I am using an AB 1305 Series C VFD and measured the actual current reading using a digital clamp meter. It reads 1.2 Amps, but the HIM displays 3.3Amps. My current meter is ok as I already tried using another current meter, and the readings are the same. What could be the cause of this? Any reaction is highly appreciated.

 

I used many of these drives, only because they were customer specified. I believe that they are now obsolete (AB now make much more European type drives and I have even found myself recommending the powerflex ) to your problem, why are you measuring the current, is the drive tripping? Where are you measuring the current? I know that this sounds counter intuitive but measuring at the motor end of a long cable can be different than measuring at the inverter.

 

Really? I've never seen that. Can anyone else comment on that one? I thought it was pretty much agreed upon that Kirchoff was right. I'll admit that cable capacitance actually provides parallel paths that would cause a higher current flow at the source end of a cable but in the context of a motor load, its not worth considering.

Assuming the VFD's parameters were set properly, perhaps the original posters problem was from using an average-sensing meter rather than an RMS meter. VFD's don't have very good sine waves.

Jim

 

We've had several hundred of these installed and insist that techs consider the current as a reference value only. I suspect that either the waveform of the drive is not what the clamp-on is expecting so it fouls up it's measurement, or the alogarithm used in the 1305 is not correct.

Regardless, consider that the current expected will be relative to motor nameplate, but the output voltage of the drive will most likely not be at the motor nameplate value. Therefore the value given may relate to conductor load but not to power used, which surprisingly is what we usually are looking for.

When checking with a clamp-on meter for wire current, check 1305 output leads. For checking motor power, check the 1305 supply leads. The supply current is always lower.

While I do look at the current display for a relative sense I never look at it without looking at output voltage also. I always reccomend that one refers to output KW for power delivered.

Note that most VFDs run real good on single phase. Therefore, when troubleshooting or during routine PM always verify that all three phases are providing power. Running near 1305 the nameplate rating hammers the capacitors so is not really a good thing to do. If running on single phase, derate or run on DC with larger capacitors. Yes, 1305, Powerlexs and all other < 10hp IGBT drives I've used will on even DC and I've routinely used them on single phase or DC as required, among other weird things not really reccomended by AB & Square D.

Slate

 

Hi

If you are measuring at the o/p please keep in mind the harmonics contained within your signal. The only true reading would be with a "true RMS digital meter" . Some people prefer to use 'analogue' averaging meter, but the
readings I believe are NOT true RMS values, due to the massive suppressing/filtering (or averaging) of the electrical signal.

Clyde September