110) as an on/off switch. Foot switch appears to be fine.

    I've pulled the relay, it's an older Cutler Hammer. I'll try to post a pic. It has two contacts on the coil, and two down below marked 'on' (a one contact relay?)

    My questions: Do these things typically wear out/break? Would a cleaning do it any good? I've just done a few minutes snooping on the web.. don't know squat about electrical matters. Appears I'm dealing with a 'contact relay' as it powers a motor? There's a dizzying aray of these things.. wish Cutler Hammer had a chart of which old part #'s translate to current #'s.

    -Mike

  • #2
    Mike,
    Relays do wear out. If the contacts are toasted you will need to either fix them or more than likely replace the relay. it sounds like the relay might have been improperly sized for the application. A motor is an inductive load and relay must be rated and sized. Given the facts that you report it sounds like the contacts are welded closed as you report it does not turn off.
    You must ensure the relay is sized properly for the load. You need to get the details off the motor nameplate to source a replacement. A sketch of the components and a wiring diagram would be useful for remote diagnosis.

    Dave

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    • #3
      Thanks. Will post pics and more info.

      The contact does not appear to be welded to the armature, if that's what you mean. Externally all looks normal except for some dust/light scorch marks.

      -Mike

      I took it apart a bit further, and the two lower (non coil) contacts are pretty cooked. Nothing welded, but corroded/melted for sure.
      Last edited by softtail; 03-09-2011, 12:20 PM.

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      • #4
        Mike,
        I look forward to some pictures.
        If you say its on all the time and the foot seems to work that seems unusual.

        I think from what you describe this is what the circuit might look like.
        If it is like this there are only a couple things that are wrong.
        Either the Relay contacts are welded together or both switches are faulted.
        Dave

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        • #5
          Here's some pics:





          Nothing is welded together that I can see, but the lower two 'on' contacts are in bad shape.. maybe they were welded together and came apart when I removed the relay.

          -Mike

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          • #6
            Mike,
            It looks like the relay is properly sized. It should be easy to troubleshoot. I expect that something on the coil side is not working.
            Do you follow the diagram I posted?
            Dave

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            • #7
              Inductive loads can draw up to 6 times more than the rated current during startup. That relay is terribly underrated for the given load. You need a contactor type relay, which is specially designed for inductive loads and which can tolerate the unavoidable arcs better.

              Here is a good document with quite detailed information:

              http://relays.tycoelectronics.com/ap...fs/13c3236.pdf
              Last edited by taydin; 03-09-2011, 02:40 PM.

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              • #8
                When it was operational, upon hitting the foot pedal, it would sometimes make quite a racket.. power being transmitted and cut rapidly.. now that I have this conductor in my hands the sound was partly the armature 'bouncing' I think, and the sound of arcing. I did see in my limited research that relays for starting for motors are unique... so this one is a 'regular' relay as opposed to a contactor relay?

                -Mike

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                • #9
                  that relay has a horsepower rating,so i can be used for motor loads.

                  Comment


                  • #10
                    Originally posted by Bmyers
                    that relay has a horsepower rating,so i can be used for motor loads.

                    This is clear as mud!

                    The motor is well w/in the ratings on the relay.

                    -Mike

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                    • #11
                      While relays rated for HP is a good indication its suitable for inductive load, It still REALLY helps relay life to have a snubber across the contacts (Typicaly an RC snubber)

                      That said, that relay looks about a billion years old, And relays are well known for not lasting forever, Especialy when used anywhere near capacity, And inductive loads (especialy without a snubber) make life time even shorter. I bet the only reason it lasted this long was because it was a very well made relay. That said, they are cheap and basicly consumable these days, Like motor start caps and such.

                      Often relays are even socketed for ease of replacement.
                      Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

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                      • #12
                        So what relay do I need given that motor? I can go by the motor plate, but folks here as well as printed materials posted previously say that start up, etc can far exceed those #'s.

                        Here's a good resource from Grainger:
                        http://www.grainger.com/tps/electric...ers_contactors

                        1) So I know the coil voltage. 110 or 110/120.

                        2) Max amp and Max volt: Things start getting murky here. Motor plate says 60vlt 15 amp, but nothing at MSC matches up with that. Can I have a relay that is over-rated in either of those two categories? Would it be beneficial for the relay to be overated?

                        3) Total number of contacts. Mine is a 2 contact right?

                        Also, I *think* I can use a 'definite purpose' relay as opposed to a motor relay. The Delta motor has not been changed or modified in any way. Is that correct?

                        Thanks,

                        Mike

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                        • #13
                          Why not move the wires that were on the contact to the other (presumably unused) contact?

                          It'll last as long as it did originally.

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                          • #14
                            Both were used, and both are cooked...

                            -Mike

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                            • #15
                              Just as a point of interest, I might guess that the contacts marked "ON" may actually be marked "NO" for "Normally Open." Could you possibly be viewing them upside down? Not that it matters in this case, but as I said, a point of interest.

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