[TeaManMachine]

I'm looking into doing a VFD upgrade on my machine which is a Shopmaster Elderado Bridgemill. I am curious on those who own a Patriot VFD, what gear ratio you have between the motor and the spindle on both the lathe and mill. What are the sizes of the pulleys on the motor and spindle in each case? Also what Horsepower and RPM are your motors? This will help me decide on how to proceed.

Thanks
TeaManMachine

[instructor37]

We are considering doing the upgrade on 1 or more of our machines as well. I spoke to JT about it at length- The current machines use about a 1.2/1 ratio on the mill so you can get the higher rpm- about 3000 tops. The lathe uses a 1.8/1 ratio so you get lower rpm- about 1500 tops with better torque. A couple of things to consider- the new motors are bigger and require drilling holes for the larger mounts. Also the shafts are larger, so your pulleys will need to be bored out. Since the older machines have multi- sheaves on the motors and drives, you just select the set which gives you the proper ratio, so no need to buy new ones.

[TeaManMachine]

Thanks for the reply, looking at the ratios you spoke of, if the motors are 1800 rpm, you would only get 2160 rpm on the mill. Most motors can only be slowed down about 30% so the slowest you'd be able to go is about 648 rpm. A machinist told me that you'd have to go a quite a bit slower to turn Stainless with HSS tools. You wouldn't be able to do it. I don't know, I've never tried it. Have no experience at all.

On the lathe, you could get 3240 rpm at the top and 972 on the bottom.

Do you know if the motors are 1800 rpm that he's using?

I was considering 1:1 ratio with 3600 rpm motors, but my bottom would be limited to around 1080 rpm.

Maybe there just isn't a perfect solution.

TeaManMachine

[instructor37]

Make sure your advisor is talking about VFD, not DC variable speed. VFD drives maintain full voltage to the motor and vary the speed by changing the frequency ( Hz), therefore they are more efficient and can run down to very low rpm with little loss of power. The Shopmaster motors are 1400 rpm at 50Hz, but can run as high as 120Hz, or up to 3360 rpm.
With the reduction of 1.8/1 on the lathe that gives you a range of 0-1867 rpm. The mill uses a 1.2/1 ratio, giving a range of 0-2800 rpm.

[TeaManMachine]

Thanks again instructor37. I was looking at AC VFD, not DC variable speed. We use inverter duty motors at work, 3 phase 480V, and don't run them much lower than 30% because they heat up and don't last very long. The fan doesn't move much air at that speed and the windings get hotter at the lower frequency. In our case, yours and mine, it may not be as much of a problem since the duty shouldn't be as severe. The motors will get hotter.

If you take a standard motor of rated speed at a rated voltage and frequency and maintain a constant voltage to frequency you get normal flux in the stator and can get rated torque at any speed. If you reduce the frequency to say 30hz, you will get the rated torque, but since power is speed X torque and you reduced the speed by 1/2 by reducing the frequency by 1/2, you also reduce the power by 1/2. If you have a 3 hp motor, you now have only 1.5hp at 30 hz. Now above 60 hz which most motors are rated for unless they are european, things change. You can't get more voltage out of the VFD than you put in so you are limited to 120V or 240V which ever you have. If you increase the frequency above 60Hz to 120Hz, the voltage to frequency ratio drops and so does the available torque. You will have your speed going up and your torque coming down. The power will remain relatively constant. Below 60Hz you loose HP and above 60Hz you loose torque.

This is sort of why I was considering going with a 3HP, 3600 RPM motor to keep relatively good HP below 60Hz, and full power and torque at 60Hz and 3600 rpm. If I go with 3600 RPM motors, I'm assured that the bearings and everything is rated for that speed.

I do like the idea of the gear ratios. Even with the 3600 rpm motors, it offers very good speeds on the lathe and mill. I would like to be able to use the drive output as the speed of the lathe and mill. This may be something I can program into the drive though. Have to look into this. If so, I like the ratios you will have a bit better. Puts more power on the tools.

Hey, thanks again for the reply. Makes more sense to me now, and the decisions are coming closer to reality.

Have a great evening, much appreciated. TeaMan

[instructor37]

It sounds like you know all you need to about the system, so you just need to choose the components that match your situation.

[TeaManMachine]

I'm getting close, and you helped a lot. It's always good to hear what others are doing and what they think, because very few designs don't change through the process.

Not to change your mind or anything, but I would be cautious to push a 1400 RPM motor up to 3360 RPM. The motor was designed for 1400 RPM with an engineering fudge factor. The bearings are not rated to double the speed, though you probably won't be there all the time. If these are Chinese motors, I'd really be cautious. Their standards are not where they should be. They fail within their duty cycle, can't imagine what happens when you double it.

Hey, just food for thought.

Again thanks
TeaManMachine

[Al_The_Man]

Not to change your mind or anything, but I would be cautious to push a 1400 RPM motor up to 3360 RPM. The motor was designed for 1400 RPM with an engineering fudge factor. The bearings are not rated to double the speed, though you probably won't be there all the time. If these are Chinese motors, I'd really be cautious. Their standards are not where they should be. They fail within their duty cycle, can't imagine what happens when you double it.
TeaManMachine

I have installed a few VFD's over the years and consistently used 120hz on a 4 pole motor, part of my reasoning is that I would imagine the motor manufacturer would use the same construction standards, bearings, balance etc, for a 4 pole as he would the 2 pole version?
Also the 4 pole has the advantage on lower rpm due to the fact there is 2 Electrical Revolutions per mechanical of the motor, compared to a 2 pole motor that only has one E.R.
Also it pays to get the sensorless vector version of VFD rather than the cheaper variable speed version.
Al.

[TeaManMachine]

Both very good points Al. If this upgrade is coming from ShopMaster, the chances are good that both the VFD's and motors are Chinese, and both these things should be investigated. Even if the parts are not coming from China, a manufacturer such as shopmaster doesn't always put the most expensive parts in the upgrades, or on the machines. Hard to make money that way and keep costs down. You may not be getting exactly what you want, or think you're getting.

TeaManMachine