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Motor control board

Here we go again!

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Macolm09/03/2021 17:11:02
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185 forum posts
33 photos

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 Baron09/03/2021 19:42:39
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520 forum posts
194 photos

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.

Macolm09/03/2021 22:37:37
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185 forum posts
33 photos

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.

Andrew Johnston09/03/2021 22:55:39
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7061 forum posts
719 photos
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

John Baron10/03/2021 07:41:44
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520 forum posts
194 photos

Hi Andrew, Guys,

The Device known as a voltage-dependent resistor (VDR), is not a resistor at all It is a semiconductor device more akin to a voltage sensitive diode. A similar mechanism as a Zenner diode but it has the same electrical characteristics for current in both directions. In other words it is not unidirectional as a diode is but Bidirectional.

FWIW a resistor is a linear device. The behaviour is according to Ohms Law.

As far as discussing motor brushes is concerned, I've better things to do than going round in ever decreasing circles !

The theory and the math is there on the Internet !

not done it yet10/03/2021 08:05:57
7517 forum posts
20 photos

FWIW a resistor is a linear device.

That’s not what I would call it. Ohm’s Law only applies at constant temperature. The actual value of a resistor will almost certainly change with temperature change. Normally a positive coefficient change but not always (and not a linear function at the extremes). Those with a negative coefficient are called thermistors.

This discussion above about brush resistance is fairly unimportant - for instance, cutting a brush in half will either halve or double its resistance - but hardly an important point with a resistance likely in the milli-Ohms range (as clearly demonstrated by half-worn brushes still being operational😀 .

One should still select the best grade of material for the installation in hand. I would expect a soft grade of brush would be used for a slip-ring application, for instance.

Michael Gilligan10/03/2021 08:45:25
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23121 forum posts
1360 photos
Posted by John Baron on 10/03/2021 07:41:44:

[…]

The theory and the math is there on the Internet !

.

Ah but ... this is on the Internet !

MichaelG.

John Baron10/03/2021 09:03:46
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520 forum posts
194 photos
Posted by Michael Gilligan on 10/03/2021 08:45:25:
Posted by John Baron on 10/03/2021 07:41:44:

[…]

The theory and the math is there on the Internet !

.

Ah but ... this is on the Internet !

MichaelG.

True !

Andrew Tinsley10/03/2021 10:06:57
1817 forum posts
2 photos

Seems to me that most people miss the point. If the motor is high current, low voltage, then the brushes need to be designed for a high current. If it is a high voltage, low current motor, then the requirements for the brush are low current. So what are people arguing about?

I don't know the ins and outs of brush technology, but I am certain that the brush composition varies with the current taken.

Andrew.

Andrew Johnston10/03/2021 11:37:53
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7061 forum posts
719 photos

It is interesting to note section 1x.2.3 of the "The Art of Electronics - The X Chapters" where non-linearity (voltage coefficient) of resistors is discussed and illustrated with measurements on a variety of fixed resistors. Of course the effects are often small, but they do exist and it helps to be aware of them when designing.

Another useless fact is that Zener diodes over about 5V are dominated by avalanche breakdown rather than the Zener effect. Avalanche breakdown gives a 'sharper' knee than the Zener effect. I avoid using low voltage Zener diodes for that reason. Although it isn't a resistor an important design criteria when using a Zener diode is the dynamic resistance.

Andrew

modeng200010/03/2021 16:03:36
340 forum posts
1 photos

The temperature coeficient of zeners is also something to consider..

Andrew Johnston10/03/2021 20:40:22
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7061 forum posts
719 photos
Posted by modeng2000 on 10/03/2021 16:03:36:

.......temperature coeficient of zeners.........

Now there's something else that is voltage dependent.

Andrew

Hadria Guy12/03/2021 15:54:08
1 forum posts

Hi...the overall things you can do are, INSPECT the board and parts on the two sides. The underside for any bind joints that may be "cold" - Also search for capacitors that may have spilled or swell. Check diodes utilizing a multimeter. Confirm anything checked F1/f2 and so forth as they are generally melds and ought to have congruity.

Keith Matheson13/03/2021 18:28:44
43 forum posts
20 photos

Hi The OP here. I’m sure we all love a happy ending and some resolution ( and a thread that comes to a satisfactory ending!) Well I put the new brushes in and Ta Daaaahh! She works again . Thanks for the advice and the useful comments. As I kept the old dead board from before I will swap over the diodes I ordered and see if I now have a spare- that would be a result. The only odd thing was that the slot that the original bushes (and the like for like replacements) went in was enormous when compared to the bushes. I would say easily 2mm all the way around. Is this normal or did the motor factory run out of the correct bushes and just whacked in any thing that would work to get it out of the door? Chinese quality assurance? You are assured it is a low price? Best regards Keith MM

John Baron13/03/2021 19:47:14
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520 forum posts
194 photos
Posted by Keith Matheson on 13/03/2021 18:28:44:

Hi The OP here. I’m sure we all love a happy ending and some resolution ( and a thread that comes to a satisfactory ending!) Well I put the new brushes in and Ta Daaaahh! She works again . Thanks for the advice and the useful comments. As I kept the old dead board from before I will swap over the diodes I ordered and see if I now have a spare- that would be a result. The only odd thing was that the slot that the original bushes (and the like for like replacements) went in was enormous when compared to the bushes. I would say easily 2mm all the way around. Is this normal or did the motor factory run out of the correct bushes and just whacked in any thing that would work to get it out of the door? Chinese quality assurance? You are assured it is a low price? Best regards Keith MM

Hi Keith,

Your motor may not be identical to mine. The brushes in mine are 3.6 by 10 mm and 15 mm long. I had similar problems getting hold of some brushes, so I made my own.

If you measure the brass sleeve that will tell you the size.

As far as the semiconductors on the power board are concerned, failed diodes almost invariably go short circuit, the more likely things to fail are the SCR's.

If its any help I'm near York.

 

Edited By John Baron on 13/03/2021 19:49:36

noel shelley13/03/2021 19:57:45
2308 forum posts
33 photos

The brushes should be a good sliding fit with little slack. 2mm all round it's no wonder they caused trouble. But the good thing is you got it going again. Best wishes Noel.

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