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, 71 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

I used CSI cable probe 341B to measure AC inputs and noticed 10.5% current variance between 3 phases while another similar one machine have only 2.5%. Dose that an indication of electrical unbalance and cause of 120Hz vibration on motor.
I am attaching the spectrum. Please take a look and comment. And is there any rules to interpret these AC current spectrum? Any useful documents you can send to me?

Thanks, Kevin Ku.

CFP-BE04-extruder-7-28-08.ppt (656 Kb, 22 downloads) Current signature

Kevin,
Confirm that the frequency you are truly seeing is 120Hz. If it is, you have a drive issue, period. No air gap issues, etc. etc. etc. There should NEVER be any evidence of AC voltage frequency on the motor frame. The AC is rectified by the DC drive. That's supposed to be the end of it. It doesn't necessarily mean you have a drive problem, you could have a grounding issue, but it is a good idea to check the drive, since things like SCR and fuse issues can cause 120 Hz to show up in the motor vibration.

I would say the new information confirms an input-side power supply problem. Unbalanced current magnitudes. Slide 5 shows the peaks repeating at 6*LF, with differing heights repeating every 3 peaks (low peak shows up every 3rd peak, high peak shows up every third peak). Every 3rd peak we come back to the same input phase. It is an indication of a problem on the input side of the power supply (output side problem like an SCR would more likely show up as differing magnitude once every 6 peaks in the TWF and spectral peak at 60hz). Possibly a supply voltage unbalance or other problem on the input side of the power supply.

120 hz would be expected if it happened to be a small dc motor fed from a power supply derived from single phase power. This extruder is obviously not one of those.

Hi,

We measured input side voltage after reactor and only got 2V difference between phases(450~452V). The input voltage unbalance is ruled out by our electrical guy. So what else might cause current unbalance? And which components we should check?

Thanks, Kevin

Have you done any infrared scans of all electrical components? Has it recently been removed and replaced?
Typically motor will run hot as stated before in this thread, speed will flucuate.
If vibration was 1/2 wave rectified is 3-60hz fired scr's=180hz full wave 6-60hz=360hz this would inidcate DC side fault. But primary vibration is 120Hz which could be loose electrical connection on AC side, soft foot on motor, poor grounding on AC side.
If you perform a voltage drop test across the series fields that will tell you if a short is starting to develop or not. Hope this helps
Dave

I have seen in the past a similar problem with a DC motor. Use the current probe and record or capture a TWF of the DC going to the motor. If 1 out of 3 negative going pulses is extreme in the negative region for a very short period, an SCR may be gating early which causes an imbalance in the AC current and causes heat in the SCR and motor. For a brief moment there is a short because the early gated SCR overlaps the previous SCR that has not completely turned off. The motor will also try to slow down every 3rd waveform of the output current TWF and the next positive half cycle will be increased to compensate (if the drive is tuned with sufficient gain). If this is the case, a gate pulse generator card or an SCR is the problem.

This message has been edited. Last edited by: joe nichols,