va of watt
dkace said:
Thanks guys.
Most helpfull.
The valve is AC solenoid. And I agree with xxargs. It is not the current that is measured in VA. It is stated that when valve is openning, the consuption will be 9.6VA, given the fact that V=24VAC.
The original question states as follows: Is the 3W max power capability , covering the specifications of my solenoid? ( the solenoid is to be controled by the circuit which is working on 24VAC.)
Thanks,
D.
Measure DC-resistance in your valve, and measure how much current valve draw on normal 24 Volt AC feed.
take measured DC-resistanse * measured AC current on 24 Volt feed and you have resistive loss in Watt, rest is reactive power.
VA^2 = Watt^2 * VAr^2
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Wanted power level = 3 Watt load
3 Watt / 24 Volt = 0.125 Ampere (R = 192 Ohm)
measured and/or know value from valve:
9.5VA /24 volt = 0.396 Ampere (or Z=60.61 Ohm in starting moment , ignoring this in later calculate)
6.5 VA/24 Volt = 0.271 Ampere ( Z = 88.56 Ohm - on going state)
Code:
Watt 3
cos(φ) = ------- = ------ = 0.461 or 62.5 degree
VA 6.5
ie loads phase angle between current and voltage to load must be higher than 62.5 degree (or resistive part lower than 0.461 * 192 = 88.62 Ohm DC-resistance) to make possible using full phase compensate parallel capasitance to make 192 Ohm or higher resitive impedance in resonant mode for <= 3 Watt consume .
Question: is your driver circurit is power limited (internal power cannot delivery more than 3 watt in long term) or current limited (depend of relay/triac, thyristor heating inside etc. above 125 mA. current), can equipment give high starting peak current without problem, starting with zero crossing in period etc.
Maybe possible to using phase compensate capacitance on valve if reactive part is big compare to resistance (see above, needed capacitance in this case with 86.2 Ohm DC-resistance is 32 uF) to take down current , but for calculate this exact value I must know DC-resistance in valve or (better) phase angle between current and voltage to load ie. this case include iron-loss in valve (copper/iron loss in AC is added resistance to measured DC-resistance).
but remember - phase compensate capasitance can give very high peak current if connected outside zero-crossing area in voltage periode.
/Xxargs