Another dirty Brian trick - reversing the chuck

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

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.

Myford lathes used to have only the Dewhurst switch, there was no alternative except the switch on the wall.

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.

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

!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.

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

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

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.

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

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

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.

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!

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

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.

The high running current is due to the very poor power factor of an induction motor running off-load.

That might depend on the breaker type fitted, a slower C type might survive the current inrush.

'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.

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

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.