Capacitive Load Dimming

[aakici]

I want to dim a capacitive load. Mains voltage is 220Vac. I am trying to change voltage across this capacitive load. I used thyristor, FET and BJT to get a variable sinusoidal voltage across load but load is capacitive. Load charges and then behaves like a power supply. How can i change voltage across a capacitor?

 

[Nigel Goodwin]

Perhaps you might try explaining exactly what you're trying to do?.

 

[Hero999]

Never use a phase control dimmer with a capacitive load - it will blow the TRIAC.

A simple way would be to use a variac but you still need to be careful that the inductance of the transformer doesn't resonate at the mains frequency or any harmonics, or high voltages and currents will be generated which will ruin the transformer and capacitor.

 

[Willbe]

I want to dim a capacitive load. Mains voltage is 220Vac. I am trying to change voltage across this capacitive load. I used thyristor, FET and BJT to get a variable sinusoidal voltage across load but load is capacitive. Load charges and then behaves like a power supply. How can i change voltage across a capacitor?

How much µF, volts, amps, Hz?

 

[aakici]

I am trying to change the AC voltage across a capacitor. Exactly a capacitor. It's a 100uF / 400V / 2000VA capacitor. I intend to use it as power factor correcter.

 

[Hero999]

A variac is the only sensible way of doing it but I don't see how this will help you with power factor correction, normally you vary the capacitance for power factor correction.

 

[Willbe]

I am trying to change the AC voltage across a capacitor. Exactly a capacitor. It's a 100uF / 400V / 2000VA capacitor. I intend to use it as power factor correcter.

i = C ΔV/ΔT. If V changes by 100v and T changes by 10 mS you need an I of 1 amp.

 

[MrAl]

A variac is the only sensible way of doing it but I don't see how this will help you with power factor correction, normally you vary the capacitance for power factor correction.

The idea could be that the transformer and cap combination becomes
a variable capacitor because the capacitance reflected to the primary
varies with the turns ratio. With a variac, i would think you could
easily vary the capacitance seen looking into the primary and this
could be used as a variable capacitance load.
He certainly doesnt give us much info however.

 

[Hero999]

I don't like the sound of that because it's possible that the capacitor could resonate with the transformer's inductance at a harmonic of the mains frequency when the transformer is set to a specific voltage.

 

[MrAl]

Hi again,

Well, that's life If there are secondary problems they would also need
to be solved.

I dont know any of his specifics, but this technique was often used on the older
type ac inverters. The output filter required an ac rated cap and at the time
the caps were very expensive and the cost was roughly linear with the voltage
rating and capacitance. By incorporating a third 2x higher voltage winding on the
output transformer we could obtain 4 times the capacitance with a capacitor that
was rated for two times the voltage. This allowed us to use a cap rated for 1/4
the capacitance (and so 1/4 the price) but twice the voltage (so double THAT
price) for a total savings of 1/2 on the capacitor by itself.

 

[Hero999]

I thought that the price of capacitors is roughtly linear to their energy storage which goes up exponentially with voltage E = 0.5CV².

 

[MrAl]

Hi again,

... and at the time the caps were very expensive...

Read this again. Either this is true or designers used a 2x voltage
rated cap (more expensive) and more windings on the transformer
(also more expensive) so they could increase the price of the
converter for no good reason at all. Maybe this happens with the
Shuttle, but not with converters in most cases

Anyway, if your new more timely formula is correct, we probably
would not see any savings and it would not be feasible to
do this anymore. The exact cost also depends on where you
are getting it from however.

 

[Willbe]

Just don't go ferroresonant on me.