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3 Phase Motor - 3 Individual Circuit Breakers - Detect if 1 Breaker Tripped And Shut Down Motor

Discussion in 'General Electronics Discussion' started by Mahonroy, Aug 7, 2020.

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  1. Mahonroy

    Mahonroy

    57
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    Oct 21, 2014
    Hey guys, I am turning on and off a 3 phase motor using a triple pole relay. I am also using 3 individual circuit breakers (one for each phase). For this application, I can only use 3 individual circuit breakers/fuses, as opposed to a single circuit breaker that has 3 poles in it.

    I am wanting to detect if a circuit breaker on any of the phases has tripped, shutdown the relay, and display a red "Fault" LED.

    My idea was to use a bridge rectifier, optoisolator, and high value resistor. I would put these across each circuit breaker.Under normal operation the optoisolator will not conduct. If a breaker trips, the optoisolator will begin conducting, and this will start generating a pulse. I can then run that pulse through a RC filter so that I get a nice steady binary 1 or 0 if the circuit breaker has tripped or not. I can then run all 3 of these binaries through a XOR gate. The relay will then have an AND gate, and the relay is activated by using a transistor on the relays coil. This way the relay will only turn on if all 3 breakers are still connected. If any individual breaker gets tripped then the relay will turn off. I can also insert the red "Fault" LED based off this.

    Does this seem like a reasonable way of accomplishing this?
    I am skeptical and curious if the optoisolator will conduct properly if the breaker trips.
    Any help or advice would be greatly appreciated, thanks!
     
  2. kpatz

    kpatz

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    80
    Feb 24, 2014
    How about connecting the coils of 3 relays, one to each phase, then wire the contacts in series so all 3 relays (phases) have to be energized for current to pass. Then run the control signal for your main motor relay (the 3 pole one) through the 3 series relay contacts. Bingo, the motor only powers up if all phases have power.
     
    hevans1944 likes this.
  3. Bluejets

    Bluejets

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    Oct 5, 2014
    In many countries, the use of 3 single phase breakers on a 3 phase motor would be illegal.

    Yes I do realise in the "old days" that three single fuses were used but times change and for the better most times.
    Just as it is illegal here in Aus to use fuses in new or replacement/repair installations.

    I did have occasion to monitor 3 single phase breakers feeding 3 lighting circuits and in this instance, three single relays were used between phase and neutral on each.

    The relay contacts used normally open with the three connected in series for the one, no-volt signal.
     
    TCSC47 and hevans1944 like this.
  4. Minder

    Minder

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    Apr 24, 2015
    Why not do it the usual way with 3ph contactors with O/L's attached, each O/L normally has a N.O. and a N.C, contact for sensing.
    The N.C. is typically used to drop the contactor coil, the N.O. is for a sensing/indicator circuit of some kind.
    When using a PLC, I used to use the N.O. as a indication in the PLC program. Or any other configuration you want.
    M.
     
    hevans1944 likes this.
  5. Kiwi

    Kiwi

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    Jan 28, 2013
    I would also like to know why you can't use a single 3 phase circuit breaker.
     
    TCSC47 likes this.
  6. Minder

    Minder

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    Apr 24, 2015
    The 3ph NFB, (non-Fused Breaker) is often used, but as opposed to the OP's method, the NFB drops all three phases.
    These can also be obtained with remote trip action or trip detection contact.
    M.
     
    Last edited: Aug 8, 2020
  7. Mahonroy

    Mahonroy

    57
    1
    Oct 21, 2014
    To answer some of the questions and to clarify - the main breaker box contains a proper 3-phase circuit breaker. Its my device that contains the relays and a much lower amperage circuit breaker for further protection. This part is mainly to protect the traces on the circuit board from excessive current draw, and to only allow all 3 phases of power to the motor if none of the onboard breakers have tripped. The main reason for not using a single 3 phase breaker is cost and size. I have been using 8 amp thermal breakers and they are only around $1 a piece and are small.... so if I can accomplish the same thing by using 3x of those thermal breakers that would be preferable.
     
  8. Minder

    Minder

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    Apr 24, 2015
    If you used rectifiers into opto IC's you could just OR the outputs, or totem pole style.
    M..
     
  9. Mahonroy

    Mahonroy

    57
    1
    Oct 21, 2014
    Would it be best to use the opto IC's across the breaker, so if the breaker trips, the opto IC would start conducting.
    Or would it be better to use the opto on just one side of the breaker along with a neutral. This would start conducting immediately and would stop as soon as the breaker trips.
     
  10. Minder

    Minder

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    Apr 24, 2015
    Either way could work, I think I would go with the output side detection, You will need a small bridge and resistor for the opto's.
    M.
     
  11. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    Connecting the opto-coupler in parallel with the breaker will also require adding a diode in parallel with the internal LED input of the opto-coupler along with a series current-limiting resistor to prevent damaging the opto-coupler by applying full line voltage across it when the circuit breaker is open. Same applies if you connect this circuit between load side of circuit breaker and neutral, assuming you aren't working with a three-phase delta instead of a wye voltage source..

    I think the load-side connection is better because you can wire the output photo-transistors in series to logically AND the individual circuit breakers. You can use the series-connected output transistors to drive the coil of a small relay that enables the coil on your motor contactor. You may need to kludge up a low-current DC supply to operate the coil of the small relay. Keep in mind the current limitations of the output photo-transistor in each of the opto-couplers when choosing a small relay. Or you can add a transistor relay-driver that is "turned on" by conduction through all three photo-transistors.
     
  12. Mahonroy

    Mahonroy

    57
    1
    Oct 21, 2014
    Hey guys, so I did get a chance to try this out. I ran each phase through a 51K resistor and through an opto-coupler (part # TLP290-4(GB-TP,E) ). The optocoupler, paired up with a capacitor and a pull down resistor generates a nice clean 0V for "phase disconnected" and a 3.15V for "phase connected".

    Here comes the problem:

    So I had this connected up to a 3-phase motor. If I manually trigger one of the 3 individual circuit breakers while the motor is running... the motor is still actively spinning and thus generates current out of the motor onto that disconnected phase. So the circuit below still sees voltage present and doesn't shut everything down like its supposed to.

    If the motor is stopped, everything works perfectly - the circuit sees that one of the phases is missing and doesn't allow the motor to run. If the motor is already running, then it doesn't work because of the generated voltage of the motor still spinning when a phase is disconnected.

    Here is the current circuit:
    upload_2020-9-16_17-0-44.png

    What do you guys think? Is there a good solution to this? Thanks again and any help/advice is greatly appreciated!
     

    Attached Files:

  13. Bluejets

    Bluejets

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    879
    Oct 5, 2014
    I think you are asking for trouble as you seem to have found out so far.
    I, for one, wouldn't want to be within cooee of you, the switchboard etc. etc. if you chose to ram that now reverse phase breaker back in with the motor running, fault or no fault.
    Chances are, before you know it, there will be an electrical explosion that will blow everything to kingdom come and back again.

    Usual arrangement ( and there are reasons why it is known as that) is to monitor motor current on the output of the 3 phase contactor and have the auxiliary contacts in the overload device trip the control circuit.

    Any indicator device could be (and in most instances are) run from an aux relay connected to this control circuit in a fail safe manner.
     
    Last edited: Sep 17, 2020 at 1:55 AM
    hevans1944 and Harald Kapp like this.
  14. Frankchie

    Frankchie

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    Nov 14, 2017
    Might a "mini" breaker something like this suit your needs? They are designed for overseas power and are rated for 400v, 50 hz, but since they are current driven I would think it can work in the U.S. at the lower voltage and 60 hz.

    Cheap enough. If it satisfies your needs maybe others here can comment on it's applicability and safety.

    https://www.aliexpress.com/item/329...earchweb0_0,searchweb201602_,searchweb201603_
     
  15. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    @Mahonroy, it looks like you are good at appreciating our advice and then ignoring it. The suggestion was to place the opto-coupler LED in parallel with the circuit breaker, not between the phase and neutral. It is also necessary to place an inverse-connected diode in parallel with the opto-coupler LED to prevent reverse line voltage from damaging it, which can occur even with the current-limiting resistor in series with both diodes. LEDs have very small forward voltage drops across a conducting LED, and virtually NO capability of withstanding reverse voltage applied across the LED. Hence the need for a second, inversely connected diode, that will conduct when the LED is reverse-biased by the AC line voltage.

    Unless I misunderstood you, the whole idea is to detect when a circuit breaker is open or closed, not whether power is present on each phase. With the opto-coupler LEDs wired in parallel with each breaker, a closed breaker will result in zero conduction of the opto-coupler transistor because the closed breaker shorts out the LED. An open breaker will cause enough current to flow through the LED (as well as harmlessly through the inverse-connected diode) to turn the photo-transistor on. Connect the transistor outputs to whatever logic you think is necessary to detect when one (or more) circuit breakers is/are open. I suggest connecting all the transistors in parallel, so all of them must be off to prevent asserting an alarm.
     
    Harald Kapp likes this.
  16. Frankchie

    Frankchie

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    Nov 14, 2017
    hevans,
    Bypassing the circuit breaker may introduce a safety issue as voltage will still be presented to the motor when it appears to be turned off.

    True the motor loading will minimize the voltage level, but if someone is working on the motor and disconnects a winding it could cause a shock hazard.

    P.S. Hopefully the Op is using a latching relay that requires manual intervention to reset. Maybe a small low current circuit breaker controlling the relay can serve this purpose, or a fuse if space is a problem. And, obviously, the relay should fail "off" upon loss of any power, etc.
     
  17. Minder

    Minder

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    Apr 24, 2015
    I am still not clear why you cannot use a simple standard 3ph O/L on the contactor output?
    They come with adjustable current range and can be had in a stand-alone type as well as fit to the bottom of a contactor.
    Commonly have 1 N.C. & 1 N.O. control contact.
    M.
     
    hevans1944 likes this.
  18. Bluejets

    Bluejets

    4,151
    879
    Oct 5, 2014
    Exactly.......while at it, dump the 3 individual breakers before someone inadvertently operates one under load.

    https://www.youtube.com/watch?time_continue=21&v=hA-w0QAaxRU&feature=emb_logo
     
  19. hevans1944

    hevans1944 Hop - AC8NS

    4,521
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    Jun 21, 2012
    Very little voltage will be presented to the motor when the circuit breaker is turned off. The motor winding resistances are much less than 51 kΩ. When the circuit breaker is open, because there is a 51 kΩ resistor (also in series with two back-to-back inversely connected diodes, one of the two diodes being the LED in the opto-coupler) now in series with the low-resistance motor winding, almost all of the line voltage will be dropped across this resistor, leaving virtually nothing to drop across the motor winding.

    There is always a shock hazard when working with line-powered equipment. The very fact that non-interlocked circuit breakers are being used to energize a three-phase motor violates code in most jurisdictions, so a minor issue like a "shock hazard" from an "open" circuit breaker that also happens to have a 51 kΩ resistor bridged across it should be of no concern to @Mahonroy, who is also apparently ignoring the advice offered by @Minder, who happens to know a thing or two or three about motors.

    Hmmm. Would it be too much to ask that you post a schematic diagram of this "latching relay that requires manual intervention to reset?" It is unclear to me how this would solve the original problem described in post #1.

    A "simple" way to do this is with a CMOS camera and some image processing software. Using IR LEDs, illuminate the three circuit breakers so all three are in the FOV of the camera. Train the software, if necessary, to distinguish between an "open" versus a "closed" circuit breaker. Using your program's logic, determine what to do if a circuit breaker is tripped to the "open" position... light a red "Fault" LED, shutdown the relay... yada, yada, yada. Inexpensive bare-bones board-cameras are readily available for this, and a Raspberry Pi small computer should have enough "horse power" to perform such a simple image processing task. Some programming required, of course.
     
    Last edited: Sep 18, 2020 at 1:04 AM
  20. Frankchie

    Frankchie

    98
    6
    Nov 14, 2017
    hevans,
    An example of a latching relay would have one pole with it's NO contacts in series with it's activating coil. A momentary switch (push button) across the normally open contacts would have to be manually pushed for the relay to activate. Once the relay opens it cannot close until the switch button is manually pushed again. Anyway that's the general concept I envisioned.

    I didn't give any thought to how/if it would solve the Op's original problem, nor was it intended to solve the problem. I just offered the latching concept as a safety feature thinking it could be incorporated without too much trouble. A relay, circuit breaker or fuse were examples of how it might be accomplished.

    The low amperage circuit breaker or fuse solution might be a little more complicated as they would have to be overloaded somehow to latch open. Thinking about that some more, it's probably too complicated to be practical.
     
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