No Jim I am not a Scott, I am a Scot
The problem I see with using a gearbox is not being able to change speeds on the fly, you will have to stop the motion then change the spindle then resume. Granted if there is no tool changer then that may not be a problem as you will have to stop for the toolchange anyway.
 My lathe has a gearbox but its controlled with electromagnetic clutches so speeds can be changed on the fly, however I would dearly love CSS so I am on the lookout for an AC Servo and drive, only problem is its a big lathe and the current motor is 11KW so there wont be many going about at the prices I like
 Hood

Hood

Sorry Hood - Finger twitch. I knew you would pick it up as soon as I saw it - at least I didn't say Scotch !!

Sorry Hood - Finger twitch. I knew you would pick it up as soon as I saw it - at least I didn't say Scotch !!

Yes that would have been war

Hood

I have a VFD and induction motor on my mill. It is a good quality Mitsubishi drive but just the standard Bridgeport 2HP motor so I dont know if its an inverter duty motor. I have heard that the inverter duty motors dont drop off so much in torque but how good they are I dont know. What I do know is with my VFD/motor combination I lose too much torque if I go down below 50% of the motors rated frequency. I still use the vari speed pulleys on the mill to get to the rough speed range and then fine tune by the VFD (external potentiometer). I was considering hooking the VFD to Mach but after using the mill for a while I decided there was no point as the drop off in torque would be no use for the varying types of work I do.
 

Hood,

Saw this post and it made me wonder. Could you not permanently set the pulleys transmission to say 4:1 reduction and thus have a low speed of 370RPM (assuming a 2 pole motor) - which is at 50% rated frequency like you said - 30Hz. Then for higher speeds, good inverters can output frequencies as high as 400Hz, which would mean a top speed of almost 5000RPM with the same pulleys. Or did you figure the motor wouldn't last long at 20kRPM. But if it were a 4 pole motor then I think it could handle 10kRPM no problem.

I am asking this because I'm considering doing this - using a 3 phase, 4 pole motor (or even better a 6 pole motor if I can find such) with an inverter to give speeds from 1500RPM (50% rated frequency for a 2 pole motor) to about 10,000RPM  (at about 160Hz). What do you think?

Daniel

I dont know enough about it Daniel, going to be redoing my Bridgeport with servos anyway , so will fit a servo to the spindle as I have a 5.5KW one sitting here doing nothing

Hood

I think you will have to look at the torque figures produced over the range of sppeds you are talking about. As I say, I use a 2HP three phase motor in place of my 1HP single phase. My Omron inverter worls up to 60 HZ. Below 30Hz the torque drops off quite quickly, so if I want to turn 6 inch diameter wheels, which need a low speed,  I must gear down the motor to the spindle shaft to maintain the torque, (and therefore run the motor faster) to keep the torque, otherwise if I take anything of a reasonable cut, the motor says "I'm not playing", and stalls.

If you can maintain the rated torque over such a wide speed rahge, then fair enough - but if you manage it, patent it.

Hi Jim,

I haven't been able to find such charts... it seems that the charts I need are torque against frequency, but those that I see published by manufacturers are torque against speed, where the speed seems to be the percent of the nominal speed - these show the startup process of the motor.

Daniel

The problems would appear to be with the motor then, which is normal. Torque is a function of the speed. DC motors tend to be higher torque at low speeds - e.g. railway traction motors, whereas I think most AC motors tend to produce their maximum torque at higher speeds.

I am not saying that your idea is necessarily bad, indeed for us it would be ideal - maximum torque over a whole range of speeds - and I am not in touch with the latest developments, it may have been that modern electronics had managed to produce more torque over a wider range of speeds, but as I say, I do not think it is necessarily achievable at the moment.

In the miniature railway scene, many years ago square wave was produced in a pulsed type of system, instead of merely increasing voltage, to produce more torque at lower speeds, and, indeed most traction motor controls use a similar system today, where full voltage in pulses is fed to the motor.

I don't think you can do that with AC yet, the speed controls using frequency control still have a sine wave type of output.

You may be right saying that at very low speeds an AC motor has very low torque, but then please note that I said that I was looking for a 6 pole motor, which means that its rated speed at the rated 50Hz is about 900RPM. Then I can decrease the frequency about 50% to 25Hz before the torque starts dropping significantly and get 450RPM, which is almost low enough for me. And then for the high speeds the frequency can be increased up to 400Hz which would give me around 8000RPM.

Daniel

Sounds pretty good to me - let us know how you go on.