Since you are currently run a 1.5hp and 5" drive wheel, do you have any problems with that setup? Does it ever bog down at low to mid speeds? I'm making the assumption you have a VFD? If not, Then you NEED a motor and VFD MUCH MUCH more than you need a different motor.
Is that with a 4 pole (1800 rpm) motor or 2 pole (3600 rpm)? If 1800 rpm motor I'm "assuming" you're using a double jumper for 3600 rpm motor? You are with VFD? AND, this is your general purpose grinder, perhaps your only grinder? Lots of folks tout the 4 pole motor with a double jumper get give 3600 rpm while other folks say the 2 pole 3600 rpm motor is just fine.
With a 4" drive wheel 3600 rpm will give you around 3700 SFPM belt speed while a 5" drive will give around 3700 SFPM belt speed. 1800 RPM will only give around 1800 SPFM with a 4" drive and 2300 with a 5" drive wheel.
I personally use a 2 hp 3600 rpm motor with a 5" drive wheel. The motor is over speed to 3850 rpm to give 5,000 SFPM belt speed.
While a 2hp 1800 RPM motor with the double jumper to give 3600 rpm will still give 2 hp at 50% speed and a 2 hp 3600 rpm motor will only give a tad over 1 hp at 50% speed. That's not really a problem for how much pressure do you put at slow speed grinding? At 100% speed the 1800 RPM motor spinning at 3600 rpm is ever so slightly less than 2 hp while the 3600 hp motor is still at rated 2hp.
All in all, it's been my experience in actual use you'll never know the difference between a 2 pole or 4 pole motor while grinding. The 2 pole motor is less expensive and a tad lighter in weight if the grinder has to be moved very often. All in all, I think you'll be happy with either motor.
On the 2hp vs 3 hp - the folks I've read that use the 3 hp motors are the full time makers who are using the 3 hp on the hard grinding at high speed, perhaps 6,000 to 7,000 SFPM belt speed. These folks will have 2 or 3 grinders lined up side by side and never change belts while grinding a blade, move from grinding station to station for each stage of grinding. The only time a belt is changed is when worn out. Once you get up to a 6 or 7" drive wheel to give4 7,000 SFPM a 3 hp motor is desired due to motor bog at heavy pressure. Takes more power to turn the larger drive wheel.
When I first started grinding I thought 4,000 SFPM was plenty fast. As I gained more experience I now find 5,000 SFPM to be a good grinding speed. I doubt I'll ever get to 6,000 SFPM comfort level, I'm just a hobby maker. AND - the idea of a 36 grit belt breaking at 6 or 7,000 SFPM is scary!
Remember, a VFD is a MUST. My first grinder was a belt driven with 3 step pulleys and I didn't see the need for VFD - cost too much money since I had 2hp 1ph motors laying around. When I built my 2nd grinder I got the motor and VFD - WOW - what a difference. A decent variable speed setup will do more for your grinding than you can imagine - until you spend a week or so with variable speed.
