Sine wave shape distorts when transformer is connected.

Thread Starter

rocon123

Joined Jun 26, 2015
27
Control algorithm contains PI regulator.
hardware contains H-bridge inverter.
(1) when resistive load is connected at inverter output terminals, Sine shape retains(which is expected.)
(2) When i am connecting Transformer to boost current from 0.5A to 5A, sine shape convert to something like bell shape.
how to overcome this issue?
 

Sensacell

Joined Jun 19, 2012
2,544
What is this mysterious load you are connecting to the output? Do you have details of that too?
Can't provide any insight until I see the whole picture.
 

Thread Starter

rocon123

Joined Jun 26, 2015
27
What is this mysterious load you are connecting to the output? Do you have details of that too?
Can't provide any insight until I see the whole picture.
I am connecting step down transformer accross capacitor, and at secondary watt hour meter is connected which has resistance in millis.
 

PeterCoxSmith

Joined Feb 23, 2015
148
Are you controlling to a sinusoidal voltage feedback from vfb1? Is that where you are recording your waveform shapes? It may help to put the screen shots of the waveform on the blog.
 

Thread Starter

rocon123

Joined Jun 26, 2015
27
Are you controlling to a sinusoidal voltage feedback from vfb1? Is that where you are recording your waveform shapes? It may help to put the screen shots of the waveform on the blog.
peter,
no, i am controlling current by giving IFB as feedback. But ultimately it will change output voltage amplitude only to keep desired RMS current.
I have attatched distorted wave.
 

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PeterCoxSmith

Joined Feb 23, 2015
148
peter,
no, i am controlling current by giving IFB as feedback. But ultimately it will change output voltage amplitude only to keep desired RMS current.
I have attatched distorted wave.
is that the voltage on the secondary? The distortion could be due to the non-linearity of the transformer if it is at all saturated.
 

Thread Starter

rocon123

Joined Jun 26, 2015
27
is that the voltage on the secondary? The distortion could be due to the non-linearity of the transformer if it is at all saturated.
peter,
IFB is current in primary side as can be seen on schematic. I can use that as feedback because after all that is only gets boosted via transformer. Please correct me if i am wrong.
Transformer saturation limit is 20A. I am checking it with much less current.
 

Kermit2

Joined Feb 5, 2010
4,163
You mentioned your sinewave comes from an inverter with an Hbridge the waveform looks like a good output from such an inverter when loaded. The "flat" spots where the wave changes polarity would indicate a time delay in the Hbridge designed to prevent overlap in the switching fets "on time"
 

Thread Starter

rocon123

Joined Jun 26, 2015
27
You mentioned your sinewave comes from an inverter with an Hbridge the waveform looks like a good output from such an inverter when loaded. The "flat" spots where the wave changes polarity would indicate a time delay in the Hbridge designed to prevent overlap in the switching fets "on time"
are you pointing at deadtime of IGBT?
 

Thread Starter

rocon123

Joined Jun 26, 2015
27
Yes.
Yes I am.
My crystal ball says you will now post the reasons this cannot be the cause?
Can you please help me understanding that transformer specifications? I am not getting saturation limit from that. And that is all i have about transformer...
 

Kermit2

Joined Feb 5, 2010
4,163
There are magnetic fields in your load which must alternate polarity. Current and voltage waves are phase shifted in transformers. Inductors "resist" any change in current flow. Those three things combined can cause what you see on the scope.
The current flow reverses in the primary BEFORE the magnetic field can follow it. The result is a back EMF created by the inductor which counters the input voltage as it crosses zero
 

Kermit2

Joined Feb 5, 2010
4,163
Transformer secondary #1 has a 10% duty cycle with a 3 minute maximum on time. Is your watt hour meter connected to secondary 1 or 2?
 

PeterCoxSmith

Joined Feb 23, 2015
148
peter,
IFB is current in primary side as can be seen on schematic. I can use that as feedback because after all that is only gets boosted via transformer. Please correct me if i am wrong.
Transformer saturation limit is 20A. I am checking it with much less current.
Distortion on a current waveform is normal, there are a number of potential sources.
The B/H curve of the iron core is not linear and there can be localised areas of saturation which you can only discover by modelling the magnetic circuit. Kermit2 has pointed out the phase shift issue. The meter, which loads the circuit will have an electronics board with a bridge and a capacitor which also distort the current waveform. I think what you are seeing is normal and not much can be done.

What is your problem? are you testing meters and worried about accuracy?

Peter
 
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