new forward stop reverse swtich

Hello everyone

I would like some help/advice please.

I recently purchased a second hand PMD milling machine. It need some tending loving care to bring it back to life.

However here lies the problem - I think the forward stop reverse switch was replaced sometime ago. It is a mechanical drum switch... not the Dewhurst model. It is rated at 15amps, the motor is a 1.1Kw at 8.7 amps running on 240 volts

The switch at times trips the RCD in the main fuse box. I have the milling machine on its own circuit on a 32amp MCB. I think the switch is at fault and would like to replace it with a new one but not a drum switch.

.I wondered if the switch needs to be 15amp rated and can I replace it with a lower amp e.g. 10amp and the push button type.

Any help would be great as the misses complains when the TV goes off.

Thanks

Colin

Hi Colin, your question doesn't have a simple and straightforward answer, but here goes.

The short answer is yes, the 10 amp switch will work OK, it'll stand the use for a time period which might be quite difficult to predict, but it's unlikely to go snap crackle pop immediately, so give it a try and see how you get on.

BUT>>>> (number 1)

Switch gear for controlling motors is rated to allow for the inrush current. When you start a motor, it accelerates from stand still very quickly, and while it is doing so it will take (if the supply will provide it) typically 6 - 9 times the "rated (full load) current". The rated current is the current while providing the full load torque relating to the power of the motor. Power equals torque times speed, so the rated current shown on the motor rating plate assumes that the motor has already accelerated up to the speed at which it runs at the corresponding load torque, But the switchgear has got to survive the shock of getting the motor onto its load curve.

This inrush current is brief, but severe. Most drum or cam switches are designed for switching a motor, so they have a rating for what is called AC3 duty - this is a number that allows for some de-rating of the switch. The start duty of using it upstream of a motor is only a brief torture, most of its life the switch will only have to carry no more than the rated motor current. But while it is carrying that start inrush current it has a hard life, and the "rated current" quoted for the switch has to allow for this. There is also something called AC1 duty - this is closer to the simple use of the switch to control a resistive (e.g. heating) load and will be a higher number that the AC3 figure specified. So which duty is the "15 amps Dewhurst switch"? Well, it is almost certainly an AC3 figure, because a Dewhurst switch's reason for being is to control a motor, but when you buy a 10 amp switch you need to be clear what the 10 amp figure actually represents. If it's a 10 amp push button switch, and the 10 amps is an AC1 duty then it is probably (guessing) only a 6 amp AC3 motor duty rated switch and the magic smoke will escape if you connect it to your 8.7 amp rated motor.

There is a complication here that the concept of AC3 only applies to 3 phase switchgear, and single phase motors are a quite different beast to start, but that's too complicated for this time of night!

Now, let's develop this a bit further. You have mentioned a push button type switch - and this also affects the answer. If your plan is to fit a forward/reverse selector switch AND a push button ON/OFF switch we're into a different can of worms. If the drum/cam forward/reverse selector switch is only ever going to connect the motor wires together without them being energised, and the switch of the inrush (starting current) is done elsewhere then you can over-rate the selector switch significantly. Nobody's going to commit to a definitive statement how greedy you can be, but your 10 amp switch controlling a 8.7 amp motor will be fine. The motor rated current is below the rated current of the selector switch, so all is well. The push button starter now has to make the motor inrush current, and break the motor voltage to stop it again, while the selector switch just has to carry the motor load current. It's the making and breaking reliably which is the arduous duty, by comparison any contacts carrying the motor current which don't have to make or break the circuit and carry current at the same time have an easier life.

However I'm bound to muse on "BUT" number two. My experience is that it is unlikely to be the switchgear that causes an RCD to trip, it is much more likely to be a fault within the motor. The current and voltage figures you quote say this is a single phase motor, and that rings alarm bells straight away. Single phase motors have a capacitor that'll happily go fault to earth, and if that's not the problem the centrifugal switch inside the motor which controls the start winding (if there is one) has a REALLY hard life, and splatters conducting metal vapour over its insulating mounting plate. So what leads you to believe that the drum switch is the faulty component? Has the switchgear been re-wired recently - and if so has the problem only arrived since something was changed? Have you had the insulation of the motor checked - it's a very easy and quick measurement for someone to do with an insulation tester that could stop you spending money on new stuff which doesn't fix the problem. My suggestion is to take it to a rewinder and get them to check the motor over before spending money on switchery. But maybe you already tried that.

Yada yada yada. Like I said, it's a simple enough question, but the answer is anything but!

But I sympathise with the aggro from SWMBO - I had to feed my shed with a separate RCD circuit for all the same reasons, though that was caused by the leakage currents of VSD's.

Hope this helps

Simon

I had a similar problem with my old S7 drum switch a few months back. Occasionaly when starting it would trip the earth leakage breaker. It suddenly got to the point that it would trip every time when trying to switch to forward, but in reverse it was OK.

After puzzling about it I decided to pull the motor and take a look.

Turned out moisture - read thrown about cutting fluid - had gotten into the motor housing and was causing a partial short circuit. The reason why it didn't trip in reverse was the reverse winding connection put the capacitor in the neutral leg and not the line leg. The rubber wiring had perished and was leaking to ground. Renewing the wiring and plugging the decayed rubber grommets solved the problem.

Hi to both of you that replied.

Sorry I did not reply last night. Went to bed early, had a hard day, too tired to keep awake for much longer.

The milling machine was kept in a shed and stood unused for at least 2 years. I did switch it on before buying it and it all appeared to be ok. It is now homed in my centrally heated utility room which I managed to persuade the "boss" to allow me turn part of it into my workshop area. The joys of not having to walk outdoors to a wooden shed like I used to have to before we moved into our new house.

Thanks for the detailed explanation.I wondered if it was the motor.

I don't believe the motor has been taken apart.

It's a Leroy Somer motor. On the motor plate it states;

120v, 1.1kW, 17.4A

240v, 1.1kW, 8.7A

It has two capacitors each rated for 120V. This leads me to believe the motor is dual voltage? therefore could I replace the two capacitors for a single 240 rated capacitor?

Or would it still be best to have the motor checked out?

Thanks Colin

Good morning Colin

Two capacitors probably indicates a start capacitor and a run capacitor, so they do different things. Not sure about the apparent anomaly with their rated voltage without knowing some more about the internal windings, but if it's never been apart it's probably right. So I think you need a motor rewind house to check over the motor for you. Once you know the answer to whether the motor is actually serviceable you can plan from there. Notwithstanding the new warm and heated environment you now have (you lucky devil) damp or debris in the windings of some kind is always your first suspicion. Unfortunately , whatever anyone else says about "cook it and it'll be OK" once the internal wiring has flashed down to the case it'll never be right. Good luck with it, keep us posted. Rgds Simon

A lot of drum switches are not intended to go from forward to reverse under power. The off on mine isn't quite off and blows the fuse

Most drums can be dismantled and sorted out.

Dont think most push button controllers will replace a drum switch. Polarites need reversing.

The motor mentioned is a dual-voltage type, which is fairly common in the USA. For 230V the windings are connected in series and for 115V they are connected in parallel. So it needs a capacitor for each start winding.

If you want to only run off one supply voltage then the best option is to change the motor for a suitable type to suit your supply mains. Check on the motor speed rating, if it is a 4-pole motor it will run at about 1450 rpm and if a 2-pole it will run at about 3,000 rpm, these being the 60 Hz values. 50 Hz is a little lower but not normally a problem for us.

hi all

Thanks for your replies.

It looks like I have 3 options

1. have the motor checked out

2. Swap the motor for a new one and loose the reverse

3. strip and clean the drum switch.

Plenty to sort out.

Although the machine is old it is in good nick and very robust, better than the Clarke mill/drill which the plastic gears smashed and the lever changing from low to high revs did not always engage as it should when the motor had stopped.

Again thanks for all you help.

Colin

When I got it my Myford 254 had single phase motor and three control buttons, fwd, rev, stop. They were connected to a single contactor. No idea how it worked, but it wasn't Myford's own. When I changed to 3 phase I sold it, but if it is of any interest I can dig out the manual and see if there is a part number.

However, if you're faced with a new motor I'd seriously consider 3 phase and VFD, so much smoother, and even on a milling machine having speed control is great.

I agree with Duncan - once you find yourself replacing the motor it's VSD and three phase motor every time. Not for the variable speed so much, but for a smooth speed ramp up and a controllable stop. Most VSD's will provide DC braking which is now mandatory in industry, and a Very Good Idea for us chaps (and chapesses) in sheds. And the controllable speed is a bonus.

BUT>>>

Not all three phase motors are VSD compatible. Quite apart from the issues over choosing a motor which can be connected in delta and is suitable for use at 230 volts supply, there is much complication in the internals of the motor to permit successful torque generation at frequencies other than 50 Hz. Some cheap (and some not so cheap!) motors are effectively useless as you depart from their nominal design frequency. All three phase motors are not created equal, so it's worth buying a motor and VSD combination from a reputable supplier. I've seen threads here recommending such.

You can buy VSD's which will generate three phase 415 volts, but that's a whole lot more expensive than doing it the simple way with a 230 volts compatible motor.

And just for my naked nosiness, what model is the mill?

Rgds Simon

Hello Simon

The Milling machine is a Pinnacle PMD30 and the motor plate is stamped 1991 so I assume the machine was made around that time. II appears to be no longer produced but Axminster Tools sells the same model ​- Axminster Engineer Series ZX30 Mill/Drill with a 1.5kw induction motor 240v and no reverse.

I could, because of its age just go all out and buy a new 1.5kw single phase 2pole motor and new switch to bring it up to current safety standards but with this idea I would loose reverse.

As I bought it second hand at little cost with various other bits and bobs thrown into the deal such as a surface plate, kennedy hacksaw, milling tools and a 6inch rotatory table - it may be a worthwhile investment.

Coin

Hi Colin, that's a very handy bit of kit, especially if the price was right. I've got one myself, albeit with a different badge on the front.

You might like to look at another thread which relates to that same family of machines, being

"RF-25 Belt-sizes and Table Stop Info Please"

Sorry but I haven't worked out how to copy the URL for the link to this thread, but if you look in Latest Posts you'll find it.

Rgds Simon

Collin, would it not be a 4 pole motor (1540rpm), or is the old motor a 2800 rpm/2 pole type?

Ian S C

.

Allow me ...

**LINK**

MichaelG.

Hi

For Ian S C - I am not sure if it's a 2 pole or not, is there a way of telling without a spec sheet?

It has 12 spindle speeds from 125 - 2500 if that helps.

For Simon - I like the idea of the screw operated belt tensioner...next improvement!

Colin

Hi

Further update. I have found a company motors-direct.co.uk selling a forward stop reverse switch for £64 - what do you think?

Colin

Michael - Thank you for the magic link button.

Colin - I think you're wasting your money until you've had someone check the motor.

If you are going to change the motor there are a couple of things you need to know. Although my machine is a lot older, and isn't the same make, (but I bet its very similar) I found that the motor was a non standard one when I came to swap it for a three phase unit. The motor shaft was a non standard size (22 mm, standard motor is 19 mm shaft) and also was longer than standard. The motor shaft carries a 4 groove cone pulley, and the pulley sits over the end of the shaft with two grubscrews (yuck!) holding it in place. At least it had a keyway!. The smallest pulley for the slowest speeds is smaller than the shaft diameter, so the pulley cone overhangs the end of the shaft. I made a new pulley to suit an IEC standard 19 mm dia motor shaft for a 80 frame 3 phase 4 pole motor. Oh, and the frame size of the original motor was an oddity as well (from memory 85 mm).

I bought a standard cast iron taper lock pulley with a single groove on it corresponding to the biggest diameter pulley closest to the motor, and turned a register in the upper face of the cast iron. I then turned a piece of aluminium up to replicate the other diameters. The aluminium is stacked on top of the face of the taper lock pulley, with a radial face key to transmit the drive. The two parts are clamped together with a central M6 allen cap head screw though the centre, threaded into the end of the motor shaft. The M6 screw doesn't carry the drive torque, it just holds the two parts of the composite cone pulley together.

The stuff about two pole (which I really don't believe it can be) or 4 pole doesn't really matter. Just buy a motor with approximately the same speed figure on the rating plate as the original. I'll bet you pretty much anything that it says something like 1420 rpm full load speed on a 50 Hz supply (could be anywhere between 1400 and 1470) - this is a four pole motor. It can't be a two pole motor (full load speed somewhere in the 2800 - 2900 range) because you'd need an enormous reduction ratio to get down to your 125 rpm bottom speed. Big reduction ratio means big pulleys driven by really small pulleys. Also if you had a two pole motor your top speed would be a lot higher (3500 rpm - maybe more) just from the geometry of how the belts and pulleys have to be configured.

There's much to be said I reckon for buying a slower motor than the original (6 pole motor running at approx. 1000 rpm) and running the whole thing more slowly, but this isn't an option on single phase.

HTH Rgds Simon

Edited By Simon Williams 3 on 19/02/2016 19:59:14

6 pole 1ph motors are made, but rare, hence expensive, a 4 pole 1450 rpm motor is the general purpose motor for the workshop machinery. 2 pole motors suffer from low torque, so poor starting, although some machines, Myford Tri lever is one that uses a motor that combines 2 and 4 poles to double the number of speeds available.

Ian S C

Ian - that's interesting about the Tri-Leva with a two speed motor. I've met two speed three phase motors on cranes for example, but I didn't know you could do the same with a single phase motor. My mind is slightly boggled with how the starter winding gets in there somehow.

Rgds

Simon

Hi all

I did strip down an clean the contacts within the switch as my first option but it still tripped the RCD.

Interestingly SW mentioned the RF25mill which I googled and it has exactly the same switch as on mine which I thought the one on my mill was a replacement switch.

Better start flicking through the yellow pages to have the motor looked at or a replacement motor and new switch.

Having a closer look on the motor plate it runs at 1500rpm on 50Hz

Colin