Electrical vibration in induction motor 2

[cad4]

Hi,
I'm a mechanical engineer investigating vibrations in small cooker hood motors(70W). The motor is a 4-pole single phase induction motor of external rotor type. This is fitted with a 2uF permanent split capacitor.
My problem is that I have a high freqency vibration(transformer vibration) in some motors, which decrease bearing life.
It's not a mechanical unbalance. I've also checked the air gap eccentricity and it's in tolerance. Could the angle of the rotor bars in the squirrel cage have any influence on the vibrations?
Do you have any suggestions what causing these vibrations and how to minimize them ?

Best regards

cad4

 

[nrupalakolkar]

NOW THAT YOU HAVE INSPECTED ALL THE MECHANICAL SIDE EFFECTS THAT RESULT IN VIBRATION. THE ONLY POINT REMAINS IS THE SKEWING OF THE ROTOR SLOTS THAT MIGHT HAVE CAUSED AN UNBALANCED MAGNETISING FIELD TO RESULT INTO A DISRUPTIVE FORCE ACTING RADIALLY OUTWORD AT A SPECIFIC ANGLE .5 TO 2 TIMES THE SKEWING ANGLE OF ROTOR. NOTE THE ANGLE OF SKEWING I AM TALKING IS IN ELECTRICAL DEGREES AND THE CONVERSION OF ELECTRICAL DEGRESS AND MECHANICAL DEGREES IS KNOWN TO YOU OR ANY ONE THERE WHO IS AN ELECTRICAL ENGINEER IN YOUR COMPANY CAN TELLIT TO YOU. ELSE IN ANY BOOK ABOUT ELECTRICAL MOTORS/DRIVES YOU CAN GET THE ANGLE CONVERSION.
THE SECOND THING THAT MIGHT BE PLAYING ROLE IS DUMMY COILS ROLE. THE CAUSE IF NOT THE ONE MENTIONED ABOVE ARE THERE THEN LET ME KNOW WHAT U DID AND WE WOULD THINK OF SOMETHING ELSE POSSIBLE.
NRUPAL!

 

[electricpete]

You have not identified the frequency. I will make the assumption since you call it transformer-humming that it is at a frequenc of twice line frequency.

Soft foot and foot-related resonance can cause 2*LF. Try loosening and retightening one of four hold-down bolts at a time and seeing if the vibration goes down.

High voltage can cause 2*LF.

A litte bit less common but still possible:
Shorted turns can cause 2*LF
Misalignment can distort the airgap and cause 2*LF (but usually does not and other frequencies 1x 2x are usually more prominent).

Was the airgap checked at both ends of the motor?

If you have not verified the frequency of the vbration then there may be a lot more causes.

 

[GusD]

Hello Cad4

I have tested a lot of electric motors and I don't recall a single time when the skewing of the rotor slots became a concern. Electric motor Slot "ripple" may be agravated by insufficient skewing,but you would have to have a very special motor to probably have to worry about it.
If the motor is as small as you mention,try to follow Electricpete's advice on the possibility of motor frame distortion that may influence you electrical frequencies.
Try to loosen one base bolt at the time and see what happens to your 2xLf.On such a small motor,soft foot is a real possibility.
Knowing your slot angle of(Eg 5/10 /15 or 20 Deg)is not going to help you much.You cannot change it,unless you want to buy a new motor .
Good Luck

GusD