Motor Power

[classicsteve]

Since fitting a phase converter and 3-phase motor to my Centec mill it has transformed it (and much quieter!). I went down from 1.5HP to 1HP but so far it is holding up OK. So I am thinking of doing the same for my old toolroom lathe - that also has a 1.5HP motor (well its 5.1A on the plate - the Centec was 5.3A).

It is only a 3.5inch lathe, though heavily built. So I was hoping to go for 1HP 3-phase motor and inverter - but I understand Myfords have only 0.5hp 3-phase motors - as there is a motor for sale locally. So why would an industrial precision lathe need a 3 times bigger motor ? I also have a Smallpeice lathe and that is 1.5HP as well.

I suppose the basic question is, what is the main factor determining the power of motor you need ? Are industrial precision lathes given larger ones for reliability, or maybe it is related to the greater mass needed to start moving or drag from large conical bearings on cold mornings ?

Has anyone agonised over this before - I didn't find a thread on it ?

Steve

[russell]

It's just that in industry time is money so it is necessary to take the biggest cuts possible. Bigger cuts take more power. With our type of work the set-up time is usually as much or even more than the machining time so we don't take such heavy cuts - hence less power.

Having said that, I replaced the 1/2 hp single phase motor on my lathe with a 3/4 hp 3 phase motor/inverter on the basis that I didn't want to loose too much torque when operating at reduced speeds. As with your mill the change has transformed the machine - much less vibration and soft starting.

Russell.

[classicsteve]

I guess I thought no one would ever be taking big cuts on a precision lathe or a toolroom lathe - I kinda thought that was for production lathes.

Steve

[jackrae]

An oversized single phase motor running light will be less prone to induce vibration than a "correctly" sized unit running under full load.
jack

[classicsteve]

Quietness is very important given the location of my workshop - although it is nice to be able to vary the speed with the VFD - the main benefit to me is that the 3-phase motor seem to cause less vibration in the first place, and the VFD lets you move away from any particular speed that causes harmonic vibration.

I have decided to stick to 1 HP - that should give the motor an easy time.

Thanks guys.

[Deleted]

I hope that I don't sound like a certain ascerbic Foreign Correspondent, but I don't understand the debate about motor power, motor type and vibration. I've never encountered a vibration problem (harmonic or not) that wasn't caused by one of the following operator errors:

Add to this list poor lathe design and some low quality manufacture, then the use of 3/single phase/oversize motors figures somewhat low down in my usual problem elimination sequence. Or am I missing something?
JB

[kneedeepinswarf]

Hello JB,
Even when all the faults you list are eliminated, you can still encounter problems with vibration when single phase motors are involved. unlike three phase motors, single phase motors produce vibration. To overcome this most small motors supplied for use with small lathes etc. are of the resilient mounted type where the motor is suspended in soft rubber rings mounted in a steel cradle. The motors supplied by Myford are of this type.
When I bought my Myford, I tried to save money by using a non resilient mounted motor I happened to have available. I used to get a most irritating harmonic pattern on the work. This was worse with a long work-piece than a short. This problem was completely eliminated by me forking out for a proper resilient mounted motor from Myford.
The same problem can be encountered when using a three phase motor being driven from a single phase supply via a static converter. In this case the three phase motor behaves somewhat like a single phase one. This problem does not occur with the variable speed inverter type of drives as these produce a very good simulated three phase output which is very similar to the three phase mains supply.
My Colchester Student is run from a static converter (a home-made one probably in need of further development!) and at first displayed similar problems to the myford. Again the trouble was cured by mounting the motor on soft rubber blocks.

Lionel

[russell]

As Lionel pointed out single phase motors inherently produce vibration. It is not caused by an imbalance of the rotating parts so, if you run the motor unloaded there is little vibration evident. The vibration is a result of the torque produced by the motor varying cyclically with the rotation of the motor. This can cause patterning on the work even in the absence of the problems outlined by JB. A resilient mount for a single phase motor motor or even a long floppy belt drive, together with the inertia of the rotating parts of the lathe, will act as a low pass filter and attenuate the torque vibration.

A three phase motor can be designed (theoretically at least) to give a torque which is independent of the rotor position. I replaced the single phase motor on my mill with a three phase motor/VFD in order to get reduced speed for use with slitting saws. I was so impressed with the result that I did the same with my lathe even though it has an adequate range of speeds and am very pleased with the resulting reduction in noise, better finish, and gentler starting (the single phase motor used to struggle when starting at its maximum 1750 rpm with my heavy 6 1/2 in chuck).

Russell.

[Deleted]

Thanks for that Lionel, what you say would be expected from hard engine mounts, but I've learnt something today. What you're basically saying is that I've been mollycoddled all my life!
Regards
JB

[classicsteve]

This is interesting stuff - when I changed my mill to 3-phase I also changed the mounting bushes for the motor plate for some old silicone rubber suspension bushes - maybe that made more difference that anything else. I think the old ones were not much more than a thin rubber washer, and the motor is mounted inside the base cabinet which I think doesn't help.

All my lathes have solid (pivoted) lathe mounts. No rubber in there at all. The problem one has a 'home-made' stand - it has a big badly fitted fabricated suds tray which I think resonates against the stand. I get vibration without cutting anything, that is just spinning the chuck. I have no vibration problems with turning - its just that it is a bit noisy, and I need to change the drive system anyway as the motor sticks out too far at the back and wastes valuable space.

I will certainly be rubber mounting the motor plate, that was in the plan anyway. Maybe that is the most important thing.

Steve