Current Ampacity Difference in Cables at two Ends

Thread Starter

coolsatz

Joined Jan 12, 2017
1
Hello,

We are using ACS 800 Drive to Run a Pump.
The Other Day, I was cross checking the Current by a Clampmeter as indicated on the Drive.
I am using the Drive to Run a Motor 0f 55 kW/1450 rpm/415 V/ 95 A, which is about say 200 metres from the Drive and using 35 sq mm Flexible Copper Cable from Drive to Motor
Now, the Current indicated on drive at full load is 95 Amps. On Checking with Clampmeter, just at the Output from the Drive Terminal is 105 Amperes, and when I checked near the Motor, ClampMeter Shows 175 Amp.
Why is there so much of difference.
I can understand Voltage Drop with increase in length of cable. But such large deviation of current from Source Point to Destination is not understood by me.
Also, what puzzles me is as I am using 35 sq mm Cable, the Cable Insulation should practically get very heated and start to degrade at such high Current at the end point of motor , but the tempaerature is just Ok at around 30 degrees.
Am I missing out some basic laws of electricity
 

crutschow

Joined Mar 14, 2008
24,929
The current at one end of a wire has to be identical to, not just close to, that at the other end, so you have some sort of serious measurement error.
Post a diagram of exactly what you have and where you are measuring.
 

LesJones

Joined Jan 8, 2017
2,638
I suspect the meter is being confused by the waveform produced by the by the inverter. The output on each phase is not a pure sine wave. It is a high frequency PWM signal and the average value of the PWM signal at any instant of time will be the same as the actual value of a true sine wave at that instant in time. The capacitance and inductance would not have much effect on a pure sine wave at 50 or 60 HZ but it will change the shape on the PWM signal. This is why I suspect the meter is giving different readings at each end of the cable.

Les.
 

Kermit2

Joined Feb 5, 2010
4,162
Clamp meters use magnetic fields to interpret current strength.

Stray magnetic fields in and around the motor may also be generating additional input to your clamp.

Insert old fashioned 50mV shunt and read on a dc multimeter or display unit.
 
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