how to tell the number of poles in a 3phase induction motor?

[shmuu102]

How do i tell how many poles a large 3 phase induction motor has?

I have come across another very large (40hp) motor 3phase 460v motor for possible conversion. Unfortunatly , i have forgo the exact rpms, but its under 2000.

[powerbuoy]

That was solved a couple times on the board, but here it is ...

n = f/p

n = RPMs

f = frequency

p = pole pairs

Example:  read the plate, might say 1725 RPM, that's in the 1800 category

(n = 1800 RPM = 30 RP sec)

p = f/n

p = 60Hz/30 RP sec

p = 2 (pole pairs), therefore 4 pole

Powerbuoy

[Flux]

"but its under 2000."

4. pole just under 1500rpm  50 Hz, just under 1800rpm  60Hz.
   
6. pole just under 1000rpm 50 Hz   just under 1200rpm  60 Hz
   
   

Flux

[shmuu102]

and generally the more poles , the better or more efficient the conversion? is that the general rule of thumb?

[Flux]

2 pole machines have small bore and little room for magnets, the core teeth relatively are too long.

Higher pole numbers have larger diameter rotors with room for more magnets and you are more likely to be able to use the existing winding.

Efficiency is a different matter, that depends on many factors.

Flux

[JW]

Figure it runs at 1750rpm's.

 This is a standard type of speed for this motor running on 3phase. It doesnt matter if its 230v or 480v or 240v and 460v. The same motor should still have the same rpm at all voltages. Unless its a known 3600rpm motor(which will still have the same rpm at all voltages, probably with less choices) it should run at 1750rpm's. I have found even with the speed grounded, to a set standard of 1750rpm the # of poles can vary. Chinese, Usa, Uk, Europe, or even different industry here in the US. 60hz and starting torque, seems to determine the strategy of the number of pole. Your results may vary. I have seen 460 3phase 30hp motors run with a stinger at 480v and it doesnt change much. Rpms are usually 1750 if thats the motors set speed. Theres only 6 wires used in parallel sets of three. The name plate,,, will specify motor type and will usually tell you the number of poles. by the way, no one ever pays attention to that, cause the diagram(in the hook up box) will tell you how to wire the motor for different voltages at 3 phase. So its 9 wire's including the source(6 in the motor). Are you shure its not a "480hz" motor? How large is it in phisical size? should weigh a couple hundred pounds at 60hz and 40hp.

JW

[ghurd]

Uhh... What?

Thats confusing to say the least.

So there are no ~850 60Hz (or ~725Hz for the same motor at 50Hz) motors?

[JW]

Hi ghurd,

 NO, I wasnt making generalitys that way. I was just saying it's probably the standard stator for 1750rpms. The number of poles is on the nameplate. And further, if it was a 460hz(480hz whatever) motor, it would be smaller in size and still put out 40hp.

JW

[fred480v]

confusing to say the least,  is to say the least. I am a motor guy & don't know what any of that means. if the rpm is not on name plate, how many coils are there? a 30 hp. should have phase paper. how many coils in between phase papers?that will tell rpm.   let me say uhh what

       fred480v

[powerbuoy]

It is very rare that the RPM info is missing on the plate or that the entire plate is missing ... anyway, if that should be the case:

Open the sucker up, remove at least one endbell, send us a pic of the coils and we'll see what we can find out by looking at the coil size, sequence, slot numbers etc.

Powerbuoy

[shmuu102]

it wasnt missing, i just didt pay enough attention to it... the "shop" that its at opens back up on tuesday.. If its still there, chances are i can get it for around $100...even though i dont plan on using it right away ( i should try a 1 hp or so conversion to warm up on) I cant see passing on that price..

[JW]

shmuu102,

 Dont get lost in the details. We often try to drive each others curiousity here. I kept getting mixed up with with the 460v thing, when I was trying to reference 415hz aircraft type of motors. If your going to convert a 460v three phase 40hp(60hz) motor, by installing some neo's on the rotor, im sure the stator has a big unuff bore to accomodate this. im sure you already plan to machine the rotor to accept the magnets. The question then becomes what speed will "you" spin the rotor. Also depending on the speed, you will determine if the original stator wiring will make the type of volts you need. just remember to configure the six wires in the box to the lowest voltage,(230v to power the motor for example, not the 460 configuration) to get your 3phase out for the rectifiers. I work with 30hp motors of this type, a 40hp will be large and heavy just as a 30hp is(there monsters like 300lbs or something).

 I have often wondered if trying to do the same thing you are, it would workout better on a 415hz(for example) motor, than with a 60hz motor. at least for the sake of salvaging the original stator winding. Especially when driving the converted motor at 1 or 2 hundred rpm.

 If this give you ideas for more questions fire away, you might be suprised how these threads can go on, long after they are off the front page. If no one responds, just make a diary entry, that will get your topic back on the front page without being too redundant.

JW