thermostat NC output for breaker system (electric motor, industrial application)

Discussion in 'General Electronics Chat' started by meld2020, Oct 3, 2016.

  1. meld2020

    Thread Starter New Member

    Mar 14, 2016
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    I am using an electric motor to actuate a drive system for a university project. This motor (7hp) has an optional thermostat relay wiring setup available that can utilize an external power source, which I'm assuming is fairly common. I am curious as to what the traditional approach is for implementing a shutoff system for this type of application? (i.e., is it just an external voltage that goes through the thermostat lines and out to the coil of a normally open relay, where the mains power is run through? Would I utilize some special breaker than can break the circuit via additional signals? I am not sure how I want to go about doing it, and what is typical for a similar industrial application. My first worry is that if I go the relay approach, the inrush current of my motor (much higher than LRA) would fry the relay!)

    Thanks
     
  2. MaxHeadRoom

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    I assume this is a type buried in the windings?
    It depends on how you want to control it, but in the case of the thermal sw I would place it series with a control relay or contactor coil.
    This would avoid the high current for that motor going through it, is this a 3ph motor being 7HP?
    Telemecanique wiring diagrams
    Max.
     
  3. meld2020

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    Mar 14, 2016
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    It's a single phase 230V.

    I'm just wondering if a relay could even handle the 140A+ inrush. And of course how that is typically implemented in an industrial setting. I'd probably place the relay in series with the breaker, but where would that go -- a separate junction box? etc. etc.
     
  4. #12

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    They are called motor contactors. Rated by the RLA (normal running) amps of the motor.
    Think, "bigger". motor starter box.jpg
     
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  5. MaxHeadRoom

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    A contactor known as a Special Purpose type would normally be used, these have special contacts and designed for repetitive starting HVAC systems etc.
    Depends on what equipement there is there already but the disconnect fuse and motor O/L and contactor could all go in one small enclosure if needed.
    You may also need a stop/start P.B. on the enclosure.
    Max.
     
  6. meld2020

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    Mar 14, 2016
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    Kind of a hack, but this is what I had in mind. I will check out the suggestions above, thanks. Haven't ever really dealt with big installations of this type before.
     
  7. MaxHeadRoom

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    Not quite right, look at the Telemecanique diagrams I posted, there is a typical stop start contactor circuits in there pages 6 & 7.
    You can go low voltage for the coil, but most special purpose contactors are 120vac or 240vac coil.
    The difference now is that the diag's show the O/L on the neutral side of the coil where the practice now is to have it on the L.H. side of the coil.
    Max.
     
    Last edited: Oct 4, 2016
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  8. #12

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    All of my residential machines use 24 VAC for control circuits, but I did have a couple of 120 VAC and 230 VAC coils find a way into my tool box. Point is, you aren't likely to find a 12 VDC coil on a contactor large enough to run your motor.

    Look in that picture I posted. The thing at the bottom is the transformer which makes the AC control voltage.
     
  9. MaxHeadRoom

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    You don't show what you intend controlling the motor with, is this part of another system or a stand-alone manual stop/start system?
    Max.
     
  10. meld2020

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    Mar 14, 2016
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    It will be manually controlled by either the master switch or a more conveniently placed wall-mount power switch inside an employee/operations cab (in which case probably another relay)
     
  11. MaxHeadRoom

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    There are certain NEC regulations for any enclosure that has conductor that are live from a separate enclosure and are not controlled by the local disconnect, they have to be colour coded accordingly,
    Depending on your final set up, this may be needed, it may be wise to submit a DWG here before final decision.
    You mention 230v installation so are you in Europe or?.
    Max.'
     
  12. #12

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    If you use 24 VAC for the control voltage, you can use...thermostat wire? Can't remember what it's called...50 volt limit, no armor required in some applications. Max is ahead of me on regulations. I'm going to butt out.
     
  13. meld2020

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    Mar 14, 2016
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    North America. Manufacturer spec is 1ph 230V for the motor.

    This has been good information so far. I am researching direct on line (DOL) starter systems, as opposed to delta star which seem to be three phase systems. It is also worth noting that there will be 3 of the motors onboard the equipment, so the enclosure must be sized accordingly.
     
  14. #12

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    Correct.
     
  15. MaxHeadRoom

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    All you really need is a motor contactor, most are three phase which is OK, they can be used on 1ph I am assuming you do not need reverse?
    You could look for a single contact relay with heavy enough duty, but it may be hard to find, you do not necessarily need to switch the neutral BTW.
    If there is any coincidental switching of more than one motor, the 3ph contactor could do the trick with its 3 or 4 contacts.

    Star delta starters are a different animal and meant for dual configuration starting for a 3ph motor, especially one under initial load.
    Max.
     
  16. #12

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    The idea that "most" are 3 phase doesn't set well with me. Consider millions of residential air conditioners running 2 or 3 HP on 240 single phase. That range covers houses from 800 sq.ft. to 1500 sq.ft.

    I assume you are speaking about what is familiar to you because I am.
     
  17. MaxHeadRoom

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    True, but we are talking 7 hp which is not really a common occurrence, even on 240v.
    Even 3ph VFD's running off 1ph 240v have a cut off around 5hp before reverting to 3ph in.
    Three motors of this size I would consider this a industrial application and should normally be a 3ph installation.
    The current of a 1ph motor is around double of the 3ph for the same HP.
    The OP has not yet revealed his C. of O.
    Max.
     
    Last edited: Oct 4, 2016
  18. #12

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    I guess I got confused when this was declared as 1 ph, 240V.:oops:
    That's a lot of horsies for single phase.
    I put a 5 hp motor on my table saw. It required 30 amp breakers and a clothes dryer outlet. I guess 7 hp would need the 50 amp crow's foot receptical for a kitchen stove, and strangely fat wire to wind the motor.:D
     
  19. MaxHeadRoom

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    Also... for a 7hp 1ph motor, when using a 7hp 3ph contactor all three (or4) contacts will be needed in parallel, alternatively if a 15hp 3ph contactor is used using one contact segment can be used or more than one motor on the other contacts.
    Max.
     
  20. meld2020

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    Mar 14, 2016
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    All, this is a 5hp motor. Sorry for the confusion. Just realized the error.
     
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