Grinder torque vs hp rating etc.

DEllis

DEllis

Part Time Steel Basher
Joined
Oct 17, 2007
Messages
966
I have a question for the motor experts.
Here is my situation. I am off grid, and have solar power with a battery bank and inverter, and a small generator for backup. I am currently(pun intended:rolleyes:) running a 1hp 1750 RPM motor, and it has plenty of torque, but it draws my batteries down real fast.
I also have a 1hp rated 3450 rpm motor which draws about 1/4 of the juice.

My inverter has a digital readout so I can see how much draw each motor takes at startup and while running. The 1hp 1750 draws 500 amps to start, and runs at 160 or so.
The other motor pulls 200 amps at startup and runs at 42.

What would be the difference in torque between the 2 motors considering they have the same HP rating? That's assuming I have them set up to run the same belt speed(about 3000 SFPM)

I can't just swap the motor and try it as the 3450 is a flange mount and the 1750 is not, so any swap will require a complete rebuild of the motor mount. I'd probably just build a new grinder with the faster motor.

If anyone can shed some light on motor HP rating, torque and RPM, and the real world relationship between these, I'd be much obliged.

Thanks,
Darcy:)
 
As I understand it, HP = torque x rpm, so wouldn't the faster motor have half as much?
 
jdm61 said:
As I understand it, HP = torque x rpm, so wouldn't the faster motor have half as much?
Click to expand...

But if geared(with pulleys or drive wheel size) to run the same belt speed??
And why the major difference in amperage draw?

What I want is to maximize torque for minimum amp draw so I can grind blades for longer periods of time without killing my batteries.

Darcy:)
 
First - Something is wrong with your amperage numbers.

Second, look at the motor plate for the current draw. That will tell you the real motor power. The HP rating can be misleading because of RPM and other things. My 5HP shop vac draws about the same current as my 3/4HP disk sander. That is because the ratings are apples to oranges. The 1750 RPM sander is truly creating 3/4HP, while the high rotation vac is only "rated" at 5HP. It probably has a real power of around 3/4HP.

Other issues are starting capacitors and starting load. The 1750RPM grinder may have much higher start up current than the low torque tool with the 3400RPM motor.

I suspect your problem is that the capacity of your solar system is too low for your current draw. You are running more on battery power than on sun power. Suggestion - get a bigger solar panel and inverter. This will run the motor much better when the sun is up. Get a bigger generator and run the motors directly when the sun is down. Let the panels charge the batteries when the grinder isn't on.
 
The dollar per watt cost of solar is very high.


You should be able to keep up better if you use wind,or waterfall waterwheel, or gas engine, or something.



Stacy's right, ther's something wrong with your amp numbers,


But I have notice that when looking at motors, 3400 1 hp motor is 12 amps on the dataplate and a 1700 1 Hp is more.
 
How hot is your motor getting? Is your current draw under load? You should be able to increase power by using a pulley system.
 
Amperage numbers are coming directly from my digital display on my inverter........

The sun doesn't shine all the time in Canada(shocking I know;))......so a bigger panel will not help. When the sun is bright, I can grind for extended periods of time with minimal battery draw.......but it's just too much for my power system to handle otherwise.

Thanks for the input. I will just have to experiment with the other motor and maybe switch out the 1750 for a smaller motor.
Anybody want to buy an almost new 1hp TEFC motor?:D

Darcy:)
 
Are both motors running under the same load when you test them?

From the description, it sounds like you might be comparing the motor/grinder combination with an unloaded motor. If so, it could easily account for the difference in current draw.
 
timgunn1962 said:
Are both motors running under the same load when you test them?

From the description, it sounds like you might be comparing the motor/grinder combination with an unloaded motor. If so, it could easily account for the difference in current draw.
Click to expand...


You are correct, the 3450 motor was not running a load at the time. But the amperage draw was basically the same when I tested the 1750 with no load as with the wheels and belt in place.
As usual, I will need to just try stuff to figure out what works.......never had great luck with theory corresponding to real life observation.
Thanks again guys.
Darcy:)
 
All motors have efficiency factors. You should get 90% to 95%+ with most electric motors. Perhaps your 1 hp 1750 motor has inefficiencies such as worn windings or other issues?
 
logem said:
All motors have efficiency factors. You should get 90% to 95%+ with most electric motors. Perhaps your 1 hp 1750 motor has inefficiencies such as worn windings or other issues?
Click to expand...
I guess that's possible.......
It has tons of power, but it uses a lot of juice to make it. I bought it brand new and it doesn't have many hours on it yet. My lathe has a 1hp 3450, and doesn't draw nearly that much. Maybe this thing is a miss-labeled 3HP?:D

If I was hooked up to the grid, I'd never notice the amount of power it uses.
I'm just going to build a new machine with the other motor.......maybe I'll hook a gas motor up to this grinder! (Rubs hands together and laughs maniacally!:eek:)
Darcy
 
I just realized that the amperage you are reading on the inverter is probably the DC draw from the batteries ... and most likely not the AC current supplied to the device. It will vary depending on lots of things like the amount of sunlight and how much is coming from the batteries and how much is coming directly from the panels. The inverter is also shows all the current being used from the batteries, which is the lights, and any other equipment running ( refrigerator, fan, AC, etc). 200 amps at 12VDC is 2400 watts. That would be about right for a high draw 1HP motor and some lights, etc.

The 42 amp reading would be about 500 watts. That is way too low for any 1HP motor, so it was probably getting most of the juice directly from the panels.
 
Lieblad said:
Perhaps you are assuming 12v system ?
Larger offgrid systems are typically higher than 12vdc battery storages.
42 amp could well be correct for a 1hp motor on a 36 or 48 volt system.
Click to expand...

Agreed. I was using a 12VDC system as an example. In the case of a 36-48VDC system, the 200 amps would produce nothing but smoke from a 1HP motor.

Either way, the readings don't correlate to the current for both 1 HP motors.
 
Comparing loaded and no load starting amps on different motors is apples to oranges. Once you have your electric motors running and are grinding with the same load at the same belt speed, the power usage should almost be the same. If not there may be an efficiency issue with one motor. Of course you'll have to build a new grinder to find out...

Hooking up a gas motor to the grinder is not such a bad idea, assuming you don't mind the noise and can work out the ventilation issues. But only slightly more efficient than just running the grinder straight off the generator. If you have more than one piece of power equipment that you want to run, you could power a line shaft set-up with a gas or diesel engine. Or use a gas powered air compressor or hydraulic power unit, and put air or hydraulic motors on everything. There are plenty of off-grid shops built by the Amish to look at for inspiration. And as wonderful as going all solar is, most power tools are energy hogs so you'll have to balance the cost of running the equipment on fuel versus the cost of more/bigger panels and batteries to keep running it on solar electric.
 
Yes, 12 volt system.....not large.
I tried both motors in the same time frame......nothing else running at the same time, I shut everything else off.
The spare 3450RPM motor as well as the one on my lathe(both 3450RPM rated 1HP) both draw about the same amount of power. The slower one draws way more at the same rating. I am baffled. That's why I asked.

I thought maybe 1750RPM motors just used more power at the same HP rating......but maybe they produce more torque as well?? If that's the case I could go to a smaller HP rating 1750 motor(1/2 or 3/4 HP) and still have the power of the 1hp 3450, with similar battery usage??
Bottom line is the 1hp 1750 is too much for my electrical system to handle, so I am trying to determine the most efficient alternative motor. I used a 2x36 attachment on a 3/4HP bench grinder for 15 years or so.......anything that can run a blaze belt is a step up!:D
Thanks,
Darcy:)
 
My only other suggestion is that torque isn't a necessary issue in a belt grinder. Use the motor that runs best on your power system. Try the 3450 RPM motor and see if it runs the grinder OK. If so, no problem.

Just out of curiosity, can you post photos of the two motor data plates? I would like to see the specs on both side by side.
 
The 3450 has no data plate on the motor itself. It was a motor from a dust collection system. It is virtually identical to the one on my lathe, including the flange mount. That one also has no data plate on the motor, but has this on the lathe.
I clearly recall the HP rating on the dust collector motor was 1HP
The lathe draws 93 amps running, but only 130 to start up.



This is the plate on the 1750


Darcy
 
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Thanks for all the input.
When you say that torque is not important for a grinder, what do you mean? I thought that was what determined the ability to remove material without bogging down.

Another question. If I use the lightest wheels possible, will that help with power to the belt since it has less mass to turn, or will the extra mass work like a flywheel and keep things rolling better while under power?

I will build a new grinder with the flange mount 3450 motor and see how it goes. Then I'll either sell the other grinder, or put a smaller motor on it.

Thanks again,
Darcy:)
 
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