Potentiometers, I'm confused!

[Wombat]

My lathe has a potentiometer to control its variable speed function. Obviously that is connected to a couple of obscure electronics boards of Chinese manufacture, not directly connected to the motor.  The motor is an 850w DC one.  In the process of being moved from one house, to storage, and to our shed, and then into the newly built workshop, the knob appears to have been bashed, shoved, or whatever, as the aluminium panel its mounted in was very slightly distorted, and the lathe will not work.  it is a 4k7 linear pot, with the usual nut to mount it in the panel hole.  I do not fully understand potentiometers, or which wire is which, but I managed to replace this one about 5 years ago after I'd mistaken the knob securing method and destroyed it when removing the knob to replace it with a bigger one.  The replacement one has often been a bit "hesitant", so if this is indeed the reason my beloved lathe refuses to function, I'd like to get a mechanically and electronically tough one, of decent quality.  I can solder stuff in place (NOT tiny stuff!) as long as I know which wire is which, this is the level of my electronics knowledge, i.e. not much...

OK, so what sort of pot should I buy, and where to get it.  RS seems a bit overkill for one weedy component, and I tend to avoid ebay, but Maplins is effectively dead, so what and where?  This potentiometer will get twiddled a lot, like up to 50 times a day, for a couple of days a week, perhaps.  I have a feeling wirewound is best, but that may be a well out of date opinion.  There aren't really any space constraints, its quite a roomy housing.

My only efforts at testing it with a multimeter (in situ) showed that resistances did indeed vary as I twiddled it.  Its quite possible that a crap Chinese electronics board has indeed failed, but I'd at least like to get it controlled by a decent pot.  My exploration of the forward/off/reverse switch (which is the last thing before the motor) puzzled me, as I was expecting to see a variable DC voltage on its output terminals, but a mains tester screwdriver lit up, and my multimeter set to a 200v DC range resolutely showed 0v.

Help!

[pcolbeck]

You need to know a few thing about a potentiometer to replace it. Its basically a resistor (thats two of your connections) with a third connection that taps it somewhere along its length (varied by twirling the knob). You need to know things like the resistance range, whether you want a linear or logarithmic scale (taper) plus physical stuff like shaft length, mounting etc. This from CTS who make great pots should give ypu an idea:

https://www.ctscorp.com/wp-content/uploads/450.pdf

[Feanor]

Most single-turn pots have 3 connections, arranged as a fan-out on the body.
The two outer ones are the ends of the track, and are a fixed resistance ( eg 4k7).
The middle one is the 'wiper', ie the contact that sweeps over the track between the two end points.

Multi-turn pots usually have a longer body, and you'd need to check the pin-out with a meter.

[pcolbeck]

And check that you don't have only two wires going to it in which case its a rheostat not a potentiometer.. The first provides a variable load in series to change the circuits behaviour whilst the second a variable voltage.

[Kim]

ETA: Crosspost with pcolbeck and Feanor

Normally three terminals on a pot:  One at each end of the bit of resistance stuff (wire, carbon, whatever) which you'll always measure the rated resistance (eg. 4.7k) between regardless of knob twiddling.  And one for the moving wiper, which you'll find has a random knob-defined value between that and one fixed terminal, and the rated resistance minus that random value between that and the other fixed terminal.

As such, it's easy to work out which is the wiper.  Which way round are the other two?  Electrically, it doesn't matter.  It's purely a function of which way you want the knob to turn to do whatever it is the knob does, and you can always swap them if it goes the wrong way.  For a bog standard single-turn pot where it's all arranged in a plane, the wiper is the middle terminal, and the resistance between that and an end terminal will decrease as you turn the knob towards it.

Wire-wound sounds sensible to me, given frequent twiddling in a dirty environment.

If precision is important and there aren't any space constraints, a multi-turn pot might be desirable.  I'm in the process of bodging some 10-turn ones into my bench power supply (where one turn for a 30V or 5A range means it's worryingly easy to let the magic smoke out with over-enthusiastic twiddling).  I've got the pots, but due to limited choice they don't fit the existing knobs, so I'm waiting for some new knobs before I start getting bodgy with the circuit board to make them fit the space available.


Seems likely that the motor speed control is by pulse-width modulation: Instead of varying a constant DC voltage, it pulses the full supply on and off at high frequency with a variable duty-cycle - the greater the time spent in the 'on' state in each cycle, the more power is delivered to the motor.  The advantage is that the circuitry to generate it is simpler and more efficient, and the motor won't really care.  This is likely to confuse the average multimeter, but you might get something on the AC volts range.

[Brucey]

if you wanted to have a few fixed increments of speed (rather than continuously variable) you could have an array of switches and a resistor network.  You could even have this as an alternative to a potentiometer, eg on a two-pole changeover switch.

  To test the motor board you could use a pair of 2.2K resistors and this should give you a middling speed setting.

BTW a (hopefully largely  theoretical) consideration is that wire-wound pots have a small amount of inductance and this can render them unsuitable for some applications  in which a carbon track pot was originally used.  Dunno if this is a real possibility here or not.

cheers

[redshift]

If the drive is the usual type of varispeed lathe PWM controller (often referred to as an inverter drive or VFD) then there shouldn't be any switching or other controls inline between the drive unit motor outputs and the motor - all the control circuitry is usually low voltage signalling and connects to a terminal block on the main drive unit.  The potentiometer wiring may have a 0V end and a +10V end, with the wiper connected to a third terminal. Forward/stop/reverse are usually another set of terminals on the same block.  The units can often be programmed with different functions and profiles, but each manufacturer seems to have their own ideas about what and how, and what the 'standard' settings might be.

CPC/Farnell should be able to furnish a new pot, but I recall seeing actual industrial control boxes with appropriate knobs and pots on eBay, too.

[Feanor]

Hmm, but VFD is used to control an AC motor.
The OP said it was a DC motor.

[redshift]

Sorry, lack of observation.

[Wombat]

Thanks people (I've been a bit preoccupied trying to finish the workshop building, so haven't replied till now) I now understand a bit more.  I have unsoldered said pot, and tested it, and it behaves perfectly, which is rather bad news, as that means that something far more expensive is amiss.  I'm wondering if the motor will show any signs of life on 12v DC, as I don't really have any way of supplying more than that (I have not yet bought my bench power supply that folk advised me on earlier, 'cos I didn't have a bench to put it on till now). 

Obviously if the motor makes some effort with what DC voltage I can supply it, which is possibly 18V DC, from a power pack from one of my railway engines (if I can find it) then its back to the favourite suspect, these Chinese power supply boards, which do not seem to be that reliable, as a friend has the same model lathe, and his died, necessitating a fairly expensive replacement, as his chose to die about a week after it went out of warranty.  At least mine has had the decency to last 12 and a bit years...

[Feanor]

Any chance of some close-up pics of the motor controller board?
And also any markings on the motor relating to it's electrical spec?

[Wombat]

I've taken one of the motor, which is made by Weiss (doesn't sound very Chinese!) which is 180VDC, but I'll have a crack at a couple of the two control boards tomorrow.  Then I have to remember how to post images hosted on Google photos...   the control boards will almost certainly be proprietary, and available only from the English retailer/importers of the lathe (Warco)  More or less the same lathe is supplied in slightly different specs to a variety of British suppliers.  Absolute worst case scenario is that it gets the guts out of a current model, one of which has an AC motor and inverter drive, which is apparently somewhat superior.  Its quite a nice lathe, and I need it to work, so whilst it'll hurt, I'll pay whatever it takes.

Bench power supply ordered, as it'll help in the diagnosis.

[Wombat]

Well that had a strange ending... but good!

In the process of wiggling the panel about to take a pic of it, I noticed it was US made, by KB electronics, and then when angling it to see it properly noticed that there were two unoccupied spade terminals, why....?   Closer examination of one of them revealed that the connector next to it was not plugged into an adjacent terminal, but was merely nestling alongside it, so I just shoved it back on to the adjacent terminal, assumed the other unoccupied terminal some 30mm+ away was for a safety interlock mine does not have, and plugged it in, and it worked.  I had to take the back panel off again to refit the chuck guard (secured by dirty great bolts into the side of the electronics compartment, not very clever) said connector casually fell off again, so I realised the connector was just sloppy as hell, so titivated it with a good squeeze with a pair of pliers, shoved it on, and then test ran the lathe through its full speed range, with various offings and onnings, and all seems well.

Well that was a relief.  Especially as my guaranteed next day delivery of a power supply ordered Sunday evening, to help diagnose it (but with numerous other uses in the future) didn't turn up till 4 o'clock this afternoon.  UPS tracking contained a lie, in that it said they'd tried to deliver it yesterday, but the van driver confirmed that they had not. (and I know they bloody didn't 'cos I was working right next to the clanky metal gate all day.)

Power supply looks nice, though 

[Kim]

While we're on the subject, my power supply now has cheap black knobs rather than even cheaper grey ones, and these ones^[1] have the wire-wound feel and go to 11 10. 


_{[1] Apart from one where I appear to have b0rked the innards while wiggling it into position (it's a tight fit), where about a third of the travel is "random voltage each time you touch it".  Replacement in the post.}   

[Diver300]

Apart from one where I appear to have b0rked the innards while wiggling it into position (it's a tight fit), where about a third of the travel is "random voltage each time you touch it".  Replacement in the post.   

I used a 20 ish year old potentiometer today, and found that the wiper goes open circuit when it's turned up to maximum.  A new one was already on it's way but it didn't arrive in time.

[pcolbeck]

Apart from one where I appear to have b0rked the innards while wiggling it into position (it's a tight fit), where about a third of the travel is "random voltage each time you touch it".  Replacement in the post.   

I used a 20 ish year old potentiometer today, and found that the wiper goes open circuit when it's turned up to maximum.  A new one was already on it's way but it didn't arrive in time.

It might have been designed to do that. On some Fender guitars they have "no load" tone pot that removes itself from the circuit on max. Your might be a similar funky pot.

[Diver300]

Apart from one where I appear to have b0rked the innards while wiggling it into position (it's a tight fit), where about a third of the travel is "random voltage each time you touch it".  Replacement in the post.   

I used a 20 ish year old potentiometer today, and found that the wiper goes open circuit when it's turned up to maximum.  A new one was already on it's way but it didn't arrive in time.

It might have been designed to do that. On some Fender guitars they have "no load" tone pot that removes itself from the circuit on max. Your might be a similar funky pot.

I had seen pots with switches attached, but they always had separate terminals.

I doubt it is intentional, but I can't remember where that one came from so I can't be sure. I'll watch out for that in future.