Race Engine Forum UK

You can buy a Metabo flexi-shaft and 6mm collet and run it off an ordinary drill, just clamp the drill in a vice. The Metabo shaft has ball-bearings so minimises vibration. I've been using them for 20 years altogether and have no finger-joint trouble!

It will take you forever with a Dremel, plus the bigger burrs and bands you can run the better finish you will get

G

Id advise against using the router motor unless you have a speed controller, it will go way too fast for normal use and can be positively dangerous if it sheds a burr at any kind of speed.

Quite so, you need high torque for burrs not high speed, the Metabo unit with variable speed control has 'torque feedback' to keep the torque high when speed is low, this is vital.

Incidentally does anyone skilled in the art of electrical things happen to know if an ordinary lighting dimmer switch (thyristor?) could be used as a basic speed controller for a single phase electric motor? ie, one like this:


http://www.maplin.co.uk/module.aspx?Tab ... &U=Strat15

G

WOW! How timely was that!

Soooo nearly ordered a flexi with machine mart this afternoon!

Re: the speed controller/dimmer switch.

That dimmer switch in the link cannot be used, theres actually a question and answer section below that addresses it in respect of motors.
It can only handle 230watts or there abouts so something like a router motor will be well in excess of that at approx 1100 watts.

Using ohms law you can work out the current capabilities and currect consumptions via the following formulas to help you rate components you might want to use.

V (voltage) multiplied by A (amps or current) = W (watts)

Conversely, Watts= Amps divided by Voltage.

For example, a 55watt bulb running at 12 volts consumes? 55/12 = 4.5 amps

Voltage divided by resistance = Amps.
Voltage divided by Amps = Resistance.
Current multiplied by resistance = Voltage.

Guy Croft wrote:Quite so, you need high torque for burrs not high speed, the Metabo unit with variable speed control has 'torque feedback' to keep the torque high when speed is low, this is vital.

Incidentally does anyone skilled in the art of electrical things happen to know if an ordinary lighting dimmer switch (thyristor?) could be used as a basic speed controller for a single phase electric motor? ie, one like this:


http://www.maplin.co.uk/module.aspx?Tab ... &U=Strat15

G

A lighting dimmer is what can be classified as a P.W.M (Pulse Width Modulated) power supply. In these the AC house voltage is turned off and on much like a fuel injector driver switched the duty cycle of an injector. More on time to off time and more fuel and the brighter the light. Typically they can handle 300 watts of incandescent load. These types of loads have as high as a 10X starting load until the filament gets hot. That will correlate to motor starting current closely.

Two questions before it can be answered. You say "single phase"and to me that infers an induction motor ( outer windings and a squirrel cage rotor of aluminum and steel. In that case no a PWM dimmer will not work as a speed control. Only altering the AC voltage frequency can change the motor speed in this case.

It will however work on a brush type of ac motor like is used in a typical hand drill or flex shaft porting tool like a Foredom so long as been already posted, the power rating of the motor is in line with the dimmer rating.

Sorry if I rambled or stated the obvious but thought I should be thorough and touch on details.

If you have a brush type of motor, I still wouldn't use a light dimmer for speed control. It controls the torque the motor is able to give, not the speed. It does manifest itself as a speed controller, since the motor will settle at a certain speed in which torque and resistance equal out, but change the requested torque and the speed changes as well. This is what you get with an ordinary DIY drill. The expensive models have a speed controller that will increase power when more torque is requested.

The feedback Guy is referring to is what you want for porting. Solid speed of the grinder, no matter how much torque you are requesting. You can have your tool spinning at a normal speed and not burn your work piece at initial contact, but still manage to do actual work once you are grinding.

Thanks very much all for the very informative replies re controller!

G

WhizzMan wrote:If you have a brush type of motor, I still wouldn't use a light dimmer for speed control.

I respect your opinion on what you would personaly use but that was not the question. "Can one control a particular type of motor (brushed) with a PWM supply". Yes.

If one buys a variable speed drill, the speed control built into them is exactly that a PWM speed control but in some cases they employ armature frequency or current feed back to govern and maintain a given speed.

A most unfortunate row has broken out here between two accredited members, Robert Kenney from the USA and Homme Bitter (Whizzman) from the Netherlands and I am the inadvertant cause.

I find myself in an awkward position (for the first time since setting up this forum) because I have asked a question and received answers I don't fully understand and moreover am so useless with anything to do with electricity that I cannot even remark on the substance of what has been said or the level of expertise of those who have said it. My instincts tell me that I should have not asked the question in the first place (as I may not understand the answer) and that I should not mess with controllers or recommend anyone else should....

I just want to say this: on engine stuff I am on reasonably good ground, but even so sometimes I get things wrong,and sometimes I can't fully follow subjects under discussion and have to step aside. There are times when I have to remind myself of Protocol 9, to which we have all signed-up:

9. Content

Keep cool at all times even when things go wrong. This is a website for informed opinion and fact, not guesswork. Stick to the facts as a rule and present them in an enjoyable readable fashion.


Who is technically right and who is technically wrong here? I am impressed with the knowledge on display but I have absolutely no idea! I guess you both figure you're right and there is nothing unusual about that - plenty of projects fall down because man 'A' believes man 'B' is wrong about something and the facts usually show in the end. But I am not qualified to comment.

I must ask you both to 'agree to disagree' and say so publicly here. Shake hands and be done with it. Please do not engage in an angry pm exchange over this, it is not worth it and I should much prefer to think we can all behave like gentlemen here, and let's not forget it's a race engine website.

That will be the end of the matter, I trust and I am sorry for asking a question which has led to acrimony.

G

No reason in my mind that members can not disagree in a civil manner. I don't think we disagree in premise. No PM's have occured between us so no worries there.
Robert

Robert, thank you.

Homme over to you please.

GC

Thank you for stepping in Guy. I was afraid that the discussion was going to be about language semantics and not about a technical question asked.

I think we both agree about the idea that using PWM to control motor speed is common practice. We both agree that an off the shelf light dimmer will use PWM to dim lights and as such, can control motor speed. My personal opinion is that, being without the feedback circuitry as used in more sophisticated variable drill controllers (as pointed out by Robert), an off the shelf light dimmer you get at the DIY store (as was in Guy's question) would not be suitable. There is no disagreement from my part regarding the use of PWM to control speed. It's just that for all practical purposes, a light dimmer would not give satisfactory control over tool speed, but merely over motor power. I'm sure that if we had this discussion face to face we would have resolved this within thirty seconds and all this would not have been happening.

My apologies for any inconvenience and misunderstanding this discussion may have caused.

Thank you Homme,

regards to all

GC

aka 'electrically challenged'

This should do the job, enclosure and control knob sold seperately - a need to screw it to the workbench and reasonably priced. The 1500 w version would be needed to run the metabo in the picture.

http://www.quasarelectronics.com/cr0008 ... -1500w.htm