Side Bands in FFT Spectrum of Faulty Motor's Current Signature

FIGURE 1

Normally FFT spectrum gives you a spectrum of harmonics which are multiple of fundamental frequency. But the harmonic you show in about 75 Hz, which is not the harmonic. Are you sure you capture the correct spectrum?

It certainly does not look like it.

Also, I am curious about motor current signature analysis. Do you have examples of what kind of motor fault give what kind of pattern of spectrum ?

The first analysis shows peaks at f/2 and 3f/2 at about -40dB. These are not coming from the motor. They are distortion from somewhere on the power system upstream.

No matter how you distort , you will not get half harmonic, the FFT spectrum will only give frequecies in multiple of fundamental. The OP's spectrum is something else.

Quite.

There are a few things that must be clarified to proceed.

• What type of AC motor is this; induction motor, universal motor, synchronous motor, single phase, three phase?
• What type of motor failure condition exists that makes this waveform; stalled rotor, shorted stator pole winding, open stator pole winding, shorted or open rotor winding (universal motor)?
• What is the rotor rotational speed during this fault condition?
• You've shown us the FFT current amplitude values. What difference do you see in the FFT current phase analysis signals?

I like PWSlack's answer. Bravo, could you maybe explain in a little more slow person friendly terms :P. Specifically how you know what the fundamental frequency and/or the harmonic is.

The OP already said about the fundamental frequency. In any case ,it is either 50 or 60 Hz, the spectrum diagram shows 50 Hz. I thought this is obvious.

Power lines are quite dirty in any office / manufacturing site. I am not believing any reading without solid proof that it is not coming from the power line from inside or even outside of the site!

He has his work cut out to convince himself, THEN us on that account. Labs and test rooms cost a bundle for a reason!

I agree. With a nearly 60db side lobe to main lobe difference I would expect that this is actually an artifact of the power main voltages. A simultaneous comparison of the supply voltage and motor current will settle that. I still say that a comparison of the AC current phase to the AC voltage phase will be more informative than the amplitude. Now what might be real useful is to take the raw LabView data and find the amplitude of the real power drawn by the motor. That should show some interesting effects as the rotor gets loaded or a winding phase opens.

Thank you for all your comments!

Firstly the two images that i've shown are reference signals taken be a PHD thesis of motor current signature analysis! I just want to know that why do the side bands even occur at various frequencies due to different faults?? For example first sideband peaks due to broken rotor bars occur according to this formula

Fb = (1± 2s) fL

where Fb is the side band frequencies, s= slip of the motor and FL is the line(supply) frequency.

So the question is that why does it follow this formula everytime for broken bar faults??.. why always at twice the slip frequencies?

Are there any physical explanations to this??. i couldnt find any answers on the internet. So looking upto you people to help me out!

Thanks once again for your replies

The levels at 25 & 75 Hz are -60 dB compared to fundamental current. That is 1 millionth the fundamental power lost in a series resistance - is that enough to regard as a fault? As has been noted, you have not given evidence that these are not in the input voltage or due to 25 Hz and its harmonics - rather than sidebands. And if it were a 1500 rev/min motor rotating at half the supply rate, components at 25Hz, 75 Hz would be no surprise. More clarity on the motor speed, if the motor is normal or faulty and a spectrum say 20 to 200 Hz are needed.