You are not registered yet. Please click here to register!


 
 
plc storereviewsdownloads
This board is for PLC Related Q&A ONLY. Please DON'T use it for advertising, etc.
 
Try our online PLC Simulator- FREE.  Click here now to try it.

---------->>>>>Get FREE PLC Programming Tips

New Here? Please read this important info!!!


Go Back   PLCS.net - Interactive Q & A > PLCS.net - Interactive Q & A > LIVE PLC Questions And Answers

PLC training tools sale

Reply
 
Thread Tools Display Modes
Old June 11th, 2019, 03:51 AM   #1
Rob S.
Member
United States

Rob S. is offline
 
Join Date: Sep 2008
Location: Maryland
Posts: 672
How can you determine the RPM of a 3 Phase motor without a nameplate ?

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 .
  Reply With Quote
Old June 11th, 2019, 04:36 AM   #2
Rob...
Supporting Member
United Kingdom

Rob... is offline
 
Join Date: Jul 2016
Location: Manchester
Posts: 219
If it's not a hermetically sealed unit.

You can open it up and take a look at the stator.
  Reply With Quote
Old June 11th, 2019, 08:30 AM   #3
Firejo
Member
United States

Firejo is offline
 
Firejo's Avatar
 
Join Date: Jun 2008
Location: Redmond, WA
Posts: 1,170
Count really fast as it's spinning
__________________
Go Hawks!!!
  Reply With Quote
Old June 11th, 2019, 09:02 AM   #4
alive15
Member
United States

alive15 is offline
 
Join Date: Oct 2015
Location: al
Posts: 294
Haha, since it's not working, I guess you can't count the RPM. Perhaps you can contact the OEM or the vendor you bought from?
  Reply With Quote
Old June 11th, 2019, 09:18 AM   #5
kamenges
Member
United States

kamenges is offline
 
kamenges's Avatar
 
Join Date: Nov 2002
Location: Brillion, WI
Posts: 3,776
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
  Reply With Quote
Old June 11th, 2019, 09:32 AM   #6
Randymauls
Member
Canada

Randymauls is offline
 
Join Date: May 2019
Location: Ontario
Posts: 2
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
  Reply With Quote
Old June 11th, 2019, 09:58 AM   #7
CapinWinky
Member
United States

CapinWinky is offline
 
CapinWinky's Avatar
 
Join Date: Aug 2011
Location: Virginia
Posts: 536
Quote:
Originally Posted by Randymauls View Post
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.
__________________
-----------------------------------------------------
Cycling power is a sign of weakness.
  Reply With Quote
Old June 11th, 2019, 07:10 PM   #8
jraef
Member
United States

jraef is offline
 
jraef's Avatar
 
Join Date: May 2006
Location: Northern California
Posts: 1,734
Quote:
Originally Posted by Randymauls View Post
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/softw...-phase-stators
  Reply With Quote
Old June 11th, 2019, 07:35 PM   #9
PapaSmiff
Member
United States

PapaSmiff is offline
 
Join Date: Jun 2015
Location: Georgia
Posts: 69
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.
  Reply With Quote
Old June 12th, 2019, 01:56 AM   #10
Ronnie Sullivan
Member
United Kingdom

Ronnie Sullivan is offline
 
Join Date: May 2010
Location: London
Posts: 510
Have it rewound.
__________________
I promise to get it right 3rd time every time.
  Reply With Quote
Old June 12th, 2019, 07:01 AM   #11
Randymauls
Member
Canada

Randymauls is offline
 
Join Date: May 2019
Location: Ontario
Posts: 2
Quote:
Originally Posted by jraef View Post
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/softw...-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.
  Reply With Quote
Old June 12th, 2019, 07:49 AM   #12
rpoet
Member
United States

rpoet is offline
 
Join Date: Jun 2008
Location: New York, NY
Posts: 488
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
  Reply With Quote
Old June 12th, 2019, 08:28 AM   #13
Firejo
Member
United States

Firejo is offline
 
Firejo's Avatar
 
Join Date: Jun 2008
Location: Redmond, WA
Posts: 1,170
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.
__________________
Go Hawks!!!
  Reply With Quote
Old June 12th, 2019, 09:43 AM   #14
balash
Member
Croatia

balash is offline
 
Join Date: Dec 2012
Location: Koprivnica, HR
Posts: 413
Quote:
Originally Posted by Firejo View Post
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
  Reply With Quote
Old June 13th, 2019, 05:36 AM   #15
ianingram
Lifetime Supporting Member
Australia

ianingram is offline
 
Join Date: Apr 2002
Location: Newcastle
Posts: 417
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..
Attached Images
File Type: jpg Capture.JPG (146.2 KB, 53 views)
  Reply With Quote
Reply
Jump to Live PLC Question and Answer Forum

Bookmarks


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump

Similar Topics
Thread Thread Starter Forum Replies Last Post
Mechanical/Material Handling and ISA 88 rlmunoz LIVE PLC Questions And Answers 7 October 11th, 2018 08:45 AM
What materials do I need to wire a VFD to a three phase motor? belae1ka LIVE PLC Questions And Answers 17 December 14th, 2016 06:15 PM
speed control split phase 115VAC snugmonster LIVE PLC Questions And Answers 15 March 22nd, 2016 03:18 PM
why the two out of phases is called single phase ?? magdyfayad LIVE PLC Questions And Answers 140 May 28th, 2014 01:06 PM
What do you call this voltage joeparrish LIVE PLC Questions And Answers 52 October 15th, 2008 02:20 AM


All times are GMT -5. The time now is 08:42 PM.


.