vfd and motor ?


Well-Known Member
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.
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.
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.


Well-Known Member
I run a 3600 rpm motor at 300 rpm a quite often - just not too long at a time (5 minutes?). If you wish to run less than 10% speed for an hour, put a fan blowing on the motor for cooling. How much cooling will be needed depends on how much load you are putting on motor. At those low rpm the motor is basically running with no load so it's not heating up near so fast as if turning a load.