# rotor bars and # stator slots in induction motor 1

[electricpete]

Are there any generalizations that can be made about the # rotor bars and # stator slots in an induction motor?

One thing I know is that most higher-speed windings are integral-slot (vs fractional-slot) windings, the number of stator slots is divisible by 3 and by the number of poles.

Are there any other generalizations that can be made?

The reason is that vibration spectra sometimes show rotor bar pass frequency and stator slot pass frequency... but may not be able to distinguish if we haven't inspected the motor to count the number of bars and slots.

 

[THooper]

Hi electricpete,

From past experience only, there is no correlation that one could reference that I know of. For example I am reviewing two 4-pole motors evaluations now, both are same hp, same design, SF, duty, both have 48 stator slots, but one motor has 46 rotor bars and the other has 40. The only defferance between each of these motors is the mfg.

We perform rotor re-barring in house about three or four times a month and have found that the mfg will use different rotor bar configurations and amount of bars will vary from one motor to another, but will have close to the same rotor resistance as its sister motor, but differnt engineering rotor designs.

It is also rare to even see the same internal rotor design from the same mfg, from one identical NEMA Frame motor to it's same mfg sister motor.

I think this is done to keep repair shops like us on our toes... You can request this bar and stator slot count from the oem, and also have your repair facility supply this information on all repair reporting for future reference.

Kind Regards,
motorhead

 

[electricpete]

Thanks motorhead1, that's good info. I'm goiong to revise our motor repair spec to count the bars/slots whenever repairs are done. Also our electricians can probably do it when changing bearings due to vibration symptoms (unfortunately that seems to occur fairly regularly on some of our motors).

It's good to see that your example matched the "thumbrule". 4-pole 3-phase motor should have number of stator slots divisible by 4x3=12. If we were looking at the spectrum without knowledge of the physical construction, we could have deduced that 48x harmonics could be stator slot pass frequency but 40x and 46x probably wouldn't be stator slot pass frequency.

 

[electricpete]

Thanks motorhead1, that's good info. I'm going to revise our motor repair spec to count the bars/slots whenever repairs are done. Also our electricians can probably do it when changing bearings due to vibration symptoms (unfortunately that seems to occur fairly regularly on some of our motors).

It's good to see that your example matched the "thumbrule". 4-pole 3-phase motor should have number of stator slots divisible by 4x3=12. If we were looking at the spectrum without knowledge of the physical construction, we could have deduced that 48x harmonics could be stator slot pass frequency but 40x and 46x probably wouldn't be stator slot pass frequency.