Grinder motor horsepower

AVigil said:
I found the key is a motor with good torque and high amps. As mentioned before if you are running a 1 or 1.5 HP motor that is only 8amps it will be a dog.

So always consider the amps when buying a motor
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Thanks for the tip. I checked mine and found it draws 13.1A. Maybe not a bad motor after all.
 
FWIW, stick with 1750RPM motors if possible.

If a 1HP motor draws less current than another 1HP motor, it is due to one of two things. Either the "rated HP" is not a true HP rating ( 5HP router drawing 12 amps), or the motor is less powerful. Amperage draw is the tell-tale of power ( assuming efficiency is equal).
3400 RPM motors deliver half the torque of a 1750RPM motor. This results in having to gear it down ( using pulleys and belts) to get the same torque output....BUT....there is some mechanical loss in gearing the motor down, so you get less power from a 3400RPM motor running your belt sander than a 1750 RPM motor. Also, most 3400RPM motors are less powerful due to the way the motors are wound. They will over-heat easier, too.

Most GRB's will run happily with a 1HP motor.
3/4HP would be fairly anemic, and stall under pressure, as well as overheat the motor.
1.5HP will run all grinding tasks most GRB units are capable of.
At 2HP and higher, the robustness of the grinder and the bearings comes into play, and many GRB units would be over-powered.

A 1HP-1.5HP 3PH motor and a VFD is the way to go if possible. ( not all 3PH motors are the same, either...but that is for another thread)
If not, a 1750RPM motor with a set of step pulleys will work quite well.
 
Dashrawder said:
Would you then go with a 4 or a 6" drive wheel?
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If you want to run ceramic belts and your grinder is built to handle the speed, go with the 6 inch drive wheel. You can always slow the belt speed by switching to the smaller pulley. I run 6's on both my kmg grinders and find the set up very adequate.

Fred
 
Thanks Fred. I'm graduating from files and sandpaper to a grinder that I am going to build. Just trying to get things as right as I can. What are the thoughts on "Grizzly" motors?
 
AVigil said:
I found the key is a motor with good torque and high amps. As mentioned before if you are running a 1 or 1.5 HP motor that is only 8amps it will be a dog.
So always consider the amps when buying a motor
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good point....

Although 8amps run off of 220v is another animal, or so I think..

lets say you have a 15amp high/low volt motor and run it off of 220v that would convert the motor to about 7.2 amps??

so would you not still receive the same hp powered by 220v? or the complete capacity of the motors hp, if not more??

I once plugged a holehog drill into a 220 outlet that was a common receptacle and WOW!!!!!!!!!!!! the power was insane!!
 
If you are grinding right, and letting the belt do the work instead of you, 1hp is sufficient for a grinder, 1.5hp is nicer because it's about as powerful as you can get on a standard wall outlet in the US. My variable speed grinder is a 2hp and is a beast. The amount of friction created is a based on the surface area of the contact as well however, so a large flat piece against a flat platen will bog down faster than the same piece against a contact wheel, etc.

If you let the belts do the work and dont try to force it, anything 1hp or above should be fine for all the work we do.
 
Motor windings, and nameplate amp draw, don't signify a conversion. When a higher potential is used (220v as stated above) the "work" is more easily done. The only conversion that takes place is the reconfiguration of the stator windings for the voltage supplied.
 
220 is drawn on two legs...110 is drawn on one leg. A 220 motor drawing 8 amps ( on each leg) is the same as a 110 motor drawing 16 amps ( on one leg). The advantage is more power from smaller shop wiring.

It gets real important when running a 7.5HP motor. It will draw about 35 amps on each leg. That will take a dedicated circuit and a 50 amp double breaker, but the panel will handle it and 8 gauge wiring should handle the load. If a 7.5HP motor was made to run on 110 volts, it would draw about 70 amps ( not gonna happen in the real world) and would require a special power panel and 2-4 gauge wiring. My first house only had a 60 amp (total) fuse box.
 
I see SOOO many people using belts that are worn past their usefulness and instead just leaning into the grinder extra hard to get them to still cut, rather than swapping to a fresh belt that will cut with far far less pressure. I see people bogging down motors not because the motor isnt sufficient for the job, but because they refuse to let the belt do the work, and insist on jamming as hard as they can. Even on a fresh belt, if you push too hard, you're going to shorten the actual useful life of your belts immensely, you'll both build more heat, and tear more grit off the belt.

I watched one person break 2 belts in a row (60 grit blue zirconia ones) and he said the joints were crap, so he gave me the other 4 that he had... They worked great for me and I never had a problem, and got a ton of use out of them (and more use out of the 2 broken belts, cut up for other jobs) The person who broke the 2 belts was using a 3hp motor and pushing for all he was worth, my grinder at the time was 1.5 horse and I don't push nearly as hard.

When it comes to abrasive belts and paper, the best advice I ever received was "use it like it's free"
 
Justin.Mercier said:
I see SOOO many people using belts that are worn past their usefulness and instead just leaning into the grinder extra hard to get them to still cut, rather than swapping to a fresh belt that will cut with far far less pressure. I see people bogging down motors not because the motor isnt sufficient for the job, but because they refuse to let the belt do the work, and insist on jamming as hard as they can. Even on a fresh belt, if you push too hard, you're going to shorten the actual useful life of your belts immensely, you'll both build more heat, and tear more grit off the belt.

I watched one person break 2 belts in a row (60 grit blue zirconia ones) and he said the joints were crap, so he gave me the other 4 that he had... They worked great for me and I never had a problem, and got a ton of use out of them (and more use out of the 2 broken belts, cut up for other jobs) The person who broke the 2 belts was using a 3hp motor and pushing for all he was worth, my grinder at the time was 1.5 horse and I don't push nearly as hard.

When it comes to abrasive belts and paper, the best advice I ever received was "use it like it's free"
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I agree with you there, don't waste time and energy with old worn out belts; put a new one on the machine.

I do believe many of the more modest priced belts, like AO and Zirconia can breakdown under excessive pressure, even when new. But ceramic belts are the exception; ceramic belts thrive on high pressure and high speed.
I've had this discussion with James Popin of POPS knife Supply; he told me about a group of makers that live around the Gulf Coast, who use a 5 foot levering system to apply pressure to the blade, while running the belt at 7,000 SFM. James said they can rough grind a hundred blades with one ceramic belt doing this.
Fred
 
Agreed, I could not get my Cubitron belts to work well until I started leaning into the belt with all my weight. It then started cutting like mad. The added pressure also creates shearing of the grit, thus constantly exposing new cutting surfaces.
I find that most shaped or structured abrasives ( cubic,and some others) work better under more pressure.

One problem with smaller motors ( actually all motors) is that they draw more current when resistive load is applied to the shaft ( like heavy grinding pressure). This causes the amp draw to go up, as well as the heat output of the motor to increase. It is hard on the motor and can pop a breaker or fuse. The bigger the motor, the less likely it is that you can apply enough resistance to the shaft to load the motor. On a 110VAC 1.5HP motor running on a 20 amp breaker or fuse, it is quite easy to spike the current high enough to make the breaker interrupt. If running a 1HP motor on 110VAC, the motor my burn up first.
If using a VS motor of less than 2HP, keeping a box of the Buss fuses for the drive handy is a good idea, as they are sized to fail before any motor damage will occur. I still pop one every now and then.
 
I'm going to build a 2×72 grinder and was wondering if a 5 hp motor with a vfd would be too much.
 
Robert01 said:
I'm going to build a 2×72 grinder and was wondering if a 5 hp motor with a vfd would be too much.
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Thats alot of power for 2x72,You wont suffer bogging down for sure. On a grinder, You cant really have ”too much” power, but you might not like paying the power bill.
Unless VFD & motor is already on hand, free or super cheap, I would not go 5hp.
 
Sturubu said:
I'm sure this has been discussed, but my searches came up dry...How low has anyone gone on motor horsepower - is it reasonable to try a fractional hp motor on a no-weld grinder (or similar) kit? Thanks.
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3/4 hp at 1720rpm. the motor was brand new and free. worked fine. I currently use a 1Hp at 800rpm. I don't use my 2x72 to 'hog' and most of my work is with 100 grit or less. another thing to look at is money. if you shop well, you can find a 1Hp 3 phase motor and 1Hp 120v VFD for less than $200.
 
AVigil said:
I run a 1.5 hp 1725 RPM motor I got from Wayne Coe. It never bogs down and I am a big man and can push hard. The only time it even thought of slowing down was with a 1.5" square bar of 6061-T6 and I was trying to bog it down . It only phased it for a moment of extreme pressure. I found the key is a motor with good torque and high amps. As mentioned before if you are running a 1 or 1.5 HP motor that is only 8amps it will be a dog. So always consider the amps when buying a motor
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I hate to tell you, but your 1.5Hp draws 4.5amps, if it drew more amps than that it would be a 1.75 or 2hp motor. if a motor draws 8 amps it is 3hp. basic electricity.
 
Robert01 said:
I'm going to build a 2×72 grinder and was wondering if a 5 hp motor with a vfd would be too much.
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Go window shopping and figure out your setup and costs first.

Higher VFD's are much more expensive
I don't know of any 5 hp vfd that takes 220v single phase input - so you need a rotary phase converter to power the VFD
You will need heavy duty wire -circut breaker and so on to feed it all.

Unless you can get it all for free as industrial surplus, it's going to cost thousands.

Have you even used a 1.5 or 2 Hp one yet ?
Start with 1.5hp, you can plug it into a normal plug.
 
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