vfd and motor ?

sgmtino

Well-Known Member
#49
So should i go with a 4" 6" or 8" drive wheel ? Can someone tell me what the max belt speed would be with each. I currently have a 1750 rpm motor and a 6" wheel with my single speed set up. I would like to know what the difference would be also with the vfd I slow can I go?
 

scott.livesey

Dealer - Purveyor
#51
to find belt speed: 3.14 x diameter, then divide by 12. take result and multiply by rpm. you end up with feet per minute.
3.14 x 4"=12.56 12.56/12 = 1.05 1.05 x 1800 = 1884 fpm this would be your belt speed at 100% or 60 Hz.
3.14 x 6 = 18.84 18.84/12 = 1.57 1.57 x 1800 = 2826 fpm
 

KenH

Well-Known Member
#52
Most folks with a 1800 rpm motor will run the VFD at max of 120 hz so they in effect turn the 1800 rpm motor into a 3600 rpm motor. Many of us simply use a 2 pole 3600 rpm motor because it's less weight and less expensive than the 4 pole motor.

For belt speed, a 3.8" diameter drive pulley will provide belt speed just about the same as the drive wheel RPM. A 4" drive pulley will be just a tad over so a 3600 rpm motor will give right around 4,000 SFPM belt speed. While some folks want 6,000 SFPM belt speed, and that is good for the ceramic belts. They like speed and pressure for best work.

Good luck.
 

scott.livesey

Dealer - Purveyor
#53
everyone seems to concentrate on highest available belt speed. while this is important, especially when using belts of 100 grit and coarser, when using finer belts and finishing, I think speed needs to be low. how long does a 220 belt last at 4000 sfpm? how long will that belt last at 500 sfpm?
 

KenH

Well-Known Member
#54
Scott, you're absolutely correct, the speed must be matched to the belt/grit. That's why a VFD is so important to me for a grinder. A 3 step pulley system can work "ok", but for the cost of 2 pulleys, bearings, and shaft you can purchase a nice VFD in a NEMA 1 enclosure. With a filter over the air inlets it should last years. Heck, I'll bet the pulleys, bearings, and shaft will cost over the $80 for a 2 HP VFD unit. If buying new the motor cost is about the same.

My first grinder was a 3 step pulley setup. I had a 1 ph motor, shaft & pulleys on hand, just had to purchase the two bearings so this was less expensive than a VFD and 3 ph motor. I used that setup for a yr or so thinking that's all that's needed. When I built the next grinder I decided to go variable speed, and BOY!!! was I surprised how much better it was.
 

sgmtino

Well-Known Member
#55
Thank you guys for all the help. I know the variable speed is going to pay off. the main reason i wanted to know about drive wheel size was to make sure i had the speed there when i need it. It will be nice to dial it back when doing more finish work too.
 

sgmtino

Well-Known Member
#56
So some of you say you use a 3600 rpm motor. from what I have read you lose power when your at less than 3600 rpm with a 3600 motor but with a 1800 you will have full power from 1800 all the way to 3600. Can yall let me know which is most desirable? I can get the package either way with a $40.00 difference the 1800 rpm motor being higher. Is it worth the cost initially I thought so and that was the route I was going . Should I go with the 1800 or 3600 motor
 

KenH

Well-Known Member
#57
Well, several of us have different opinions on 1800 vs 3600 rpm motors. You're right, the 1800 rpm motor that is VFD'd to run 3600 rpm does have full power at "half speed" at 1800 rpm, and for all practical purpose will have around same power at 3600 rpm as the 3600 rpm motor. At half speed the 1800 rpm motor will have full 2 hp of power which is a good bit more than the 3600 rpm motor will have at 1800 rpm. BUT - think of what you're using the grinder for at half speed (1800 rpm) and even at 10% speed of 300 SFPM (yes, I use that slow speed often). It's NOT for hogging where you need all that power. It's for the lighter touch of fine grit belts where not much power is required. Heck, 1/2 hp would provide all power needed.

The 1800 rpm motor is quite happy running at 3600 rpm, or even 4,000 rpm. I max my 3600 rpm motor out at 4,000 rpm (70 cycles). Regardless if you choose a 1800 rpm or 3600 rpm, the 4" drive wheel works good. With the 1800 rpm motor you'll wish to max it at 3600 (or 4,000) rpm. With the 4" drive wheel it lets you get down to 300 SFPM easy which is nice for making the bevel on a beveled scale. Just touching to remove a very small amount of material. Also slow speed is really good for sharpening knives.

Either motor will give you good service and once installed, I doubt you'll ever know the difference in use.
 

John Wilson

Well-Known Member
#58
It’s always important to consider the application. A belt grinder application is not a high torque application as defined by motor/vfd manufacturers.

A motor running below about 20% of rated speed does lose power, but in reality if you are running a grinder that slow you don’t need power. Keep in mind that motors and VFDs are primarily used in industrial applications for two things:

1. Constant Torque applications. This is for machines, conveyors, etc where the goal is to maintain a very tightly controlled speed. Imagine a full pallet of product weighing several tons traveling on a roller conveyor. You want to start it moving slowly and then increase the speed. To stop the load you need to decelerate safely and then stop it without making it tumble. Then slowly move the load to a set position and hold it there.

That kind of operation requires a lot of torque and good control.

2. Variable Torque. This is where you want to be able to control the speed of something but you don’t need finite control. Typically this would be large fans and pumps- loads which generally ramp up and coast down without a load which would overdrive the motor via inertia (like a heavy pallet would do- it would want to keep rolling, thereby turning the motor into a generator and back-feeding the VFD). A fan or pump won’t do that. When you turn off the motor the stuff being moved simply stops moving.

#2 is where belt grinders come in.

To hog off steel with a coarse grit you need speed. As your belt gets finer you slow down. As Scott and Ken have said fine belts don’t last long at speed. Fine belts also generate a lot more heat. Speed is not your friend as grit gets finer.
 

scott.livesey

Dealer - Purveyor
#59
buy a motor/vfd combo and get going. save pennies and get:
a 3" drive wheel
an 800 rpm motor
a 6" drive wheel
the direct drive grinders i have or have used changing drive wheel will take about 5 minutes. changing the motor will take about 10 minutes. so to 'hog' you use 6" wheel with 3600 rpm motor. to finish and shine, you use 3" wheel and 800 rpm motor. VFD lets you find the best speed to do these jobs and all the in between stuff. going from 3" to 6" will double belt speed. so start with a 3600 motor and 6" wheel, then add a second wheel and second motor as you get the money.
 
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