Originally Posted by
joeavaerage
Hi,
depending on where in the world you are, or which engineering school you went to there are two different terminologies in use with respect to pole numbers.
Americans and many Europeans use the idea of poles per phase. A two pole motor has a synchronous speed at 400Hz of 400 revs/sec or 24000 rpm.
A four pole motor has a synchronous speed at 400 Hz of 200 rev/sec or 12000 rpm. You can see that doubling the number of poles halved the synchronous speed but will
double the torque. In general the more poles the slower the motor but increasing torque.
In New Zealand Universities its common to use the term 'pole pair per phase'. Thus an American two pole motor would be called a 'one pole pair per phase' motor here,
and a four pole motor would be called a 'two pole pair per phase motor'. Pay close attention to the language used in advertising, it may illuminate which terminology is in use.
I used an Allen Bradley servo which is an eight pole motor (four pole pair per phase), which is common for servos. An AC servo has a rare earth permanent magnet rotor
and they exhibit class leading torque for a given current. An asynchronous motor like those you are looking at do pretty well but still not up to servo levels.
You may note the many industrial mills have spindles (asynchronous) of 22kW,25kW,30kW at 12000 rpm or so. These spindles have good torque at low speeds by virtue
of their huge power rating. You'd never run one on single phase power.
By all means try one of the spindles you've linked to, they will be great in many situations and you'll use it a lot, but it achillies heel will be high torque at low revs for steel, and
even then it may surprise you.
I got lucky and found a second hand 1.8kW servo and drive on Trade-me for $900NZD. I still had to buy the set-up and tuning software, another $200NZD. I bought/made two cables,
and as the cable plugs are rare as rocking horse poo, they weren't cheap either at $300. I paid $450NZD for a genuine Rego-Fix ER 25 toolholder and $200NZD for P4 matched pairs of angular
contact bearings. As you can see making your own spindle is not cheap, but I'm happy with the result, and its probably better than I could have bought outright.
The truth is though that for every hour I use it I use my highspeed spindle for 20 hours. I would suggest invest in a good highspeed spindle.......experiment with it at low speed
BEFORE deciding on what you want. For the amount I use mine it was not worth it. That may change with my new build mill which is many MANY times more rigid than my mini-mill
and so I will be able to extract everything from the low speed spindle that I've been unable to tap into with the mini-mill.
Craig