variable speed control

[Electric Comet]

some say not to add variable speed control to single phase induction
motors

but it is done for consumer products so the key seems to be that you
want to avoid varying the speed to the point where the starter winding
circuit closes and you burn up the starting winding


how to determine the speed of the starter winding

there is usually a click when it opens so the speed could be
measured at that point but are there other ways to calculate
that using the motor specs

fairly certain that the starting winding circuits open around
1500 to 2000 rpm

the cost for a vfd variable frequency drive must be the reason they
are not seen more in consumer products

or are there other reasons

want a grinder that can go 200 rpm or so but also do 3500

prety sure that to get that low and maintain some torque will
require a vfd

[whit3rd]

On Wednesday, October 19, 2016 at 8:06:10 AM UTC-7, Electric Comet wrote:
> some say not to add variable speed control to single phase induction
> motors

That's because running slower than design speed for most AC motor
types causes excess heat. They burn up.
Some very small motors (for fans) are intended to tolerate this.

> want a grinder that can go 200 rpm or so but also do 3500
>
> prety sure that to get that low and maintain some torque will
> require a vfd

Not necessarily; stepped pulleys, or a cone drive, or a gearbox can
do the job. Or, you could use a universal motor, or a pneumatic one.

[Electric Comet]

On Fri, 21 Oct 2016 03:11:32 -0700 (PDT)
whit3rd <whi...@gmail.com> wrote:

> That's because running slower than design speed for most AC motor
> types causes excess heat. They burn up.

finding out it is more complicated than that and as i said if there
is a starter winding it is more complicated

you can look at some variable speed tools and see they just avoid
going near the rpm where the starter circuit will close

> Not necessarily; stepped pulleys, or a cone drive, or a gearbox can
> do the job. Or, you could use a universal motor, or a pneumatic one.

pulleys would work but not what i want to do nor a gearbox

will have to research a cone drive but i think i know what that is
also will need to check for pneumatic and universal

pneumatic probably not going to be the choice but have never looked
into that so maybe i find different


have found VFD and motor combos that look good and are not crazy
expensive and the VFD gives a lot of flexibility

[krw]

On Fri, 21 Oct 2016 10:57:14 -0700, Electric Comet
<electri...@mail.invalid> wrote:

>On Fri, 21 Oct 2016 03:11:32 -0700 (PDT)
>whit3rd <whi...@gmail.com> wrote:
>
>> That's because running slower than design speed for most AC motor
>> types causes excess heat. They burn up.
>
>finding out it is more complicated than that and as i said if there
>is a starter winding it is more complicated
>
>you can look at some variable speed tools and see they just avoid
>going near the rpm where the starter circuit will close

Variable speed tool, sure, but not a simple speed control on an
induction motor. Induction motors are synchronous machines. If you
reduce the voltage, the current will go up to try to maintain RPM.
Whit is correct, they'll burn up (nothing to do with starter
switches). They way to vary the RPM of an induction motor is to
change the frequency of the drive, which is rather expensive but
do-able these days. The cheap way is to use a universal motor.

[ads]

You need a DC motor, a DC power supply and a PWM (Pulse Width
Modulation) controller. That's how most variable speed hand tools get
their unlimited variable speeds. Many of the drill/driver units have
a speed switch for low (below 500-600RPM) and high (above whatever the
max speed is for "low" speed.

---
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[krw]

Close. Universal motors are the same thing as DC motors but there's
no reason to use DC. Triac control of the AC line is perfectly good.
The switches on my drills are transmission (gear ratio) switches but
perhaps that's what you meant.

[John McGaw]

I would think from an engineering standpoint that simply adding
variable-frequency electronics to a single-phase motor would not be
satisfactory. When they are used with three-phase motors, the normal
situation, the rotor is essentially locked solidly to the rotating magnetic
field which maintains torque over a wider range of speeds. With a
single-phase motor there would be a greater chance of the rotor not keeping
up with the field when high torque is called for, reducing the available
operating range. I only have one stationary machine with variable speed, my
lathe, and that uses a DC motor with a feedback loop which maintains torque
down to ridiculously slow speed.

[Electric Comet]

On Fri, 21 Oct 2016 22:42:48 -0400

ads wrote:

> You need a DC motor, a DC power supply and a PWM (Pulse Width
> Modulation) controller. That's how most variable speed hand tools get
> their unlimited variable speeds. Many of the drill/driver units have
> a speed switch for low (below 500-600RPM) and high (above whatever the
> max speed is for "low" speed.

dc not an option

[Electric Comet]

On Thu, 27 Oct 2016 10:47:28 -0400
John McGaw <Nob...@Nowh.ere> wrote:

> I would think from an engineering standpoint that simply adding
> variable-frequency electronics to a single-phase motor would not be
> satisfactory. When they are used with three-phase motors, the normal
> situation, the rotor is essentially locked solidly to the rotating
> magnetic field which maintains torque over a wider range of speeds.

so far all the vfd motor combos i see use a three phase motor

single phase into the vfd and three phase out

but the one limitation i am seeing is that the motor have only
a single outboard shaft

[cl...@snyder.on.ca]

Why not?
>
>
>
>
>
>

[krw]

Why not? DC motors (with a diode or four) are also known as
"universal motors".

[Electric Comet]

On Tue, 01 Nov 2016 17:12:57 -0400
cl...@snyder.on.ca wrote:

> Why not?

have not looked and want to stick with ac

also i am guessing a dc setup will be more dollars


so no dc because it means more research

[John McGaw]

How serious are you about this and how much work are you willing to put
into it? The old-time solution to setting up a grinder/polisher/sharpener
was usually to put a motor on a benchtop arbor. Something like this comes
to mind.

http://www.brownells.com/gunsmith-tools-supplies/metal-prep-coloring/polishing-tools-accessories/buffer-arbors/330-arbor-sku177300330-4988-11235.aspx

I had a similar one, vintage 1930s, that was solid cast iron with babbit
bearings but I left it behind somewhere in my travels way back when because
it weighed so much.

[Electric Comet]

On Wed, 2 Nov 2016 15:18:07 -0400
John McGaw <Nob...@Nowh.ere> wrote:

> How serious are you about this and how much work are you willing to
> put into it? The old-time solution to setting up a

just researching what my options are for a variable speed grinder

off the shelf variable speed choices are very limited and even the
king or is it queen baldor have none as far as i can see

there is very little innovation going on with equipment makers

hand held tools seem to innovate but stationary does not

they have to be conservative as it is a competetive market and profit
margins are slim and they seem to not spend much on r and d and probably
use only customer feedback as the research and broken returns for new
design inspiration

[krw]

Oh, good grief. Too lazy to do a *little* research[*]. No problem.
You don't need a solution, evidently.
>
[*] Not surprising; too lazy for punctuation or shift key.

>

[John McGaw]

On 10/19/2016 11:06 AM, Electric Comet wrote:
>

Do you have a lathe that covers the range of speeds you desire? If so, it
would be a simple matter to add a shaft to hold 2 or 3 or 4 or more
grinding wheels of different grits to the lathe. You can use the tool rest
to brace the tool you are working with if you are doing freehand sharpening
or you could easily fabricate an angled rest for more controlled work. I
have seen this trick used with MDF disks coated with lapping compounds for
sharpening carving tools but there is no reason it wouldn't work with
abrasive wheels as long as you gave the lathe a good wipedown after sharpening.

[whit3rd]

One cannot save money by 'guessing', but you can do so with 'more research'.

The brushed motor tools you see around you (Dremel, many AC drills and saws)
are all 'universal motor' types and can be speed-controlled. These are a different
type than 'AC motors' even though they may have AC cords...

DC controlled motors are trivially speed-controllable, and many such motors with
control boxes are available off-the-shelf.

[Electric Comet]

On Thu, 3 Nov 2016 12:35:09 -0400
John McGaw <Nob...@Nowh.ere> wrote:

> Do you have a lathe that covers the range of speeds you desire? If
> so, it would be a simple matter to add a shaft to hold 2 or 3 or 4 or
> more grinding wheels of different grits to the lathe. You can use the
> tool rest to brace the tool you are working with if you are doing
> freehand sharpening or you could easily fabricate an angled rest for
> more controlled work. I have seen this trick used with MDF disks
> coated with lapping compounds for sharpening carving tools but there
> is no reason it wouldn't work with abrasive wheels as long as you
> gave the lathe a good wipedown after sharpening.

brought the topic up not so long ago on rec.woodturning

but the lathe does not go slow enough for this

there are some ready made solutions for this and they look decent
but i am going for a stand alone grinder solution