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Old 02-22-2011, 04:58 PM   #1
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Three Phase Motor Overload Protection Control


Hi there,
Howcome in a ladder diagram the overload NC contact is placed after the coil and not in series directly beside the stop button?

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Old 02-22-2011, 05:32 PM   #2
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Three Phase Motor Overload Protection Control


have you ever followed the actual circuit is a motor starter?

Do you have one handy where you can?

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Old 02-22-2011, 07:18 PM   #3
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Three Phase Motor Overload Protection Control


Hi there "nap".
How is your reply relevant to the question?
I think the reason the O/L contact is placed where it is for safety purposes, but I'm not sure why.

Last edited by parkysay; 02-22-2011 at 07:23 PM.
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Old 02-22-2011, 09:23 PM   #4
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Three Phase Motor Overload Protection Control


Quote:
Originally Posted by workfuture View Post
Hi there,
Howcome in a ladder diagram the overload NC contact is placed after the coil and not in series directly beside the stop button?
Short et sweet answer.,, safety issue when the O/L coil kick out it will shut off the contractor coil it always wired in series in this circuit.

For the stop button function it can be wired in series with O/L coil so when you hit the stop button it will " force " the contractor coil to drop out that for basic set up however there are other means but will cover at other time but for now let stay with basic first.

If you want to study more on the motor control circuit set up there are few books on market you can or look up in the internet to search more details and they will cover very well with their function.

parkysay.,

To find this answer just look above.

And Parkysay let you know Nap and myself and few other members in here is electrician by trade so we will steer you right with the answer.


Merci.
Marc
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Old 02-22-2011, 11:15 PM   #5
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Three Phase Motor Overload Protection Control


Quote:
Originally Posted by parkysay View Post
Hi there "nap".
How is your reply relevant to the question?
I think the reason the O/L contact is placed where it is for safety purposes, but I'm not sure why.
because if you look at most starters, you will find that the overload contact is actually between the second leg of the control circuit and the coil while the start stop contacts are in circuit between the first leg of the control circuit and the coil. It isn't always that way but most starters I have worked on have been.

I cannot remember any offhand that actually had the stop start contacts on the same leg of the control circuit as the overload contact.

so, it's relevant because I figured the OP could actually trace the control circuit and realize that for himself. While I and many others here have no problem answering questions, when we see somebody like the OP with a question such as he has asked that appears to be in school or maybe an apprentice program, I would rather help him learn how to figure things out for himself rather than simply giving him the simple answer.

You know, the old "give a man a fish and he eats for a day; teach a man to fish and he eats for life"
If you would rather be given a fish, I am more than capable of doing so. If you want to learn how to fish, you are more than welcome as well.

Now, with your response, you effectively asked a different question

Quote:
I think the reason the O/L contact is placed where it is for safety purposes, but I'm not sure why.
In the first question, he asked why the contact was placed where it was in a ladder diagram and I gave him the answer. Now, if you want to know why it is in the second leg of the control circuit I can answer that as well.
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Old 02-23-2011, 05:46 AM   #6
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Three Phase Motor Overload Protection Control


Quote:
Originally Posted by nap
I cannot remember any offhand that actually had the stop start contacts on the same leg of the control circuit as the overload contact.
Starters with a 120v coil?
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Old 02-23-2011, 07:34 AM   #7
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Three Phase Motor Overload Protection Control


I also believe it is better to let a person figure things out .. much better to know why.


You might enjoy and learn a bit about a motor starter by going through this interactive lesson on control circuits and motor starters.

http://www.wisc-online.com/objects/V...spx?ID=IAU9007


If your in a class or if your the inquisitive type pick up a magnetic motor starter with the overload block and teach yourself to wire it utilizing the auxiliary contact on the coil body to latch the circuit. You will see the M1 contact in the diagrams. Trace the circuit from paper to the device. You teach yourself a lot that way.
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Old 02-23-2011, 08:45 AM   #8
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Three Phase Motor Overload Protection Control


Quote:
because if you look at most starters, you will find that the overload contact is actually between the second leg of the control circuit and the coil while the start stop contacts are in circuit between the first leg of the control circuit and the coil. It isn't always that way but most starters I have worked on have been.

I cannot remember any offhand that actually had the stop start contacts on the same leg of the control circuit as the overload contact.
If I remember correctly and I believe it is mentioned in the link I posted that if you place the overloads in the first leg of the ladder when they are reset on an overload the motor will start. Because in that series placement in the circuit (1st rung of the ladder) you would bypass the m1 contact and the coil would stay engaged after the overload occurred.
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Old 02-23-2011, 12:01 PM   #9
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Three Phase Motor Overload Protection Control


Quote:
Originally Posted by Stubbie View Post
If I remember correctly and I believe it is mentioned in the link I posted that if you place the overloads in the first leg of the ladder when they are reset on an overload the motor will start. Because in that series placement in the circuit (1st rung of the ladder) you would bypass the m1 contact and the coil would stay engaged after the overload occurred.
if used in a 3 wire control, as long as the opening of the OL contact causes the holding circuit to drop out, it wouldn't matter. The circuit could not be re-energized unless the start button was pressed again and reactivating the MS coil.

The OL's will act in the same effective capacity as the stop contact as long as it is placed in series, anywhere, in the single conductor between the M1 coil and L1. That is why you can have multiple stop contacts that all provide for a complete shutdown of the control circuit without automatic restart upon closing of the stop contact.

In the link, the addressed the presence of the overload contacts being required in the control circuit as opposed to simply having an overload contact in the power circuit. They did not address the placement of the OL contacts any more specific than that that I saw.

Nice site though.

what I believe is:

by placing the OL contacts in the second leg, it simply removes the possibility of an accidental wiring that would bypass the OL's. While a simple start/stop control is unlikely to be wired so as to bypass the OL's contacts (if they were in the first leg of the circuit), when you start adding in 27 different control stations and multiple control devices intended to shut down the circuit, the possibility of an errant design or installation allowing a unintentional bypass of the OL;s increases. It is simple and effective means of ensuring a positive means of opening the control circuit so that it drops out the holding circuit that will not be unintentionally bypassed or wired so that the circuit could be re-energized by simply resetting the OL's.

in other words; a failsafe


the funny thing about putting it in the second leg of the circuit is; if this is a 120 or 277 volt control (any system utilizing a "neutral"), it is contrary to typical NEC requirements. NEC requires a neutral to not be interruptible without also interrupting the hot conductors as well when controlling a circuit. I know, not applicable because of what it is. It just is funny to see it allowed when the NEC is so particular about it in other areas.
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Old 02-23-2011, 12:05 PM   #10
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Three Phase Motor Overload Protection Control


Quote:
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Starters with a 120v coil?
just think of the neutral as the second leg. I figured somebody would think of what I was saying in the manner you have and should have clarified it previously.

When I speak of the first or second leg of the circuit, I am simply referring to the circuitry on either side of the device being powered. In the case of a 120 volt control, that would be the 120 v hot leg powering the control circuit and the neutral on the other side of the coil. In a circuit design, they are typically designated as L1 and L2 even if L2 is a neutral.
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Old 02-23-2011, 04:40 PM   #11
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Three Phase Motor Overload Protection Control


Quote:
if used in a 3 wire control, as long as the opening of the OL contact causes the holding circuit to drop out, it wouldn't matter. The circuit could not be re-energized unless the start button was pressed again and reactivating the MS coil.

The OL's will act in the same effective capacity as the stop contact as long as it is placed in series, anywhere, in the single conductor between the M1 coil and L1. That is why you can have multiple stop contacts that all provide for a complete shutdown of the control circuit without automatic restart upon closing of the stop contact.

Yes your correct .. I'm not sure what I was thinking but I'm getting a little rusty at this stuff that's for sure. If the op comes back I'll see if I can put together a drawing of the actual connections as they relate to the ladder circuit.

Then you can find my mistakes ....
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Old 02-23-2011, 06:09 PM   #12
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Three Phase Motor Overload Protection Control


Quote:
Originally Posted by Stubbie View Post
Yes your correct .. I'm not sure what I was thinking but I'm getting a little rusty at this stuff that's for sure. If the op comes back I'll see if I can put together a drawing of the actual connections as they relate to the ladder circuit.

Then you can find my mistakes ....
ya, what can we do about the drawings you had done. I am sure you spent considerable time and they were nice to have around. It's a shame to not have them available anymore.
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Old 02-23-2011, 08:55 PM   #13
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ya, what can we do about the drawings you had done. I am sure you spent considerable time and they were nice to have around. It's a shame to not have them available anymore.
The drawings are a long story but I'll PM you with the details.
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Old 02-23-2011, 09:48 PM   #14
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The drawings are a long story but I'll PM you with the details.
Publicly telling someone that you'll PM them about a potentially juicy topic only breeds extreme curiosity.
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Old 02-23-2011, 10:14 PM   #15
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Three Phase Motor Overload Protection Control


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Publicly telling someone that you'll PM them about a potentially juicy topic only breeds extreme curiosity.
True .. but that isn't my intention. Nap and I have been posting here for a long time. I have a lot of respect for him and he gets to be in on the juicy details.

The goal is to get the drawings back on the forum if possible.

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