View Full Version : 3 Phase Motor

Jim Wilson
08-30-2006, 09:11 PM
I have a Hardinge split bed lathe 9 x 20 I guess. I got it with a 3 horse power 3 phase motor which I recently replaced with a variable DC motor. In removing the old motor I discovered that the high/low rpm range selector lever controlled a drum switch which switched the wires to the motor of which there are many. Does anyone know what is going on with this setup? I am not well versed in 3 phase technology and am curiuos.

Thanks for any info,


Bruce Griffing
08-30-2006, 09:59 PM
Three phase power has 3 power leads and a ground. Switch any two of the power leads and the motor will rotate in the opposite direction.

08-30-2006, 10:16 PM
Hmm This is not easy to explain but I'll give it a go. First simplify it a bit, Three phase is going positive an negative but lets just focus on one side only say postive. Now label the phases A B C.

Then we can say A goes High then B then C then A,B,C,A,B,C etc etc Now imagine a clock face (no an analogue not a digital clock sheesh) Now attach coils at every second number. Let the armature (the revolving bit) have sticky out bits and pick one. Lets say it points at Twelve, Now Attach The A phase to the coil at 12. B to 2 oclock, C to 4 Oclock A also to 6 oclock B also to 8 oclock C at also to 10 oClock.

Start with the point we chose, pointing to 12 when A is High, Now B goes high and it turns to point at 2, Now C hots up and in turn the point points to 4, Now A fires Up again Points to 6 etc. Round and Round we go. Now in this example As A goes positive the bit we are following points To 12 on the next time it points to 6 and the next back to twelve, now since the mains it going up and down 50 times a second or 50 x 60 = 3000 times a minute and it takes two pulse to do a complete turn, it turns at 1500 Rpm.

Now add more pointy bits so that as A Fires Up the Twelve Oclock and the Six oclock coil there are pointy bits attracted to Both. Add some coils and some more pointy bits so that when A (and the other phases) go Negative there is a pointy bit waiting to be given a boost. Now as it turns there are a whole lot of pointy bits at all different angles getting a boost, but its still turning at the same rate.

Sitll here? Now consider what happens if you throw a switch so instead of as above, A now Fires Up 12 oclock B fires 1oclock C fires 3 oclock A also Fires 4 Oclock B 6 oclock C 7 etc. Now starting with our chosen Pointy Bit pointing to 12 with A hot. Next B goes hot our pointy bit turns to 1 oclock, then C so 2 Oclock. It now takes Four Pulses of A to turn round. A is still positive 3000 Times a minute, so now it turns at 3000/4 or 750Rpm.

Ok so we just doubled the coils and slowed it by half. Can we use the extra coils in the faster speed, well yes, instead of one coil at 12 we have A fire up the coils as 12 and 1 to make a pretend single coil at halfway between. B we wire to fire the 2 and 3, C to fire 4 and 5 etc.

If you have time and aspirin, you can figure out a scheme where the pointy bits and coils and everything are spaced and wired to work at both speeds.

Ok hopefully that all makes some sense - even though I have sort of simplified and glossed over the neccesary extra figuring out required. If not ask away.

J Tiers
08-30-2006, 11:23 PM
Or, the short story....... ;)

Motor speed is directly related to the number of poles in the motor.

A speed change switch "rewires" the motor to change the number of poles, and thus the speed.

It isn't unique to 3 phase, it can be done with single-phase also. But it takes more contacts to do it for 3 phase.

Rich Carlstedt
08-31-2006, 12:32 AM
I may add that the Hardinge has 18 wires, since it is a two speed motor(9 in each). You did good wiith the DC change, but can forget about the drum switch since you no longer have two speeds.

08-31-2006, 02:04 AM
Ahh, yeah ASparky, yeah that's it. I don't understand what you said but it's clear as mud. Yep, got it:eek: :confused: :D