Motor control board

[Keith MathesonParticipant @keithmatheson47708]

  After  6 years of happy usage with my mill (Same as the wm18) I fear I could be about to be on my third control board -aaarrrgggghhhhhhh.

Display lights up but nothing happens when the pot is turned. When switching on it makes my LED batten lights flicker – I don’t recall that happening before. It also hums when switched on but it may have done that before and I ignored it. The last one cost over £100 and unhappy I may need to do this again. I did find a you tube video where someone claimed the diodes often go. I did find a source for a few quid so will try replacing these on the current and old board and see what happens. It also did that hunting business whilst cutting 18mm into steel for about 5 minutes of use before giving up.. I know there are lots of very knowledgeable people on this site and would appreciate any wise words as I’m getting pretty fed up with this. Many thanks in advance.

Edited By Keith Matheson on 07/03/2021 19:52:04

Edited By Keith Matheson on 07/03/2021 19:53:31

[Keith MathesonParticipant @keithmatheson47708]

Here we go again!

[John BaronParticipant @johnbaron31275]

Hi Keith,

Before doing anything else check the motor brushes ! These very commonly give problems, sometimes damaging the control board.

I've just made a pair of new carbon brushes for mine simply because it stopped rotating when it shouldn't have.

The removed brushes !

The springs.

Note that this is the remains of the previous pair of brushes that I have made and the springs are ones that replaced the previous ones.

There are a number of issues with these motors all mainly due to getting hot !

 

Edited By John Baron on 07/03/2021 20:27:31

[Keith MathesonParticipant @keithmatheson47708]

Thanks John, good suggestion. I know my lathe is definitely brushless but no idea on the mill. I will check tomorrow. If it is, that’s an easy fix

best regards

Keith MM

[Keith MathesonParticipant @keithmatheson47708]

Thinking on John with regard to your comments on getting hot. I did hammer the mill in a way I have never done be for the day previous. I was cutting 20 grooves in aluminium with a 4mm cutter on a rotary table. I’ve never had it running so fast for so long in all the time I’ve had it. I did notice the whole head had got uncharacteristically so had to have a break. After 6 years of normal hobbyist use maybe this marathon session finally finished off the brushes? I will be soooooo pleased if it is that. As an aside where did you get the material to make the new brushes?

best regards

keith Mm

[noel shelleyParticipant @noelshelley55608]

No little copper wire down the middle of the spring to carry the load ? It might be worth wipping the mosfets or switching devices of the heat sink and checking them. Nothing ventured nothing gained ? Noel

[Anonymous]

It doesn't look like a brushless controller to me. The board looks a little rough on the upper left side of the two preset pots top left – almost as if something has leaked out? There are also several light brown areas on the right side of the board. Are these real or a trick of the light?

Andrew

[John BaronParticipant @johnbaron31275]

Posted by noel shelley on 07/03/2021 21:04:38:

No little copper wire down the middle of the spring to carry the load ? It might be worth wipping the mosfets or switching devices of the heat sink and checking them. Nothing ventured nothing gained ? Noel

Hi Noel, Guys,

No ! The little pigtails don't exist on my home brew brushes, they did on the originals, but they simply fell out allowing the motor current to destroy the springs.

[John BaronParticipant @johnbaron31275]

Hi Keith,

Good question ! Those big brushes were given to me by a motor re-winders in Sheffield quite a while ago. They came out of their scrap bin. I haven't a clue what they would have belonged to, but they are enormous compared to the ones that I make from them.

The one on the right is the one that I've made, it also requires cutting in half. That big brush is 50 mm by 32 mm by 6 mm thick. Manufactured by Morganite I believe. The brushes in the mill motor are 3.6 mm thick by 10 mm wide and 15 mm long. Everybody I've tried for replacements wants to sell me an expensive motor costing an arm and a leg !

[oldveloParticipant @oldvelo]

Hi

" It also did that hunting business whilst cutting 18mm into steel for about 5 minutes of use before giving up "

The setup of the Current Limit and Intensity Regulator pots are not adjused properly.

KB Controls is an excellent source of information on DC Motors. Scroll to Page 27 for correct setup

Control Board Setup

Eric

[not done it yetParticipant @notdoneityet]

I don’t know whether it is appropriate, but perhaps a read through THIS thread might be enlightening?

https://www.model-engineer.co.uk/forums/postings.asp?th=149361&p=1

Edited By not done it yet on 08/03/2021 18:11:56

[Keith MathesonParticipant @keithmatheson47708]

Thanks for all the suggestions and guidance so far. Gratefully received. It makes problem solving soooooooo much easier and less time consuming – and hopefully less expensive. Anyway, onto current matters. I took the brushes out of the motor and found one broken in two (with the associated wire unattached) and the second looking somewhat short. I have found the same cross sectional area brushes on evil bay so have ordered a pair. I will report back what happens (I do so much hate a thread that doesn’t reach some sort of resolution)! I will also pop the new diodes into the previously died board and see if I now have a spare. Thanks again. keith MM ps like the idea of a pc fan attached to the motor housing- will do this as a sensible addition.

[not done it yetParticipant @notdoneityet]

Do get the right brush hardness. DC and AC brushes are not usually of the same material.

[John BaronParticipant @johnbaron31275]

Posted by Keith Matheson on 08/03/2021 21:15:20:

Thanks for all the suggestions and guidance so far. Gratefully received. It makes problem solving soooooooo much easier and less time consuming – and hopefully less expensive. Anyway, onto current matters. I took the brushes out of the motor and found one broken in two (with the associated wire unattached) and the second looking somewhat short. I have found the same cross sectional area brushes on evil bay so have ordered a pair. I will report back what happens (I do so much hate a thread that doesn’t reach some sort of resolution)! I will also pop the new diodes into the previously died board and see if I now have a spare. Thanks again. keith MM ps like the idea of a pc fan attached to the motor housing- will do this as a sensible addition.

Hi Keith, Guys,

Thanks for the picture, that is what I expected to see ! just curious, did you measure the dimensions of yours ? I wonder weather they are the same size.

As far as the brush hardness is concerned there is no difference between AC and DC brushes, as seen in universal motor use. There are however quite marked different grades and composition of brushes intended for different applications. For instance, some brushes are copper loaded and some are very soft, like the carbon material that I am using for making my replacements.

I'm not saying that the stuff I'm using is correct, but it works for me. The wear on the brush is basically due to two causes, brush pressure and arcing. Higher pressure on the brush reduces arcing but increases wear hence the reason for harder brush material.

I would remove the brush cap and inspect the commutator for wear and blow out any dust. Inspect the wiring to the brush holders for any heating damage, particularly loose push on fix connectors. Any looseness will cause heating leading to failure of the insulation of the brush holder.

DO NOT attempt to remove the armature !

Apart from the fact that it will be very difficult, it will cause the field magnets to loose their strength.

HTH.

[not done it yetParticipant @notdoneityet]

the brush hardness is concerned there is no difference between AC and DC brushes, as seen in universal motor use

You may know better than I, but on the two occasions (over the years) – when I have had need to obtain non-standard brushes – from CBS Tools at Peterborough – the fellow there has asked me whether these brushes were for a DC or AC motor. I only enquired ‘why?’ on the first occasion. The reply was that brushes for AC motors used a harder carbon grade than those for DC motors. Can’t say more than I accepted his word for it – he was my expert on those occasions as he was the main repair man from the back room repair section, so should have known.

Edited By not done it yet on 09/03/2021 09:12:27

[noel shelleyParticipant @noelshelley55608]

If your up the creek and you've broken the paddle, one source of brushes that might work is to cut down Lucas dynamo brushes, 22amp ones. Most old garages would have kept them as spares. H.C. Cargo, will almost for sure have them. Noel

[MacolmParticipant @macolm]

The brush material resistivity is mainly a function of the supply voltage. A brush bridges two commutator segments so partially shorting the relevant winding, but theoretically at the commutation point there should be no coupled flux. However nothing is perfect.

I suspect the “AC or DC" question was really “mains or battery”. Certainly, some heavy portable tools like angle grinders have different brushes for 240v and 110v versions.

[V8EngParticipant @v8eng]

You could try these people for carbon brushes.

Brushes

Never actually used them but been in my favourites page for ages.

[Tim ChambersParticipant @timchambers76147]

I bought brushes for my 210 lathe from Amadeal for a fiver just a couple of weeks ago, turns out it was the control board had released the magic smoke. I've now gone brushless with a kit from Aliexpress.

[John BaronParticipant @johnbaron31275]

Posted by Macolm on 09/03/2021 10:30:35:

The brush material resistivity is mainly a function of the supply voltage.

No it isn't ! In fact it has absolutely nothing to do with voltage. Material resistivity is a function of the material ! It is a measurement of conductivity, the ability to conduct electricity.

 

Edited By John Baron on 09/03/2021 14:54:09

[SamsarandaParticipant @samsaranda]

Just as an aside, to prevent the motor on my Champion 20 mill from overheating when pushing it hard, I have fitted two small 12 volt computer type fans in the motor shroud because air wasn’t circulating around the motor and it was getting very hot. The computer fans are driven by a small transformer used for driving led lights which gives the required 12 volts. No overheating problems since. Dave W

[MacolmParticipant @macolm]

Perhaps I should have made it clearer – “The choice of brush material resistivity is mainly a function of the supply voltage”. To make it clearer yet, it is down to how it affects allowable current density in the brushes, so for similar size motors, the current will increase as the supply voltage decreases. This is why battery tools will usually have copper loaded graphite brushes, whereas small mains universal motors will use straight carbon/graphite.

In practice, choice of best brush composition is much more complicated. There is plenty on the Internet about this, but mostly oriented towards large machines rather than typical diy motors. If cutting down brushes to a smaller size, selecting them with the same supply voltage is a way to reduce the risk of problems.

[John BaronParticipant @johnbaron31275]

Hi Malcolm, Guys,

Resistivity "resistance" is a constant ! It does not vary with voltage. There are however resistive devices that increase or decrease in resistance with temperature.

For any given resistance the current through it will increase with voltage.

I do agree that the current drawn by a motor tends to increase as the motor load increases. Motor brushes are very low resistance devices, fractions of an Ohm in most cases, loading them with copper increases the hardness of the brush, in turn improving the wear resistance, usually at the expense of the copper segments of the commutator. Actually the arcing that one often gets at the brush/commutator interface can wear the brush as much as just the friction between them.

But as you imply quite a complex subject, much of which has been alleviated with the advent of brushless DC motors.

[MacolmParticipant @macolm]

Agreed that the brush resistance is (necessarily) very low. However, the brushes inevitably short circuit the commutator segments as they bridge them, though there should be very little back emf between them at the point of commutation. Choice of brush material is thus a compromise between the series resistance introduced in the main current path, and the losses due to any residual circulating short circuit current (which higher brush resistance would reduce).

If the motor were rewound for, say, half the supply voltage, the current would double for the same power, and the back emf, per above, would half. Thus the optimal brush material resistance would need to be four times less for the losses to remain the same.

[Anonymous]

Posted by John Baron on 09/03/2021 19:42:39:

It does not vary with voltage.

Voltage dependent resistor? There's one on the board in question – the blue disc above the rectangular yellow capacitor.

Andrew

[Author]

Posts