Dear All,
I want to check for rotor bar problem of a 3.3KV SCIM but our current probe's (AEMC Instrument, Model MR411) maximum operating voltage is only 600V.
Is it OK to take data for MCSA in the CT?
I already tried it using SKF CMVA65 data collector. However, data is fluctuating considerably. Is this fluctuation expected?
Please share me your experience.
Thanks.

MRTA

It would be helpful to post a link to the other thread on same subject:

http://maintenanceforums.com/eve/forums/a/tpc/f/7161085...131012763#2131012763

MRTA,
It is perfectly acceptable to use the ct to collect data. I have successfully tested this several times with repeatable results.
And FYI, the 600v rating is for the insulation quality of the amp clamp if it were to come in contact with exposed terminals. It will accuratly measure higher voltages so long as it is around properly insulated wire. I DO NOT RECOMMEND DOING THIS. I just though you might like to know.
As for the fluctuation, how is your resolution?
good luck,
Nick

MRTA,
As Nick said, you can clamp onto the CT wire. If you know the CT ratio, then you can have your ananlyzer display close to the same current value as the display meter or PLC process value. For example if the current clamp has a sensitivity of 1 mv per amp (0.001 volts/amp) and the CT has a ratio of 10:1 (acutal current is 10x higher than CT) then divide the sensitivity by 10 to get the actual volts per amp. If you use RMS units, then the spectrum value at line frequency should be the actual amps in the circuit. If not, then there is a measurement issue.

Regarding the fluctuating level: if the level has a periodic fluctuation, then there may be an electrical fault in the rotor. There is a lot of info on the Web and within several discussions on this Board (use the Find function) about motor current spectrum analysis and interpreting the data.

Walt

I thought I understood that looking for a problem I viewed the signiture in DB and look for a ratio between Line Freq and sidebands. I thought actual amplitude did not matter. Or are you discussing a different idea all together.

jbalco,

Yes, you can scale the current in dB units and look for the dB difference (not ratio) between the slip frequency side bands and line frequency. I also like to scale my current measurement to verify/confirm the actual current.

Walt