Does it matter where the overloads are in a motor control circuit?

[nogard1126]

I always wondered why the overload relay contact is placed after the contactor coil in the control circuit. Does it really matter, as long as it is in the circuit to open the rung?

Also on the actual overload (heaters), why are they always placed after the contactor just before the motor. Could they be put anywhere in the circuit as well?

I have a basic motor control circuit. It is 120VAC into a rheostat with a transformer, then into a bridge rectifier, then 160VDC to the DC motor. Does it matter if the overload heaters are placed in AC part of the circuit before the rheostat or on the DC motor leads?

I am using a three phase contactor/overload. I am using the looping/series method of wiring by going into the top of phase 1 out the bottom to phase 2 out the top to phase 3 then out the bottom of 3 to the motor.

Thanks.

 

[ericsherman37]

I've always wondered that too. Electrically I don't think it would matter - if the overload relay contacts broke the rung then that's all that needs to happen. Maybe it's wired that way, and diagramed that way, to differentiate it from regular contacts. Good question, maybe someone smarter than me can explain it

 

[benaround]

The overloads are to protect the motor, that's why they are right before the motor.

How are you going to protect the motor from the AC circuit, with all that other stuff

before the motor.

#2 Why are you using a 3ph starter on a 120v circuit?

 

[quogueelectric]

The overloads are to protect the motor, that's why they are right before the motor.

How are you going to protect the motor from the AC circuit, with all that other stuff

before the motor.

#2 Why are you using a 3ph starter on a 120v circuit?

He is not asking why the overloads are placed where they are placed. He is asking why the nc contact is always located after the coil in the control ladder diagram. It is kind of a practice that we have been taught not to do as in switching the neutral. I personally think that it was put there to be sure that the control ckt drops out and is not involved in the complications of the sequence of operations of the motor. I am past wondering and just wire acordingly at my stage of the game. I will soon be 50 and am tired of taking on the world.

 

[ericsherman37]

He is not asking why the overloads are placed where they are placed. He is asking why the nc contact is always located after the coil in the control ladder diagram. It is kind of a practice that we have been taught not to do as in switching the neutral. I personally think that it was put there to be sure that the control ckt drops out and is not involved in the complications of the sequence of operations of the motor. I am past wondering and just wire acordingly at my stage of the game. I will soon be 50 and am tired of taking on the world.

Good answer

So maybe my hypothesis has some oomph then: placing the overload relay N.C. contacts after the coil on the diagram kinda differentiates them from the multitude of other contacts in the rest of the diagram.

 

[benaround]

He is not asking why the overloads are placed where they are placed. He is asking why the nc contact is always located after the coil in the control ladder diagram. QUOTE]

Oh, that's the way they come wired from the factory. coil to overload contacts to # 95 ?

 

[Besoeker]

I always wondered why the overload relay contact is placed after the contactor coil in the control circuit. Does it really matter, as long as it is in the circuit to open the rung?

I suppose not although we generally have them before the coil.

I have a basic motor control circuit. It is 120VAC into a rheostat with a transformer, then into a bridge rectifier, then 160VDC to the DC motor.

Sounds like you are using single phase. If that's the case, you won't get 160Vdc from 120Vac. Unless you have a huge bank of capacitors across the DC then the supply current will be distorted.....

Does it matter if the overload heaters are placed in AC part of the circuit before the rheostat or on the DC motor leads?

I am using a three phase contactor/overload. I am using the looping/series method of wiring by going into the top of phase 1 out the bottom to phase 2 out the top to phase 3 then out the bottom of 3 to the motor.

If it's an AC contactor, puting it in the DC is not a good idea. I'd have the contactor/overload in the AC feeding directly into the rectifier.

 

[Jraef]

The only reason why JIC (Joint Industries Council, the predecessor to NEMA) wiring standards have the NC OL contact on the right side of the coil is, as previously mentioned, to separate it from any other control elements. That makes it easier to troubleshoot. The JIC standard dictated that the OL contact(s) was to be the ONLY thing on the right side of the coil.

But it really makes little difference. In fact, the IEC standard is to have it on the left side, along with all other control elements. But an argument FOR doing it that way is that if a wire goes to ground, the starter will NOT work. On the JIC method, when using 120V control with a grounded neutral, if a wire ahead of the OL contact breaks off and goes to ground on the neutral side, the OL relay may trip but would not drop out the coil.

As to why the OL sensors are downstream of the contactor, that's just convention. It makes no difference and n fact on a lot of Soft Starters they are on the line side because of the need to have them in the circuit when the SCRs are bypassed.

By the way, an AC contactor s probably NOT rated for switching DC. DC switching is a lot harder on contactors. Put it on the AC side of the rectifier f you can, or buy a true DC rated starter.

 

[nogard1126]

Jraef,

Thanks for the answers I was looking for. That makes sense.

Also to answer the other questions, I'm using single phase 120V but there is a huge rheostat I've never seen something like this, it looks old. And there must be a transformer built into. 120VAC goes in and 160VAC comes out. Then that goes into a rectifier. The D.C. motor is 180VDC 7 amps. I'm using a three phase contactor and OL because that is what was available. I never check to see if they make a single phase version. I've always seen three phase using on single phase applications with the the three phases wired in series.

If the contactor and overloads are rated to handle more than the DC part of the circuit uses would they have to be specially rated for DC?

Putting the OL contact after the coil does make sense for trouble shooting like was mentioned. If there is a pilot light in the circuit paralleled off the coil but not going through the OL contact. Then if the OL is tripped and the start button is pushed the pilot light will come on, but not latch in, and the motor will not run, until the OL is reset.

I am going to put the OL contact after the coil where it normally goes, for ease of troubleshooting purposes. And I'm going to keep the OL sensors just before the rheostat.

 

[petersonra]

I always wondered why the overload relay contact is placed after the contactor coil in the control circuit. Does it really matter, as long as it is in the circuit to open the rung?

Also on the actual overload (heaters), why are they always placed after the contactor just before the motor. Could they be put anywhere in the circuit as well?

I have a basic motor control circuit. It is 120VAC into a rheostat with a transformer, then into a bridge rectifier, then 160VDC to the DC motor. Does it matter if the overload heaters are placed in AC part of the circuit before the rheostat or on the DC motor leads?

I am using a three phase contactor/overload. I am using the looping/series method of wiring by going into the top of phase 1 out the bottom to phase 2 out the top to phase 3 then out the bottom of 3 to the motor.

Thanks.

Why would anyone do such a goofy thing in the first place? Is this some kind of test question?

 

[nogard1126]

It is not a test question. I came across this at work, and while I was trying to figure out the best way to set up the circuit, I realized that there were a few ways. And that got me thinking why do we always wire overloads (sensors, and contact) this way in a control circuit?

So I signed up for this forum and asked the question. Jraef nailed it when he said the JIC predecessor to NEMA were the ones to come up with it for trouble shooting.

And I'm sure I'm not the only one who has seen a three phase contact or overload used for a single phase application.

The questions I asked where because I wanted to know the history of why we always to it this way. A few people missed what I was asked and a few got it. If there wasn't a reason I would have been fine with that. But most of the stuff I come across that I don't understand that I think doesn't matter I know there is a reason that someone originally decided to do it that way.

Thanks to all for you input on this matter.

 

[Jraef]

Why would anyone do such a goofy thing in the first place? Is this some kind of test question?

If you are using an IEC 3 phase overload relay, one that has what is called a "differential trip bar" that will bias the trip curve under single phasing conditions, you must pass current through all 3 phases of the OL relay. Otherwise it may give you nuisance trips. So looping back through like that is the required wiring configuration.

If it's a NEMA starter, you may not need to do that because NEMA OL relays don't have that differential trip bar inside. But some mfrs have now taken to adapting their IEC OL relays to fit onto their NEMA contactors, so you have to be aware of the wiring requirements for single phase use by reading the instructions. However if an OL relay has replaceable heater elements (as opposed to only a dial adjustment), then it is for sure a NEMA OL relay and you don't need to do the loop back.

 

[Besoeker]

By the way, an AC contactor s probably NOT rated for switching DC. DC switching is a lot harder on contactors. Put it on the AC side of the rectifier f you can,

Glad you agreed with my previous post.

 

[Jraef]

Jraef,

Thanks for the answers I was looking for. That makes sense.

Also to answer the other questions, I'm using single phase 120V but there is a huge rheostat I've never seen something like this, it looks old. And there must be a transformer built into. 120VAC goes in and 160VAC comes out. Then that goes into a rectifier. The D.C. motor is 180VDC 7 amps. I'm using a three phase contactor and OL because that is what was available. I never check to see if they make a single phase version. I've always seen three phase using on single phase applications with the the three phases wired in series.

{snip}

If the contactor and overloads are rated to handle more than the DC part of the circuit uses would they have to be specially rated for DC?

Putting the OL contact after the coil does make sense for trouble shooting like was mentioned. If there is a pilot light in the circuit paralleled off the coil but not going through the OL contact. Then if the OL is tripped and the start button is pushed the pilot light will come on, but not latch in, and the motor will not run, until the OL is reset.

I am going to put the OL contact after the coil where it normally goes, for ease of troubleshooting purposes. And I'm going to keep the OL sensors just before the rheostat.

So if the contactor is ahead of the rectifier, meaning it is in the AC side of the circuit, then you don't need to worry about the DC switching issue. That "rheostat" is most likely a "variable transformer", a.k.a. "Variac" which is a brand name. It would be used to provide a variable amount of AC going into the rectifier, which will of course provide variable DC to the motor as a form of manual speed control. That is a really old way of doing it, you can get a very inexpensive 120VAC input DC speed controller that does all of that electronically now.

Your Pilot Light, to be accurate, should be wired off of a Normally Open aux contact of the contactor. That way the light is only On if the contactor is closed, not just whenever there is power to the circuit.

 

[Besoeker]

I'm using single phase 120V but there is a huge rheostat I've never seen something like this, it looks old. [/qote]
Are you sure it's a rheostat and not actually a variable transformer (Variac)?

And there must be a transformer built into. 120VAC goes in and 160VAC comes out. Then that goes into a rectifier.

If it is a Variac, it could possibly be arranged to give higher output voltage than the input.

The D.C. motor is 180VDC 7 amps.

Even with 160Vac into a single-phase rectifier, you won't get 180Vdc. The mean value of the output would be about 145Vdc. It just means you won't get rated maximum speed which may or may not be a problem for your application.

If the contactor and overloads are rated to handle more than the DC part of the circuit uses would they have to be specially rated for DC?

Many AC contactors do have a DC rating but it is usually much lower for both voltage and current. And, obtaining this data can be difficult - if it is available at all.
Do what both Jraef and I have suggested - put it in the AC supply to the rectifier bridge.

And I'm going to keep the OL sensors just before the rheostat.

Then they will sense the current into the rheostat which is fine if it is a rheostat and it it directly feeds the rectifier. If the rheostat feeds into the transformer you mentioned, then it wouldn't be fine unless you set the overload trip level to take account of the transformer ration.
If it's a Variac....
It's not such a complex circuit. Maybe you could draw it out.
We could then take out the "ifs".

 

[petersonra]

It is not a test question. I came across this at work, and while I was trying to figure out the best way to set up the circuit, I realized that there were a few ways. And that got me thinking why do we always wire overloads (sensors, and contact) this way in a control circuit?

So I signed up for this forum and asked the question. Jraef nailed it when he said the JIC predecessor to NEMA were the ones to come up with it for trouble shooting.

And I'm sure I'm not the only one who has seen a three phase contact or overload used for a single phase application.

The questions I asked where because I wanted to know the history of why we always to it this way. A few people missed what I was asked and a few got it. If there wasn't a reason I would have been fine with that. But most of the stuff I come across that I don't understand that I think doesn't matter I know there is a reason that someone originally decided to do it that way.

Thanks to all for you input on this matter.

I was not refering to using a 3 phase OL on a single phase circuit. The whole setup just seems screwy to me. So much simpler to buy a $50 or $100 DC drive.

 

[Wes G]

The only reason why JIC (Joint Industries Council, the predecessor to NEMA) wiring standards have the NC OL contact on the right side of the coil is, as previously mentioned, to separate it from any other control elements. That makes it easier to troubleshoot. The JIC standard dictated that the OL contact(s) was to be the ONLY thing on the right side of the coil.

But it really makes little difference. In fact, the IEC standard is to have it on the left side, along with all other control elements. But an argument FOR doing it that way is that if a wire goes to ground, the starter will NOT work. On the JIC method, when using 120V control with a grounded neutral, if a wire ahead of the OL contact breaks off and goes to ground on the neutral side, the OL relay may trip but would not drop out the coil.



The trouble shooting ease could be debated. As to positioning of OL'S, I agree with jraef. I worked for a number of years at a plant with Klockner Mohler buckets and the OL'S were the first item on the control rung. I actually prefered it that way, because when they tripped the whole control circuit was dead. The NO contacts on the OL unit were wired to show on the program of the PLC and also to an LED indicator lamp on the MCC bucket door. Trouble shooting was a breeze!

 

[Jraef]

The trouble shooting ease could be debated. As to positioning of OL'S, I agree with jraef. I worked for a number of years at a plant with Klockner Mohler buckets and the OL'S were the first item on the control rung. I actually prefered it that way, because when they tripped the whole control circuit was dead. The NO contacts on the OL unit were wired to show on the program of the PLC and also to an LED indicator lamp on the MCC bucket door. Trouble shooting was a breeze!

LOL,
I started out in this business as an apprentice working for an industrial OEM wiring up machines. Everything we did was Allen Bradley, NEMA controls, JIC wiring and drawing standards. When I first learned that people in other countries did it differently was when I took an engineering job at Klockner Moeller. I argued the placement was wrong and should have been on the neutral side of the coil. I was summarily de-programmed by my Engineering Manager as to the fact that not everyone used JIC wiring and drawing standards. He's the one who pointed out the benefits of doing it their way.

 

[quogueelectric]

LOL,
I started out in this business as an apprentice working for an industrial OEM wiring up machines. Everything we did was Allen Bradley, NEMA controls, JIC wiring and drawing standards. When I first learned that people in other countries did it differently was when I took an engineering job at Klockner Moeller. I argued the placement was wrong and should have been on the neutral side of the coil. I was summarily de-programmed by my Engineering Manager as to the fact that not everyone used JIC wiring and drawing standards. He's the one who pointed out the benefits of doing it their way.

It was that moment when you crossed over to the "Dark Side"???

 

[Jraef]

It was that moment when you crossed over to the "Dark Side"???

Yes, they said I had to be "deprogrammed" after that. They made me read what the K-M Germans called their "Planning Manuals" from cover to cover. They were a set of 16 bound volumes of nothing but engineering rules and "rationalizations" for every single little nit picky rule. Mostly they involved the IEC standards and the histories behind them. But they even had rules as to what I should wear or how the drafting tools on my desk were to be laid out, AND they had rationalizations as to why they thought so. It was one of the most tedious things I have ever done.

Now I kind of ride the fence on design specs. I think the best way to approach it is to assess what the capabilities are of the people who will be maintaining it and cater to that. Forcing someone t learn something new on the job is often asking fr trouble.