AC motor runs ok but showing wrong RPM

Thread Starter

apanmal

Joined Mar 18, 2022
9
Hi ,

I am new here so hello and I have a problem to solve if anyone has any idea :) I have an AC motor in a machine that has the option to run 60RPM or 72 RPM , on 60RPM it does 60 rotations and the counter shows 60rpm but at 72 rpm motor does 72 rpm but the counter shows cca half less than it should be showing. Any idea what has happened? Machine has some inner calibration option which is done at 60 and 72 rpm but this issue at 72 rpms stays present.
 

Reloadron

Joined Jan 15, 2015
6,585
Motor part numbers? Speed sensor used? Speed sensor correctly setup? Since 60 RPM works fine I would start looking at the sensor. Anyway, less part numbers and drawings of what you have it's about impossible to answer anything.


Ron
 

Ian0

Joined Aug 7, 2020
5,147
Are you sure it has the option to run at two speeds? Doesn’t it just mean that it runs at 60rpm on a 50Hz supply, and 72 rpm on a 60Hz supply?
I‘d be less suspicious if the two speeds weren’t in a ratio of 60:50.
 

MaxHeadRoom

Joined Jul 18, 2013
25,212
It would be helpful to know the type of motor and the reduction method?
What does the form of the speed option take ?
The description details are sparse, to say the least!
 

Thread Starter

apanmal

Joined Mar 18, 2022
9
Hi guys,

I know i went short on details I will provide more details on monday. Its a single phase motor (im from eu) that drops grid voltage from 230V to 120V.
Machine has two speeds yes 60rpm and 72 rpm...
No optic sensor or anything like that to count rpms.. all other info Ill look into on monday :)
 

Thread Starter

apanmal

Joined Mar 18, 2022
9
It has a tranformer that drops 230 to 120V, I dont know how it regulates speed of motor i belive through voltage change? Like i said i will make more pictures on monday here is already 5pm and weekend started :)
There was something on motor about induction but something like that induction has nothing to do with this motor.. This is from my head what i remember..
 

MisterBill2

Joined Jan 23, 2018
11,888
A 72 RPM motor would most likely be a "So-Syn" motor on 60 Hz. Those motors, from Superior Electric, used an RC phase shift arrangement to develop torque. So 72 RPM is the basic shaft speed, no reduction needed.
But those motors are not so very common presently.
So certainly we do need at least a description of the motor and the drive system. If it is a servo type motor then the issue is quite different.
Every 72 RPM motor that I have seen was a synchronous motor, with the speed determined by the supply frequency.
Based on the additional information in post #7 it may also be used to provide three-phase power from a singe phase source, which is a totally different application. So if you are attempting to run a synchronous motor at a non-synchronous speed, the results will not be predictable.
 
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BobTPH

Joined Jun 5, 2013
4,907
What do you do to change the speed? Change the wiring? Use an electrical switch? A mechanical means to change gearing? Or other?

Bob
 

Thread Starter

apanmal

Joined Mar 18, 2022
9
I will attach link below to this instrument... Its basicly a turntable on top ju put a product (piece of woden floor for example) and on this piece you drop a grinding wheel that simulates abrasion. The bottom turns with 60 or 72 rpm (it used to be different US and EU) because of different standards and while turning material is abrased. Before test you weigh your sample and after and than you have formulas to calculate this loss and etc (not my field) :)


https://www.manualslib.com/manual/1557252/Taber-5135.html
 

Thread Starter

apanmal

Joined Mar 18, 2022
9
No it used to be like that and because of that they used to write different standards that is why you have two different standards one with 60and other woth 72 rpm and now you have both options on one machine so you can make both test :)
 

MaxHeadRoom

Joined Jul 18, 2013
25,212
It is not a 'Standard' . It is how the motors are designed, So they will operate at different speeds in both jurisdictions because of the mains power Frequency difference.
i.e. Synchronous Motors!
 

MisterBill2

Joined Jan 23, 2018
11,888
I read a lot of the attached manual. This system uses a multi-speed drive, which I think is probably no longer a synchronous motor, but rather a servo controlled variable speed system. A dual speed synchronous motor would be very expensive.
The drive may also be a brushless DC drive that is out of calibration, or possibly defective, but what it is certainly not is a simple AC motor.
And as the problem is in the displayed RPM, it is probably a display software problem.

This thread is a good example of why we need more information at the start. We were lead to believe that it was a motor by itself, and that is very far from the case. The system is a computerized testing system that includes some sort of two speed motor drive package. Very much different.
 
Last edited:

MisterBill2

Joined Jan 23, 2018
11,888
It is not a 'Standard' . It is how the motors are designed, So they will operate at different speeds in both jurisdictions because of the mains power Frequency difference.
i.e. Synchronous Motors!
Actually, Max, it became a standard when the machine for the test had those speeds and nobody else had a different test machine. So as the test was then described that became the standard by definition. A reverse of the more standard scheme, but very valid.
 
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