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Old 08-04-2011, 10:05 AM   #1
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Default "Simple" E-Stop circuit

I have been asked to install two emergency stop buttons.

The first one is to be added to an existing 230V/30A circuit that feeds a hydraulic pump motor on an auto hoist. At first I thought I could just get a DPST E-stop button and put it in line but I have yet to find one that is rated for more than 10A. My next thought was that the E-stop button would control the coil of a contactor but if the E-stop button is NC (normally closed) and the contactor is NO (normally open) then that would mean it was constantly applying voltage to the coil of an electronicaly held contactor. Is this acceptable? It would seem to me that contantly applying voltage to a NO contactor would be excessive wear. Is there a better way to do this?

The second E-stop button is to kill power to a pair of DC fuel pumps. This is not existing except for the pumps which are currently being powered by a large car battery on a hand dolly. My plan here was to run one 120V circuit to two switches. Each switch controls a power supply that provides the DC to each motor. The E-stop would need to kill power to the switches to ensure that both fuel pumps were off. I am still waiting on quotes on a power supply that would give the 12vdc/20A output needed so that I can know what size AC circuit is needed. Assuming that I can make all this work as easy as it sounds I am still left with the same concern about having to apply constant voltage to a coil just to keep the circuit closed.

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Old 08-04-2011, 11:24 AM   #2
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For the first one, put the estop in series with the contactor control circuit, if there is none you'll need one. How is the pump being operated now? With a mechanical disconnect? The coil and estop will only be energized when the pump is running anyway so its not going to be pulled in constantly, even so many contactors stay pulled in all the time with no issue.

As for the batteries on a hand cart it sounds hack rigged to me, but the same thing can be done using an appropriately sized contactor

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Old 08-04-2011, 11:27 AM   #3
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You don't want to use the starters to control your power sources, you want them to turn the motors on as needed, and the estops will be there to drop coil power in the event that something happens while they're running
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Old 08-04-2011, 11:55 AM   #4
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Get out the drawings and find the control power source. Since this is an E-Stop you want to kill everything. (ask someone if you are not sure).
Find a point after the control transformer and install a E- Stop button/switch in series with this wire.
This will stop everything on the control circuit.
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Old 08-05-2011, 05:07 AM   #5
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What the others are telling you is that an E-Stop button is a CONTROL device, not a power device. The contacts are what is referred to as "Pilot Duty", they are not intended to be used directly on a motor circuit, AC or DC. So something else needs to be doing the heavy switching in both cases. Often that is done with an electro-magnetic (a.k.a."mag") contactor that has a coil and is controlled by other pilot devices, such as a Start / Stop push button or another pilot duty switch of some sort. If that's the case, you simply put the E-Stop in that control circuit to drop out the coil regardless of the status of the other pilot devices.

But if you do NOT have a mag contactor somewhere in the circuit, and your motors are being controlled by a mechanical switch directly, then you cannot use an E-Stop all by itself, you need to ADD a contactor to the circuit and put the E-Stop in the contactor coil circuit.

If your motor HP is low enough, there are also some small push button style Manual Motor Starter (MMS) devices out there that have an E-Stop style accessory that can be added to the front, as in the picture below. But if you were to have to add this, you could probably add a mag contactor too for the same price.

MMS with E-Stop style operator (Allen Bradley Bul. 140).


Be careful with the DC circuit however, AC motor ratings are very different from DC motor ratings when it comes to switching devices. It's much harder to switch DC so the ratings are lower for the same device. Read the data carefully.

Last edited by JRaef; 08-05-2011 at 05:26 AM.
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Old 08-05-2011, 08:01 AM   #6
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My next thought was that the E-stop button would control the coil of a contactor but if the E-stop button is NC (normally closed) and the contactor is NO (normally open) then that would mean it was constantly applying voltage to the coil of an electronicaly held contactor. Is this acceptable? It would seem to me that contantly applying voltage to a NO contactor would be excessive wear. Is there a better way to do this?



What do you mean by NO contactor ?? A contactor has a coil which usually closes the Contacts when power is applied. Most, as in industry, have power applied to them 24/7 with no problems.
so that being said, You can use an E-stop in line with the coil voltage, whether the voltage to that coil is fed directly, or from a control transformer as John mentioned..............
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Old 08-05-2011, 11:27 AM   #7
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Thanks guys. Now I know the principal is sound. My biggest hesitation was having voltage applied to the coil 24/7 thinking maybe it would chatter or wear or something. I'm more a Retail & Resi guy and usually do contactors only for P-lot lights and the like.
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Old 08-05-2011, 01:19 PM   #8
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Quote:
Originally Posted by XCasper View Post
Thanks guys. Now I know the principal is sound. My biggest hesitation was having voltage applied to the coil 24/7 thinking maybe it would chatter or wear or something. I'm more a Retail & Resi guy and usually do contactors only for P-lot lights and the like.
I think you are over-interpretting something here. The coil is only going to be energized WHEN YOU NEDD TO RUN THE HYDRAULIC PUMP. So as was said, SOMETHING ELSE has to turn the pump on and off, all the E-Stop N.C. contact does is interrupt that OTHER circuit if it is pushed in.

let's start over. How is the hydraulic pump turned on and off right now?
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Old 08-06-2011, 07:57 PM   #9
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Find someone that knows what they are doing. You have a good chance of screwing something up here. If you have to conduct an Internet forum to ask how to do an install, maybe you shouldn't do it. Asking for pointers is one thing, but c'mon man.
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Old 08-06-2011, 09:08 PM   #10
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Put it in series with the other stop buttons. Then test when you're done. Make sure your logic was good. You should be able to draw out the circuit and see how it works. Research some basice stop/start circuits. It's not that complex really.
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Old 08-07-2011, 12:29 AM   #11
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Start-stop w/E-Stops - picking a proper contactor - setting-up a control transformer - picking the RIGHT overload by reading the motor for the appropriate data to set it all to their proper levels - this, I agree with Sparky, is more than can be handled on this forum. You're apparently short of Motor Controls 101, "The Basics" and need to get this and many more instructions if you're faced with these kinds of challenges on any kind of regular basis. Please do your client and yourself a favor and get some hands-on help this time, do a LOT of learning about this stuff before the next time and come back to us with a success story - okay? Thanks!!!
You CAN pick it up - it's really not all that tough once you've been shown the ropes. Best of luck!!
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Old 08-07-2011, 12:38 AM   #12
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Oh yeah, Casper, for the DC loads, especially at low voltages, the currents are quite high usually so some diligence is needed there as well. Tricks sometimes involve running 2-contacts of a starter in series to handle the current "break" issues with DC loads. They LOVE to weld them together so creating a HUGE instant contact gap is an old-timer's trick - but, again, get some help this time - next time, be ready to handle these yourself by boning-up on it....a LOT!
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Old 08-07-2011, 12:44 AM   #13
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Anyone that's pondering estops would be well-advised to bone up on NFPA-79.
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Old 08-08-2011, 03:10 PM   #14
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Default Clarification needed perhaps

Quote:
Originally Posted by JRaef View Post
I think you are over-interpretting something here.

let's start over. How is the hydraulic pump turned on and off right now?
I think if anything I have been over interpreted, perhaps because I gave too much info. The one question I wanted answered has been. Namely...

" ...that would mean it was constantly applying voltage to the coil of an electronicaly held contactor. Is this acceptable?"

At least 3 folks have indicated yes, this is normal and common place.

I included the full description so that if someone had a better idea or something was glaringly wrong they could ring in. Everyone keeps talking about the heaters and motor controls which in this case are a non-issue. An auto hoist is a simple and common place system used in most if not all auto and tire repair shops. These are self contained systems with one manual lever to control up and down, nothing else. What I am installing is a "Category 0" "kill the power" e-stop that is controlling or in this case removing the power to the motor and not controling the motor itself. In this case there is no need for anything more.

Again, my error for giving too much info. All I am doing in this application is having an E-stop button break voltage to a coil to open a contactor to kill power.
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Old 08-08-2011, 07:35 PM   #15
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Okay. You want simple? Find one of the secondary conductors leaving the control transformer. Remove it from its terminal and run it to a E-Stop NC button. Then take a wire from the other side of the NC E-Stop button and run it back to the empty terminal.
Now, How easy can that get.

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