Another dirty Brian trick – reversing the chuck

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Another dirty Brian trick – reversing the chuck

This topic has 31 replies, 13 voices, and was last updated 29 June 2021 at 17:00 by Georgineer.

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25 June 2021 at 17:19 #551313
brian jones 11
Participant
@brianjones11
My treasured ML& didnt have its Dewhurst capstan switch. Monster thing can cost £75 used on EB. The roller blades would lose their spring temper and become lose with vibration

The cct meant to confuse those not skilled in the arts is actually very simple, it used a DPDT switch to change the polarity (ie direction) of the starter winding,

It must be a break before make design, in fact i got a 3 position with middle = off in the middle to be safe

So I got a chunky toggle switch rated 15A 250v from EB ca £3.50 and the rest was a bit of nifty wiring. Of course the caveat is dont try and change the state while the motor is moving and energised, it must be completely at rest and switched off first, (same applies to the Dewhurst)

i have a 1/2 horse motor 375W so starting current is ca 10A and it only momentary <1 sec. rest of the time the switch is not passing any current or doing any live switching (which is what wears out the contacts) so its quite a benign duty. just dont get it wet with mistic

Ive hardly ever used mine and have note got round to making a chuck clamp to stop the chuck flying off in reverse
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25 June 2021 at 17:19 #20347
brian jones 11
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@brianjones11
25 June 2021 at 18:32 #551328
noel shelley
Participant
@noelshelley55608
Your right, there are numerous ways to make a reversing switch IF you know what your doing ! The lack of knowledge of many in safely dealing with 240V power means trying to do this sort of thing is potentially dangerous. The dear old DEWHURST usually failed due to improper use – being used to stop and start the motor rather than being used to simply reverse the start windings. Noel.
25 June 2021 at 18:48 #551332
old mart
Participant
@oldmart
Myford lathes used to have only the Dewhurst switch, there was no alternative except the switch on the wall.
25 June 2021 at 18:54 #551336
Maurice Taylor
Participant
@mauricetaylor82093
Hi,A 1/2 hp Brook motor on an ML7 takes nearly 50A for 100mS on start ,this will sometimes trip a domestic mcb but probably wouldn’t trip an inductive load mcb ,as previously mentioned .An earth leakage fault would trip the elcb not the mcb.
25 June 2021 at 20:53 #551346
brian jones 11
Participant
@brianjones11
Wow rule of thumb is that motors use 6x steady current on no load start up. Where did you get your figures. surely this would blow a 13 A fuse
25 June 2021 at 21:06 #551349
Andrew Tinsley
Participant
@andrewtinsley63637
!3 amp fuses, as in plug tops are not quick blow. They will also pass a tad more than 13 amps on a steady basis, without blowing. Did some real world tests to check this, so don't throw specifications at me!

Andrew.
25 June 2021 at 21:48 #551354
Robert Atkinson 2
Participant
@robertatkinson2
A "13A" fuse will by specification carry 50A for at least 100ms (1/10 of a second) without fusing.

See the curve here

https://pat-testing-fss.blogspot.com/2014/02/whay-13-amp-fuse-does-not-seem-to-blow.html
25 June 2021 at 21:52 #551355
Anonymous
Posted by brian jones 11 on 25/06/2021 20:53:03:

………..surely this would blow a 13 A fuse

Maybe, maybe not. In the UK domestic plug fuses are to BS1362 and are normally fast or medium speed. The BS specification states that the fuse should carry 160% rated current for a minimum of 30 minutes, but should blow within 30 minutes at 190% of rated current.

Fuses are thermal devices; they rely on heating of the element to melt the element and break the circuit. The current and time needed are dependent on the exact resistance of the element (which is not well controlled) and the external environment.

Looking at the datasheet for a 13A UK domestic fuse it will blow at 4 times overload (52A) somewhere between ~15ms and ~800ms.

Andrew
25 June 2021 at 21:57 #551356
noel shelley
Participant
@noelshelley55608
Never mind6X ! The starting current in some starting configurations can be as high as 1200%. A 180w induction motor would stop DEAD a 2000w generator ! Yet same would drive a 1200w brush motor. A 13A fuse is a crude device thought cheap that has caused a few house fires and is a slow blow ! As per Andrew ! Noel.
25 June 2021 at 22:06 #551357
brian jones 11
Participant
@brianjones11
Agreed with your fuse breaking chart – its well established

the current of a 1/2hp motor is ca 3A, so the starting (no load) current should be say 20A and indeed a 13A plug can carry this without blowing for a long time

If you have figures to dispute this pls show source and reputable evidence

for now I am simply quoting the rule of thumb

for those who dont want to spend silly money what c/o switch would you suggest

here I am quote Brooks catalog

http://www.brookcrompton.com/upload/files/products/2227e_1phase_v1.1e.pdf

375kW cap start load current 4A, start current 4,1x load ie 16.4 A so I was being conservative and the 6x probably relates to old designs post war stuff. Its what I was taught as an apprentice

Pls quote sources not just opinions off the top of heads, its not helpful or credible.

Edited By brian jones 11 on 25/06/2021 22:26:19
25 June 2021 at 23:24 #551359
Emgee
Participant
@emgee
This discussion has shown the benefit of a DOL starter fitted with an overload device to prevent continued overcurrent to the motor until the 13A fuse ruptures.

Remember that of course the starting current will be increased dramatically if a load is present, such as if a lathe clutch is not used.

Emgee
25 June 2021 at 23:48 #551363
brian jones 11
Participant
@brianjones11
You mean the typical old MEM Motor starter. This has a built in thermal trip on prolonged overload. Early ones didnt have a no volt trip feature and should be changed out as the motor could start up again after a trip = dangerous.
26 June 2021 at 00:25 #551367
ALAN MOORE 5
Participant
@alanmoore5
I recently installed a circular saw with 3HP Capacitor start motor, 2.2 Kw, 10A. Finish installation, start motor, instantaneous trip of 16A breaker. Tried starting with extension lead from a different circuit, ran nicely.

Electrician said 'long lead, high resistance makes a difference'.. .. Anyway, I advised changing the breaker to 32A; cant have long trailing leads in a workshop!
26 June 2021 at 00:36 #551369
brian jones 11
Participant
@brianjones11
Erm so you had a 2,5mm2 wire connected around your workshop with all your outlets tapped off, protected by a 16A mcb

did you complete the cct and turn it into a ring main? which must complete directly at the dis board

I hope thats the case, or you have a dangerous fire hazard installation. your leccy should know this unless hes a cowboy

your insurance would be invalid
26 June 2021 at 06:50 #551377
not done it yet
Participant
@notdoneityet
Posted by Maurice Taylor on 25/06/2021 18:54:26:

Hi,A 1/2 hp Brook motor on an ML7 takes nearly 50A for 100mS on start ,this will sometimes trip a domestic mcb but probably wouldn’t trip an inductive load mcb ,as previously mentioned .An earth leakage fault would trip the elcb not the mcb.

I’m just a bit mystified of that graph/screenshot of AC current.

It would appear, from the y axis, that the current after half a second is approx 5A (likely close to 6?).

As it is peak current RMS current (equivalent to DC) would be 5A/root 2 = 5A/1.414 = 3.5(4)A. At 230V (RMS value of the grid) that equates to 813W If the motor is rated at 370W, it shows that the motor is only 46% efficient.

That would be 370W at full load, not at idle – like these figures may actually be (motor achieves full speed in only slightly over 1/4 of a second).

I am wondering what the current (and therefore power) would be at full motor load?

Now, if the peak current was actually 6A and the typical UK grid voltage is 240V (often it exceeds that) the input Wattage would be in excess of 1000 which would indicate a motor efficiency of around only 1/3rd.

I reckon that 1/2HP at, say 0.8 Power Factor and 70% efficiency, would draw only 660W (including the Wattless power) at full load.

Perhaps the power factor is making that great discrepancy? Anyone out there explain, please?

Re stopping a much larger generator – I expect the generator would be a stator wound machine with a small electronic AVR. An armature wound (slip ring) genny would easily cope with such transient spikes, wheras the new fangled things are pretty hopeless on that score.
26 June 2021 at 14:54 #551459
Robert Atkinson 2
Participant
@robertatkinson2
The high running current is due to the very poor power factor of an induction motor running off-load.
26 June 2021 at 16:35 #551473
Dave Halford
Participant
@davehalford22513
Posted by ALAN MOORE 5 on 26/06/2021 00:25:01:

I recently installed a circular saw with 3HP Capacitor start motor, 2.2 Kw, 10A. Finish installation, start motor, instantaneous trip of 16A breaker. Tried starting with extension lead from a different circuit, ran nicely.

Electrician said 'long lead, high resistance makes a difference'.. .. Anyway, I advised changing the breaker to 32A; cant have long trailing leads in a workshop!

That might depend on the breaker type fitted, a slower C type might survive the current inrush.
26 June 2021 at 17:01 #551476
ALAN MOORE 5
Participant
@alanmoore5
'did you complete the cct and turn it into a ring main? which must complete directly at the dis board '

Thanks, Brian. Yes it was a complete ring main, though this was not my own workshop (where I also have a complete ring main) but I was installing the machine for a friend.

Emgee, So far as I am aware the term 'DOL starter' properly referrs to a 3phase starter.
27 June 2021 at 17:10 #551623
brian jones 11
Participant
@brianjones11
Good show AM

One thing Im going to be sure never to raise on this board, is anything to do with electrics. The armchair opinination Ive seen spouted herein – well life's too short.

your right about DOL as opposed to star delta (and soft start VFD etc)

A little knowledge and a smattering of buzz words = plausible conjecture aka BS

I wish I'd never raised the topic

I recommend the Mod close this thread – preferably delete it
27 June 2021 at 21:20 #551652
old mart
Participant
@oldmart
Back nearer the topic, what do you want to reverse the spindle for? I have left hand boring bars and singlepoint threading bars which have justified a lock for the Smart & Brown model A, but the conversion was deemed much too sophisticated for anybody to copy when I posted a thread. 4 three jaw chucks, 1 four jaw and one er25 collet plate can be locked.
27 June 2021 at 21:22 #551653
Anonymous
Posted by brian jones 11 on 27/06/2021 17:10:09:

A little knowledge and a smattering of buzz words = plausible conjecture aka BS

Oh my word, we've failed to genuflect in the presence of genius.

The OP isn't the first new member to start by mouthing off about the stupidity of other forum members, and I don't suppose he'll be the last. Experience indicates that they normally calm down, or quickly disappear.

Andrew
28 June 2021 at 18:03 #551729
Georgineer
Participant
@georgineer
Posted by old mart on 27/06/2021 21:20:10:

Back nearer the topic, what do you want to reverse the spindle for? I have left hand boring bars and singlepoint threading bars which have justified a lock for the Smart & Brown model A, but the conversion was deemed much too sophisticated for anybody to copy when I posted a thread. 4 three jaw chucks, 1 four jaw and one er25 collet plate can be locked.

This sounds interesting Mart. Can I take the idle approach and ask you where the thread is to be found? (The alternative is to search for it, which might take me for ever.)

As for the matter of reversing the motor, I use an 'intermediate' light switch which cost a couple of pounds from an electrical wholesaler. It's rated at 10A and is never switched while the motor is running. Well, it was once by my carelessness, but the motor didn't even notice.

George B.
28 June 2021 at 18:14 #551730
brian jones 11
Participant
@brianjones11
Well done george, my point entirely. I think 10 A is a bit tight as the start current is likely 16A momentarily, I use a 20A DPDT 3 way (centre off) toggle switch. Its only meant for dead ccts use. It is NOT a cct breaker. Your m/c wouldnt notice the difference when you accidentally switched the c/o while running because the motor centrifugal switch would have disconnected the start winding. However I strongly advise against such fooling around. After all you would drive your automatic car at 70mph and then throw it in reverse. Clowns do it and ruptured gearboxes dramatically
28 June 2021 at 19:53 #551742
old mart
Participant
@oldmart
Posted by Georgineer on 28/06/2021 18:03:58:

Posted by old mart on 27/06/2021 21:20:10:

Back nearer the topic, what do you want to reverse the spindle for? I have left hand boring bars and singlepoint threading bars which have justified a lock for the Smart & Brown model A, but the conversion was deemed much too sophisticated for anybody to copy when I posted a thread. 4 three jaw chucks, 1 four jaw and one er25 collet plate can be locked.

This sounds interesting Mart. Can I take the idle approach and ask you where the thread is to be found? (The alternative is to search for it, which might take me for ever.)

As for the matter of reversing the motor, I use an 'intermediate' light switch which cost a couple of pounds from an electrical wholesaler. It's rated at 10A and is never switched while the motor is running. Well, it was once by my carelessness, but the motor didn't even notice.

George B.

I cannot find the thread, but if you can access my album, the basic pictures are from the 3rd to the 8th.
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