DC link capacitor
DC link capacitor
(OP)
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.
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.





RE: DC link capacitor
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)' ?
RE: DC link capacitor
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(2B)+(2B)' ?
RE: DC link capacitor
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
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: DC link capacitor
Did you remember to multiply the three phase current by √3 ?
Bill
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"Why not the best?"
Jimmy Carter
RE: DC link capacitor
David Castor
www.cvoes.com
RE: DC link capacitor
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!
RE: DC link capacitor
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.
RE: DC link capacitor
Thanks everybody!
RE: DC link capacitor
The input and output currents (instantaneous, average or RMS) DO NOT have to be equal.
Guess you didn't read my first post...
RE: DC link capacitor
RE: DC link capacitor
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
RE: DC link capacitor
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.
RE: DC link capacitor
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.
RE: DC link capacitor