A potters wheel need to repair - Newbie help with electrics.

MaxHeadRoom

Joined Jul 18, 2013
28,757
At that point it would be simple to understand where each connection point should be connected, given that the only output is to a variable speed DC brush type motor,
. The whole system can't be that complicated, can it????.
No, in the proper hands on site it would probably be fairly trivial to solve, but as the O.P. says, he is a newbee to electrics, so it is doubly difficult when solving or directing someone from a distance.:(
Max.
 

MisterBill2

Joined Jan 23, 2018
18,920
If the TS gave some idea of his location in the world there is a remote possibility that someone on the forum lived close enough to look at the problem.

Les.
I think that he is in the UK. Still a fair sized realm. Since I have repaired equipment via fax and email I am convinced that it is often possible. There is certainly enough talent available from the collection of participants here to work out a solution.
What would be handy to find is somebody who uses one of these potters wheel systems who coulkd describe ho it works and what controls are present in an unmodified correctly functioning system. And if they were willing to take a photo of the overall assembly that also could be handy.
 
Looks liek teh switch with red and green buttons is a standard on/off and the circuit breaker might be an earth leakage breaker as well as standard breaker (red button to test). Yellow box is definitely some kind of speed controller. Small boxes look like crimp type joiners.
Brown over here (Australia) is phase (live) and blue is neutral, your diagram seems to indicate neutral is switched, which would seem wrong to me.
 
So the green and red buttons are momentary contacts. Green puts power onto the "stopcircuit" thingy, which is a relay with power dropout capability. In other words, if you plug the appliance in, you have to press the green button to start it. Red button breaks the relay hold circuit to stop it. If power is removed, the relay drops out so it doesn't start on its own if plugged back in.
Two yellow wires on circuitstop have to be the switched contacts to feed power to the yellow box.

How it all wires in is not obvious to me. One of the plastic blocks has two yellow wires, maybe they attach there. The plastic blocks are definitely joiners by the way, Googled them.

We need to know which wires are the feed wires to the yellow box. Are the two yellow wires in the same side of the joiner?
 

MisterBill2

Joined Jan 23, 2018
18,920
Yes, it would be handy to know the correct connections to that yellow box. Presently there is no actual proof that the two disconnected items were ever part of the system, that is the challenge. We are told that the condition is "damaged" with no clue as to the nature of the damage, mechanical, electrical, flood, or fire. And my mind reading skills are so poor that I can't see what the author is seeing. I lost many of those super-powers back in 1969.
I have suggested that with an adequate photo of the circuit board we should be ale to create the board circuit and probably sort the whole system back to working. That has not happened yet
 

MaxHeadRoom

Joined Jul 18, 2013
28,757
So the green and red buttons are momentary contacts. Green puts power onto the "stopcircuit" thingy, which is a relay with power dropout capability.
From what I have gathered the C.B. functions as a dual function stop/start P.B. as well as a breaker, now discontinued by the manuf., the speed controller is a Variac, I believe the board is mainly a full wave bridge rectifier and electrolytic for the DC motor, the board being fed from the Variac as well as the control relay.
Apparently the C.B. was a weak point and was fairly expensive, it would normally be very easy to sub these for common discrete components.
The two yellow are the main breaker conductors and the three centre terminals are X-S = N.O. and X-T = N.C. as low current aux control contacts.
The latest versions are DIN rail and PCB mount.
Max.
 
Last edited:

jevpots

Joined Jan 15, 2020
14
Hi Guys

I realise that this thread has been dead a while but as so much of the info and photos in it relate to my problem I thought it was worth starting up here again. Apologies if I should open a new one.

My problem concerns a similar potters wheel with a similar Rilton Control box. However this wheel has a DC shunt motor manufactured by Normand Electrical Co rather than the French Leroy DC motor. Image attached – 4 wires plus earth going to the motor.

This wheel, which was recently purchased second-hand, is working sort of OK but the problem with it occurs when wanting to stop the wheel. The motor doesn't come to a dead stop and as the end of motion micro switch is activated (at the bottom end of the Variac position) it actually speeds up a bit before gradually coming to a stop. For precision work on the wheel this is annoying but not a game stopper!

I noticed that at item 31 in this thread MisterBill2 said, ‘The microswitch turns off the DC and short circuits the motor to provide a quick stop when the pedal is released.’. This gives me a clue as to where the solution may lie in fixing my machine. The micro-switch is clearly doing something sso possibly the solution lies in the associated circuitry which it is activating. I haven’t removed the control box yet and, before I do, wondered if you had any suggestions of whether there may be other components I should look at as well as the micro-switch.

Thanks in advance for any helpIMG_20200212_151319085.jpgIMG_20200212_151348443.jpgIMG_20200212_151649041.jpgIMG_20200212_151846490.jpgIMG_20200212_153958756.jpg
 

LesJones

Joined Jan 8, 2017
4,206
It would probably stop quicker if the field winding was still powered at the same time as shorting the armature. Post a schematic of how your machine is wired.

Les.
 

jevpots

Joined Jan 15, 2020
14
It would probably stop quicker if the field winding was still powered at the same time as shorting the armature. Post a schematic of how your machine is wired.

Les.
Thanks Les - I'll have to wait until I get the Control Box off to see exactly how the motor connections are made to the switch and the board. I have another of these machines which does stop dead so am thinking that something has gone wrong with the control mechanism on this one and in its original state it would have functioned properly.
 

MaxHeadRoom

Joined Jul 18, 2013
28,757
If the power is being reduced to the armature as well as the field then you may be getting a speed-up condition due to field weakening.
Ideally the field should be held at a constant and the armature voltage reduced, this could be a design fault.
In a wound shunt field motor, the armature can speed up if the field goes O.C. or decreases in RPM.
In large DC motors of this nature, they posses a field loss protection circuit that shuts all power down in cases where a field loss can cause catastrophic damage due to run-away.
Ideally the field on your motor should be fed a separate constant DC voltage.
Max.
 

jevpots

Joined Jan 15, 2020
14
If the power is being reduced to the armature as well as the field then you may be getting a speed-up condition due to field weakening.
Ideally the field should be held at a constant and the armature voltage reduced, this could be a design fault.
In a wound shunt field motor, the armature can speed up if the field goes O.C. or decreases in RPM.
In large DC motors of this nature, they posses a field loss protection circuit that shuts all power down in cases where a field loss can cause catastrophic damage due to run-away.
Ideally the field on your motor should be fed a separate constant DC voltage.
Max.
Many thanks Max - this helpfully points me in the direction of things to check. I'm pretty sure that originally it would not have speeded up before shutting down (as this makes it awkward to use for fine work) so maybe something around that micro-switch is causing the field to go OC. I'm not familiar with a field loss protection circuit. If this circuit had a fault could this lead to speeding up instead of stopping dead?
 

MaxHeadRoom

Joined Jul 18, 2013
28,757
It is not all that common to find field loss protection on the smaller motor ranges. So I would suspect it is likely yours would not have it.
The degree of speed up when this occurs is dependent on motor and if a any load is applied at failure time.
As you mention, I think the course of action would be to measure both field and armature voltage when the speed-up occurs and detect why.
A little reverse engineering is needed if you do not have the schematic.
Max.
 

jevpots

Joined Jan 15, 2020
14
It is not all that common to find field loss protection on the smaller motor ranges. So I would suspect it is likely yours would not have it.
The degree of speed up when this occurs is dependent on motor and if a any load is applied at failure time.
As you mention, I think the course of action would be to measure both field and armature voltage when the speed-up occurs and detect why.
A little reverse engineering is needed if you do not have the schematic.
Max.
Thanks again Max - Time to take it apart and see what's going on.
 

MisterBill2

Joined Jan 23, 2018
18,920
Max is correct in that reducing the field without removing power from the armature will make the motor speed up. So what is probably wrong is that the field and the armature are both being fed by the variable voltage. That may have been the result of a change when some part of the system was replaced.
The correct operation would be for the armature voltage to be reduced to zero before the field was switched off. It may be that there is a switch that is supposed to switch off the field when zero speed is selected has become mis-adjusted so that it cuts off too soon.

In addition, the brake shoe that is supposed to stop the wheel when the pedal is released is missing. That shows in the first photo.
 

jevpots

Joined Jan 15, 2020
14
Max is correct in that reducing the field without removing power from the armature will make the motor speed up. So what is probably wrong is that the field and the armature are both being fed by the variable voltage. That may have been the result of a change when some part of the system was replaced.
The correct operation would be for the armature voltage to be reduced to zero before the field was switched off. It may be that there is a switch that is supposed to switch off the field when zero speed is selected has become mis-adjusted so that it cuts off too soon.

In addition, the brake shoe that is supposed to stop the wheel when the pedal is released is missing. That shows in the first photo.
Many thanks Mister Bill. I hadn't spotted the missing brake shoe. That certainly won't help
 

MisterBill2

Joined Jan 23, 2018
18,920
The shutoff arrangement is important too, and that may be the solution before anything else. Do you have a means to monitor the shunt field voltage? I think that in the pictures there was a terminal strip with labels for Armature and Field, and that would be a good place to check voltages. The field should stay on until the armature drops to zero, or maybe even is short-circuited. A switch that operates when zero speed is desired could be the best approach, there may not be one, though.
 

jevpots

Joined Jan 15, 2020
14
The shutoff arrangement is important too, and that may be the solution before anything else. Do you have a means to monitor the shunt field voltage? I think that in the pictures there was a terminal strip with labels for Armature and Field, and that would be a good place to check voltages. The field should stay on until the armature drops to zero, or maybe even is short-circuited. A switch that operates when zero speed is desired could be the best approach, there may not be one, though.
Many thanks. That's really helpful. Yes, it would be quite easy to put a meter on those terminals before I even remove the control box. Much appreciate your thoughts on this
 

MaxHeadRoom

Joined Jul 18, 2013
28,757
See post #53 also explained there, I would try to maintain the field voltage, even if it means a small mod.
You just need to turn off the field only when the motor armature is turned off by the relay etc.
Max.
 
Last edited:
Top