pf and current question

On math problems involving power factors less than 1, I saw the solution is that the current increases in order to maintain the same output. Are there fixtures that can change their amp draw and how is it done?
 
It's not so much that the fixtures change their draw. It's that some devices have 'non-linear' impedance. Look up "reactance"

Capacitors and inductors react differently to AC than to DC. Both 'charge' and discharge as the voltage changes, 120 times a second. This causes extra current in the supply without affecting the total power used. If you try to imagine it in simple water flow terms it seems crazy.

Autotransformers can change their power factors as the control changes the size of an inductor. Variable capacitors are often found in radios as tuners but that's usually DC.
 
It's not so much that the fixtures change their draw. It's that some devices have 'non-linear' impedance. Look up "reactance"

Capacitors and inductors react differently to AC than to DC. Both 'charge' and discharge as the voltage changes, 120 times a second. This causes extra current in the supply without affecting the total power used. If you try to imagine it in simple water flow terms it seems crazy.

Autotransformers can change their power factors as the control changes the size of an inductor. Variable capacitors are often found in radios as tuners but that's usually DC.
Electricity providers don't like you if your loads are making their generators work harder than your local meter indicates. This is why they levy an additional charge if they don't like your power factor.
Toodleoo!
Ron Hebbard.
 
We need to be clear, a low power factor fixture IS drawing more current and therefore more heat is developed in the wiring and both the supply breaker and wire gauge must be adjusted. The reason the wattage does not change is that the current is being drawn at a lower voltage, often, later in the AC cycle. Therefore amps times volts equals watts still apply. Lets look at two loads each 100 watts: One is incandescent, (pf=1 or 100%) and at 120 volts and is drawing 0.83 amps. The second is an HPS lamp ballast with a pf of .47 (or 47%) Its drawing 1.81 amps! Why? Because it is drawing later in the waveform and is effectively running at 55 volts even though the supply voltage is 120. (55 x 1.81 = ~100w) We can fix this by putting the proper capacitor across the fixture. Basically, the capacitor is storing power from earlier in the cycle and helping to supply the ballast later in the cycle, thus moving us closer to the pf 1 number.
 
... Are there fixtures that can change their amp draw and how is it done?
Many. Almost all LED fixtures and most smaller moving lights are auto-ranging; will work with any voltage from 100-240VAC. In the US, we will most likely be dealing only with either 120V or 208V bi-phase.

Take for example the VL 2500Spot, a 700W short-arc gaseous discharge fixture.
vl2500_VoltAmps.jpg


I can't think of any incandescent fixtures that do this, except for perhaps those with their own onboard dimmer (Jarags, Sunset strips, TW1, etc.).

See also SMPS.
 
I'm still a little confused. I don't think PF affects the magnitude of the amperage. Either the amp or the volts lags due to it having to first fight the charge/magnetism held in the capacitor/inductor when changing polarity. The peak of the amp no longer meets the peak of the volts, but somewhere lower, meaning the average output is lower.

I'm guessing magnetic ballasts can let more volts in to compensate for less power so the amps increase in relationship with the volts. But I'm not sure about auto-ranging lights with auto-transformers. Transformers work by keeping the same VA. Power remains the same. Volts can increase but the amps decrease keeping the same power. I don't think it's a solution for low power caused by a low power factor. Right?
 
What derek posted above has more to do with auto-ranging electronic ballasts, not magnetics. In the case of the magnetic ballast, current draw increases later in the cycle so as you indicated, the peak current draw occurs after the peak voltage in the waveform, thus the low power factor and the need to correct via a capacitor.
On electronic ballasts, the input voltage is converted to DC and that voltage drives a small transformer at a very high frequency. The output drives the lamp and an optical feedback loop informs the driving circuit how hard to drive the transformer. Also known as Pulse width modulation, as the width of the driving pulse is varied. These ballasts can operate over an extremely wide range of voltages and will draw less current at higher voltages. Because the transformer operates at a very high frequency, it can be very small, so these ballasts weigh a lot less than magnetic ballasts.
The only way to achieve multiple voltages on a magnetic ballast is to have multiple winding taps which must be manually changed if you change the operating voltage.
 

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