DC motor controller output

Thread Starter

Peterw1966

Joined Feb 18, 2021
6
Apologies if I am posting this to the wrong audience, but I am struggling to think where else to ask.

I am trying to fix a pottery wheel. Problem is that it runs intermittently and the speed controller doesn’t vary the speed smoothly.

Thought I’d take a logical approach and start with checking power to the motor. I think it’s a DC permanent magnet motor. I put a multimeter across the 2 wires into it and WOW 300v DC! Actually it varies and can show 400v or more. Logic would tell me this can’t be right. I have tried 2 multimeters, which show the same. Also I have a neon test probe and this lights up considerably brighter than when I test the 240v mains.

I have tested with both the motor connected and disconnected, as initially without the motor I thought it might be something to do with no load applied. But both tests show the same.

This is a cheap Chinese hobby pottery wheel (not an industrial unit). The wiring is certainly not spec’ed for very highly voltage. The controller also doesn’t look like it’s built for high voltage (similar to a treadmill controller)

Can anyone help me identify what I am assuming must be a basic error in my testing approach?

many thanks
Peter
 

MaxHeadRoom

Joined Jul 18, 2013
22,823
For one, a bit better information on the controller and motor is needed.
Is this a UK or Australian unit? 240v mains?
Some pic may help including any part No.s and motor plate details.
Don't expect NFPA wiring spec from Chinese equipment.
 

Thread Starter

Peterw1966

Joined Feb 18, 2021
6
@MaxHeadRoom,
Thank you for the reply.
I am in the UK, so UK 240v AC supply.
I have attached a picture of the motor and a picture / diagram of the control board.
No data plate on the motor. Part number on the controller board doesn't show up on a google search. As you say, no schematic in the case anywhere.
The diagram is simplified and doesn't show the RCD circuit breaker on the input or the reversing switch on the output (although I have tested with the latter removed)
Hope that this may help in explaining what I am seeing when I stick a voltmeter across the motor.

Motor.pngController.png
 

kennybobby

Joined Mar 22, 2019
75
The motor has a large number of turns of very small gauge wire, which is an indication that it was wound for high voltage. Some motors require the use of a "keeper" ring during disassembly/assembly or the magnets in the stator will become de-magnetized and weakened. Might this be the case with your motor? Once weakened the motor will no longer operate.

The controller doesn't show any obvious indication of failure or blown components. It has a little bit of filtering on the 240vac input, then a rectifier bridge and big storage capacitor (any markings visible?), there is a low voltage transformer and opto-isolation of the command signal and control chip. A big transistor on the heatsink to regulate the current to the motor.
 

Thread Starter

Peterw1966

Joined Feb 18, 2021
6
The motor has a large number of turns of very small gauge wire, which is an indication that it was wound for high voltage. Some motors require the use of a "keeper" ring during disassembly/assembly or the magnets in the stator will become de-magnetized and weakened. Might this be the case with your motor? Once weakened the motor will no longer operate.

The controller doesn't show any obvious indication of failure or blown components. It has a little bit of filtering on the 240vac input, then a rectifier bridge and big storage capacitor (any markings visible?), there is a low voltage transformer and opto-isolation of the command signal and control chip. A big transistor on the heatsink to regulate the current to the motor.
Thank you for the response @kennybobby

The motor still runs, so I don't think it is an issue with that. I disassembled it just to confirm the type and put it straight back together. Hopefully I have not caused any lasting damage as I didn't use a keeper ring.

There is no sign of damage on the controller. No scorching on the board or any of the components. The capacitor looks intact and is a 150µF 400v. Transistor seems to be G20T650, if that is any help.

Without a large transformer, I would have assumed that the output to the motor should be 240v, albeit DC (if there is a bridge rectifier in the circuit). I don't understand why I am seeing really high voltages (the motor runs fine, although the speed is not controlled correctly)

The only information I have found on the pottery wheel is a similar looking model which states it has an output of 36v. Whilst not a very sound assumption, this made me think the motor would likely be 36v or similar in my model.
 

kennybobby

Joined Mar 22, 2019
75
240vac is the RMS value, which is 0.707 times the peak value; so peak is about 340 Volts and this is what will be passed by the rectifier and stored in the big cap to filter the ripple of the sinusoid resulting in about 300 VDC.

The speed adjustment potentiometer signal coming off the wiper (center tap) looks to be passed thru the optocoupler to control chip. If that signal is not varying when the pot is moved, then maybe the pot is defective?

Just the action of sliding the stator off of the rotor without a keeper (if reqd) is enough to permanently damage a motor. Depends upon the type of magnets used.
 

Thread Starter

Peterw1966

Joined Feb 18, 2021
6
240vac is the RMS value, which is 0.707 times the peak value; so peak is about 340 Volts and this is what will be passed by the rectifier and stored in the big cap to filter the ripple of the sinusoid resulting in about 300 VDC.

The speed adjustment potentiometer signal coming off the wiper (center tap) looks to be passed thru the optocoupler to control chip. If that signal is not varying when the pot is moved, then maybe the pot is defective?

Just the action of sliding the stator off of the rotor without a keeper (if reqd) is enough to permanently damage a motor. Depends upon the type of magnets used.
So, a bit more detail on the how the issue manifests itself...

As the pot. is turned there is a "sound" from the controller board (very faint squeak!). This starts as soon as the the pot. is turned (from 10K) and the pitch changes slightly with the movement of the pot. However the motor does not start to turn until the pot is almost fully turned. I haven't measured it accurately but would guess that from 10K to 8K nothing happens. Then the motor will start to turn. It will get faster as the pot is turned further but not in a smooth manner. However, at full turn (0 ohms approx.) the motor stops and there is a regular clicking sound from the controller. As the pot is wound back again, the motor will start at a fast rate and slow down, again only over a small percentage of the pot. range.

Before the fault, the motor speed varied smoothly thoughout the full range of the pot.

I have tested the pot. completely disconnected and it smoothly varies from 10K to 0.

I will have to put everything back together to retest the motor to see if I have bricked it!

Thanks again for you guidance.
 

bwilliams60

Joined Nov 18, 2012
1,391
This won't be the answer to your problem but you may want to seat your brushes to the armature a bit better. It doesn't look like they are making good contact. I would fit a light piece of sandpaper to the commutator, slide the brushes over and work it until the brushes have the contour of the commutator
Perhaps look at the IC and get a datasheet and see if it is contrlling the board as it should?
 

kennybobby

Joined Mar 22, 2019
75
The noise is likely ringing of the transformer windings from the frequency of the ic regulator of the low voltage supplies (viper chip?)

Can you read part numbers on all the ICs?
 
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Thread Starter

Peterw1966

Joined Feb 18, 2021
6
I can't see any part numbers on the IC's. Tried with my best pair of reading glasses as well a zooming in on the iPhone.

On the brushes, I am assuming that as this motor is almost brand new the brushes will bed-in in time, but a good suggestion if the motor does not run smoothly once the controller problem is sorted.

Is this a common type of control board? I would like to retain the motor (assuming I haven't broken it already) and the pot. for control of the speed. So a 240v AC to 300v DC control board would do the trick if it was a cheap option. Most controllers I have seen seem to want DC in and DC out and at much lower voltages (up to about 60v).
 

kennybobby

Joined Mar 22, 2019
75
If the repair route doesn't suit ya, Check out many like this on abay,
dc motor controllers

That motor will run on lesser voltage, you could even connect a car battery to run it at low speed on the bench to seat the brushes. i wouldn't get sandpaper anywhere near a motor commutator, it could embed particles in the copper, not worth it.
 
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MaxHeadRoom

Joined Jul 18, 2013
22,823
I don't think you would have a problem of not using a keeper on modern motors, as most now use Neodymium materials, I have some field magnets from a dismantled motor I use on the bench for holding parts etc and they are still as strong 15 yrs hence.
The thing you do have to watch if you did use a iron keeper, is to avoid chipping the magnets, as their first tendency is to attract the bar. Likewise, take care when refitting the armature.
The best thing to bed brushes is a comm stone, but not many have access to one.
I would let them bed of their own accord and the comm looks clean.
The presence of a large electrolytic points to a PWM controller of some kind.
Normally the automotive battery test confirms the state of the motor, should be minimum current when up to speed from that source.
You could also try a 240v lamp in place of the motor and check the voltage throughout the range.
Max.
 

Thread Starter

Peterw1966

Joined Feb 18, 2021
6
Ok, so rather embarrassing.
I put everything back together on the bench. Same problem but at least the motor works still.
So I thought I’d try swapping the pot. connections around and guess what...they were connected incorrectly! Put back correctly and all works fine.
Given that I took a photo of the connections before I started, it can only mean someone had fiddled with it before I had a look. Will be speaking to my son and his girlfriend (who’s wheel it is)!
All that said...I got some great advice on this forum that stopped me chasing my tail with regards to the output voltage. Thanks to all that helped me.
I am working on a couple of little arduino projects at the moment (more my area of knowledge than all this high voltage stuff!). I will keep an eye on the forum and hope to be able to reciprocate the help I have had.
Thanks again.
Peter
 
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