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Measuring current drawn by vsd compressor


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i have attached an inverter to a 22kW compressor. I'm having problems measuring the current and cos phi (before the inverter). Having connected two different instruments i have similar current readings in the range of 50 Amps (the current drawn without inverter is measured at 30Amps), while i have one device measuring a cos phi of 0.65 whilst the other measures a cos phi of 1.00. I know that theoretically the inverter tries to rectify the cosphi bringing it close to 1.00, but this would mean my machine is absorbing about 34kW, thats 50% more than the rated power of the compressor!... the inverter manufacturers give an increase in 8% max on the rated power so i find these measurements hard to believe. On the technical side i know the inverter mounts a RC filter (resistance capacitance i think!). Please help me understand which device reads correctly and why i have such high Amp readings. thank you! (btw very nice forum)

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Hello binobong62

Welcome to the forum!


The problem that you are having is a very common problem and is due to the fact the the current flowing into the inverter is non sinusoidal. Most instrumentation assumes that the current is sinusoidal and there a errors immediately the current deviates from this.

The current into an inverter will depend very much on the desing of the inverter. I assume that you are talking about a standard PWM voltage source inverter, in which case the current can be very narrow current pulses on the crest of the waveform, or if there is either a DC Bus choke fitted, or AC Line reactors, could be much wider current pulses of up to 120 degrees conduction per half cycle. If you current measuring device is responsive to the peak current, and assumes that this is a sinewave and gives a reading of rt of the peak, then you will be terribly wrong. Likewise, if the instument measures the average value, there will be a big error relative to the true RMS value.

In the case of cos phi, this imediately assumes that the current is sinusoidal. The figure for cos phi would typically be better than 0.95 because the maximum current will only flow when the voltage is maximum. This does not however indicate the true power factor. Power factor is KW/KVA. Cos phi is totally meaningless for non sinusoidal currents and/or voltages.

If you wish to know the true KW consumed, you must use a true KW measuring device. P=VxIxCos phi is only correct for sinusoidal currents and voltages.


Best regards,

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