Here is a tabulation of rotor bars/stator slots for Reliance NEMA frame motors:http://www.reliance.com/mtr/pcrssmn.htm
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).