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If your grizzly is an AC motor, you will kill it, maybe not now, maybe not tommorow, but you will kill it.

 

If it is a DC and had decent hardware on it, you can get a control with variable speed.

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Like I said, pardon my total ignorance here, but is there a reason I couldnt put a rheostat dimmer type thing on my Grizzly 2x72 grinder? It would be nice to slow the thing down sometimes......

 

You will eventually burn out your motor and here's why: The formula for Power is P= I(current)*E(voltage) Now lets say you have a One Horse motor 120 volts 1750 RPM and with some substutions we have have the formula solved for current we would have P/E=I

 

746 Watts/120volts = 6.21666666amps

746 Watts/60volts = 12.43 amps

 

As you lower the voltage you will draw more current and eventually you will burn out your windings on your motor.

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Ed, I have a Delta variable speed wheel grinder/buffer, it was made that way, is that thing designed to fail or have they figured out a way to "beat the system"?

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I believe the difference is in the type of motor. Universal motors can handle variable speed devices. Induction motors can't. That's why machinery built with induction motors often relies on pulley systems or the like to achieve different speeds. (Multi-speed induction motors do exisit, but AFAIK they're not infinitely variable. They're built with a different internal circuit for each speed -- basically multiple motors in one housing.) I'm sure Delta chose an appropriate motor for your machine.

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A couple questions on this subject:

 

What about using a bona-fide voltage transformer like a Variac unit? Would this cause damage to an AC motor?

 

Also, has anyone ever tried using the motor from a treadmill on a grinder? I believe they are transformed to DC; I know the speed is variable.

 

Thanks,

 

Don

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A couple questions on this subject:

 

What about using a bona-fide voltage transformer like a Variac unit? Would this cause damage to an AC motor?

 

Also, has anyone ever tried using the motor from a treadmill on a grinder? I believe they are transformed to DC; I know the speed is variable.

 

Thanks,

 

Don

 

 

Well a Variac is basically the same thing as a light dimmer just heavy duty. The ones I have seen inside contain a coils wrapped around a horse shoe shape with a wiper that taps the voltage at any location. This would still cause the voltage to drop causing the current to raise.

 

I have seen quite a few people use treadmill motors on lathes and such, but these motors are pretty open. I would worry about getting grindings inside the motor but they are DC motors. In any case, one looses torque if the proper voltage is not attained since the motor turns because of an elctromagnet acting on the rotor will not have as much influence. The reason DC motors are more desirable for speed control is because they retain thier torque at slower speeds. I bet your wondering how....well they use something called Pulse Width Modulation where the same amount of voltage is applied to the coils but the current is turned on and off very quickley so the motor still has the same torque (maybe over simplifying here but that's the jist of it). Three phase motors powered by three phase power speed is controlled by varying the frequecy of the power as well as a three phase motor run on Single phase 220 curcuit. The only difference is your speed controller makes the third phase electronically. Without getting too deep in motors no, don't try to control your speed on a single speed Induction motor since as voltage decreases current increases.

 

Having said all that, if I were to try to experiment I would take a $1-5 dollar garage sale motor or one from a gas dryer and refer back to the formula P=IE and Ohms LAW E=IR. AS you notice if R increases so does E. Since you only have say 120 volts available, that couldn't happen so the current would have to decrease. You will also notice that when the current lowers in the power formula the Power will follow. Having said that, if I HAD to try to slow down a motor that wasn't designed to go slower, I would try adding resistance to each pole (four for a 1750 rpm if I remember correctly) You must add to each pole equially. This can be accoplished by several different ways, with capacitors inductors or just a good old resistor. Further, you could rewind the motor with different amount of poles....there are some on the net that have :wacko:

 

A more realistic but not nearly as convinient approach would be to change the size of the drive wheel. Lowering the diameter of the drive wheel should slow the feet per minute passing the platten. As a bonus, your motor will be supplied with the proper cooling from the fan, current in specs, increased torque since the motor isn't working against such a long lever (wheel) no motor melt down.

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Gents,

 

I'm not an electrical expert, but I thought I would chime in and add that a variable speed motor is a *wonderful* thing on a grinder.

 

I purchased a variable speed Burr King grinder last year and have been amazed at the control it permits me.

 

You know that teeth grinding feeling you get when you've moved to the 600 grit belt on a hollow ground knife and it slips just slightly right as you're trying to merge the polish into the curve of the blade? On my old single speed, 3000+ RPM grinder that was a "throw it in the scrap bin" moment. Now that I have a variable speed motor I just go slower and slower the finer the grit gets. Small slips result it minor dents, not catastrophes.

 

IMHO it's worth the extra effort/money to get the variable speed feature.

 

Best of luck in your quest!

 

-Dave Stephens

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I agree that speed control would add a whole new level of pleasure grinding, but I hope I have talked the group out of the light dimmer idea. Motors are too expensive to chance burning them up and in my way of thinking, if the motor in a store bought tool were designed to support variable speeds chances are they would be available already geared up to accomplish it out of the box.

 

I'm pulling for ya, keep your stick on the ice......

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Yeah, thanks Ed... I think I'll just hold out for a good motor and a couple step pulleys.

 

I think I'm gonna try the Variac on an old electric blower for my coal forge. The whole rig cost nothing, so I'll be out nothing if it fails.

 

I appreciate it,

 

Don

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Yeah, thanks Ed... I think I'll just hold out for a good motor and a couple step pulleys.

 

I think I'm gonna try the Variac on an old electric blower for my coal forge. The whole rig cost nothing, so I'll be out nothing if it fails.

 

I appreciate it,

 

Don

 

While I was in the Navy when I calibrated test equipment occationally the proceedure would call for the use of a Variac to vary the input voltage to make sure that the item under test would still read within spec... Just might be overkill for only a blower.

 

Happy grinding!

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  • 5 weeks later...
You will eventually burn out your motor and here's why: The formula for Power is P= I(current)*E(voltage) Now lets say you have a One Horse motor 120 volts 1750 RPM and with some substutions we have have the formula solved for current we would have P/E=I

 

746 Watts/120volts = 6.21666666amps

746 Watts/60volts = 12.43 amps

 

As you lower the voltage you will draw more current and eventually you will burn out your windings on your motor.

 

 

I'm back after a long hiatus. College is a handful. Finally something on this website I know something about. This is kinda right, but there is still Ohm's law. Voltage = Amperage * Resistance. The voltage in will always be 120 volts, so as you turn up the rheostat, resistance increases and amperage drops. The first problem with the rheostat is I^2*R (formula for electrical power converted to heat) I've managed to melt a few rheostats. The real problem with why it does not work is that AC motor speed is controlled by frequency, not voltage. (DC motor speed is generally voltage controlled) Alternating current is just that, a current that alternates. Any current carrying wire has a magnetic force. Imagine you have this wire wound inside the motor and run an AC current through it. Your magnetic field strengths would look something like a sine wave. The magnets in the motor lock into one of the waves and it more or less surfs on the wave. The rheostat basically just makes the waves smaller, causing the magnets to "slip." So yes, it kind of works, but the reason motor controllers are so much more expensive then rheostats are because controllers need to increase or decrease the frequency of the wave and rheostats just change the amplitude.

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Not real sure I am following you on this one and on one point I completely disagree. Current in a series circuit remains constant, only the voltage drops as described on http://buphy.bu.edu/py106/notes/Circuits.html . After rethinking things though I will have to admitt my power calculation and line of thinking was not correct. I sat down and did a circuit analysis and found that with a 1 horse motor it would consume 746 Watts at 120 volts and draw 6.2 Amps. If you add a resistor into the circuit say 5 ohms, your current would drop to 5 amps sure enough. But you also have to consider the added resistor is using 25 volts up leaving only 95 volts for the motor. Having said that, the motor will still have the same 5 amps but at only 95 volts so it will not draw 746 Watts any more it will draw 475. Somewhere close to a 40% loss in power at the motor. So I guess what I am saying is I don't see where the motor should heat up and burn, but I know it should. Where is my error, my calculations just don't make sense to me.

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Ok, I just might have the heating up and burning out answer, but it's not explained my Ohms law directly. I found you can determine for 60 hertz electrical systems: 7,200 divided by the number of poles in the motor gives you the no load RPM. So a four pole motor at 60 Hz will turn 1800 and a two pole at 3600. The formula does not take into account the voltage, only the frequency. I am convinced the reason a motor will melt down is because you are more likely to stall the motor since my above example shows you loose almost 40% of the power with a 5 ohm resistor. The motor can draw somewhere around 6x the current in locked rotor condition, sort of like a solinoid that can't pull in fully.

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Not real sure I am following you on this one and on one point I completely disagree. Current in a series circuit remains constant, only the voltage drops as described on http://buphy.bu.edu/py106/notes/Circuits.html . After rethinking things though I will have to admitt my power calculation and line of thinking was not correct. I sat down and did a circuit analysis and found that with a 1 horse motor it would consume 746 Watts at 120 volts and draw 6.2 Amps. If you add a resistor into the circuit say 5 ohms, your current would drop to 5 amps sure enough. But you also have to consider the added resistor is using 25 volts up leaving only 95 volts for the motor. Having said that, the motor will still have the same 5 amps but at only 95 volts so it will not draw 746 Watts any more it will draw 475. Somewhere close to a 40% loss in power at the motor. So I guess what I am saying is I don't see where the motor should heat up and burn, but I know it should. Where is my error, my calculations just don't make sense to me.

 

Sorry, I think I worded what I said poorly/over simplified, looks like I need to get more technical. Voltage into the system will always be a constant, Kirchhoff's circuit law says the number of electrons entering a node must be the same as leaving a node, so when resistance is added, it is voltage drops that will increase. However, all of the voltage drops must equal the starting voltage of 120 (Kirchoff's voltage law). So yes, adding a 5 ohm resistor will drop the voltage at the motor to 95 volts, but the input voltage is still 120. I don't know how to quote twice, but yes, no load RPM is controlled by frequency and the number of magnetic poles in the motor. Lastly, you're right about the motor burning up. Lenz's Law and conservation of energy. When a motor is spinning at full speed, the voltage into the motor might be 90, but because the wire is moving in a magnetic field, it is also inducing a current in the opposite direction (back EMF), so the voltage in the coil might only be 1 or 2 volts. At zero rmp, there is no back voltage, so all 90 volts pass through the motor and all the amperage that goes with it. This is why motor controllers also act to limit starting current. Hopefully this makes more sense.

 

*Edit/re-read original post*

Ya, my original post was pretty ambiguous/crappy.

Edited by Tony C
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  • 3 years later...

im very sorry for bringing up such an old topic but, cant i just buy the grizzly, and then get a nice motor that can handle the variable speed modifications?

 

the electrical jargon is a bit over my head...lol

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A lot depends on your inginuity. Most of those grinders have the business end mounted directly to the motor and probaly won't easily fit onto a different motor. There are several ways around this problem, but unless you already have the Griz, it would most likely be easier/cheaper to invest in one of the other grinders or kit grinders that lend themselves more towards a step pulley system.

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I think the easiest, but not necessarily the cheapest, way to go is a

3 phase motor and a Variable Frequency Drive. The motors are inexpensive,

but the drives are not.

 

Besides, if you hook a a single phase motor to a variac or dimmer, eventually

the sacred smoke will leave the motor :angry: , and we all know when that happens

you can't put it back. :lol:

 

Bill

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im very sorry for bringing up such an old topic but, cant i just buy the grizzly, and then get a nice motor that can handle the variable speed modifications?

 

You could but, why? The Grizzly has quite a few shortcomings and if you wish to buy a grinder without a motor you can get a much better one for about the same cost. You will have to put some labor into it but, you would have to do so swapping out motors on the Grizzly as well. USA Knifemakers Supply sells plans (I thought they had kits too?) for a No-Weld Grinder. Jamie Boley sells a Grinder in a Box, which is most of the parts to build a grinder. Both of these are much sturdier, heavier, machines than the Grizzly - more akin to a KMG. One of the biggest failings of the Grizzly (at least to me) is lack of clearance on the left hand side of the belt because the motor is there. If I was to replace the motor with a better one I would want to move the motor out of the way, add a shaft and pulley to the grinder attachment, slap a pulley on the motor shaft, and add a v-belt. At that point you are buying parts and adding labor so, why not just get one of the above kits (which are better grinders to begin with) and put the money and effort into it?

 

~Bruce~

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