Recently, I asked a question concerning harmonic specifications and a system containing a 120W switching supply in parallel with an intermittant 450W resistive heater. I came up with two positions that support harmonic measurement while the resistive load is powered on (the system passes test under these conditions).
Annex C discusses test setup conditions. The first paragraph states that: " ... controls or automatic programs shall be set to produce the maximum harmonic components under normal operating conditions ... " Case 1 - 'Control setting' corresponds to cycling between 120W and 570W. Considering the radical change in input power (120W -> 570W), there might be enough parasitic effects that increase harmonics slightly during the high power heating period. Inherent distortion of the AC line voltage alone would be magnified during the high power operation. The harmonics are higher during high power operation, therefore harmonics are measured at 570W. The less-stringent Class A limits are applied. A difference in harmonics could be generated on purpose by switching a 5W dummy load onto the DC output of the switcher, concurrently with the 450W resistive load operating. Again, because the harmonics are higher during the 570W period, this causes the Class A limits to be applied. Case 2 - Normal operating condition: 120W or 570W? The time when the resistive heater is on, and the system input power is 570W, represents normal operation. This causes the Class A limits to be applied. This is supported by the safety report and nameplate rating that reference an input power of 570W. The time when the resistive load is off represents idle time, and is not representative of normal operation (this product does function as a heater, after all!). Comments?
