DC link capacitor 1

[youngeth]

Hello, I have a very basic question that I can't seems to find out why.

I am trying to analyze a induction motor fed by a PWM controlled voltage source inverter, which is supplied by a diode rectifier connected to a 3 phase generator. Examining the current waveforms, I see that the current drawn into the inverter is higher than the output of the diode rectifier...It seems that the capacitor is supplying the extra current.

My question is: how is this possible? Doesn't this violate the conservation of power, as the power consumed by the induction motor must equal the power output of the 3 phase generator?

Many thanks for any help.

 

[electricpete]

There is nothing that says the peak instantaneous output cannot exceed peak instantaneous input (difference would be cap discharge current).

Also it has been mentioned before, you can get fooled if you are not understanding what your instrument is telling you (true rms, true peak, estimated rms based on peak, and is it fast enough to respond to that voltage).

You are certainly right of course that the output power cannot exceed input power (accounting for all phases).

If you share as many details of the specific measurements you are looking at, I'm sure there will be more useful comments from all.



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(2B)+(2B)' ?

 

[electricpete]

You are certainly right of course that the output power cannot exceed input power (accounting for all phases).

I left out a qualifier: This would apply to time-averaged real power.

=====================================
(2B)+(2B)' ?

 

[Skogsgurra]

You need to make clear to yourself that there is nothing like conservation of power.

Energy is conserved, power is something different. As Pete says, discharging the capacitor can peoduce lots of power, but it cannot produce more energy than you have put in it.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[waross]

How are you measuring the current and with what specific instruments?
Did you remember to multiply the three phase current by [√]3 ?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dpc]

Also, current does not equal power.

David Castor

http://www.cvoes.com

 

[ScottyUK]

Think of the PWM conversion as being vaguely akin to a transformer: you are delivering higher current at the output but almost certainly at a lower voltage, and likely at a different frequency too.

Power (in kW) going in to the DC link will be equal, neglecting losses, to power coming out of the DC link. The actual AC currents you measure taking in to account power factor and harmonics could be significantly different.


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If we learn from our mistakes I'm getting a great education!

 

[LionelHutz]

Also, current does not equal power.

Exactly, don't try to equate current to power. When dealing with any AC system, current and power will often appear to be proportional to each other BUT don't ever assume they are proportional to each other.

If you are just beginning to work with VFD's then wait until you run an unloaded motor on a VFD and you see the typical no-load motor current on the output side yet only see a much smaller current on the input side. For example, a 100hp unloaded motor could show 45A on the output side and 4A on the input side.

 

[youngeth]

I think I got it now. The average current supplied by the source equates to the average current drawn by the converter. It all makes sense.

Thanks everybody!

 

[LionelHutz]

No, you don't "got it".

The input and output currents (instantaneous, average or RMS) DO NOT have to be equal.

Guess you didn't read my first post...

 

[davidbeach]

Not only do they not have to be equal, they don't have to have any discernible relationship.

 

[electricuwe]

As youngeth is talking about the current from the rectfifier into the DC-link cap and out of it to the inverter the following is correct:

average - has to be identical
instantaneous: no simple relation
RMS: no simple relation

Keep in mind that you even can not apply Kirchhoffs law to RMS currents

 

[LionelHutz]

I more or less read the post about measuring current and then that conservation of power comment and I then tried to address those comments.

I did read comments about the current measurements before and after the DC link capacitor so maybe I should have addressed that more directly. When dealing with a typical AC line power to DC capacitor link to AC motor converter, or a typical industrial VFD, I highly doubt the average current into the DC buss will equal the average current out of the DC buss. From what I have observed, the reactive motor current only flows between the DC capacitor link and the motor.

 

[electricuwe]

LionelHutz,

please consider that I refered to average current not Abs(average current). In an inverter supplying an idleing motor (rsp. supplying reactive power) there will be nearly no current from the recifier, but the current to the inverter will alternately flow forward and backwards ( resulting in zero average).

So math and experience are not in contradiction if applied properly.

 

[LionelHutz]

electricuwe - very good point.