VFD & Inverter questions

I have a large Boxford CNC lathe. The spindle is driven by a massive DC motor.

Post a picture of the rating plate of the existing DC motor, as it is difficult to make recomendations when you don't know the specification that you are trying to duplicate.

On many occasions at my last employment (CNC machine tool rebuild & retrofit) we ended up keeping DC spindle motors rather than replacing them with a new AC solution, as it was just not possible to duplicate the original motor characteristics with a new AC one.

AC motors usually have a much wider speed range than DC, but the down side is that they usual had a much higher base speed (the point at which the motor goes from constant torque to constant power). In some cases the base speed of the AC motor was twice that of the DC original & would have required an expensive reduction gearbox between the new motor and the machine. So although getting the DC motor professionally refurbished & driving it with a modern microprocessor thyristor drive was expensive, it was cheaper overall & retained the original machine performance.

This was on larger machinery than usually gets into model engineering workshops, but still something to bear in mind.

Is your machine a Boxford ACL ?

Nigel B.

Thanks for your helpful replies.

Nigel - yes it is a Boxford ACL - although the rest of the cnc is now Mach3 .

Steve - amazing YouTube clip. Thanks.

I use the lathe (and my cnc mill) for small runs of parts and for one off designs. Neither machine is worked on an industrial scale.

I tend to run the X and Y axis quite slowly - because I prefer it that way and feel a little more in control.

I recently had some blind holes to tap (M8) and a spindle speed of 150 rpm was comfortable, but could be a little faster I guess.

Nigel - the spindle motor is an SEM one and weighs a ton. The issue really is whether to repair the DC drive boards or start again. The motor itself is old but OK.

John

Hi John,

A long time since I last saw an ACL - I wired the first one (prototype), some time in 1982, I think. Norwin drives & an ANC control on that one IIRC

An SEM motor will probably be a permenant magnet servo motor of the MT series - "weighs a ton" suggests maybe an MT40 or MT52 (these had double armature brushes, the MT40s were singles). There should be a rating plate on the motor with the model number, speed & torque rating. WIth that info, should be easy enough to work out a suitable replacement.

Nigel B.

Hi Nigel.

My ACL is now a Mach3 lathe. But when I got it the internal wiring had been well messed up - I think it had had a swarf conveyor fitted at some time, and then removed without care!

I still use one relay per Z and X axis limit switch as in the original - and these wired in series.

Here is a picture of the Motor plate.

The Parker SSD drive has blown quite a few resistors and a capacitor - not for the first time either.

So I was wondering - I can gat a replacement for about £350, or go AC with a VFD as discussed above.

Any advice gratefully received.

John

John.

You can access a .pdf copy of the SEM MT range technical manual here.

That motor requires 20A to generate it's rated torque, so quite a meaty drive on a single phase supply + it is a constant torque motor, where there is no constant power range - much like the DC motors fitted to Chinese mini-lathes but much, much gruntier & more robust.

A 3 phase 1 KW 4 pole motor generates approx 6.6 Nm & a 1.5 Kw motor approx 9.9 Nm at rated speed, so such a replacement would be either a bit less or a bit more than you have at the moment. Runnning a 50 Hz 4 pole motor at around 138Hz would get you the same 4000 rpm top speed. The DC motor is capable of sitting stalled all day generating it's rated 8.5Nm with a suitable drive, which a 3 phase motor on an inverter would be decidedly unhappy doing - if you do a lot of lower speed running, fitting a separate blower to cool the motor in place of the rotor fan would help here . The reality is that the Parker SSD drive will probably be a fairly basic device (though I note that there are wires to the tachogenerator for speed feedback) & probably will have a miniumum speed setting above stalled.

I'm not up on how much a 1 / 1.5Kw 3 phase motor & inverter would cost at the moment, though probably not much more than your quoted DC drive replacement. As I am aware that spares for the SEM MT range are OOP & increasingly difficult to obtain now, and that may help sway your decision - and when parts were readily available they were pricey. The last tachogenerator armature I bought (15 years or more ago) was over £350 +Vat IIRC (they can go open circuit on one or more windings - only solution when that happens is to fit a replacement).

HTH

Nigel B.

Nigel

Many thanks for all this information which is very helpful.

It reminded me that my Harrison L5 lathe (now long gone) was powered by a 1.1kw motor and was plenty powerful enough - so a 1.5 kw will give me power to spare.

Now I just have to work out how to fit the motor in the lathe...

Thanks once again

John