One DC motor was noticed with high 120Hz harmonics at different running speed. I checked with books and said one bad firing card might cause it. How can I verify it? Any other possibility?

BE04-extruder_A_motor-3-13-2008.ppt (547 Kb, 43 downloads)

I don't do that much with dc motors and haven't looked that closely at your data. But some general comments:

120hz can be associated with normal ripple frequency of a dc supply if fed from a single-phase-ac-input power supply.

Could also be the ripple frequency of a dc supply fed from 3-phase ac input when the ac input is unbalanced.

Kevin,

Not a lot to go on here with the absence of phase data, but looking at the spectra and the significant difference in amplitudes between horizontal and vertical planes at both the DE and NDE of the motor I suspect that you have a misalignment problem. 2x Line Frequency is associated with an uneven air gap between rotor and stator which again suggests an alignment abnormality.

Thanks Guys,
We will go for electrical approach first by verify AC 3 phase balance and DC ripple pattern.

It seems that x2 rpm is higher, Miss aglinment and if 120hz is high i would suspect that fundation is to low or to high in one of the feets of motor so air gap is not ok.

Had a simular case some year a go.

With full bridge 3 phase rectifire normally producing 360 HZ ripple, I agree with Pete, 120HZ is a result of 2 ripples out of 6 being different - indication of electrical unbalance -which is a serious problem.

Are there any other malfuntion symptoms such as elevated temperature?

Mechanical vibration at 120 HZ IMO results from an oscillating torque at 120HZ rather then misalignment.

David