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Wiring Diagram for Potter&Brumfield PM-17AY-120???

Discussion in 'Electrical Engineering' started by John, Nov 26, 2003.

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  1. John

    John Guest

    I bought a used table saw that has an elaborate switch involving this
    magnetic starter. I want to change it to 240v from 120v, but this device is
    120v only.

    Someone here suggested putting the coil from one leg to ground so that I
    could salvage the starter rather than dropping $90 on a new 240v one.

    I ran it by an industrial electrician friend. He initially said it was
    improper to connect the coil to ground, but then agreed that the 100ma in
    the ground couldn't do any harm. So, if I can get him a wiring diagram, he
    will help me rewire it.

    I have contacted the manufacturer (Magncraft) and the distributors they
    suggested, but no one will give me the diagram. If anyone here can figure
    out how to get one, I would be extremely grateful. Thanks.
     
  2. John

    John Guest

    Sadly, I don't have a neutral, just two hots and a ground.
    But, the way I see it, maximum current through the coil is only 128ma, so
    that is the maximum running through the ground. Since the ground is a much
    better conductor than a person under even the worse scenerio, if you touched
    the ground wire (or the grounded body of the motor), you wouldn't get a
    measurable shock.
    And if the ground connection was bad, so that you would be the only
    conductor, the relay would open because the coil would lose power, so there
    would be no current at all.
    I just don't see any way this is dangerous. My kitchen oven has no
    neutral, the body is grounded, yet when the ground is carrying 7a (oddly,
    the bottom oven is designed to put only one leg of current though the
    broiling element when set to bake) I don't even get a voltage reading off
    it.
    It is the same situation isn't it? If it were hardwired it would be a code
    violation, but I just don't see how it could be dangerous.

    If I am confused over this, please set me straight, because I don't need to
    save the $90 badly enough to risk an electrocution or fire!
    Someone from the company that makes the ON/OFF switch on the saw was kind
    enough to talk me through it.
    The first row of 4 contacts is where the line connects.
    The second row of 4 is normally on, switched off when the coil is energized.
    The third row of 4 is normally off, switch on when the coil is energized.
    The fourth row of 2 is the coil.
    I connect the two hots to the first row, and the motor wires to the same
    contacts in the third row.
    They explained how to wire the ON/OFF switch, so I think I have that.
    I just can't figure out how it is now wired for 120v! It should be the same
    as the 240v, only with the neutral bypassing the relay completely, but they
    have wires all over the place. I am not changing anything until I figure
    that out.

    If I have problems I will get in touch with you.
    Thank you, and please let me know if you find errors in my safety reasoning
    or my understanding of the relay!
     
  3. John

    John Guest

    I examined the existing installation more carefully, and the main thing is
    that they jumpered two parts of the relay.
    The relays says 1 hp max, 25a @ 277v max.
    I have a 2hp 16.6a motor. If my ampmeter is reading correctly, it drew 81a
    for a split second to start. (I wouldn't have thought you could force that
    much through 12 gauge) Presumably that is why they jumpered to two parts,
    to reduce the load on each.

    Now, my current will automatically be cut in half when I go to 240v, so the
    load on the device won't be any greater; it just won't be much less.

    What are the consequences of using a 2hp motor when it says 1hp max?
    I don't really care if the life is reduced, as 1) it has no other value to
    me, so I might as well replace it when it burns out as now, and 2) it has an
    expected life of 100,000 cycles, so 5% of normal life is still enough to
    last me a few years.

    Are there more serious problems that could result from the overload besides
    shortened life? I really don't want to start a fire, though it is in a
    steel box.

    I thought the price I paid for the saw was too low; but of course I can
    alway chuck the whole magnetic starter and put on simple double pole switch.
     
  4. Guest

    Don't wire it to ground. Only??? 128 mA?
    128 mA will kill you! GFCI's protect against a ground fault
    of only 5 mA! DON'T WIRE IT TO GROUND!!!!!!!!!

    That said, you have no problem. Just add a resistor in the
    coil circuit and you're good to go. At ~128 mA on a 120 volt
    circuit, the coil impedance is ~1000 ohms. If you put a 1000
    ohm resistor in series with the relay coil, it should limit
    the current through the coil to ~120 mA. The resistor will
    get hot - it will dissipate about 15 watts of heat. To give
    yourself a bit of help in getting rid of the heat, I'd use
    4 250 ohm 10 watt resistors in series. 1/4 of the heat will
    be dissipated in each resistor. I'd install them in a separate,
    ventilated enclosure with air space around each resistor,
    and install the enclosure under the bed of the saw after
    testing. To test, you would connect a voltmeter across the
    coil and look for about 120 volts when you turn the switch on.
    Precision is not important, but it is better to have the
    voltage across the coil lower than 120 versus higher.
    Adding more resistance in series reduces the voltage, adding
    resistance in parallel with one of the existing resistors
    increases the voltage. 10 watt cement power resistors sell
    for 55 cents apiece at Mouser.

    Let us know how you make out. Here's a diagram:

    hot-----Resistor---Coil----hot
     
  5. John

    John Guest

    Don't wire it to ground. Only??? 128 mA?
    I appreciate your concern, but I am unable to come up with a situation in
    which I could contact any current, let alone 128ma.
    If the ground is intact, I get no current because the ground is a much
    better conducter.
    If the ground is broken, I get no current because the relay opens.
    When could I get any current?
    Even if simultaneously the relay froze closed and the ground opened (both
    unlikely events), I still wouldn't get much current because I would be in
    series with the coil.
    What am I missing?
     
  6. John

    John Guest

    There is a very confusing dialog happening in this thread. Anyway, I'm
    going
    No, other way around; now set for 120v and I want to change it to 240v.
    They have the hot paralleled. I am intending on leaving it that way, and
    running the new hot through the remaining contact. (I can't parallel the
    second hot because I need a contact for the coil)
    So, will the unit be okay at 8a/240v with a 2hp motor, even though the
    specify 1hp max? I suppose it ought to be, since it worked at 16a/120v, and
    that was worse. But, I am mostly concerned about some failure more serious
    than simply wearing out.
    I read that section about adding resistors as being for a DC coil. With an
    AC coil they want you to use their unit with a 240v coil. Presumably the
    difference in inpedence between a 120v coil/resistors and a 240v coil is
    sufficient to affect operations.
    Am I reading it wrong?!

    Thanx
     
  7. John G

    John G Guest

    DO NOT use the ground wire under A NY circumstances
    A. It is a code violation
    B If the ground goes open then you will have 240 volts on
    the frame thru the relay coil and if the frame is at all
    insulated you will be dead
    Thats why it is a code Violation.
    Please do not tell me the GFCI will save you . It could fail
    too.
     
  8. John

    John Guest

    My real problem? I want to get a meaningful answer to my question.

    1) Since only 120v is going through the coil, I can't possibly get 240v on
    the frame under any circumstances.
    2) If the ground goes open, then the coil will lose power and the relay will
    open, so there won't be any voltage anywhere. I would have have the ground
    suddenly open (a most unlikely event) at the same time the relay got stuck
    closed (another most unlikely event).
    3) Ignoring #2, I would be in series with the coil, so it would almost
    impossible to "be dead" even if I were standing in salt water when all this
    happened.

    So I can't think of any situations under which it would be dangerous.
    Recognizing that I might have overlooked something, I am asking for advice.
    Just telling me it is dangerous without explaining how is not at all
    dangerous.

    I suppose I could take the precaution of grounding the table saw frame
    independently to a water pipe, but I don't see the point of it.
     
  9. John G

    John G Guest

    I am sorry to say your real problem is trying to do
    something beyond your current training so the proper thing
    to do is get a licensed professional to do it for you.
    One relay and one change of voltage is really too
    fundamental to have elicited all the waffle that has gone on
    here.
    Get a 240 volt relay and someone who understands electricity
    and is licensed and all will be well.
    I am sorry if I am too direct but it is
     
  10. John

    John Guest

    I am sorry if I am too direct but it is
    Can't say if you are direct or not, because you haven't said anything
    intelligible.
    If you either don't understand the question, or don't know the answer, then
    just don't answer it.
    To babble something extraneous and then insult me is pointless.
     
  11. John G

    John G Guest

    You want to connect your 120 volt machine to 240 volts.
    Right?
    You do not know how to connect the existing 120 volt relay.
    Right?
    Then--------
    Get a 240 volt relay and someone who understands electricity
    and is licensed to do the work and all will be well.

    What did I get wrong or what can you not understand????
     
  12. John

    John Guest

    You got it wrong when you told me:
    1) I will have 240v on the frame, which I will not because there is only
    120v going through the coil. In fact, it would be impossible to have 240v
    on the frame, no matter how I wired it!
    2) That I should not rely on a gfci for protection because it could fail.
    Uh, if I am deliberately leaking 128ma to ground, how is a GFCI going to
    work? It couldn't, or don't you even know how a GFCI works?
    3) You say that I would die if the ground opened. That means you don't know
    how a magnetic starter works, because the power would be cut off as soon as
    the ground opened. (The OFF switch works by cutting power to the coil, which
    also happens when the ground opens. Duh) It also means you don't understand
    anything about electrical safety, because there is virtually no chance of
    death from touching a 120v line after it has passed through a coil that only
    draws 128ma; especially when the machine frame provides a better alternate
    ground than a person.

    Are you aware that most electric dryers and ovens use a common
    ground/neutral? I didn't think so. How many die annually from being
    electrocuted from open grounds? Geez, and they don't even have relays that
    would cut the power if the ground opened.

    So, what did you get wrong? Everything.
     
  13. John G

    John G Guest

    Yes I said 240 when it should have been 120 but remember
    the civilised world uses 3 phase 415/240 volts.
    Sorry for my typo.

    I will just be brutal.

    IF you are such a KNOW ALL how come you did not know
    how do do this and break some safety rules without asking
    us??
     
  14. John

    John Guest

    So; you concede you don't know your ass from a hole in the ground, but think
    you have a right to be brutal? Think again.
     
  15. John G

    John G Guest

    I made an assumption or typo that had no real effect
    on the outcome so why be abusive??

    As soon as the argument exceeds your intellect you descend
    to abuse.
    Too bad So Sad.
     
  16. Wade Lippman

    Wade Lippman Guest

    Someone suggested putting a 3k resistor in series with the coil.
    I looked at the spec sheet, and it looked to me that it was for a DC coil
    only. I posted that as a reply.
    No one has commented otherwise.

    Have you read the sheet and thought it was for A/C coil also? Please let me
    know.
     
  17. Guest

    What are you missing?
    1) You are fighting against the advice you have been given.
    That advice is 100% correct. You are, in effect, saying
    you know better than the people who give you the advice
    AND the National Electrical Code. And you are wrong.
    Unfortunately, you could end up *dead* wrong. You are
    placing a killing level current on ground if you follow
    your scheme.

    Current should never be on ground intentionally. Ground is
    for safety.

    Hot--->relaycoil--->brokenground--->you--->realground.

    The coil is going to try to draw 128 mA through you if you
    get between the ground and it.

    It is illegal and could kill you or someone else.
     
  18. John G

    John G Guest

    Thank You.
     
  19. Guest

    Don't know if you are reading it wrong.
    See my earlier reply, where I gave you the solution.
    Here's the existing and new circuit analysis:

    Hot----coil---ground. You said it draws ~128 ma:
    "But, the way I see it, maximum current through the coil
    is only 128ma". Assuming that is true, the relay coil
    impedance is E = IZ; 120 = .128 Z; Z = 120/.128 or 937.5
    ohms. I told you to use a 1000 ohm resistor in series
    with the coil.

    The new circuit:
    hot---coil---resistor---hot.
    Figuring the coil as 1000 ohms, and the resistor as 1000 ohms,
    the voltage will divide equally.

    Using the exact numbers:
    240 = I * (937.5 + 1000); 240 = I * 1937.5; 240/1937.5 = I
    so I = .1238 At I = .1238, the voltage across the coil
    will be E = IZ; E = .1238*937.5; so E = 116.129

    The resistor will have to dissipate some heat. The formula
    for that is P = I^2*R; P = .1238*.1238*1000, so P = 15.32644
    watts. My recommendation was to use 4 250 ohm, 10 watt
    resistors in series to spread the heat out. You always want
    to give yourself a good margin on wattage. 4 250 ohm resistors
    will give you 40 watts capability - and spread the heat out
    "geographically". You could use a single 25 or 50 watt resistor,
    but you would not gain the spreading out effect.

    Let us know how you make out.
     
  20. daestrom

    daestrom Guest

    True, you *can* get 120V on the frame to ground if the ground line from the
    frame opens.
    This is where you are mistaken. The relay contacts opening will not remove
    the 'hot' from the relay coil. If they did, then you could never get the
    thing to start in the first place. The relay coil 'hot' is probably from
    the on-off switch *upstream* of the relay contacts. Normal operation: Close
    the switch, energize the coil, picks up relay, closes contacts, energize
    motor. With open ground, close switch, put 120V on coil, no current through
    open ground, but frame now @ 120V. You touch frame, bad day at the
    workshop.
    If the coil can pass 128 mA, then you most certainly *can* be dead. That is
    enough current to contract the muscles in your had (can't let go) and if
    passing from arm to feet, constrict your chest (can't breath). And heart
    can go into ventricular fibrillation. Granted, several things would have to
    'align' to give you all this, but that's why its a code violation.

    A series resistor to drop the voltage from 240 to 120 could work. The data
    sheet that Alan posted a link to earlier seems to show for 220V operation,
    'use the 120V relay with a series resistor'
    Easy, no violation, no arguments.

    daestrom
     
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