[How-To] 5hp 3-phase AC motor & VFD control question

[rabler]

Ok guys I've been fiddling with graphing as Rabler showed as it helps me see the comparison better. Here's what I come up. If correct, I need to change my current 6.5" OD drive pulley to a 4" and retain my 5:1 reduction gearbox. These plots are based on my current chinese 5hp AC motor, so they might change a little if I am able to find a better 5hp inverter duty motor around 1800 rpm. Am I right? Bill

Looks good to me, although I didn't re-crunch all the numbers. Basically an inverter motor is going to buy you better performance the farther you get from 60Hz, but will match the chinese motor torque numbers at 60Hz.

It doesn't really change anything in your end conclusion, but DC motors behave quite a bit differently than AC/VFD motor combos. So the graph for the DC motor is not what I would use. (But none of this is exact so you're not out of the ballpark). The key difference is that the DC motor torque will actually INCREASE a bit as you drop below 690 RPM on the motor. This helps the motor compensate for a load. It's ability to create torque increases as the RPMs drop, thus it does a better job of fighting to keep the RPM stable. The VFD can also do this temporarily (which is what mksj is talking about in the 150% overload), a DC motor does it indefinitely. In many ways this is similar to gas vs diesel engines. Pure HP rating doesn't tell the whole story. But I'm in academic discussion level details at this point

I'm sure mksj (and others) are more familiar with your specific model VFD. The VFD I used on a different lathe was initially configured to be very rigid about RPM, and would trip off-line if it couldn't keep the RPM very tightly controlled, which would happen if I engaged the clutch in that machine too quickly in higher gears. Changing some of the VFD parameters allowed the VFD/motor to temporarily drop the RPM a bit when hitting full load on the motor, so it wouldn't trip on aggressive clutch engagement. I mention this to point out that it pays off to spend some time getting the VFD tweaked to run well. You've certainly shown the willingness to understand what is going on and make the most of it, so don't be afraid to continue to do so once you get the mechanical setup on the new system.

 

[Nightshift]

Thank you both Rabler and Mksj for your insight and advise. I've been putting this off on the 10EE for a while now but its become far more troublesome recently and its time to pull it apart and resolve. Everything else on the lathe works fantastic so getting a reliable drive system back in it will make it a joy to use again.

I should have mentioned I'm not a stranger to VFDs and 3-phase motors as I put them on my Bridgeport vertical mill, surface grinder, 3 belt grinders and 4 other pedistal grinders and buffers. I also put one on an old Southbend 11x60 lathe which actually made that 90 yr old machine quite nice to work on. And yet another on a 1hp electric chain hoist. So once I finalize on the motor going in the 10EE I feel confident I get get everything running, but may be back here to pick your brains on some setting tweaks. Appreciate your time and help to steer me thus far on this project. Cheers Bill

 

[Nightshift]

I called some local motor shops and one had this new WEG pump motor on the shelf which they will sell for C$300. Seems like a better motor choice than my chinese induction one as this is inverter duty. I'm thinking a c-face mount would make installation and connection with a lovejoy coupling quite easy. What do you think guys? Bill

 

[mksj]

Should be better, they are rated to 3600 RPM or 2X the base speed. Typically I see them painted blue and a bit heavier cast iron. I would check if it is footed, often pump style are not footed, and do not know if they have different performance characteristics vs. standard duty motors. If you have the specific model number you can match it up to the specs. see attached document.

 

[Nightshift]

I don't think its footed ... c-face only. I've asked the seller to send me a better pic of the motor plate without the reflection so I can see the model number.

 

[mksj]

I would stick with what you have unless you can find something that is a significant improvement. Looking at several sites in Canada, was pretty dismal. Only one I saw that looked decent was this one for SKU 70043 for a Baldor EM3665T through Lenmark. They are very good motors at the price point, should be good for 2X their base speed and they are rated for constant torque from 6-60 Hz.

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EM3665T