Converting a 12x36 lathe to 3-phase power.

[DanH]

Willie,
Do you still use the different gears or just stick to one setup?

Dan

[Highpower]

(05-23-2012, 12:31 PM)DanH Wrote: Willie,
Do you still use the different gears or just stick to one setup?

Dan

I still use the gears and the high/low speed ranges (pulley ratio) to set my highest RPM range that is anticipated, and use the speed knob on the VFD to make any adjustments to the spindle speed from there. I know a lot of folks like to select one range and only use the VFD to make all their speed changes.

To me that is fine if you have an inverter duty motor that is built for that. But "overclocking" a standard motor (running it at a higher frequency than what it's rated at) to get to a higher RPM when the lathe is in a lower "gear" is hard on the motor. You CAN run a standard 50/60 Hz motor at 100/120 Hz with a VFD and double it's speed - but it won't last long.
Likewise, having the lathe in a high "gear" and slowing the motor way down to do thread cutting for example - can also overheat it.

I choose not to run my lathe above it's normal 60 Hz, and only use the VFD to reduce the speed when needed. I'm not in a production environment and so hard pressed for time that I can't change a couple of levers or a belt position. I'm only doing it for my own benefit and enjoyment so I tend to "baby" my machines anyway.

So basically I set my gearbox for the high end (smallest diameter), and then slow it down with the VFD to start off with (largest diameter). Then as my stock gets smaller I can increase the speed with the knob, topping out at near 60Hz when I hit the final upper RPM needed to maintain the SFM for the material I'm working with.

I'm not very good at explaining the things in my head most of the time, so hopefully some of that made sense....

[starlight_tools]

Scavenge a cooling fan from an old computer power supply or check out the fans at the local electronics repair place and attach it too the fan cover of the 3 phase motor. Hook up a DC 12V power supply to power it and then it will keep your motor cool.

Walter

(05-22-2012, 01:37 PM)Hopefuldave Wrote:

Highpower Wrote:Yes, that is a good point John.
If you run the motor too slowly - for too long, they will overheat since the motor cooling fan blade slows down as well. Work on and off for short periods of time in order to let the motor cool down between times, or set up an additional external fan to help cool the motor as it runs.

They do make special inverter duty motors that have a separate motor and fan built in to handle that, but they are somewhat expensive.

Just use common sense and keep an eye on the motor temperature if you are using a standard TEFC motor.

Wot he said...

A lot of VFDs have configurable relay outputs - on my ABB inverter I have one configured as Normally Closed, opening when the VFD output frequency goes above 30Hz - that way it switches an external mains-powered fan *OFF* when the internal fan's running fast enough to cool the motor, back *on* when the motor speed drops (including when the motor's at rest or the VFD's powered off for a cool-off!) The fan at the moment is a cheap desk-fan-in-a-plastic-cage that cost 50 pence at a carboot sale hung on some sheet ally brackets, I'm hoping to find a way to fit something a bit more, er, industrial :)

A lot of the DC motors used in smaller lathes could benefit from a separate fan, too - often they get used at low speeds with heavy loads, a recipe for overheating!

Dave H. (the other one)