Broken Pottery Wheel Speed Contoller

Hello everyone,

I'm having a lot of trouble understanding how this circuit works. It's an old analog speed controller board to a pottery wheel with a dc motor.

I hope I recreated it correctly but this is what it looks like to me

http://www.digikey.com/schemeit/#gz6

The problem is when the petal is in the off position (petal resistance really high) it will stop the wheel but after a little while the wheel will creep back on to a slow crawl, even when I open circuit the petal.

I wish I had a scope to see what's really happening but for now I'm just trying to figure out how it works on paper. It would be great if I could figure out how to simulate but it seems like triac/diacs might be a little complicated to simulate (was having trouble finding triac/diac models for ltspice).


Any input on how this circuit is suppose to function would be great. The part that confuses me the most is the zener/resistor bridge like thing. I figure it has to be some type of feedback control to keep the wheel speed the same under different loads but I'm having a hard time seeing exactly how that works.

Any input would be greatly appreciated.

Thanks,
-Tom

There is a bit missing there, the slider of the pot should go to a point in the circuit?
There are some DC controllers you can pick up on ebay rather reasonable made by KB/Baldor that are a little more sophisticated than that one.
Min spd, max spd, current limit etc.
That one is a little primitive in the features.
Max.

The slider on the pot is controlled by the mechanical foot petal.

I over simplified the petal unit as it is really 3 pots. both the tuning pots were busted so I couldn't measure their values but the main petal controlled pot is 0.7M ohms.

R_petalunit = R_hi + ( R_lo // R_petal )


I'm really interested in fixing the original controller/ understanding how it works before I go out and buy a new one to replace it. It might come to that but for now I'm hoping someone can try to explain to me how it's suppose to work.

But the way it is drawn the slider does not do anything as it is unconnected?
Unless it is connected as a variable resistance ( two leads) instead of a potentiometer (3 leads)?
Are you sure of that pot value?
So if there are 3 pots, we are only seeing one??
Max.

My mistake didn't realize how i was suppose to draw them in the schematic.

Here is a link to the updated schematic that I believe now correctly represents the petal unit.

http://www.digikey.com/schemeit/#gz6

I believe the end result is still R_petalunit = R_hi + ( R_lo // R_Petal)

either way since the hi and lo pots are busted right now the whole unit is disconnected and i have a 1M ohm pot in place that I've been playing with and I get the same results. I can control the speed and shut it off but after a little bit of time the wheel will kick on to a slow crawl. This makes me believe the problem is elsewhere in the circuit.

-Tom

Try removing the snubber, temporarily.
Max.

Well I ended up destroying one of the Zener (D6) before I got to test the circuit without the snubber.

But the circuit still seems to work without that zener, the motor just has a lot of extra vibrations in the lower speeds (guessing it's do to a not so symmetrical waveform). The motor still seems to hum a bit when it's suppose to be stopped but it looks like it is no longer false kicks back on to the slow crawl.

I tried removing the snubber with the D6 zener still missing and that completely got rid of the hum while the motor was off.

My question is if it is the snubber at fault how would I fix it. I tested the snubber capacitor and it's capacitance is good (.462uF, rated for .47uF @ %10 tolerance).

Thanks for all the help so far,
-Tom

IMO that is a very poor design, I still think you would be ahead to pick up something like this ebay# 400579925144 there are quite a few of them.
This will give you much better control.

What size is the motor, BTW?

As to the snubber, I thought that you may be getting some leakage there causing the creeping.
You just need a 10Ω resistor and ceramic .47μf 250v rated cap.
Max.

Are you sure 'R3' is really a resistor? If the circuit is indeed as per the schematic then there is a current path through R1/R3/R2 and the triac gate which will cause the triac to trigger even if the diac is disconnected and the pedal pot will have no effect.
If D2 and D5 are 1N4326 (9.1V zener) then the voltage across C1 will be clamped to ~± 9.7V, which is insufficient to trip the diac (a 32V type).

Alec_t said:

Are you sure 'R3' is really a resistor? If the circuit is indeed as per the schematic then there is a current path through R1/R3/R2 and the triac gate which will cause the triac to trigger even if the diac is disconnected and the pedal pot will have no effect.
If D2 and D5 are 1N4326 (9.1V zener) then the voltage across C1 will be clamped to ~± 9.7V, which is insufficient to trip the diac (a 32V type).

Click to expand...

Pretty sure R1/R3/R2 are resistors they all have the bands and look like resistors.

But I'm wondering if the zeners are really 9.1volt zeners. I broke the D6 zener and tried to replace it with a new 9.1volt zener (NTE139A) but the wheel is always on when I put that zenner in there.

Is it possible to have miss labeled parts? the label reads "GI 1N4 326" and the only data sheet I can find says 9.1volt.


I need to get myself a little lab setup with a dc powersupply or maybe just a battery array for now to test these zeners out and see what there real value.

IMO that is a very poor design, I still think you would be ahead to pick up something like this ebay# 400579925144 there are quite a few of them.
This will give you much better control.

What size is the motor, BTW?

As to the snubber, I thought that you may be getting some leakage there causing the creeping.
You just need a 10Ω resistor and ceramic .47μf 250v rated cap.
Max.

Click to expand...

Thanks for all the help so far. That ~$50 controller on ebay is probably going to happen but I still would like to get this controller working.

The Motor is 1/3HP not sure on the volt/amp ratings, I'm sure there is a spec sheet on the motor I just havn't disconnected it yet.

Circuit was still working with that broken zener (D6) and the snubber disconnected. I bought a new cap for the snubber and hooked it up but it gives the motor a noticeable hum when in it's in the off position like it's never really truly off. I wonder if this is just part of this bad controller design or if something else is going on since I have the missing zener right now.

-Tom

it gives the motor a noticeable hum when in it's in the off position like it's never really truly off.

Click to expand...

Simulation shows there is always ~45mA RMS (4.5A brief spikes) through the motor via the snubber. That probably accounts for the buzz.
As far as I can interpret the schematic the zener diodes are intended to clamp the voltage at the triac gate if the diac should fail to conduct. The sim works perfectly well without any zeners.
One notable omission from the circuit seems to be EMI suppression components (apart from the snubber). I would have expected at least chokes in the mains supply lines.

MaxHeadRoom said:

IMO that is a very poor design, I still think you would be ahead to pick up something like this ebay# 400579925144 there are quite a few of them.
This will give you much better control.

What size is the motor, BTW?

As to the snubber, I thought that you may be getting some leakage there causing the creeping.
You just need a 10Ω resistor and ceramic .47μf 250v rated cap.
Max.

Click to expand...

I'm looking to buy one of these KBIC controllers but I'm curious about the motor voltage rating. The motor on the wheel says 1/3hp (though there is no data sheet or model number listed on the motor). When I measure the voltage at max speed I see ~120v across the rectifier on the motor terminal side.

The KBIC controllers say they are 90v controllers. So my question is if I use one of these controllers will my max speed be limited? Or is there someway I can identify what type of motor I got and what controller would work best for it?

Thanks,
-Tom

Most of the KB's start at around 1/3rd HP, the technology is virtually the same as what you have, except more features, they just have a W.W. resistor fitted to match the motor for current limit feature.
Yours has a Triac operating into a bridge, The KB has a 2 quadrant SCR bridge, both units operate across the incomming 120vac supply.
They do also have PWM models which are a little more sophisticated, but cost a bit more.
You just need a 5k - 10k linear pot for control.
Max.