Servo vs 3 phase for mill spindle?

[coolidge]

I'm toying with the idea of converting from the 5hp 3 phase mill spindle motor over to a 5hp servo motor. I found a few threads where guys converted their lathes to servo motors. The mill has a VS belt drive now ala Bridgeport type Reeves drive.

I'm just not sure if a rated 5hp servo is really the same as a 5hp 3 phase power wise. In the same threads it seemed like guys were going with much more powerful servo's. 5hp servo to replace a 2hp 3 phase. 15hp servo to replace a 5hp 3 phase.

Price is not that far apart for a new 5hp 3 phase vs a 5hp servo but if I had to double the size of the servo that's different. Ignoring the additional $1k or so in related electronic gear the servo would require.

Since this is a cnc mill the option of CNC control of spindle speed is attractive.

In related disbelief of 'rated' power I heaved the 5hp 3 phase motor off the mill today. Expecting it to be quite heavy. It weighed significantly less than the 2hp Baldor motor I just installed on the lathe. How is that?

 

[Karl_T]

I have two CNC mills with VFD/3 phase and one CNC lathe with VFD/3phase. All three machines also have spindle encoders. The mills have rigid tapping with electronic gearing using the encoders. The lathe uses the encoder to make sure RPM is accurate and then an index mark to thread.

Spindle speed is controlled with analog 0 - 10V outputs from the control on all three machines

Anyway, see no need for the more expensive servo spindle.

 

[GL]

I’m not an expert by any means, but I have questions. My understanding is servo motors step through rotation based on angular control, but lag behind slightly. 3phase motors are frequency controlled to make a speed. I have read that 3phase motors are smoother than 1phase motors and in some cases can affect surface finish. I would think a single phase motor would be smoother than a servo motor. Maybe in a CNC mill or lathe, the controls are good enough to make the spindle smoother if they use servos? On the surface, it would seem the cogging of a servo on a spindle would be the least desirable option.

 

[coolidge]

Thanks guys. Follow up question, does a 3 phase motor need to be inverter rated to use a VFD?

Now that I have removed the original motor I'm inclined to retain the Reeves type variable speed pully drive and bag the servo idea. It does have an exceptionally long motor shaft, 7 inches. The last 3/4 inch or so is turned down to fit into a 3rd bearing the direct drive version of the mill doesn't have, no doubt due to the length of the shaft and the heavy cast iron Reeves pulley. I'm not seeing this motor with that shaft length for sale anywhere in 5HP. The original 1997 3 phase motor doesn't not note whether its inverter rated, so I'm guessing no.

 

[DavidR8]

Thanks guys. Follow up question, does a 3 phase motor need to be inverter rated to use a VFD?

Now that I have removed the original motor I'm inclined to retain the Reeves type variable speed pully drive and bag the servo idea. It does have an exceptionally long motor shaft, 7 inches. The last 3/4 inch or so is turned down to fit into a 3rd bearing the direct drive version of the mill doesn't have, no doubt due to the length of the shaft and the heavy cast iron Reeves pulley. I'm not seeing this motor with that shaft length for sale anywhere in 5HP. The original 1997 3 phase motor doesn't not note whether its inverter rated, so I'm guessing no.

They don't have to be inverter rated but it's my lay understanding that inverter rating will get you (in theory) better performance and longevity if running at less/more than 60Hz is a regular thing.

FWIW I am putting a 1.5Kw servo motor on my PM30 class CNC mill conversion. Bench tests with the motor show it to be butter smooth, much smoother than the brushless DC motor that it's replacing.

Others with more/better knowledge will be along to help

 

[Karl_T]

Inverter rated motors have more range. The inverter rated black Max on my 10EE runs from 10 to 5000 RPM.

I have standard 3 phase motors on three mills and two lathes. Not an issue on any of them in decades. I cap all of them out at 4000 RPM max and 180 min. (1800 RPM four pole motors)

 

[matthewsx]

Is the motor you pulled completely toast? It might be worth having it rebuilt since it sounds like a replacement may be hard to source.

John

 

[coolidge]

Inverter rated motors have more range. The inverter rated black Max on my 10EE runs from 10 to 5000 RPM.

I have standard 3 phase motors on three mills and two lathes. Not an issue on any of them in decades. I cap all of them out at 4000 RPM max and 180 min. (1800 RPM four pole motors)

That sounds perfect as the factory motor is 1800 rmps with a max spindle speed of 4,000 via the variable speed belt drive, which has a high and low range. I was hoping to push things to a 5,000 rpm max spindle speed.

 

[coolidge]

Is the motor you pulled completely toast? It might be worth having it rebuilt since it sounds like a replacement may be hard to source.

John

It looks fine and I was told it was running prior to the owner passing away. But the mill was not under power for inspection when I bought it so TBD. The motor is fairly clean and the bearings feel pretty good. With a bearing on both the motor side of the output shaft and another bearing at the end of the output shaft it was well supported from side loads from the belts/pulleys in-between the two bearings.

The motor with this particular shaft looks to be unobtainium. Purpose built for that kind of Reeves drive. Plenty of motors in the 1.5-2.0hp range typical of Bridgeports but not at 5hp.

That said I know an old school motor shop in the area that still rebuilds electric motors. If it has issues and I could not find a new motor those guys could rebuild it.

 

[DavidR8]

It looks fine and I was told it was running prior to the owner passing away. But the mill was not under power for inspection when I bought it so TBD. The motor is fairly clean and the bearings feel pretty good. With a bearing on both the motor side of the output shaft and another bearing at the end of the output shaft it was well supported from side loads from the belts/pulleys in-between the two bearings.

The motor with this particular shaft looks to be unobtainium. Purpose built for that kind of Reeves drive. Plenty of motors in the 1.5-2.0hp range typical of Bridgeports but not at 5hp.

That said I know an old school motor shop in the area that still rebuilds electric motors. If it has issues and I could not find a new motor those guys could rebuild it.

Rebuilding would be my first line of attack.