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]

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.

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]

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

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]

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.

[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]

dc not an option

[Electric Comet]

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

[clare]

Why not?

[krw]

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

[Electric Comet]

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.

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]

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]

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]

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