Motor speed for grinder

T

Technohedge

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Sep 9, 2018
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I have currently a 2hp 4 pole motor that Im using on my grinder. It runs fine however im wondering if I need a 2 pole instead for hogging off material.

The grinder has a 4 inch dia direct drive wheel, which at 1200 rpm (running at 50hz from vfd) gives a quiet 1300 SFPM for the belt speed.

Now i can crank it up to 100hz and it gets the speed up but i cant go too mental with pressure due to the reduced torque at that frequency.

Today i did what i would consider a big blade (23cm length, 4cm width and 3.7mm thick and pre hardened stainless) an it took me hours to grind the main bevel (at 100hz vfd) with a fresh 40 grit belt (ceramic).

Im just wondering do my ceramic belts blunt with the lower speed quite quickly and would I be better with a 2 pole motor? Thoughts?
 
You loose torque once you go over 60hz. And it’s not wise to over drive the 3450rpm motors. If you want to over drive a motor use a 1725rpm. If I was using a 3450 I would set up the drive wheel so at 60hz you get around 3500-4,000 SFPM.
 
Something is not making sense here.

1200 RPM at 50 Hz. 50 Hz time 60 seconds is 3000 cycles/min. Each pair of poles "uses" one cycle per revolution. A 4-pole motor has 2 pairs of poles and will run at a synchronous speed of 1500 RPM. It will slow down under load and I'd expect the rated speed to be somewhere between 1400 RPM and 1470 RPM at rated load/power. Newer, higher-efficiency motors tend to have faster rated speeds.

If your (2HP, 4-pole) motor is running at 1200 RPM, either it is massively overloaded, it is pitifully inefficient, or something is wrong with your setup.

Where in the world are you and have you checked the wiring?

It sounds like the problem a few folk have had over here (UK/Europe) with 230/400V motors connected in Star (Wye) instead of Delta, when run on 230V 3-phase from a single-phase-input VFD.
 
timgunn1962 said:
Something is not making sense here.

1200 RPM at 50 Hz. 50 Hz time 60 seconds is 3000 cycles/min. Each pair of poles "uses" one cycle per revolution. A 4-pole motor has 2 pairs of poles and will run at a synchronous speed of 1500 RPM. It will slow down under load and I'd expect the rated speed to be somewhere between 1400 RPM and 1470 RPM at rated load/power. Newer, higher-efficiency motors tend to have faster rated speeds.

If your (2HP, 4-pole) motor is running at 1200 RPM, either it is massively overloaded, it is pitifully inefficient, or something is wrong with your setup.

Where in the world are you and have you checked the wiring?

It sounds like the problem a few folk have had over here (UK/Europe) with 230/400V motors connected in Star (Wye) instead of Delta, when run on 230V 3-phase from a single-phase-input VFD.
Click to expand...

VFD's have overdrive features where you can give them up to double speed.
 
There appears to be something wrong with your setup. A 2 hp 4 pole motor running frequency doubled (100 hz) will have the same torque and power as a 2 hp 2 pole motor running at 50 hz. You won't gain anything by switching to a 2 pole motor. The 4 pole motor will have more torque at 50 hz than at 100 hz but don't concern yourself with that. Power is the metric that matters most when you're hogging at high speed. Power is torque times rpm. If you can't grind efficiently with a 2 hp motor then something else is your issue.
 
That doesn't sound right. I use a 2x 1725 rpm motor with a 7" drive wheel and it has loads of power as fast or slow as I want to run it. What kind of VFD are you using?
 
Absolutely.

I would expect a 2 HP, 4-pole motor to be turning at “about” 1450 RPM at 50 Hz, giving either 1.667 HP or 2 HP (depending on whether the motor rating is at 60 Hz or 50 Hz), if everything is as it should be.

It should then be possible to increase the frequency and get more speed out of it (at constant power).

It sounds from the OPs description like there is currently something not quite right, such that it is not making rated power at rated speed, and rendering any attempt at increasing the frequency pretty fruitless.

My first thought was that it was wired for “about” 400V, but being powered at “about” 200V. For most small IEC metric motors, that would with be the motor wired in Star instead of Delta. I gather many NEMA motors have dual windings which connect in series for high Voltage (400V-ish) or parallel for low Voltage (200V-ish).

Once it is running right, the maximum speed can be increased no problem.
 
Apologies guys I feel I havent been clear. My motor isn't running at 1200 RPM its running at 1500 or whatever speed a 4 pole at 50hz is running at. 1200RPM was just stuck in my head for some reason as the speed a 4 pole runs at when powered on base frequency.

Of course I haven't measured the actual RPM of the motor, but judging by belt speed compared to my last fixed speed grinder I have no reason to believe that the speed and setup of the motor is off. Its wired correctly for 230V. I can happily increase speed to double that and grind with no real issues. If I try and shove hardened steel into it at that speed (100hz) it will handle it but I do get a slight bogging (depends on your definition of shove I guess).

My question was more around the speed needed for ceramics, as Im not getting much more than 3000SFPM with the 4in drive wheel at 100HZ is that causing my belts to glaze and if I ran a faster motor would the higher speed element be more favourable for ceramic belts? (appreciate that the torque will be less, however a 3000 rpm motor driven at 125% will give me more SPEED that the 1500 rpm motor driven at 200% which is the recommended for each motor type from what I have read)

I did a little more research on belt life expectancy and have seen there is a massive range that people report for various reasons and grinding methodologies.

Grinding a big flat grind I think and being careful not to overheat the blade may be blunting my belts prematurely?
 
You could also just go from a 4" drive wheel to a 5" drive wheel to increase your speed by 25%. That'd be cheaper than purchasing a new motor. (I run a 1-1/2 hp 4 pole 1750 rpm motor at 120 hz with a 5" drive wheel and get about 5000 sfpm.)
 
Ok so Iv researched a bit on here and found a good way to resharpen glazed ceramic belts by jamming summat hard in there and re fracturing the grit. Tried it out today and seems to work well! The belt not only cuts better but you can feel the sharpness return to the touch.

I think i was blunting my belts grinding a large flat too conservatively and trying to keep it cooler.
 
The ceramic belts love high pressure that’s why thy work do good on my surface grinder. But you have to be careful because knife grinding does not require high pressure. A lot of guys switch to cheep ceramic belts so o e that fresh new sharpness is gone thy toss it.
 
Technohedge said:
My question was more around the speed needed for ceramics, as Im not getting much more than 3000SFPM with the 4in drive wheel at 100HZ is that causing my belts to glaze and if I ran a faster motor would the higher speed element be more favourable for ceramic belts? (appreciate that the torque will be less, however a 3000 rpm motor driven at 125% will give me more SPEED that the 1500 rpm motor driven at 200% which is the recommended for each motor type from what I have read)

?
Click to expand...

Get a larger drive wheel. It's a ratio.

Six inch is one and a half times the speed as the four inch.
Eight inch drive wheel is double

Wilton square wheel has the largest oem drive wheel I've seen at seven inches (memory?)

Six inch seems to be the practical limit of most grinders.
 
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I agree with The Count. You need a bigger drive wheel, especially with a 4 pole motor. 6" minimum. A 7" would be my recommendation.
I don't follow the logic of running slow speeds for grinding steel. Every grinding machine available, from cheap harbor freight angle grinders, to 12" bench grinders from Baldor are running 6k sfpm or more.
 
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