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Old 15th Nov 2018, 10:48 pm   #1
squegging
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Default DC motor speed control

I am trying to fix a potters wheel for a friend. Basically, it runs, but at the slow setting it still runs too fast.

The motor has EMD marked on it and has brushes / commutator. So I suppose it runs on DC? There is also a tacho.

The control pcb has mains to it, and has I think, two 2N3525 SCR's but no makers name/ part number to identify it. The speed control is by a 5k pot mechanically linked to a foot pedal. The pot track / wiper checks out ok.

I attach some pictures in the hope someone may recognise the kit and perhaps provide a circuit to enable diagnosis?

Any help most appreciated....
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Old 15th Nov 2018, 10:51 pm   #2
Radio Wrangler
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Default Re: DC motor speed control

Could be a universal motor. Nothing unusual. Most washing machines have motors with brushes running on AC controlled by a tacho.

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Old 15th Nov 2018, 11:00 pm   #3
squegging
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Default Re: DC motor speed control

I'm not too hot on motors....

Apart from the tacho (2 wires) it has 4 wires to from the controller. Is that any sort of clue?
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Old 15th Nov 2018, 11:26 pm   #4
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Default Re: DC motor speed control

EMD is Electric Motor Developments, no longer trading. Several potters wheels here in Staffordshire used them, though I've never seen one like that before with a tacho. All the ones I've fixed (and I've fixed very many!) had dc shunt motors on them, no tacho. A universal motor of course is not so easily reversed!
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Old 15th Nov 2018, 11:48 pm   #5
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Default Re: DC motor speed control

Well, this setup has a DPST switch connected to the controller which reverses the motor. So does that suggest its a DC motor?

Also what sort of speed would be acceptable on slow setting?

Last edited by squegging; 15th Nov 2018 at 11:49 pm. Reason: extra question added about speed
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Old 16th Nov 2018, 12:18 am   #6
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Default Re: DC motor speed control

Quote:
a DPST switch connected to the controller which reverses the motor
Don't know how that would work but if the motor reverses then it must be either an ac induction motor (no brushes) or a DC motor.

I'd have guessed all the way to standstill with the pedal backed off.
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Old 16th Nov 2018, 1:56 am   #7
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Default Re: DC motor speed control

universal motors reverse if you reverse the connactions to either the brushes, or to the field (just not if you do both!)

4 wires from the motor to the controller would allow this.

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Old 16th Nov 2018, 10:56 am   #8
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Default Re: DC motor speed control

I have just measured 110vdc (slow) to 220vdc (fast) across the brushes, and 17vdc (slow) to 18vdc (fast) across the field.

When the direction switch is operated, the field changes polarity.

So now to start tracing out the circuit.... could be a while. Watch this space!
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Old 16th Nov 2018, 12:59 pm   #9
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Default Re: DC motor speed control

Two pots on the control board, min and max speed?
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Old 16th Nov 2018, 3:16 pm   #10
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Default Re: DC motor speed control

The photo of the pcb, top left, shows a transformer that looks like it is designed for 50 Hz operation. Just above it, I think I can see 4 diodes. Also, there a few electrolytic caps. in that region too. To me, all that suggests that the controller - and thus the motor - is operated from d.c.
With that in mind, merlinmaxwell's above comment seems possibly relevant.
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Old 16th Nov 2018, 5:29 pm   #11
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Default Re: DC motor speed control

Is this motor directly coupled to the wheel, or through a belt/ gearbox system?
If that is the case and you do not need the max speed, alter the drive ratio.

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Old 16th Nov 2018, 5:51 pm   #12
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Default Re: DC motor speed control

Hello.

Before delving in too deep, try removing the 1/4 watt resistor from the choc block connectors. I agree with Merlin, the two presets are for minimum and maximum speed settings.

It could be that someone has modified the unit by fitting this extra resistor.

Good luck!!



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Old 16th Nov 2018, 6:14 pm   #13
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Default Re: DC motor speed control

Two pots on the board, both set at the end of their travel. Suspicious?
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Old 16th Nov 2018, 7:27 pm   #14
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Default Re: DC motor speed control

The resistor fitted in the terminal block could just be a pull down resistor just in case speed control input goes open circuit.
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Old 16th Nov 2018, 8:34 pm   #15
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Default Re: DC motor speed control

Check that the tacho magnet is properly secured, we have found many that are loose so the tach doesn't get up to speed.
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Last edited by Boater Sam; 16th Nov 2018 at 8:37 pm. Reason: correction
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Old 17th Nov 2018, 11:44 am   #16
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Default Re: DC motor speed control

Quote:
Originally Posted by Ed_Dinning View Post
Is this motor directly coupled to the wheel, or through a belt/ gearbox system?
If that is the case and you do not need the max speed, alter the drive ratio.
Ed,
There is belt dive reduction using two "fan belts". Smaller pulleys x2 64mm running dia, larger ones x2 241mm running dia. So I reckon overall the ratio is 14.18:1. So plenty of torque on the turntable. Not so easy changing gear!

And thanks everyone else for your interest and suggestions. There are indeed two pots L & H which I haven't yet touched, but will give them a good twiddle to ensure their wipers are ok when I can get back to the job.

Last edited by squegging; 17th Nov 2018 at 11:50 am. Reason: correction
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Old 17th Nov 2018, 11:48 am   #17
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Default Re: DC motor speed control

Quote:
Originally Posted by Boater Sam View Post
Check that the tacho magnet is properly secured, we have found many that are loose so the tach doesn't get up to speed.
Yes, The magnet is firmly secured.
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Old 20th Nov 2018, 11:49 am   #18
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Default Re: DC motor speed control

Merlinmaxwell hit the nail on the head. When I originally got the job, there was no speed control at all. The wiper of the main speed control pot linked to the foot pedal was not making contact. After cleaning it up, there was some reduction in speed, but not enough.

Tweaking the two pots on the pcb meant that I could now bring the motor to a standstill. Problem solved.
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Old 20th Nov 2018, 3:26 pm   #19
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Default Re: DC motor speed control

Thank you, I am glad it worked!
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