1. If a 3-phase AC motor happen to have the bad design where rotor bars and stator slots are the same number (28), will it be more sensitive to broken rotor bars sound wise? Vibration wise? Cast aluminum rotor that is prone to rotor bar problems. VFD drive 4200 RPM 142 Hz drive freq. Rotor bars are slanted at an angel so you would get overlap anyway but it can´t be that good?
2. Would a hi resistance rotor bar be hotter or colder than the others in a cast aluminum rotor? Olov

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1,If a 3-phase AC motor happen to have the bad design where rotor bars and stator slots are the same number (28), will it be more sensitive to broken rotor bars sound wise?

Same number of rotor bars and stator slots without skewing would cause torque oscillation at bar pass frequency. With skewing, I'm not sure. I have a hard time seeing how it would change sensitivity to broken bar.

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2. Would a hi resistance rotor bar be hotter or colder than the others in a cast aluminum rotor? Olov

If you are applying current or doing loop test, I think it could go either way. Certainly for a complete open circuit, it will be cooler. For a high resistance it is going to be hotter right at the hot location but not necessarily all along the bar. Look for differences.

There are quite a variety of tools available to check rotors. A pretty comprehensive discussion here:
http://www.pumpingmachinery.co..._29/rotortesting.pdf

The 3-phase motor with 28 stator slots is very unusual. It has to have some kind of fractional winding. Normally, the number of stator slots is divisible by 6.
The motor with identical number of the stator slots and rotor slots never starts rotating unless the rotor slots are skewed.
If it has really 28 slots, I would not expect bad design, but lots of brainpower.
1) I do not believe it would vibrate more or emit more noise than other motor with broken bar. The broken rotor bar can cause some modulation, but does not introduce any new frequencies.
2) At full speed the frequency in the rotor bar is so low, that the current distribution is governed by the resistance of the bar, not the reactance. Hence from the formula: P= (V^2)/R, the losses will decrease with increasing the resistance. The bar with higher resistance will be cooler. (V is the voltage from end ring to end ring).
jank

Thank´s for the input. Test was done by full load in the stator with normal operation 4200 RPM, 142Hz LF, VFD drive and stripping rotor and therm cam photo directly, that is tricky due to shiny surface. I was also betting on bad bar being cooler. I wanted to confirm what I was looking for, on the other hand yo can see hot spots by the eye on the rotor surface, eg. burn marks that is close to hi temp on therm cam photo. Yes stator got 28 slots (as I have been told) and it´s a special application not a industrial motor and the rotor bars are skewed. Problem is the intermittent noise from the assy including gbx, in some cases, it may be the other way around so that "good motors" rotorbar wise give the noise and the bad does not? It is also in most cases different running forward or backward, that I don´t see a reason for, part from that the gbx is designed to run best in one direction and axial postion change due to spiral gear. So the equal no of bars and slots is disturbing me but not all units have hi noise, only some and they also have hi gearmesh vibration from the connected gbx. Gearmesh can always be heard but the intermittent noise is only in some units and in some a constant grinding/growling. Good assys give good wear patterns on gbx teeth (eye shaped and centered on teeth), bad give a axial and radial dispersed wear pattern. I will try to confirm torsion "vibration" data on Monday to possibly confirm rotational irregular behaviour. Or not. Olov

Well as usually this was wrong information, 28 rotor bars, 36 stator slots so yes it was divideable by 6 but both also both divide by 4 is that good? Motor is 1kW. I did the torsion test using installed tacho with 48 pulses per rev. and only got 1xRPM with some multiples and the 2xGearmesh from second part of gbx that is attached, on the noisy ones. So I guess the part influenced by possible rotorbar problems are limited. Olov