Building a Double 2X72" Need advice, 1725rpm or 3450rpm with the VFD??

O

Original

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I have read that an VFD can make a motor go over the rated rpmX3 is this correct?
What I need to know is a 1725 rpm motor OK or would the 3450 rpm be a better choice in 2HP? This double grinder will work off of shafts and can vary the speed with 4X4 pulley arrangement and one side or the other can be brought to run with an idler pulley and lock lever. I have lots of machining to do, but the whole unit will be made from scrap, quality scrap... lol..:)

Thanks for your help in advance, will post some pics in a WIP for this Project.
 
I'm tyring to learn about this subject too, but from what I gather (for a single grinder) if it is a direct drive (like Poloar Bear Forge's Grinder in a Box) use a 3450 rpm 3 phase with VFD. If the grinder is belt driven, like a KMG, then 1725 rpm 3 phase, with VFD is best. But, maybe someone who knows more will correct me and help us both out.

Jay
 
I have never seen a VFD that will overdrive the RPM on a motor. It doesn't mean they are out there but I haven't ever seen one. Overdriving a motor will eventually spin the wire armature right out into the housing. I used to be a firm believer in 1750RPM motors for grinding but I'm switching *officially* to 3400RPM.
 
1750 doesn't hog fast enough for me any more. With a VFD, I have infinite speed so when I want metal gone, faster is better. Now I will still slow it down by huge amount as I am working on the grind line and around 240 to 600 grit and then I'll take it back up to hurry the polish along with Norax 16x and 5x.
 
Ok, Thanks Boss, Sticks, Wayne, I will go 3450 rpm then, and 2HP should be all I need for this grinder. Does the VFD keep a pretty good amount of torque to the belt, even at a very slow speed?
 
A direct drive 3400 rpm motor is equivalent to a 1700 rpm motor with the 3 step pulley on the high speed

On the 3 step pulley you have 3 ratios

twice as fast = 3400
1:1 = 1700
half as fast = 850

So running the KMG, or NWG with a 1700 rpm motor on high speed is equivalent to the direct drive 3400

I believe that it is also the standard on the Wilton, Bee, Bader, & Dozier direct drive grinders.
 
I can get you a 3 phase 2 hp, 3400 rpm motor for about $200.00 plus shipping, if you are interested. On this type order I now have them shipped directly to the customer from the company and they use UPS.
 
You can get a VFD that will "overdrive" your motor. I wouldn't do it, however, exceeding a motors rated speed will just wear it out faster.
 
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