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Old 04-20-2017, 01:56 PM
Brice Turnbull Brice Turnbull is offline
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Soft start grinding wheel with a variac?

So my 14" Covington flat wheel grinder from about 1994 needed a new motor, as the old one was very tired, and I could pretty easily stop the wheel if I pushed glass down on one of the magnetic grinding or polishing pads from HIS Glass.

I've replaced the motor once or twice over the years, with the same 1/4 horse power motor as the original. This time I decided to go with a more powerful motor, and got a 1/2 horsepower motor.
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

It is mounted now, and works great!

Except that when it starts, the entire grinder jumps about 4 inches sideways from the torque.

I'd like to soft-start the motor somehow so it doesn't break the shaft pin or any other parts over time.

Will this do the job? I'm thinking I'd dial it up from 0 to 100 to start, and back down to stop.
http://www.ebay.com/itm/Variac-Varia...38.m2548.l4275

I looked briefly at 'soft-starters', which I don't understand and which cost several hundreds of dollars, and have too many and strange electrical connections to look appropriate to me. Is there a different device I should use instead of a variac?

The motor I'm using is 8.8 amps at 115 volts.

Thanks all!!

Last edited by Brice Turnbull; 04-20-2017 at 02:07 PM.
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Old 04-20-2017, 02:21 PM
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Pete VanderLaan Pete VanderLaan is offline
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Loosen the v belt.

Since my grinders are all 2HP with 1.75 inch shafts and 31-38 inch dia , 2 inch thick wheels I have a lot of trouble with this. It needs to be big and sturdy.
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Old 04-21-2017, 01:40 PM
Dave Bross Dave Bross is offline
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I'm assuming AC motor?

Variacs essentially control voltage.

Cut the voltage and you need to "make it up" in amps . . .which strains and heats up the motor.

AC motor controllers work like the fancy furnace controls, cutting off the power for microseconds in the course of one cycle of the AC power. They still deliver full voltage.

What Pete said seems the least expensive/best/easiest way.
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Old 04-21-2017, 02:33 PM
Brice Turnbull Brice Turnbull is offline
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Thanks Pete!
That seems entirely too simple

I will buy the next longer belt and try it out - hadn't thought of loosening the belt, as I'd been working to tighten the whole operation up...

Thanks for the thoughts Dave - I didn't understand about the operating process for a variac.
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Old 04-21-2017, 03:31 PM
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I think it's better to have the motor for the pulley be on an adjustable hinge that tightens on a piece of all thread. One inch is a lot of difference in a belt.
It may be jumping like that for a very different reason. You may have a bearing that's seizing on the shaft.
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Old 04-21-2017, 05:28 PM
Brice Turnbull Brice Turnbull is offline
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Do you mean a bearing on the shaft of the grinding wheel? If so, I'm sure that is alright, as at one point in this process the pulley wheel fell off that shaft, and it spun for about 30 seconds without seeming to slow down, and there are no squeaks.

The motor turns smoothly by hand, so I don't think there's any problem from the bearings over there either.

It's just a sudden lurch when I flip power to it. It seems to me it's the torque of the immediate start - thanks for helping me think though. I want to solve it whether it's the obvious answer to me or something else.

I've got the motor mounted on bolts, and could move it out for the new belt to have better tension. It's currently 'in' all the way on the bolts.
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Old 04-21-2017, 06:07 PM
Michael Ahlefeldt-Laurvig Michael Ahlefeldt-Laurvig is offline
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What about simply bolting down the machine to whatever its standing on?
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Old 04-21-2017, 09:31 PM
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I'm assuming you turn it on and leave it on for quite a while. If that's the case, and (as you say) it all spins quite freely by hand, why not just spin it up by hand to something near the rotation speed and then turn on the motor? As it's already spinning, there should be no torque bump when you do that.

I'd say at least give it a try, as the cost is pretty much 0 (just some elbow grease).
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Old 04-22-2017, 02:09 AM
Brice Turnbull Brice Turnbull is offline
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Thanks Richard,
I tried spinning it by hand, but it is quite stiff when the belt is hooked to the motor and I can only get it up to (?) 15% of the speed the motor takes it to.

Hi Michael, yes, I could bolt it to the table it's on, but I am concerned the pulley wheel being started so violently will eventually snap the pin that holds the pulley wheel in place on the shaft.

I'll try a longer belt and see what happens with that this next week.

Thanks for all the input! I appreciate it
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Old 04-22-2017, 07:20 AM
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That sounds like the belt is too tight.
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Old 04-22-2017, 09:43 AM
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Quote:
Originally Posted by Richard Huntrods View Post
I'm assuming you turn it on and leave it on for quite a while. If that's the case, and (as you say) it all spins quite freely by hand, why not just spin it up by hand to something near the rotation speed and then turn on the motor? As it's already spinning, there should be no torque bump when you do that.

I'd say at least give it a try, as the cost is pretty much 0 (just some elbow grease).
I was going to suggest this. I was in a position once where I was given instruction to do this when starting. I can't remember where I was. All the shops are starting to blur into one another. I thought it might have just been one of those voodoo things that we all do.
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Old 04-22-2017, 11:52 AM
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I suggested this as it was rather common practice on smaller (1/4hp or less) AC motors some years ago. As the motors aged various things would wear to the point they would just hum instead of starting. Giving the pulley a quick turn would start the thing running again. Most common in homemade shop equipment powered by scavenged furnace fan motors.
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Old 04-22-2017, 01:07 PM
Michael Ahlefeldt-Laurvig Michael Ahlefeldt-Laurvig is offline
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The motor is about 320 Watts - its not going to break anything--its tiny
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Old 04-22-2017, 01:11 PM
Michael Ahlefeldt-Laurvig Michael Ahlefeldt-Laurvig is offline
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Whats this with longer belt- make it slip as it starts? Thats great for the belt- not
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Old 04-22-2017, 01:34 PM
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Nothing is great here. It's an undersized machine from top to bottom. I'm not advocating total slip, i'm suggesting it should not be so tight that Brice has trouble turning the wheelhead. Rather than a different belt, it would be nice if it could be adjusted.
For me the notion of powering a rougher with a 1/4 HP motor is not something I can fathom. I've seen the Covington machines and the shaft is quite capable of flexing if you lean into it. I'm a grinding fool and I want machinery that never slows down and never flinches. Turned off? My 38 inch head takes two full minutes to stop turning.

This thing is what it is.
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Old 04-22-2017, 03:51 PM
Michael Ahlefeldt-Laurvig Michael Ahlefeldt-Laurvig is offline
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Yep exactly
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Old 04-23-2017, 08:06 PM
Brice Turnbull Brice Turnbull is offline
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You know, I wondered if the belt is too tight. It seems too tight to me too.

I'll report back after I try a longer one and let the bolts on the motor mount out enough to be a little snug but not tight.
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