Motor control board

[John BaronParticipant @johnbaron31275]

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 yetParticipant @notdoneityet]

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 GilliganParticipant @michaelgilligan61133]

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 BaronParticipant @johnbaron31275]

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 TinsleyParticipant @andrewtinsley63637]

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.

[Anonymous]

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

[modeng2000Participant @modeng2000]

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

[Anonymous]

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 Guy 1Participant @hadriaguy1]

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 MathesonParticipant @keithmatheson47708]

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 BaronParticipant @johnbaron31275]

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 shelleyParticipant @noelshelley55608]

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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