Questions about a VFD for Lathe spindle drive

[gfull60]

Eldorado Bridgemill (model previous to Tri-power)

I have a few questions as I explore the upgrade to a VFD.

1) Could I use a 2 HP motor
(would the stock bearings handle this HP upgrade)?

2) Are VFD motors reversible
(Do the controllers have a reverse function)?

Thanks in advance for any input along these lines.

Gary

[gfull60]

Answering my own question regarding the motor reverse function.

As I am looking into a DC powered motor, reversing the polarity via contactors achieves the reverse function, but the Drive control will not directly do this.

If I was to use a 3 phase A/C motor then the Drive would be able to reverse the motor.

Bearing question remains and now...

New question : What are the the merits of using a 3 phase A/c motor over a DC motor?

Gary

[Al_The_Man]

There have been reports of the bearings getting warm/hot over 2000 rpm, there are upgrade bearings to suit if needed.
You can get the DC SCR drives with reversing capability without relay (4 quadrant drive).
A VFD is usually reversed by Contact input although some do have ±10 analogue input if needed.
The merits of VFD are brushless motor over DC, some complain of lack of torque at low rpm, this can be overcome by getting a good quality vector controlled VFD, they can also be obtained with an encoder input which not only controls speed accurately but provides torque at lower speeds.
Al.

[dahui]

Hi Gary,

I think you sent me an email, but my outlook has been eating my outgoing messages.

I actually did my VFD upgrade on the cheap because I am friends with an industrial electrician and he sold me the motor and VFD at cost. I’ll give you my best guess.

VFD: Mine was about 350 but you could find a different brand or a cheaper one. If you are going to buy nice motors, it makes sense to get a nice VFD in my opinion. It will help the motors last longer and provide a cleaner control signal. The VFD can either display volts, amps or frequency but none of that gives you RPM. A good industrial tachometer is expensive, in the several hundred dollar range but it will probably read down to 1 RPM. I got a 40 dollar cheapo Chinese one on ebay for 40 bucks that only resolves 100 rpm increments but it seems fine to me.

Motors: the ones I used retail for about 400 each. I know some people have used DC motors from tread mills and SCR speed controllers. You could put any HP motor on the machine but physical size will be a limitation. As far as the lathe, the bearings aren't intended for more than 2500 RPM (same for the mill) but I don't really have any need to run the lathe that fast. I've actually set mine up for a max of 1500 rpm and replaced the bearings anyway.

AC vs. DC: I'm not an electrical engineer or anything, but I don't know of any machine tool (commercial) that is powered by DC motors. I guess for me the main advantage was that you don't need a rectifier for AC. And, as mentioned, a DC motor has brushes that need to be checked, maintained and broken in and pre-seated if you are picky. DC motors do have "better" low RPM torque but I think we are talking about running less than 20% of full power. An AC motor should really be run at above 50% anyway.
Step pulleys for the motors: about 40 each on ebay, the only place I could find good quality ones (made in Canada). I think it's called "E motor store" or something like that.

25 Feet of Fenner Link Belt: about 250

I got all the material for the motor mounts for free and they would be difficult, but not impossible, to make on the shoptask.

I also got the control box for free, but you could make one or use any kind of enclosure.

The allen Bradley push buttons and switches retail for about 50 bucks each but are readily available on ebay for much less.

Hope that helps and let me know if I left anything out.

Cheers,
Jason

[Al_The_Man]

Just to clarify a few issues.

. The VFD can either display volts, amps or frequency but none of that gives you RPM.

There are several models that can give RPM as a readout, and if you have the encoder option are extremely accurate.

AC vs. DC: I'm not an electrical engineer or anything, but I don't know of any machine tool (commercial) that is powered by DC motors.

For many years, probabally 30~40 or so, CNC spindles and servo's were mainly all DC motors, it wasn't till the advent of sophisticated micro-processor control That AC VFD came into its own.
In fact there are many of the older DC machines that are still going strong.

DC motor has brushes that need to be checked, maintained and broken in and pre-seated if you are picky. DC motors do have "better" low RPM torque but I think we are talking about running less than 20% of full power.

??Torque is torque (current), DC motors usually exhibit max torque at zero speed and are fairly flat over the speed range, typically DC servo's/spindles in the CNC production environment would go 10~15 yrs without brush replacement.
I am not disputing the fact that VFD's are a boon, I have installed several makes and models, but there is nothing wrong with a good quality DC motor with SCR drive if it is the right price for spindle control.

I guess for me the main advantage was that you don't need a rectifier for AC.

Actually VFD's use a three phase rectifier bank and a large capacitor DC supply which SCR controls do not need.
Al.

[gfull60]

Thank you both for your insight, I fear I have missed an opportunity as the 2HP Baldor with Rotary Encoder and Fenner M Drive is long gone...could have gotten that for about 400 including shipping.

Sounds like I could have done it and it would have been a good upgrade from the stock machine from what I have read...will keep my eye out for another deal.

The price seems right based on your investement Jason.

Gary

[gfull60]

Found a great supplier of new surplus VFD and Motors.

I got 2 1.5 hp constant duty 3 phase motors and a TECO Westinghouse Fluxmaster model 202 rated at up to 2hp.

I will switch between lathe and mill motors and use the one controller.

Motors are 1800 RPM @ 240 3 Phase.

VFD is rated full power (No HP loss) at 240 1 Phase input.

I think I have only 230v from my local utility so that calculates out to 1725 RPM based on 60Hz. Although the digital readout cannot show RPM (without an outside signal) the calculations are easy enough to build a small chart to keep with machine...I am always looking at charts to figure feed speeds and depths of cut anyway, what's another chart. In time everything will be second nature I am sure.

Found this formula:
Speed (RPM) = (120 * F)/P

where F = Frequency (Hence the VFD)
and P = Number of poles the motor has.

Working backward my motor is rated 1800 RPM at 60 Hz so I deduce that I have a 4 pole motors.

So lets say I want a spindle speed of 100 and I am using a 1:2 pulley setup ( I will be working out want I want to us here as I want direct drive to the quill without running through the intermediate pulleys. Will use a link belt and think I can use a setup like the following 2:1 and 1:1 setup.

Using the 2:1 reduction pulley setup I would need a motor speed of 200 RPM.

200 = (115*F)/4

So... the frequency setting for 100 RPM on the spindle using the 2:1 reduction setup would be 6.96.

I may have to opt for a 3:1 reduction setup although the drive says it can provides constant torque, this remains to be tested on the machine. If nothing else I will be able to turn step pulleys in my sleep.

An interesting thing is that I can overdrive the motor without damage so if I have a direct 1:1 setup I can get ~2585 rpm using 90 Hz for the F setting.

Looks like I will be having fun.

Will let you all know how the install goes.

Gary

[gfull60]

I will be doing the VFD on the lathe first I think.

Getting rid of all that mess in the left side of cabinet will be a dream.

Gary

[sharpshooter90]

On the lathe, you shouldn't have any bearing heat issues at high rpm because the spindle is splash lubricated with oil. On the mill spindle, I would recommend you remove it , clean the bearings and re-pack with a high quality moly grease. Some guys will replace the bearings with timkens at this point, and its not too expensive- about 15.00 each. But if you are going to re-use the original ones be careful with the bottom one. When pressing it out if you catch the edge of the cage it will pull it apart and you will end up buying new ones- been there -done that.

[digitalmdj]

Most bridgeports only have 1.5 hp motors for their mills so 1.5 hp is all you need. Also you will have to deal with the extra weight of the bigger motor on the mill head. The 3 phase motors are lighter than the single phase ones that I used. I went with the 1.5 hp so I could use 110volt single phase input on the VFD drives. If you go more hp you have to go up to 220 single phase. Thats where the cutoff point is. The lathe can handle more HP and it might be an advantage in some machine operations. As for the mill spindle bearings their is nothing for the bottom bearing to catch on it comes straight out the bottom of the spindle. It is just a piece of junk and should always be replaced if you are serious about your machine and a VFD drive is only for the serious user.

[sharpshooter90]

We ran our machine on the chinese bearings for about 5 months with no problems- the TIR was under 0.001". But we had a real problem with fine sand- it seemed to be every where no matter how you tried to seal up the area- computer keyboards were always gritty- even in the coffee. During one of the quiet times we decided to re-pack the spindle as "preventative maintenance" ( had nothing else to do). We pulled the quill and took it to maintenance to press out the bearings- those guys were used to doing a lot of heavy stuff and not re-using things, so this little item was just slapped in the press- unfortunately they did not get it clear of the press plate and when the shaft came out it pulled the cage off the bearing. All the races were fine, and I actually considered putting it back together, but it so happened that it was a stock size we had in parts. Once it was all back together, there was no real difference in TIR or heat, but we managed to keep busy for a day.

[digitalmdj]

There was nothing in the way when I removed these bearings. The cage was already distorted before I removed the spindle from the spindle sleeve. I used a plastic block and taped the spindle out. The bearings just fell out along with the cage. As for the Lathe spindle bearings the rear bearing had to much ID clearance and the front one was so tight it took forever to get off. The new bearings fit just fine. I feel that the mill spindle sleeve could have been machined better. I had some rough machining marks not as smooth as it should have been. I do have a question What are they using to lubricate between the spindle sleeve and mill swing arm. It seem to me that it needs a seal at the botton of the spindle sleeve to keep out metal shavings.