Are the Electrics Right in ME4575?

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?

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

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

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.

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

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?

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 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

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

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

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

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

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

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

While the circuit shown in M.E will function

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

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

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.

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

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

this diagram goes with Neil's

post

John

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

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.

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

Regards Nick.

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

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

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

(deleted as on reflection possibly not helpful)

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