DC supply for a shunt wound motor

[JohnHewes]

Hi hope you can help me.
I bought an old potters wheel for my wife, it has a 0.33HP DC Shunt motor but the supply is fried.
what supply unit would be appropriate to buy, please?
The motor plate says 220, 2.5 Amp, it is a Normand motor.
Currently the speed control is from a foot pedal connected to a rotary control.
Thank you in anticipation
John

 

[Lucien Nunes]

It would be helpful if you could post pics of the existing setup including the speed control, motor rating plate and circuitry. I know nothing of potters wheels specifically but I would expect that the field is energised with rectified unsmoothed mains, and the armature likewise but via a phase-angle control. If the motor is capable of actively braking the wheel as the pedal is released, then it's a bridge rather than just a single chopper. There might be other tricks such as a field relay to turn it off at zero speed. All of this should be apparent from some pics.

Perhaps the existing controller can be fixed? If it runs at full speed all the time it may be nothing more than a shorted SCR. In what way is it 'fried'?

 

[JohnHewes]

Thank you Lucien for your comprehensive answer, and I would rather repair if possible. The coil shown on the board is the one which is “fried”, but I cannot identify it. I have left a message for Goodwin Controls who are still active and I hope that next week may see a response from them, fingers crossed!
The other photo shows the speed control, a foot pedal moves the quadrant gear, opening the micro switch to start the motor, then I assume it speeds up from there on. Thanks again, Stay safe, John

 

[Lucien Nunes]

Cool, late 60's / early 70's by the look of things (date will be on the caps and SCRs.) Appears to be a full-wave half-controlled bridge plus an extra leg for the field supply. If the burnt coil is wound around a reed-switch like the one above it, it is a reed-relay; whether it simply operates as a relay and could be replaced with any ordinary type, or whether it has specific characteristics, would have to be determined by tracing the circuit. From the number of turns I would guess at it being field-current sensing, to prove that the field is on before energising the armature to prevent overspeeding. If that is the case, it does need to be of the correct spec, and the burnout could be a warning that the motor field is shorted (hence might be worth waving a meter at that before anything else.) The reed relay above is probably armature current limit sensing, given the distinctly small number of turns.

I am going to guess that the SCRs are 2N4443s. If I am right, you get to drink a beer on my behalf later (I'm driving so can't.)

 

[R-fur]

Contact a power tool maker such as Axminster tools, they provide speed controls for things like lathes so should be able to help.
I have no connection with Axminster, just used them as an example.

 

[JohnHewes]

Cool, late 60's / early 70's by the look of things (date will be on the caps and SCRs.) Appears to be a full-wave half-controlled bridge plus an extra leg for the field supply. If the burnt coil is wound around a reed-switch like the one above it, it is a reed-relay; whether it simply operates as a relay and could be replaced with any ordinary type, or whether it has specific characteristics, would have to be determined by tracing the circuit. From the number of turns I would guess at it being field-current sensing, to prove that the field is on before energising the armature to prevent overspeeding. If that is the case, it does need to be of the correct spec, and the burnout could be a warning that the motor field is shorted (hence might be worth waving a meter at that before anything else.) The reed relay above is probably armature current limit sensing, given the distinctly small number of turns.

I am going to guess that the SCRs are 2N4443s. If I am right, you get to drink a beer on my behalf later (I'm driving so can't.)

Thanks again Lucien, I am a mech eng so most of what you said is over my head. However I have the motor and I measured 1ohm on the field coil, and 7-8 ohms across the commutator segments. How does that sound to you, please? Kind regards
John

 

[Lucien Nunes]

Definitely 1 ohm, not 1 kilohm? Definitely on the field (Z-ZZ) with the controller disconnected? Because on a 220V shunt field, one ohm is undoubtedly a shorted winding. Before pursuing the controller further, let's double-check the motor. Post pics of the terminals in case anything unusual presents itself. Pref higher res pics than above if poss.

 

[JohnHewes]

Definitely 1 ohm, not 1 kilohm? Definitely on the field (Z-ZZ) with the controller disconnected? Because on a 220V shunt field, one ohm is undoubtedly a shorted winding. Before pursuing the controller further, let's double-check the motor. Post pics of the terminals in case anything unusual presents itself. Pref higher res pics than above if poss.

Thanks, I will check again in the morning, but definitely the field coil direct to the bare wires. Not sure about the measurements on my meter, i will photograph the test tomorrow.
John

 

[JohnHewes]

Thanks, I will check again in the morning, but definitely the field coil direct to the bare wires. Not sure about the measurements on my meter, i will photograph the test tomorrow.
John

Hello again, please see the attached photo I was reading up on how to measure resistance with a multimeter last night and am now doubting my 1 ohm figure.
Regards
John

 

[James]

That’s really useful, turn the dial on the meter from 2k to 200
You will then be reading resistance in ohms, not Kohms

 

[Lucien Nunes]

indeed, that was 0.985kΩ which seems about right. The reading on the armature circuit isn't very significant, because the voltage drop at the contact interface between the carbon brushes and copper commutator is non-linear and the meter can't properly read the armature resistance through it. Armature faults can be hard to detect with a meter anyhow; the best quick guide is the appearance of the comm because most armature faults cause visible comm damage before the motor stops working. If there is no scorching / pitting / erosion of the segment edges and it's a uniform brown colour, the armature is probably OK.

Returning to the controller, this is the sort of thing I would repair to component level because I restore old equipment; many outfits don't hold spares or service info for anything over 20 years old. It will be interesting to hear what Goodwin say.

 

[JohnHewes]

indeed, that was 0.985kΩ which seems about right. The reading on the armature circuit isn't very significant, because the voltage drop at the contact interface between the carbon brushes and copper commutator is non-linear and the meter can't properly read the armature resistance through it. Armature faults can be hard to detect with a meter anyhow; the best quick guide is the appearance of the comm because most armature faults cause visible comm damage before the motor stops working. If there is no scorching / pitting / erosion of the segment edges and it's a uniform brown colour, the armature is probably OK.

Returning to the controller, this is the sort of thing I would repair to component level because I restore old equipment; many outfits don't hold spares or service info for anything over 20 years old. It will be interesting to hear what Goodwin say.

So, could you repair this for me, please?

 

[Lucien Nunes]

Sorry, I meant that figuratively rather than practically. I don't generally take on repairs these days, other than as part of a support contract that we provide. If someone turned up at the workshop brandishing a PCB I would not turn them away, but if I couldn't fix it over a cup of coffee and a natter I would try to dissuade them from leaving it in the in-tray.

 

[JohnHewes]

Sorry, I meant that figuratively rather than practically. I don't generally take on repairs these days, other than as part of a support contract that we provide. If someone turned up at the workshop brandishing a PCB I would not turn them away, but if I couldn't fix it over a cup of coffee and a natter I would try to dissuade them from leaving it in the in-tray.

Ok, understand. You have been a great help, John

 

[JohnHewes]

Sorry, I meant that figuratively rather than practically. I don't generally take on repairs these days, other than as part of a support contract that we provide. If someone turned up at the workshop brandishing a PCB I would not turn them away, but if I couldn't fix it over a cup of coffee and a natter I would try to dissuade them from leaving it in the in-tray.

Do you know somebody else who could repair it, please?
John

 

[James]

It’s probably quicker and cheaper to replace it with something like this

507-00-20-00 | Motor,speed,controller,dc,dual voltage,110/120-220/240Vac,507,parker ssd eurotherm,6A | RS Components

uk.rs-online.com

Than it is to find someone to repair it.

 

[JohnHewes]

It’s probably quicker and cheaper to replace it with something like this

507-00-20-00 | Motor,speed,controller,dc,dual voltage,110/120-220/240Vac,507,parker ssd eurotherm,6A | RS Components

uk.rs-online.com

Than it is to find someone to repair it.

Thank you. I would probably need someone to fit it for me too.
John

 

[James]

I am in coventry.
About an hour away.
Schedule is full this week and nearly full next week.
I can do it but there might be a member closer that can do it quicker for you

 

[JohnHewes]

I am in coventry.
About an hour away.
Schedule is full this week and nearly full next week.
I can do it but there might be a member closer that can do it quicker for you

Thanks, I will let you know how I want to proceed, John