Number of rotor bars in a squirrel cage induction motor

[MotorPump]

I have a data sheet for a squirrel cage induction motor.

Among info. in the sheet, I found the followings.

...
No. of leads : 9
MTR lead size : #18
Slot/Grade of steel : 303/29
...

Does this mean that the motor has 9 rotor bars in the rotor?

Thanks in advance.

 

[waross]

Three phase induction motors.
No. of leads: 3, a single voltage induction motor.
Three wires in the junction box.
No. of leads: 6, a star delta motor that may be suitable for two voltages in the ratio of 1.73:1, OR a star delta motor for reduced current starting, OR a part winding start motor for reduced current starting.
Six wires in the junction box.
No. of leads: 9, a motor suitable for operation on two voltages in the ratio of 2:1
Nine wires in the junction box.
No. of leads: 12, a motor that may be suitable for any of the above. The manufacturer should be contacted to determine suitability before using the motor in reduced current service.
I have never had to know how many bars were in the rotor, however with the widespread use of VFDs I am sure that the day will come when I find a motor displaying unexpected behavior because of an interaction between the frequency and the number of rotor bars.
If you had posted as electrical you may have not have received an answer, but I may have to ask a pretty basic question in the mechanical forum some day.
Glad to help.
yours

 

[electricpete]

Knowing the number of rotor bars can help in predictive maintenance. Mostly current signature analysis and to a lesser extent vib analysis.

9 leads refers to the number of leads (wires).

It does not refer to number of rotor bars.
I'm sure your motor has more than 9 bars.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[petronila]

Hello Motor Pump,

I am agree with above comments: 9 Leads are the number of leads available for power connection.(Two voltages but can´t start Wye-Delta)

The MTR Lead Size is the lead motor AWG: 18. and the " Slot/Grade of steel : 303/29" I think could be: 29/303: 29 Rotor´s slots - Squirrell cage and Stator magnetic Core builded with steel 303.

Regards

Petronila.

 

[aolalde]

Motor Pump.

The information you had is not related to the rotor.
As above, "9 leads" are the quantitity of terminal wires (18 AWG)from the stator winding. this allows for dual voltage wye connection (eg 230/460 Volts).
The last data "slot/grade of steel 303/29" is very starnge, not a comon nameplate data. Who is the motor manufacturer?.

 

[MotorPump]

Thank you for all kind responses.

I found out that my data sheet is about "stator core, wound".

I'm really sorry about that.

So, I dissembled my motor, then counted number of bar-like dimmed marks on the rotor surface.

BTW, the marks are not parallel with the rotor shaft. They slanted 10~20 degree from the rotor shaft line.

To me, it looks like succession of bar-slot-bar-slot.....

The number of marks (bars) are 34. I beleive This is the number of rotor bars. (If it is not, please let me know ^o^)

PS. Dear aolalde
I checked data sheet (not name plate) again,it says slot/grade of steel: 303/29. I don't know they made some typo or something like petronila indicated right above.

But I thought it may not be right to mention about specific manufacturer in open public forum. It seems like the manufacturer is one of the big companies, because their website says..."*** Motor Technologies is the largest electrical motor manufacturer in the world, producing more than 330,000 motors every day."

Anyway, I really appreciate for your help.

 

[foxbat1979]

Hello Motorpump,

You're probably right about the 34 rotor bar count, that's quite normal. The rotor slots are usually skewed one stator slot in order to prevent cogging of the motor.

Sr. Design Engineer
Solidworks 2006/DBWorks 2006 user
Special electric motors & centrifugal fans

 

[mdubs]

Hello all! New to this site and I think that you all may be able to solve my problem for me.

Is there some way to determine the number of rotor bars in a motor without taking it apart? You see, I have a 150hp motor that is currently running and can not be shut down. I have checked the manufacturers web site and they do not supply the info that I need.

Any help would be greatly appreciated.

 

[electricpete]

The vibration spectrum if you put it on a log scale will almost always have peaks spaced at RBPF+/-K*2*LF (rbpf = Rotor Bar Pass Frequency and LF = line frequency). You can recognize these as a series of peaks spaced at exactly 2*LF. Initially you don't know which is RBPF. If you have a very good frequency resolution you can identify that only one of these peaks is an exactly multiple of running speed. That multiple is the number of rotor bars.

I assume you are familiar with predictive maintenance tools such as vibration. Can't think of any other reason that you would want to know the number of rotor bars.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[mdubs]

Actually I am not very familiar with the subject but I just got thrown onto this particular project! The senior engineer on the project and myself have been attempting to use Motor Master +4.0 to determine the number of rotor bars for this motor but it is not listed.

I thank you very much for the information. This should make the entire process much easier.