By continuing to use this site, you agree to our use of cookies. Find out more
Forum sponsored by:
Forum sponsored by Allendale Jan 24th

Are the Electrics Right in ME4575?

All Topics | Latest Posts

Search for:  in Thread Title in  
SillyOldDuffer04/12/2017 17:12:07
5127 forum posts
1073 photos

I've just been reading Part 2 of 'Lathes and more for Beginners' starting Page 832 of ME4575, which arrived today.

I'm not sure I understand the Figure 2 circuit diagram - is it me, or have the gremlins struck again?

dsc04973.jpg

Firstly, the switch I've marked A and B in the photo. From the text which says 'For the on-off control do not under any circumstances use a single pole switch...' I believe A and B to be a two-pole linked switch. I'm uneasy that 'B' is downstream of the contactor. Wouldn't it be better for the contactor neutral to be disconnected entirely by the on-off switch?

Secondly, the 'A' part of the on-off switch doesn't remove Live from the contactor once the contactor's Normally Open contact is closed.

Thirdly, does the circuit as drawn work at all? Doesn't putting power on the contactor coil as shown via the normally closed contact mean the contactor immediately disconnects itself rather than latching ON?  In other words, the contactor doesn't do anything - it's the on-off switch that does all the work.

Happy to be told I'm wrong!

Dave

 

Edited By SillyOldDuffer on 04/12/2017 17:15:58

Thor04/12/2017 17:22:40
1158 forum posts
33 photos

Hi Dave,

I am no electrician, but if the N.O. switch is pressed then the contactor coil is energized and operates the A and B switch and the machine is powered. When you release the N.O. switch the contactor coil is still powered via the N.C. switch. When that switch is pressed the contactor loose power and A and B releases and powers off the machine. Does that make sense?

Thor

Martin Cargill04/12/2017 17:24:26
117 forum posts

It does work but its an unusual way to wire a contactor. A and B are the contactors contacts. N.O. and N.C. are separate start and stop buttons.

Ian P04/12/2017 17:27:55
avatar
2299 forum posts
93 photos

This looks like it could have been in the Morecombe and Wise sketch of 'playing all the right notes, but not in the right order'!

I have not seen the ME article so cannot add any valid comment other than its not something I would want in my workshop

Ian P

Andrew Johnston04/12/2017 17:28:24
avatar
5110 forum posts
594 photos

It only tells part of the story.

The "switches" A & B are actually the contactor contacts. The NO and NC switches are momentary pushbuttons. So with power applied nothing happens until the NO button is pushed. Then the contactor closes A & B, and the machine is powered. The NO button can be let go as the contactor is now powered via the NC button. To turn the machine off the NC button is pushed. If, while the machine is running, power is disconnected the contactor will open. When power is reapplied the contactor will remain open, and the machine will not start. Which is what you want.

The implication is that mains isolator is elsewhere either on the machine, but is not shown. This is more akin to industrial practise.

Confusing for a beginner? smile o

Andrew

Edit: Ian P - this is a standard industrial setup. It is how most of my machines are wired, via a contactor and momentary pushbuttons.

Edited By Andrew Johnston on 04/12/2017 17:32:02

Ian P04/12/2017 17:36:17
avatar
2299 forum posts
93 photos

I assume the contactor is only being used as a relay to perform the no-volt release function.

The momentary 'On' button carries (for a short time) all the surge current that the machine draws when it starts up, that in itself is not good practice but not as bad as leaving the live connected and opening the neutral line.

Ian P

Ian P04/12/2017 17:41:04
avatar
2299 forum posts
93 photos
Posted by Andrew Johnston on 04/12/2017 17:28:24:

It only tells part of the story.

The "switches" A & B are actually the contactor contacts. The NO and NC switches are momentary pushbuttons. So with power applied nothing happens until the NO button is pushed. Then the contactor closes A & B, and the machine is powered. The NO button can be let go as the contactor is now powered via the NC button. To turn the machine off the NC button is pushed. If, while the machine is running, power is disconnected the contactor will open. When power is reapplied the contactor will remain open, and the machine will not start. Which is what you want.

The implication is that mains isolator is elsewhere either on the machine, but is not shown. This is more akin to industrial practise.

Confusing for a beginner? smile o

Andrew

Edit: Ian P - this is a standard industrial setup. It is how most of my machines are wired, via a contactor and momentary pushbuttons.

Edited By Andrew Johnston on 04/12/2017 17:32:02

I have machines wired up with contactors and momentary switches, but not like that.

The control switches do not normally handle the motor current, that is what the contactor is for.

Ian P

SillyOldDuffer04/12/2017 17:44:53
5127 forum posts
1073 photos

Thanks Thor and Martin : I misread Fig 2 big time! I thought the NO/NC were the contactor contacts and A/B the ON-OFF switch. Good job I wasn't wiring it up.

Dave

JasonB04/12/2017 17:47:46
avatar
Moderator
17052 forum posts
1828 photos
1 articles

I think it is the way it is drawn

 

When NO button is pressed A & B close so the motor draws from the live top left straight through contactor contact "A" through the motor and then back out through Contactor contact "B" to Neutral Bottom left.

Switching current goes from live through NO and activates the contactor and then back out to neutral so does not see full load

Edited By JasonB on 04/12/2017 17:48:49

Journeyman04/12/2017 17:48:00
avatar
677 forum posts
108 photos
Posted by SillyOldDuffer on 04/12/2017 17:44:53:

... I thought the NO/NC were the contactor contacts and A/B the ON-OFF switch. Good job I wasn't wiring it up.

Dave

Problem is that it is only part of the circuit and doesn't appear to use standard circuit symbols.

John

Edited By Journeyman on 04/12/2017 17:49:11

Martin 10004/12/2017 18:24:19
252 forum posts
6 photos

Posted by Andrew Johnston on 04/12/2017 17:28:24:

this is a standard industrial setup.

I beg to differ. Good (aka sane) practice would have the contactor coil energised from live by just a push button switch (start) , the latching action by a dedicated normally open contact on the contactor (either a dedicated main contact or an auxilliary contact) connected across it and the release (stop) by another push button switch that is normally closed with an emergency stop (also normally closed) in series from the other side of the contactor coil back to the other supply leg.

The load current for the motor goes from the supply, through the main contactor contacts to the final load and nowhere else.

Sharing the contacts on the contactor between the final load and the control is questionable and 'cheap' but wiring the control circuitry such that ANY final load current passes through the start button, even for a short period until the contactor latches, is beyond ridiculous to suggest as being "a standard industrial setup"

If you are switching a single phase load then three normally open contacts on the contactor are sufficient, (live, neutral and 'control'  ) for a three phase load then you'll need four nomally open contacts (three for the phases and one for the 'control'  )

Also just using a contactor without any motor overload relay is just plain cheap, many connect direct to off the shelf contactors, cost next to nothing and should, if set correctly protect your motor from overheating.

But single pole switches, even when wired correctly are single points of failure and for emergency stop functionality may, by themselves, not fully satisfy the requirements in an industrial installation.

(spelling!)

Edited By Martin 100 on 04/12/2017 18:37:17

Emgee04/12/2017 18:24:24
1346 forum posts
212 photos

Contacts at A are the control circuit Hold contacts but they are also switching motor FLC, the contacts at B are not required as the neutral from the coil can be direct connected.
IMHO the circuit as drawn is not only misleading it is incorrectly drawn.
The idea of a contactor is for no voltage release if power fails, also to switch the motor power through contacts that are rated to safely pass the motor FLC,
Tthe control cct is switched via NO contacts which close when the contactor is energised, the control cct will also be switched off by the current overload being operated, there is no O/L shown in the cct drawn.

Emgee

Martin pipped me to it by a few seconds.

Edited By Emgee on 04/12/2017 18:26:19

john swift 104/12/2017 18:57:49
avatar
318 forum posts
183 photos

While the circuit shown in M.E will function

me4575  page 832   fig 2.jpg

I would not use it because the Live supply is connected to the machine when you press the START button before the Neutral !!!

 

this is what I would of expected 

using two main contacts to switch the Live & Neutral to the machine and a low current N/O auxiliary to maintain the supply to the coil

 

revised   stop  start circuit.jpg

with this circuit both the Live & Neutral will be connected to the machine at the same time

 

John

 

 

 

Edited By john swift 1 on 04/12/2017 19:01:22

Neil Wyatt04/12/2017 19:52:46
avatar
Moderator
17063 forum posts
690 photos
76 articles
Posted by john swift 1 on 04/12/2017 18:57:49:

revised   stop  start circuit.jpg

With a 2-pole contactor I would have one wired as 'A' and the other as 'Aux'.

Things like drill presses often use a single pole contactor, wired as the published diagram with just switch 'A'.

N.

john swift 104/12/2017 20:35:39
avatar
318 forum posts
183 photos

my second diagram is correct when your using the contactor in figure 5 with 4 N/O contacts

figure 5.jpg

 

if your using a contactor with only 2 N/O contacts 

this diagram goes with Neil's

post

revised  me  stop start circuit using  2 contacts.jpg

John

Edited By john swift 1 on 04/12/2017 20:36:36

Nicholas Farr04/12/2017 21:54:53
avatar
2067 forum posts
995 photos

Hi, I agree that the circuit will work, but it is not drawn very clearly to my mined and like John Swift 1, I would not use it. John's circuit is basically the same as one that I drew up and used for a small sensitive bench drill I made up 10 years ago. Mine has a couple of extra safe guards in the circuit and uses a three pole relay. While I don't claim it to be using all the standard symbols, I think it is clearly understandable.

nvr01.jpg

drill07.jpg

Works OK and is fairly compact and was not very expensive.

Regards Nick.

Edited By Nicholas Farr on 04/12/2017 21:59:10

Brian Sweeting04/12/2017 22:21:22
398 forum posts
1 photos

Looking at Fig 2 in the original post I would consider it dangerous because even in the 'off' position there is still the potential for a voltage to reach the motor.

Although the neutral should be at 0 volts a fault upstream would feed through the relay coil to the live feed to the motor or your fingers if you are during some wiring mods at the time.

Brian G05/12/2017 08:45:28
650 forum posts
25 photos
Posted by Brian Sweeting on 04/12/2017 22:21:22:

Looking at Fig 2 in the original post I would consider it dangerous because even in the 'off' position there is still the potential for a voltage to reach the motor.

Although the neutral should be at 0 volts a fault upstream would feed through the relay coil to the live feed to the motor or your fingers if you are during some wiring mods at the time.

Isn't that what the isolator is for? I wouldn't like to work on anything only separated from the mains by a contactor - if the neutral breaks correctly but the live contacts have welded, the motor would be live even though it had stopped. (Although this wouldn't happen in Fig.2 as the contactor is fed from the live contacts, so if the live contacts had welded the motor would re-start immediately the N/C button was released.)

Brian

Clive India05/12/2017 10:11:41
avatar
207 forum posts
Posted by Brian G on 05/12/2017 08:45:28:

........Isn't that what the isolator is for? I wouldn't like to work on anything only separated from the mains by a contactor - if the neutral breaks correctly but the live contacts have welded, the motor would be live even though it had stopped. (Although this wouldn't happen in Fig.2 as the contactor is fed from the live contacts, so if the live contacts had welded the motor would re-start immediately the N/C button was released.) Brian

Yes, agree, it is what the isolator is for.

Bikepete05/12/2017 10:34:48
227 forum posts
16 photos

(deleted as on reflection possibly not helpful)

Edited By Bikepete on 05/12/2017 11:22:15

All Topics | Latest Posts

Please login to post a reply.

Magazine Locator

Want the latest issue of Model Engineer or Model Engineers' Workshop? Use our magazine locator links to find your nearest stockist!

Find Model Engineer & Model Engineers' Workshop

Latest Forum Posts
Support Our Partners
Warco
emcomachinetools
Eccentric July 5 2018
Ausee.com.au
Allendale Electronics
ChesterUK
cowells
Eccentric Engineering
Subscription Offer

Latest "For Sale" Ads
Latest "Wanted" Ads
Get In Touch!

Do you want to contact the Model Engineer and Model Engineers' Workshop team?

You can contact us by phone, mail or email about the magazines including becoming a contributor, submitting reader's letters or making queries about articles. You can also get in touch about this website, advertising or other general issues.

Click THIS LINK for full contact details.

For subscription issues please see THIS LINK.

Digital Back Issues

Social Media online

'Like' us on Facebook
Follow us on Facebook

Follow us on Twitter
 Twitter Logo

Pin us on Pinterest