Waveform Type

Perhaps this explains the issue -

Figure one graphs about 1000 volts on the 200 volts per division scale but the scope is reporting 107 volts - is there a decimal there that we cannot see in the pictures?

The sine wave period 4 divisions * 5ms / div = 20milliseconds.

Current decreases as voltage increases. This should be correct.

Figure two shows positive-going 250 volt (at 1.2 to 1.5 divisions * 200 volts/ division) sawtooth/spike with period of 1.2 microseconds. But the voltmeter is not reading the spikes, probably because they are too short for the counter to summarize, so the voltmeter reports a blank.

my guess is you have zoomed in far more than 1,000 times and now see some trash on the line and its resultant microscopic impact on the amperage sine-waveform shown in figure one.

"Please discuss why there is a difference in shape of voltage & current waveforms."

They are different because you are driving an inductive load, not a purely resistive one.

Check out AC circuit theory 101.

You interpret OP's question as

• why the voltage and current waveforms are different to each other.

Is that right?

I interpret the question as

• why are the voltage and current waveforms in the first display different to the voltage and current waveforms in the second display.
  

Correct.

OP can pick an answer then....

Agreed.. I am driving an inductive load in which current lags the voltage. But my doubt is voltage waveform is pulse width modulated (if i am not wrong) but not the current wave. why?

I hope my question is in right way.

Thank you

As I said, it's just the effect of a non-steady voltage applied to an inductor.

Imagine the current in the inductor having a kind of inertia. The initial push from the high voltage gives it enough of a shove to get it going (the steep-ish rise of the current waveform "steps"). When the voltage turns off, the current slowly decreases (the more gentle slope of the current falling - rather like an inertial mass given a hard push to get it moving then being slowed by friction).

It'll be more obvious if you turn up the gain on the current display.

You have done more than just zoom in ie change from 5ms/div to 100µs/div.

The first is DC coupled

The second doesn't say...I'll guess it isn't.

What's in the waveform options tab?

I can't tell what the Volts/div is on the second....I'll guess its smaller than the first.

I suspect that the second is the noise (furry edge) on the primary waveform, AC coupled and the the amplitude is set to auto range.

Second waveform is also DC coupled. Nothing is changed.

I have done zoom in upto that much only to trace the exact shape of pulses.

Since you have changed from 5ms to 0.1ms, you have stretched the top display 50 times horizontally! What you are seeing in the 2nd display is just 1/50th part of the wave seen above!

You will notice that the peak (red trace, Volts) keeps going higher at each pulse: Keep going /scrollin to the right and the peak will reach the maximum amplitude then start to decrease again, which is what is displayed on the top display (previous).

if you cram the 2nd display, compressing the pulses together( going back from 0.1ms to 0.2, 0.3, 0.4 ... back to 5ms, you will see the how the pulses get back together to give the 1st display.

{Same thing with the Blue trace} the smoother blue trace is due to the fact that the curret keeps moving and does not go down to zero all the way, between voltage pulses (Induction ... etc.)

I hope you will see that

JohnDG is right! - and there is nothing wrong with the scope setup.

The VFD is driving a train of ~20us pulses at 8Kpps into an inductive load and the V/I traces look accordingly...

Sharath4,

VFD's typically use pulse width modulation of pulses that can turn on/off at multiples of a carrier frequency. This frequency is usually in the range of 4,000 to 20,000 Hz. At 8,000Hz, this works out to about 125μsec per pulse. On your second scope picture, each major horizontal line is 100μsec, so I believe the spikes you are seeing are related to the carrier frequency. Since the indicated scales for the voltage and current on the second picture are "----V" and "0.12A", I would interpret this as a very low voltage and non-zero current consistent with a "snapshot" taken close to when the voltage crosses the 0 line with the current being a few degrees off from this crossing point because of the motor's being an inductive load.

On the first picture, the major horizontal divisions are 5ms, which is close to an output frequency of 50Hz. The "choppiness" of the waveforms is because the drive output is not a pure sine wave--you are seeing a filtered output with residue of the pulse-width modulation visible. Note the phase angle shift between the zero points of the voltage and current. This is typical of a non-resistive load (wherein the current and voltage are not in phase). However, I believe that your voltage and current inputs to the scope do not agree--one of them is backwards. The phase angle between positive peaks of the current and voltage is far too much (about 220 deg. instead of the 25-40 deg. I would have expected).

--JMM

Thank you all for sharing your knowledge