AC brushed motor, speed controller?

[Konstantin]

Hello.
Does anyone have links to information about speed controller for an AC brushed motor, specially those controllers that promise hi torque at low speeds (I wonder how achievable is that). I would really like a DIY project, since international shippings add up to costs but if there is no DIY then I will probably purchase one.
There are few links about dimmer type speed controllers and I have used one of these for my soldering station and Dremel (single speed model), actually it was based on a quadrac design that I took from a broken kitchen blender, but for a router I would like a more robust design, I have this Bosh 1hp palm router/trimmer that I would like to spin at lower revs.



Thank you.
Konstantin.

[NC Cams]

Here's the problem with speed controls:

To vary the motor speed, you need to adjust the voltage up or down.

As you reduce voltage across the windings you reduce current flow potential. Thus you lose torque.

The trick is to keep the ability to flow current to maintain torque (which is what you want/need) while still adjusting the voltage to regulate speed (what you hope to do).

PMDC drives do this by taking the pure DC and chopping it into short duty, full voltage pulses. These pulses result in a lower AVERAGE voltage but, since current flow while voltage is on remains full on/high, torque is not diminished appreciably if at all while current flows.

Lionel trains at one time (my youth) were all powered on low voltage AC. Their train transformers were actually variacs which used a wiper to sweep across the trans former windings to vary the AC from low to high.

Probably a VFD would be what you're in need of. Perhaps someone with more variable AC experience than a Lionel train can help you more here than I.

I suspect something more sophisticated than a rheostat is what you're ultimately going to need....

[Al_The_Man]

A VFD will not work in this case as the motor is a Universal AC/DC, if you are interested in building one I have some links I can lookup, the best speed control type for Univ. motor uses a simple digital tach feedback.
There an IC made by Motorola IIRC that was developed for Univ. Washing machine motors, a Treadmill manufacturer also used it, I got hold of one for my 1.5hp Bandsaw which works great. TDA1085C http://onsemi.com
Otherwise go to the onsemi (Motorola) site, Texas Instruments or Allegro and other IC manuf have application notes for this type of controller.
Al.

[kreutz]

You can control the speed of a universal AC/DC motor by different ways; using a rheostat in series with the motor, using a variac transformer, using phase angle control (the way a TDA1085C based controller works), or using PWM High frequency chopper speed controller. The first two ways are open loop, the last two are used on closed loop speed control (use a tachogenerator or other encoder type as a speed feedback).

Phase angle control has the following dis-advantages:
• high brush noise
• short lifetime
• high current ripple
• poor EMI behavior

and advantages:
• simplicity
• cost effectiveness
• full speed control

PWM chopper has the following advantages:
• higher motor efficiency
• lower acoustic noise
• smooth current behavior

and dis-advantages:
• more components needed
• price disadvantage
• filtering of chopper frequency required

[davidmb]

Hi Konstantin,

I have been toying with the idea of using a pic micro to create a PWM chopper drive as I also have a spindle that is an AC universal motor, as I understand it, these motors can also run from DC, I have looked at Microchip's website and there are several methods outlined, if you are interested I can publish the design that I am working on, do you or can you make circuit boards?

The design is basically a bridge rectifier for the main voltage supply, this is chopped by a 17A 600v IGBT transistor that is itself controlled by a 12f series pic micro that uses an analogue 0 - 5v input to control the speed of the motor, it is open loop so no feedback control.

[Al_The_Man]

Did you see this post?
http://www.cnczone.com/forums/genera...nt_torque.html
Al.

[RomanLini]

There's a product currently being released that does what you need;
http://www.cnczone.com/forums/diy-cn...ontroller.html
and also has the benefit of some high level closed loop performance.

If you only need open-loop performance (ie only high RPM range) than many routers already have open-loop speed controls built in.

[marzetti]

Just thought I'd add what I discovered about my Hitachi 12VC router. The speed control module appears to be a closed loop setup judging by the presence of a rotating magnet at the top end of the armature shaft. Beyond that I can't tell anything as the electronics are totally sealed in silicone. I do know that it seems to have good torque even at its lowest speed. I'm thinking it should be possible to replace the pot with a photo resistor coupled with an LED controlled by EMC2's PWM spindle control. Do any of you electronics gurus have an opinion on the feasibility of such a scheme?

[RomanLini]

Thanks for that information Marzetti. That does sound extremely unusual, do you have a photo?

I'm not sure you could easily replace the pot with a led+photo resistor. Both components have very non-linear characteristics of voltage->light and light->resistance and you may get some adjustability but I think it would be pretty unusable.

[marzetti]

Here are a couple of pics of the Hitachi speed control. The magnet is the black button on top of the hex shaft showing in the second pic. The magnet seems to have 12 poles arranged radially. It spins inside of the cylindrical cavity which I assume houses a hall sensor. As you can see in the first pic the entire electronic module is sealed so I can't tell any more about it.
You may be right about the non-linearity issue using a photo-resistor. I may be able to use some of EMC2's HAL components to correct for this. The other solution that comes to mind is a digital potentiometer controlled through my Arduino board. Another obstacle is that the potentiometer leads are not readily accessible which would mean somehow opening the plastic enclosure thereby voiding my warranty. So I'll ponder this further while I wait for my warranty to expire. LOL

[RomanLini]

Thanks for that it is very interesting information!

I'm not sure I can offer any helpful suggestions, it looks like the best option would be just to replace the entire control module rather than try to hack and decipher the module in silicone.

[Jetrobin]

Hello

There is a very simple speed controller for drills, routers and the like.
If I have got the technique of adding an attachment then the circuit digram is in it. Almost any SCR will work if it has the right current rating. I have made several and they give very good results.If the diagram does not show you may have to scheme a way to e-mail me.

Jetrobin