AC Theory questions

[Stoday]

I can see that you guys have never installed or bought power factor correction capacitors.

1) Correction capacitors for power systems (rather than appliances) are 3-phase, so have at least 3 capacitors.

2) The capacity is never quoted in Farads (or microfarads). Always kVAr.

3) You only put in as much PF correction as is economic, so you'd normally not correct much above 95%.

 

[Pensdown]

If your a Diy'er reading this post you must wonder what the hell everyones talking about

However, you can see your in safe hands with these guys and there advise is free

 

[Pensdown]

I can see that you guys have never installed or bought power factor correction capacitors.

1) Correction capacitors for power systems (rather than appliances) are 3-phase, so have at least 3 capacitors.

2) The capacity is never quoted in Farads (or microfarads). Always kVAr.

3) You only put in as much PF correction as is economic, so you'd normally not correct much above 95%.

In reality, I cant see why unless you were forced to, why you would ever install any.. Whats wrong technically with a lagging power factor?

 

[plugwash]

more current into your supply which means bigger cables and/or more loss.

theres also other issues due to the relatively high inductance in large supply cables.

also somethings got to soak up those vars and your supplier would rather not be doing it. so they create a large financial incentive to put in power factor correction by charging a fortune for VAr hours (they only tend to do this on commercial/industrial installs though)

 

[Pensdown]

Your last comment is dead right and thats why it's normally done. Due to the capital cost, I've never seen someone volunteer to install them (unless they have a big site with there own geny backup)

 

[Stoday]

You have a point, Pensdown. 30 years or so ago, kVA demand charges were so high there was a considerable financial advantage to install PFC. Since privatisation, until recently, the only demand-related charge is the kVA Availability charge and as that dosn't vary with demand in the short term, it's not a good cost incentive.

Most DNOs have now introduced a kVArh charge which kicks in at power factors of 95% and less, but only for those consumers with metering that measures kVArh, i.e. just the big ones.

 

[ebee]

How come this last mesage was posted a 12:00 am?
There`s no such time as 12:00 am or 12:00 pm .
We had that fact stamped into our heads at school.

Just like lots of people say one pence it`s one penny two pennies or twopence (tuppence for short).

Nobody says one apples or one oranges it woul be daft .

Likewise in shops/car boot sales everything in box £1 they mean anything not everything.

Agghhh,

Signed
Angry of Mayfair

 

[olly_k]

Try something I just made up on the spot

I have a 5v 20ma LED and a 240V AC 50hz power source, I have a full wave recifier and a capacititor so I can run my LED, what value is the capacitor?

Considering a voltage of 120v (consider diode caractoristics) I get 6.4uF but I could be wrong? (relying on windows calc and my short term memory )

 

[Stoday]

Try something I just made up on the spot

I have a 5v 20ma LED and a 240V AC 50hz power source, I have a full wave recifier and a capacititor so I can run my LED, what value is the capacitor?

It all depends on the circuit.

Diode in series, capacitor in parallel with LED, Led anode connected to diode cathode
LED explodes.

Diode in series, capacitor in parallel with LED, Led anode connected to diode anode
Nothing happens

LED in series, capacitor in parallel with diode, Led anode connected to diode cathode
LED explodes

LED in series, capacitor in parallel with diode, Led anode connected to diode anode
If value of capacitor is low, LED breaks down, otherwise explodes

Capacitor in series, diode in parallel with LED, Led anode connected to diode anode
Nothing happens

Capacitor in series, diode in parallel with LED, Led anode connected to diode cathode
0.55 uF capacitor allows 20 mA to flow

All in series, Led anode connected to diode anode
Nothing happens

All in series, Led anode connected to diode cathode
0.56 uF capacitor allows 20 mA to flow