How can you determine the RPM of a 3 Phase motor without a nameplate ?

Rob S.

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Sep 2008
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Good Morning ,

We had a 3 phase motor go bad yesterday . The motor had no nameplate .
How can you determine what RPM ( or how many poles ) without a nameplate .

Any links or videos ? Good teachable moment yesterday.

Thanks so much in advance .
 
There MAY be just a little bit of residual magnetism left in the rotor bars from the last time the motor ran. If you get a scope and hook it to two of the motors leads and spin the rotor at a known-ish speed you may be able to determine the pole count from induced voltage at the motor leads.

Or you could try to get a feel for what the driven load speed was and work it back through the drivetrain. You aren't looking to be exact; you just need the pole count.

Keith
 
RPM = (120*f)/p

f = frequency of line
p = number of poles per phase

Example - 3 phase motor with 24 poles connected to 60Hz line

24/3 = 8 poles per phase

So

RPM = (120*60)/8
RPM = 900
 
RPM = (120*f)/p

f = frequency of line
p = number of poles per phase

Example - 3 phase motor with 24 poles connected to 60Hz line

24/3 = 8 poles per phase

So

RPM = (120*60)/8
RPM = 900

Just to add to this a bit, when you get a motor that is 1750RPM, that just means they are adjusting for the slip in the rating and that zero slip would actually be the perfect 1800RPM of a 4 pole motor.
 
RPM = (120*f)/p

f = frequency of line
p = number of poles per phase

Example - 3 phase motor with 24 poles connected to 60Hz line

24/3 = 8 poles per phase

So

RPM = (120*60)/8
RPM = 900
But with no nameplate, you don't know the number of poles...


If you knew what to look for in a stator, you could disassemble the motor and dissect the coils to see how many poles it has per phase by looking at the coil groups. But that's not easy to do for the uninitiated. This will give you some idea of what that might be like.
https://www.easa.com/resources/software/how-wind-three-phase-stators
 
Do you have reason to believe that the RPM is something other than 1800 or 3600? If so, Kamenges idea of backtracking through the drive train is the way to go. If not, a gearbox connected directly to the motor may have a max RPM rating that would point to the answer.

Is the motor driven by a VFD? If so, the parameters in the VFD may shed some light on the problem.
 
But with no nameplate, you don't know the number of poles...


If you knew what to look for in a stator, you could disassemble the motor and dissect the coils to see how many poles it has per phase by looking at the coil groups. But that's not easy to do for the uninitiated. This will give you some idea of what that might be like.
https://www.easa.com/resources/software/how-wind-three-phase-stators

Oh definitely! It was a little easier to do on the trainers in trade school where you could clearly see into the motor.
 
Without a nameplate, how do you know required HP for your replacement motor? It seems RPM might be only part of your problem.

Relying on frame size alone isn't a good idea; multiple HP ratings are available in each frame size, and oversizing may be as detrimental as undersizing the replacement.


-rpoet
 
You might be in a situation where it will be easier to reverse engineer the application. Whatever the motor is/was driving has requirements that the motor satisfied so if you start there (I.E. how fast does whatever the motor was driving need to run, what kind of power does it take to run it, etc...) it may be easier to spec the replacement motor rather than figuring out what the old motor was.
 
You might be in a situation where it will be easier to reverse engineer the application. Whatever the motor is/was driving has requirements that the motor satisfied so if you start there (I.E. how fast does whatever the motor was driving need to run, what kind of power does it take to run it, etc...) it may be easier to spec the replacement motor rather than figuring out what the old motor was.

This! It's like broken belt on conveyor. If you can't find the length when you can measure it
 
Do you know the kW rating of the motor? You might be able to have an educated guess by looking at the motor over load setting.


I say this because if you know the know the kW rating and you can measure/calculate the frame size, then you could possibly find a table that shows you the number of poles


I found this on the ABB website... Too easy if it is a 90L frame..🍻

Capture.JPG
 

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