1725 rpm motor & VFD drive-4" or 6" drive wheel?

J

Justin King

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Nov 8, 2009
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I have a Dayton 2 hp 3ph motor that I picked up for free some time ago. I finally have some funds to get a VFD drive and set up one of my grinders for variable speed, but I had thought the motor was 3450 rpm and it turns out it's only 1725. Would I be better off with a 6" drive wheel to get the upper speed range?
The motor has a 3/4" shaft so I will have to get a new drive wheel either way, I can go with a 4" or a 6".
 
It is direct drive, one of the GIB kit grinders. I'm currently running a 3450 rpm single phase motor and a 4" drive wheel, I want to be able to reach at least the same belt speed with the VS. The obvious answer would be the 6" drive wheel. However, I read in another thread that the VFD drive will allow the 1750rpm motor to run at higher rpms than rated, so I'm not sure whether the 6" drive wheel is necessary or not.
 
Here's my understanding and experience.
I have a GIB, KBAC VFD and a 1.5hp 1725 3ph motor.

I run mine on 2x speed on the VFD. This gets you 3450 RPM.... BUT, that's at 120hz. The motor only uses 100hz efficiently before losing torque/hp and the motor does slow slightly when really hogging. Might not be as much of an issue with a 2hp motor. I spent.a few hours researching VFD/HZ/HP relationships on the electric motor forums and learned a lot. That means at 100hz you only get around 2900 RPM before losing power/efficiency.

I hope my explanations are worded/expresse properly. Any electricians who want to clarify feel free.

I find my Blaze belts cut better at the high end. I'd get the 6" wheel if I had it to do again, but, not enough of a difference with the amount of time I spend on it....
 
So you are using 72" belts or something smaller? 3450 Rpm is very very fast.
 
ron_m80 said:
So you are using 72" belts or something smaller? 3450 Rpm is very very fast.
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Yes, 72" belts. The grinder is mechanically identical to a KMG for the purpose of this discussion and is compatible with KMG tooling and rolling stock. 3450 rpm with the 4" wheel is pretty fast, it works well for hogging annealed stock but my finishing abilities on the grinder are obviously pretty limited at that speed, and it takes a toll on small wheel bearings, which is why I want the VS.
 
Brian Ayres said:
Here's my understanding and experience.
I have a GIB, KBAC VFD and a 1.5hp 1725 3ph motor.

I run mine on 2x speed on the VFD. This gets you 3450 RPM.... BUT, that's at 120hz. The motor only uses 100hz efficiently before losing torque/hp and the motor does slow slightly when really hogging. Might not be as much of an issue with a 2hp motor. I spent.a few hours researching VFD/HZ/HP relationships on the electric motor forums and learned a lot. That means at 100hz you only get around 2900 RPM before losing power/efficiency.

I hope my explanations are worded/expresse properly. Any electricians who want to clarify feel free.

I find my Blaze belts cut better at the high end. I'd get the 6" wheel if I had it to do again, but, not enough of a difference with the amount of time I spend on it....
Click to expand...

I think that was the info I was looking for, the part about the 2x speed setting. Assuming that the VFD allows the motor to run well at very low rpms, I don't see any reason not to get the 6" drive wheel. I don't have much problem with it bogging down with the 1hp motor/4" drive wheel, so I don't think I will suffer much in the power department with the 2hp motor/6" wheel. My biggest concern now is that I don't know of any way to hook to motor up and test it without shelling out for the VFD first, but that's the risk that comes with a free used motor, I guess.
 
I use a 1750 motor and have used both a 4 and 6 inch wheel. And while the 6 inch has a higher sfpm, The grinding speed seems to be roughly the same with the 6 inch being slightly quicker. If I purchased one though i would take the 6 inch. But like i said the difference isn't large enough to warrant purchasing a 6 inch if you have a 4 already.
 
Sounds good. I thought sanding belts were rated at some FPS sped?

You can go wild with your rpm if you like, using the pulley and VFD. Or you could set it up for a target speed and build it for that. Discussing sheave sizes, and using VFD's at 120hz are both seperate discussions. Some might argue not, but I'm pretty sure that they all effect speed and power in a very non linear wierd relationship. Something a novice looking for reliable use might consider in my opinion.
 
6" drive wheel increases sfpm by roughly 51% compared to 4" drive wheel. If your motor is inverter ready/capable then you would a higher natural top speed without doing the 2x setting. I would check with motor manufacturer that the motor would perform well at hz up to 100. You would loose some lower speed maybe considering that inverter ready motors are capable of a 20:1 turn down. I discussed this at length with Baldor when finding a great deal on a 3hp 1750 rpm motor. I ran many numbers and felt that the 6" wheel gave me the numbers that i wanted. I looked at the max numbers for the wilmont grinder and the tw90 and they run up to 5652 sfpm and 4400 sfpm respectively, so why not have the capabilities and just turn the vfd down a bit when you needed to. Many belts are supposed to be run at very high speeds but many folks run them at slower speeds and they still work. Some of the newer belts are designed around very high sfpm specs.
 
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The ceramics like Blaze benefit from high speed and high pressure. I just downloaded the specs for the Blaze. I couldn't find a max or recommended SFM but they did show them tests done at 5900 and 6663 SFM.
I spoke with the guys at Trugrit and they said guys who use push bars and jigs and screaming speeds can grind more blades/parts per belt. If you use ceramic belts that's something to consider.

I still say go with a 6" wheel and 2x speed then you for sure will never exceed 100hz even when screamin speeds are used.
 
His 2hp motor should be fine to run up quite the overdrive on.

I just didn't get the bit about going to VFD settings to output double the motor nameplate rating, while he has the opportunity to set everything up within proper limits. The VFD is probably providing the third phase leg for his motor, and he has the time/ oportunity to set up a sheave ratio now that will deliver his high end belt speed. Also regardless of the method of the motor modulation the sheaves need to be rated for the speed you will be spinning them as well. I could check, but maybe 3000+ rpm is a bit much for cast iron sheaves, and that is something to consider too.
 
ron_m80 said:
His 2hp motor should be fine to run up quite the overdrive on.

I just didn't get the bit about going to VFD settings to output double the motor nameplate rating, while he has the opportunity to set everything up within proper limits. The VFD is probably providing the third phase leg for his motor, and he has the time/ oportunity to set up a sheave ratio now that will deliver his high end belt speed. Also regardless of the method of the motor modulation the sheaves need to be rated for the speed you will be spinning them as well. I could check, but maybe 3000+ rpm is a bit much for cast iron sheaves, and that is something to consider too.
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I agree with setting things up ideally so that you get desired effect. With a belt drive setup from motor to drive shaft you can easily get pulleys in such small incremental changes that almost any rpm at the drive is attainable. One neat thing that a drive belt allows for is a dramatic rpm change with a new $8 sheave. Using smaller sheaves keeps the costs down also, yet the ratio of motor pulley to drive shaft can be achieved with numerous pulley combinations. I'm pretty sure that low to mid 3000 rpm is reasonable for TD Woods sheaves. Their number is 888.829.6637 if someone is curious.

He did state that be is running direct drive so he will not have this benefit but it might be worth considering. On my vfd setup, I ran the following:

Using id of sheaves for the following calculations:
Motor sheave: 4.2 id
Drive shaft sheave: 2.3 id
Ratio: 1.83
Motor rpm: 1760
Drive shaft rpm 3214
Using 6" drive wheel, factor of 1.57, delivers 5045 sfpm

Using od of sheaves for the following calculations:
Motor sheave: 4.95 od
Drive shaft sheave: 2.55 od
Ratio: 1.94
Motor rpm: 1760
Drive shaft rpm: 3416
Using 6" drive wheel, factor of 1.57, delivers 5363 sfpm
(I don't which is the more accurate sfpm, maybe someone can chime in.)

Per Baldor, motor is rated for 20:1 turndown so I can reasonably slow it down to around 260 sfpm. Should be fine for me. Just an FYI, but Rob Frinks KMG using factory specd pullies runs at 3675 sfpm(using 2:1 motor to drive shaft ratio) by my calculations. He made the 6" drive wheel available since folks coming from other grinders that preceded his KMG were running even higher sfpm. Wilmont grinder runs at 5652 sfpm. TW90 runs at 4400 sfpm.

Hope this helps someone a bit.
 
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ron_m80 said:
Also regardless of the method of the motor modulation the sheaves need to be rated for the speed you will be spinning them as well. I could check, but maybe 3000+ rpm is a bit much for cast iron sheaves, and that is something to consider too.
Click to expand...

Just looked up the rpm for a TD Woods AK2534 sheave that I have a my 3/4" drive shaft.

http://www.amazonsupply.com/tb-woods-ak2534-outside-diameter/dp/B003N17JU2

Max rpm is quoted to be 9810 rpm so for most belt grinder applications the safety margin is there.


My larger motor sheave is a TD Woodsn BK52.

http://www.amazonsupply.com/dp/B003N17SOO/ref=sp_dp_g2c_asin

Max rpm is quoted as 5060 rpm, again based on 1760 motor rpm, so again plenty of safety margin barring a manufacturing defect.
 
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Just to give a a comparison to other well made grinders:

KMG grinder specd as sold by Beaumont Metal Works: 1750rpm motor; resulting in 3675 sfpm with 4" drive wheel. 6" drive wheel would result in 5549 sfpm with standard 2:1 sheave ratio.

TW90: 1725 rpm motor with 50% overdrive resulting in 4400 sfpm using 6.5" drive wheel.

Wilmont: 1800 rpm motor with 2x vfd setting resulting in 5652 sfpm with 5" drive wheel.

I see no reason to use a 4" drive wheel when you have to buy one anyways. Go 6" and you will have good adjustability. On your direct drive setup, you would have 2708 sfpm in standard mode and 5416 in 2x mode. I know that the KB ac drives have this 2x setting but not sure about other brands, but you really should go with a nema 4 and the options for nema 4 ac drives in a small package are slim. Hope this helps.
 
Just out curiosity, I ran some numbers and a 9.75" drive wheel using your 1725 rpm motor in standard mode would give you 4400 sfpm. Maybe Rob would make you a one off but at what price? :)
 
I appreciate all the responses, I'm fairly sure I have the info I needed. The greater efficiency of the ceramic belts at higher speeds is one of the reasons I was concerned about the 1725 rpm motor. I think the KBAC VFD and 6" drive wheel will put me in the speed range I'm looking for.
 
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