Electric Motor Grinder Info

[monkeh]

After i get my new gas forge built and running my next project will be making a belt grinder/sander.
Id love to make a kmg clone with a 3 phase motor, vfd etc but there is no way i can afford it atm so I'd like to make something similar to this

I'm planning on just using a single phase 3hp 4 pole motor and run it direct drive like in the above picture,Or maybe a 2 pole would be better as 3hp should have enough torque
Is this feasible or am i missing something? I'm not bothered about single speed atm.
I'd be running this off a 13 amp plug. What would be the best motor type?
Cheers

 

[knighty]

3hp is a bit big - most 9inch grinders are about 2hp (some big ones are 2.5, I just googled it)

a 3ph motor pulls a lot of power when it's starting up, which can be a pain to work with sometimes

if you imagine how hard you can work a 2ph 9inch grinder... you couldn't work the sander that hard, you'd tare the sanding band thingie before you managed to put that much strain on it


EDIT: 3ph is about 2200watts.... have a look at some other bench belt sanders and see how many watts they're rated for)

 

[brightspark]

a 3/4 hp motor is well adequate on a 4inch belt sander

 

[insomniac]

1/2 hp per 1" of belt width according to the you tube info ive been watching

 

[pedrobedro]

1hp will drive a 2" belt well enough for you to lean on it when grinding without stalling.

 

[monkeh]

The rule of thumb I've seen was 1hp per 1 inch. I've used a grinder with a 1hp motor and it's ok for sanding wood or rust removal but for removing stock or beveling knives it's underpowered. 3hp it's probably a bit much on a small grinder though i agree 2hp will probably do. Looking on eBay the price of 2nd hand motors is pretty equal for 2 or 3hp.
What I'm not sure of is if it will trip the breaker when it starts up. I see there is a few different types of motor and I'm not sure which is the best.

 

[piman]

Hello Knighty,

both three phase and single phase imotors pull a lot of current on start, 7 to 10 times full load current was the figure I was taught at college. It is for a very minute amount of time and quickly drops off as the motor gets to speed.
As matter of trivial interest, very large motors have a time limit before a second start can be attempted, my last employer had a 6 Megawatt 11Kv motor, reduced voltage start and with a 20 minute time before a second start could be attempted.

Alec