Plug packs: Difference between revisions

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[[File:Plugpack harmonics.xls]]
[[File:Plugpack harmonics.xls]]


Failures of generators have been reported from OLPC deployments. The following is a likely explanation.

<i>Harmonic currents produce high frequency flux change and
cause heating in stator cores. Rotor losses also occur
because harmonic currents in the stator will induce
currents in the pole faces and amortisseur windings. Higher
magnetic core temperatures result in a higher winding
temperature.</i> (1)

Power factor has been suggested as a cause and power factor correction discussed, but power factor correction is likely to produce more problems than it fixes.

<i>Caution must be exercised where power factor correction
capacitors are used. A resonant condition at one of the
harmonic frequencies with some part of the line inductance
such as a transformer, on-line motor, or the generator is
possible. Excessive and possibly damaging currents at the
harmonic frequency can flow through the equipment. Power
factor correction capacitors are used primarily for
economic reasons; however, they can also be effective in
reducing waveform distortion.</i> (1)


==Refs==
==Refs==

Revision as of 22:15, 4 December 2010

Waveforms

Charger input waveforms, 0.2A/square, screen edges and centre are the input voltage zero crossings


Chargerwaveform1.jpg

Green wallwart AD5953 ALF, XO1.5 running 93% charge 240V input


Chargerwaveform4.jpg

as above but run on battery power down to 80% charge then plugged in


Chargerwaveform2.jpg

Green wallwart Delta ADP17FBA, XO1.5 running 93% charge 240V input


Chargerwaveform3.jpg

White wallwart Bestek NA0241WAA, XO1.5 running 93% charge 240V input


Pcwaveform.jpg

For comparison a PC power supply. Vertical scale is 0.4A/square, the waveform has a wider pulse and could be expected to be less stressful on generators and transformers but not much better.

Harmonics

The load is well balanced with respect to leading/lagging power factor, showing only a slight leading or capacitive power factor. The waveform is very rich in harmonics. This could lead to overheating in transformers and generators due to eddy currents.

A harmonic analysis on the following approximation of the waveform

Assumed waveform.jpg

yielded this result

Fourier analysis.jpg

File:Plugpack harmonics.xls


Failures of generators have been reported from OLPC deployments. The following is a likely explanation.

Harmonic currents produce high frequency flux change and cause heating in stator cores. Rotor losses also occur because harmonic currents in the stator will induce currents in the pole faces and amortisseur windings. Higher magnetic core temperatures result in a higher winding temperature. (1)

Power factor has been suggested as a cause and power factor correction discussed, but power factor correction is likely to produce more problems than it fixes.

Caution must be exercised where power factor correction capacitors are used. A resonant condition at one of the harmonic frequencies with some part of the line inductance such as a transformer, on-line motor, or the generator is possible. Excessive and possibly damaging currents at the harmonic frequency can flow through the equipment. Power factor correction capacitors are used primarily for economic reasons; however, they can also be effective in reducing waveform distortion. (1)

Refs

http://www.idc-online.com/technical_references/pdfs/electrical_engineering/Generators_and_nonlinear_loads.pdf

http://sureshks.netfirms.com/article/upsgen/upsgen.htm