Number of Rotor Bars and Stator Slots!!

Hi guys,

Is there any formula/technique(other than counting Razzer)for calculating the number of Rotor Bars and Stator Slots of an AC Induction Motor?What are the design parameters they will consider in calculating the same?

Please forgive me if it is an old wine in a new bottle!! Smiler

Have a nice day!!
Original Post
Here is a tabulation of rotor bars/stator slots for Reliance NEMA frame motors:

Jan K has also posted another list for a variety of manufacturerers which I have attached.

There are a number of rough thumbrules used in motor design, but they won't get you very far since one manufacturer may build a motor differently than another manufacturer even when the specs are the same.

Stator slots will usually be a multiple of 6. The exception may be slow speed motors which may be a fractional slot configuration.

Generally speaking, as the torque rating (horsepower/speed) goes up, the number of stator slots goes up.

For a given number of stator slots N1, there are general restrictions on number of rotor slots N2:
The following general "thumbrules" are taken from Electrical Apparatus Magazine June 2005 "The importance of Slot Combination on Motor Design" by R. Nailen:
  • |N1-N2| < > P +/- 2 where P is number of poles
  • |N1-N2| < > 3 * m * P where p is integer
  • |N1-N2| < > 5 * P
  • N2 should be above or below N1 by at least 15 bars or 25% of N1.
  • N2 is generally not odd
  • N2 should not be twice a prime number
  • N2 should not be divisible by the number of poles
  • Some designers prefer N1 > N2 although there are lots of exceptions
  • N1/N2 = 24/19 is a relatively common ratio

Again, there are very few guarantees of what we can predict just based on the horsepower, speed etc. Your best bet is having a count or access to a list or info from the manufacturer.

One other method to determine the number of rotor bars it to examine the the RBPF +/- K*2LF pattern in vibration: take vibration using high Fmax (above RBPF) and high # lines (to get good frequency resolution) - something like Fmax=300,000, lines =6400. Look for the set of peaks separated by by 2*LF. The one within that set that is exactly a multiple of running speed is likely the RBPF. (Peak label feature is important to determine actual frequency more accurately than the bindwidth... also may help to have more accurate estimate of 1X through strobe or another higher resolution spectra at lower Fmax).


A few other guidelines:

1. The no. of stator slots is normally a multiple of the no. of phases & the no. of poles. This, it generally increases for lower speeds (for the same kW rating).

2. While looking for the RBPF in the vibration spectrum, bear in mind that the 2xFL sidebands can often be larger in amplitude that the RBPF peak itself. However, the 2xRBPF & 3x RBPF peaks will generally be much stronger than their sidebands. So, it is a good idea to keep the Fmax high enough to look for those peaks also. I normally take data at 10 kHz acceleration frequency with 6400 lines the first time I test any motor so as to calculate the no. of bars correctly. Not difficult to estimate the no. of stator slots after that.

A list of slot & bar combination is attached.




Hi Pete & Aditya

Thank you for your valuable information...It will help us a lot!!

So there is no direct formaula from which the numbers can be derived?

Pete, what you had given were general restrictions for the number of Rotorbars and Stator Slots.. Do you have any documents about their design criteria? Like, how it is related to the capacity of the motor, size of the motor etc.

Have a nice day!! Smiler
There is definitely no direct formula that converts motor ratings (horsepower and speed) into number of stator slots or rotor slots. It is a choice made by the individual manufacturer.

As I said, number of slots generally goes up as machine size goes up.

Off the top of my head, I think for a 50 2-pole hp motor, you might have in the range 20 - 50 slots. For a 7000 hp 2-pole motor, you might have in the range 150 - 200 slots. Maybe someone else can provide a better guess. Either way, you need more info to determine exact number (info from manufacturer, or from count, or from looking at vib to find RBPF pattern).
50hp 2 pole generally 48 slots, sometimes 36 slots or 60 slots. Common slots for 2-poles are 18 & 24(fractional hp up to 15hp), 30(usually 10-20hp designs) 36, 48, and 60.

4 pole motors will be same, but will include 72, 84(rarely), 90 (common on changeovers) or 96 (common on changeovers).

6 pole motors include all the above mentioned, but also include 54 slot designs. I've seen 108 slot motors as well.

Anything above an 8 pole design will be above 72 slots.

These are the most common I've run into for motors from fractional up to 1000 hp. I hope this helps.

You have discussed using the vibration spectrum to identify the rotor bar number but could you use the current spectrum to do estimate the rotor bar number?

Also, Aditya, please can you explain how once you know the number of rotor bars it is easy to estimate the number of stator slots?


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