speed control board

Hello friends, I have a tiny homemade lathe. There is the following motor on it and I use this motor with dc voltage with pwm driver and bridge diode. But I want a cleaner dc voltage. Ready-made drivers are very expensive. Is there a circuit I can make? By the way, I do tube radio repair as a hobby.

I am assuming that is a universal brushed motor, as it's marked for mains AC power, but you say you are running it on DC.

Why not as a first (cheap) step simply smooth the DC output of your existing power supply. If you repair tube radio's you probably have a suitable choke and some reasonably big electrolytic capacitors with an appropriate voltage rating in your bits box.

All the commercial circuits will be the same basic design as the one you have - the cost just reflects the level of smoothing applied to the output. Making one yourself will almost certainly cost more for the components than you can buy a ready made one for.

Speaking from a fairly low level of expertise (when did that stop me), isn't there a risk that putting a large capacitor across the output of the speed controller will cause a large inrush current and pop the controller

And the capacitor will remove the considerable benefit of using PWM.

PWM delivers full voltage to the motor in pulses, allowing the motor to take full current for as long as the pulse lasts. Speed control is achieved by PWM reducing the time energy is available to the motor, not by limiting the current that can flow by reducing volts. Much better than speed control by reducing Voltage/Current because that causes the motor to operate inefficiently at a low-output/low-torque point of it's power curve, so it gets hot and is less good at driving a load.

Adding a capacitor to the output of a PWM smooths out the pulses, converting an efficient waveform into a poor one.

How does the motor misbehave Şaban?

Might be the wrong type of motor for DC - it's labelled for 230V 50Hz. Peter suggests it's a Universal Type but maybe not. Or the wrong type of controller for the motor: does the controller have a specification or part number?

If the controller is the right type for the motor there should be no need for a capacitor or rectifier.

Dave

Duncan, it would if you used a simple capacitor, but a smoothing circuit consists of a series choke (inductor) or resistor followed by the capacitor. The inrush to the capacitor caused by voltage spikes (or on start-up) is "dropped" across the inductor/resistor and if appropriately sized should give no problems.

Dave - I would agree that it should be unnecessary, but he was asking how to "clean" the DC.

Looking at the controller he has I suspect it is one of the cheap brush motor speed controllers available for <£10. They are not true PWM controllers (which usually run at high frequencies) but simply half wave rectify the incoming mains then chop each cycle short to give output voltage control. I have one I use to speed control an old Black & Decker drill . It works fine but has "ringing" spikes in the output on each half wave that cause a significant hum. I assume that is what he is trying to clean up.

Typo

Edited By Peter Cook 6 on 29/08/2022 15:31:54

This motor is an universal motor.I want a cleaner DC voltage because the motor have loud. I have tried with drill battery(20 vdc) and the motor runs very quiet.

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If the motor is designed for 230volts [as per the label] you must be under-running it enormously at 20volts

Some PWM controls will struggle with that, because it requires a tiny Mark/Space ratio and the pulsing is then obvious … [others, running at high frequency, effectively mask this effect]

Any decent ‘lab-bench’ DC power Supply should do the job better

… or maybe consider using a Variac ?

MichaelG.
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P.S. __ Does the motor produce sufficient power at 20volts to drive your tiny lathe ?

Edited By Michael Gilligan on 30/08/2022 08:01:07

You won't need 230v DC, probably only 2/3 that at most. Consider just designing a standard old fashioned DC psu with a big transistor to vary the output. It used to be a 3055 that was the workhorse for that sort of thing before everything went FET.

Looking at that circuit board, I'm with Peter Cook - it isn't a "real" PWM controller, but a variable phase angle thyristor circuit: basically a dimmer switch. It will operate at 2 x the AC supply frequency, rather than the tens of kHz that a true PWM would use.

To make a true PWM, it would be necessary to rectify and smooth the incoming AC, then design / copy a control circuit running at PWM frequency (with its associated power supply) to drive some power switching semiconductors that are capable of handling ~400V (xx3055 isn't it!) Ideally, there would be an isolating transformer in there somewhere too. Not impossible, but not a trivial task, either.(And there's the business of making sure it doesn't radiate interference to everyrhing around it).

I would be looking for a ready built unit (I build tube amplifiers as a(nother) hobby )

Try running the motor directly on 230v AC without the controller in the way. It will run at full speed, if it still runs noisily, you have a motor problem. If on the other hand it runs at full speed fairly quietly, then the noise you are getting is an artefact of the waveform generated by the PSU.

If its the latter I would at least try a simple low pass filter. Put a series resistor ( 5w 50ohms which will limit current inrush to about 5A) in the positive line and then a 500v 100µF capacitor across the +/- lines. That should supress frequencies above about 40Hz and give you a smoother DC supply.

What do people mean by a "universal" motor? It is normally taken to mean a series-wound motor as used in domestic appliances like vacuum cleaners. These have a wound field and a very high no-load speed, indeed usually it is considered dangerous to run them without a load as they can spray their commutator segments around the room (or worse). Looking at the picture of the motor it looks more like a permanent magnet motor. Though usually made for low voltage I have come across these in things like food processors, and actually the Denford mills usually use permag motors in around 1/2 HP size. These larger ones work from up to 230 v typically and for variable speed are driven from PWM variable voltage. The original Denford system had a very large choke in series but this isn't necessary.

If you have some caps lying around rated at high voltage it would be worth a try to see if it makes a difference to the noise level. Always worth trying something out.

I'm not sure of your definition of expensive - there are quite a few 220V input speed controller at less than £30 on ebay, including ones with field drive if you need it, and I believe people on here have used these with success.

**LINK**

Quick question :

Does the ‘mains’ information on that rating label actually make sense ?

… or is it really a 9volt DC motor ?

MichaelG.

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Think that might be 80ma - 9.0W @ 230V/50Hz

Dave

If the controller is as others suggest a triac, then it outputs AC in a chopped form. Might be a bit iffy putting electrolytic capacitors across it, and I'm not sure what it would achieve. Might it be possible to full wave rectify the mains input to the controller, or the output, then a low pass filter with a choke

Edited By duncan webster on 30/08/2022 13:45:00

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Mmm … a better photo would clarify that

MichaelG.

250V at 80 mA is 20W input, so 9W output sounds about right for a little motor. Here's a link to someone selling the motor. If you google translate it it says 'working with bridge diode' so my earlier worries about electrolytics are probably unfounded

Decent size 400v electrolytics are not going to be cheap

Thanks for the link, Duncan

MichaelG.

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Edit: __ it also says “KBPC 3510 is shipped together”

… and that is a decent bridge rectifier

https://docs.rs-online.com/96a5/0900766b814bbf42.pdf

Edited By Michael Gilligan on 30/08/2022 15:38:55

Andy G, 2N3055, the dear old 3055. But 9W ? really ?. Noel.

Yes, indeed, a true stalwart, but Vce max of 60V.

A motor with same part number is shown with a bridge rectifier here:

https://www.cnc.ist/makina-aksesuarlari/mini-torna-motoru-2500-devir.html

(Mini lathe motor 2500 RPM)

(In Turkish!)

FWIW I suspect it's a permanent magnet motor.

If that motor is only 9 Watts as per label, why not just get a lamp dimmer from B&Q?