THD analysis in EV charging test system - HELP

Thread Starter

Cajmccormick

Joined Jan 24, 2018
4
Hi guys,


I have a few questions if y’all could help me out! I’m testing an EV simulator with a few different battery chargers. Motomaster Eliminator, Xantrax True Charge 2, and a Motomaster manual Charger. I’ve been asked to explain why the THD is different in each but I am having a hard time..


A little more background:

The simulator is able to simulate different line lengths to the distribution station Aswell as “equivalent” transformer sizes. As my testing has gone, the Xantrax has a THD of 9%, motomaster manual THD of 30% and the motomaster eliminator a THD of almost 100%.. can anyone explain why this would be? What components would cause such differences?


Thanks in advance!


If any other info is needed, I will add as requested.
 

The Electrician

Joined Oct 9, 2007
2,971
I assume the THD is measured on the grid input side and not the DC output to the batteries. :)

Using an oscilloscope have a look at the current waveform drawn by each charger from the grid. Use a clamp-on current probe for safety. The ideal is that the current waveform should be a sine wave. The more the current waveform deviates from a sine wave, the greater will be the THD.

The specs for the Xantrex (not Xantrax, by the way) indicate that it is power factor corrected. A power factor corrected charger would be expected to draw a current waveform more nearly a sine wave than if it's not power factor corrected. The waveform of the current drawn by each charger will be more a characteristic of the design of the charger rather than other parameters that are varied by the simulator, such as line length, etc.
 

Thread Starter

Cajmccormick

Joined Jan 24, 2018
4
I assume the THD is measured on the grid input side and not the DC output to the batteries. :)

Using an oscilloscope have a look at the current waveform drawn by each charger from the grid. Use a clamp-on current probe for safety. The ideal is that the current waveform should be a sine wave. The more the current waveform deviates from a sine wave, the greater will be the THD.

The specs for the Xantrex (not Xantrax, by the way) indicate that it is power factor corrected. A power factor corrected charger would be expected to draw a current waveform more nearly a sine wave than if it's not power factor corrected. The waveform of the current drawn by each charger will be more a characteristic of the design of the charger rather than other parameters that are varied by the simulator, such as line length, etc.
It is actually set up to already display the waveforms of the current via Labview. So I have seen the waveforms and it makes sense as so why the THD %’s are what they are, I’m just having issues on comparing why. As you mentioned the Xantrax is power factor corrected so it will most resemble a sine wave. Just can’t figure out the Motomaster types.
Thanks for the reply!
 

The Electrician

Joined Oct 9, 2007
2,971
It is actually set up to already display the waveforms of the current via Labview. So I have seen the waveforms and it makes sense as so why the THD %’s are what they are, I’m just having issues on comparing why. As you mentioned the Xantrax is power factor corrected so it will most resemble a sine wave. Just can’t figure out the Motomaster types.
Thanks for the reply!
It might be possible to give you some more info if you would take a picture of the waveforms for each charger and post them here.
 

The Electrician

Joined Oct 9, 2007
2,971
Well, there's the answer to your question. The THD of the two Motomasters is high because their design doesn't do a good job of drawing a nearly pure sine wave of current from the grid.

The Motomaster manual also doesn't bother to filter the DC output current at all. It applies a large ripple current to the batteries. This causes unnecessary heating in the batteries and the cable connecting the charger to the batteries. The extra heating means that the cable/connector system can't supply as much charging current before their ratings are exceeded. It's much better to supply pure DC to the batteries.

How is the THD being measured/calculated?

You might find these references useful:

https://en.wikipedia.org/wiki/Total_harmonic_distortion

https://en.wikipedia.org/wiki/Ripple_(electrical)
 
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