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Author Topic: For all of the EE types of people?  (Read 6826 times)

Offline frank mccune

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For all of the EE types of people?
« on: July 05, 2015, 10:36:38 AM »
     Hello All:

     I would like to know if there is a device that I may plug my power tools into to limit their speed.  What I have in mind is a Dremel Tool, a rotary buffer and angle grinder. 

     There may be a device that I may purchase that will enable me to slow my tools a bit so that they are more  versatile.

      Suggestions?

                                                                                                       Tia,

                                                                                                        Frank McCune

Offline EddyR

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Re: For all of the EE types of people?
« Reply #1 on: July 05, 2015, 11:18:59 AM »
a simple light dimmer will work. that is what i use.
Ed
Locust NC 40 miles from the Huntersville field

Offline frank mccune

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Re: For all of the EE types of people?
« Reply #2 on: July 05, 2015, 02:34:59 PM »
     Hi Eddy:

     Thanks for the tip! I wonder if it can handle the current draw of the buffer?  I will check!

                                                                            Be well my friend,

                                                                            Frank

Offline Tim Wescott

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Re: For all of the EE types of people?
« Reply #3 on: July 05, 2015, 04:11:59 PM »
There are speed controllers like that which are specifically for power tools.

AFAIK they won't regulate speed -- they just limit the power to the tool, which sorta-kinda slows it down.
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Offline Tim Wescott

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Re: For all of the EE types of people?
« Reply #4 on: July 05, 2015, 04:19:14 PM »
I forgot to mention: the simple ones that are already out there probably work GREAT, at least as long as your tool uses a universal-wound brushed motor (in other words: darned near anything hand-held, but probably not a bench grinder).

You don't want to ask this sort of question on an engineering group, because the first thing an engineer will do is to think of the problem that would be fun to solve, then they try to talk you into that solution.  Since "fun" equals "lots of the boss's money" to an engineer, this isn't always the best solution if you just want to grind something without burning it up.
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Offline Jim Carter

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Re: For all of the EE types of people?
« Reply #5 on: July 06, 2015, 08:37:03 AM »
 :)  Howdy!  Maybe you can get your hands on one of either of these!  I use the Solid State Speed Control for my fixed speed Dremel and the variable allows me to work away from the bench without toting too much stuff around.   ;)

Jim

Online Brett Buck

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Re: For all of the EE types of people?
« Reply #6 on: July 06, 2015, 01:28:45 PM »
For many small hand tools, you can just adjust the voltage with a simple dimmer switch or even a variac, causing power loss and thus slowing the tool down. For anything using a synchronous AC motor (a typical small industrial motor like a table saw, etc), this will not work very well, or not at all, because it drives the tool at some multiple of the AC cycle rate (60 Hz). Typically this is either 1725 or 3450 (2x 1725) rpm. If you slow it down very much it will overheat, and if you slow it down enough, it will have to revert to the "start windings" which are not intended for prolonged use, just enough to get it up to speed. More than a few seconds and the motor will start smoking and burn out. There are ways to slow it down (by altering the AC frequency) but they are not really practical for this application.

   This sort of thing is why a lot of variable-speed industrial applications (elevators, diesel-electric trains) use DC motors. Until fairly recently, the power company in New York City (Consolidated Edison) was still supplying DC power to run elevators.

    Brett

Offline Phil Krankowski

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Re: For all of the EE types of people?
« Reply #7 on: July 06, 2015, 01:42:59 PM »
There are two types of motors used is small power tools.  Universal and synchronous.  

Universal motors have brushes, and will run on DC if the correct requirements are fed to them, but manufacturers don't want you to do this and won't give the DC requirements.  These motors can "safely" use a rheostat to reduce the voltage and slow the motor.  It is harder on the motor, and the correct rheostat needs used.  A Dremel may be fine with a dimmer switch.  Anything larger, probably not on a light dimmer, but speed control rheostats are available.

Synchronous motors require the AC frequency to be altered to alter the speed.  This is not too different from the brushless motors used in electric airplanes, and the speed control is pretty much the same function.  These devices are also available if required.  (Variable Frequency Drive VFD)

Phil

Offline Tim Wescott

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Re: For all of the EE types of people?
« Reply #8 on: July 06, 2015, 02:33:00 PM »
Just a very minor nit, because some days it seems that God has led me to Stunthanger for the express purpose of disagreeing with Brett Buck over totally insignificant issues (but, this is an engineering group, and disagreeing over nits is one of the things that engineers do):

The "other kind" of motor commonly found in power tools isn't synchronous.  They're called induction motors and they're almost, but not quite, synchronous -- this is why you see speeds that are almost, but not quite, 3600 RPM (60Hz), or 1800 RPM (60Hz/2, because it's a multi-pole winding), or (more rarely) 1200 RPM.  Here's the skinny: https://en.wikipedia.org/wiki/Induction_motor.

However, both Brett and Phil are entirely correct that an induction motor is going to be very unhappy running on reduced voltage: it'll be trying it's best to run at full speed, which means using it's usual amount of electrical power.  But the voltage will be lower, which means it'll have to suck more current, which means that it'll get hot and, in the worst case, it'll let out the magic smoke and cease to function (and stink up your shop).  Phil is also correct that a VFD will solve that problem, although I've got zero experience with it.

There are such things as synchronous motors.  Old electro-mechanical clocks use a kind of self-starting synchronous motor called a hysteresis motor.  Synchronous motors were used in mills and factories for bits that needed to be constant speed and that didn't have to have controlled speed.  Such motors might have power ratings up to the hundreds of horsepower, and been as large as a car.  The big synchronous motors weren't self-starting: they either had a pilot induction motor to get them going, or they were designed so that they would operate either as an induction machine for starting, or as a synchronous machine for running.  Getting them running involved switching them over to being an induction machine, waiting until they were up to speed, then throwing a honkin' big switch (and probably ducking -- the closest I got to these things was in engineering lab in school, and even a 1HP synchronous machine makes some pretty alarming noises when you switch it over).

Just for grins, consider the fact that a brushless DC motor is essentially a synchronous machine with a permanent magnet providing the field (which means it can't be run as an induction motor).  Instead of starting on induction and switching to synchronous operation, however, the ESC synchronizes itself to the motor, driving at a frequency that's tailored to the need of the machine it's turning.  Brushless DC motors with Hall-effect commutation, similar to RC car motors, are making their way into high-end power tools for the same reasons of weight and battery economy that we like using them in our planes.
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Online Brett Buck

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Re: For all of the EE types of people?
« Reply #9 on: July 07, 2015, 04:20:37 AM »
Just a very minor nit, because some days it seems that God has led me to Stunthanger for the express purpose of disagreeing with Brett Buck over totally insignificant issues (but, this is an engineering group, and disagreeing over nits is one of the things that engineers do):

   Thank goodness someone is keeping me straight, what with the endless stream if misinformation I emit. However, for once, you are actually correct, I used "synchronous" when I meant "induction".

     Brett

Offline dave siegler

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Re: For all of the EE types of people?
« Reply #10 on: August 27, 2015, 02:51:50 PM »
No one really answered his question.

Most of the motors in your shop are either universal, and they will respond to speed controls, or some form of single phase induction, that will not respond well to speed controls. 

Single phase induction motors are not easy to control the speed.  
Single phase motors need some way to start them, either capacitor, split phase winding or shaded pole for very small one.  There are some special drives that will control the speed of special single phase motors.  It is still a trick to get them to start. 


Induction motors run at a synchronous speed less a little slip ( due to a 50 or 60 hz line)  which is where everyone gets messed up.     


Industrial induction motors are 3 phase, even in real small sizes.  By using a controller that varies both the frequency and voltage, you will get very good control out of a 3 phase motor.  

Synchronous motors can take a lot of current to get them moving when line started.  The back EMF is real large when that are out of step.  A industrial 3 phase drive can usually get them moving, but at reduced starting torque. Permanent magnet motors run well on speed controls.
« Last Edit: August 28, 2015, 04:01:50 AM by dave siegler »
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Offline Fredvon4

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Re: For all of the EE types of people?
« Reply #11 on: August 27, 2015, 03:44:47 PM »
My grizzly 1" belt and 5" disc bench sander does not have speed control

I use a relatively inexpensive Harbor Freight Router Speed controller. It works fine for the short use (less than a few minuets) that I need to slow it down to do delicate shaping

http://www.harborfreight.com/catalogsearch/result?q=router+speed+control

$20 and rated at 120V 15A

I have used this el cheapo on my fairly good Porter Cable routers and it does OK, but I only use it for short periods

Most all my other airplane shop tools are variable speed: scroll saw, 9" band saw, Mini Grinder/buffer,

Additionally if you can find a purely analog battery charger (12Vdc 2 to 10 Amp out) this router speed control can drive it for a foam cutting bow and let you set the correct heat for the length and diameter you are using.

It will not work on any charger that has a sensing circuit to control the charge rate

Any way this thing is much cheaper that an variac
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