Birdies Par 16s

[Brian]

Mr Hippy's reply is spot on, I'll just add something.

 

defininition - 'reactive load' - a load which is not a pure resistance ie one containing inductors (transformers/chokes) or capacitors (switch mode supplies).

 

In an AC circuit with a reactive load, an AC voltmeter across the supply will read V, the RMS voltage, and an AC ammeter in the circuit will read I, the RMS current. The product of these readings, VI, is termed the 'Apparent Power' since it is the power that is apparently being expended on the circuit.

 

However, basic AC theory tells us that in a reactive circuit the current and voltage are not in phase, in an extreme the voltage could be at a maximum and the current at zero. So the actual power or 'True Power' can be a very different figure to that found from our voltmeter or ammeter. This is where the 'Power Factor' figure comes in, it is the ratio of True to Apparent power and is defined as...

 

Power Factor (PF) = cosine theta where theta is the phase difference between the voltage and the current.

 

True Power = cos theta x V x I = cos theta x Apparent Power

True Power = PF x V x I = PF x Apparent Power

 

So True Power is always less than or equal to Apparent Power (PF is between 0 and 1)

 

You might ask 'Does all this matter?' Well it does, especially to the people who generate our electricity or install our sub-station.

 

An alternator is rated in VA and not Watts. The machine will generate a constant voltage when rotated at a constant speed and will supply a certain current without overheating. It's the current rating that limits the machine because it's the windings that get hot as current flows through them. The same is equally true of things like transformers.

 

For example, if an alternator, rated at 5kW at 200 volts, was connected to a load with a power factor of 0.7, it would have to supply an apparent power of 7143 watts. This means that the current flowing would be 7143/200 = 35.7 amps instead of the 5000/200 = 25 amps figure you would get if you didn't include the power factor in your calculation. That's a 43% current overload and the generator would run very hot.

 

So, a generator could be labelled as 5kW, 0.8 PF, 6.25kVA, 200 volts - this is a machine that will deliver 25 amps at 200 volts into a purely resistive load or 31.2 amps, without overloading, into a 0.8 PF load. The windings in the machine will be designed around the 31.2 figure and it's copper that costs the money.

[Ellis]

If I'm screwing to a stage, or wooden plate, I use a penny washer with a hole small enough to hold the head of a number 8 wood screw. Screwfix do a long screwdriver bit (Meant for the auto feed screwdrivers) - drive the screw at a slight angle and voila the birdie is fixed without the expense of a special base plate.

 

As for transformers, mine came from Screwfix - 150va will power 3 50w birdies at the limit of their power handling - I try to use 2 per transformer. These look like switch-mode power supplies and do have a soft start facility that cannot be disabled. This makes the birdies come up slightly slower than the rest of the lx.

[PeterT]

The product of these readings, VI, is termed the 'Apparent Power' since it is the power that is apparently being expended on the circuit.

 

However, basic AC theory tells us that in a reactive circuit the current and voltage are not in phase, in an extreme the voltage could be at a maximum and the current at zero. So the actual power or 'True Power'

My Old Electrics lecturer had a great analogy for apparent power and true power:

 

Imagine a pint of bitter (or stout if you prefer) (apologies for the drawing)

 

\HHHHHHHHH/ The whole glass is the apparent pint

\TTTTTTTTT/ The bit marked T is the true pint

\TTTTTTTT/ & the bit marked H is the head and is waste

\TTTTTTT/

\TTTTTT/ Now substitue Power for pint and you can see that the true

\TTTTT/ power you get from a reactive circuit is going to be less than the

\TTTT/ apparent power you're putting in,as it's got to overcome

\___/ the power wasted by the reactive circuit hence your power factor.

 

Well it makes sense to me and I always like it when when Beer can be used to enhance the learning process

 

I'll get me coat now...

[Andrew C]

brain, for the interested among us, could you please explain the difference, and maybe an example of what situation it would be different in

freudian? ;)

[robloxley]

As for birdie bases, the best I've seen were - I think - Stage Electrics' ones, but you could easily make your own. They are basically a 3" square, roughly 3mm thick steel plate with an M8 stud sticking out to attach the birdie to. They're heavy enough not to necessarily need screwing down, but big enough that you can screw them down through the corners without taking the yoke off the birdie.

[egri]

PAR 16 barn doors here for £6.17 Thomann

 

PS

I know I often mention Thomann, I don't buy much stuff from them but they do seem to have most things on their website. It's a good place to find a rough price.

Where can I get black par 16 barn doors?

 

I found where you get the stands I was talking about from: Henley theatre services.

[Bryson]

Could you not just spraypaint the Thomann ones?

[Stu]

If they are nice and shiny 'silver' barndoors, they might need a primer coat first in order for the black paint to take...

 

Just an idea

Stu

[Thirdtap]

Going back to Peter's post.

 

I have found that some of our hire stock of transformers whilst rated correctly for the load where breaking down after a while. I believe this is from the amount of heat generated whilst they are used and in a conference setting they may get left on all day. So on the last lot of transformers I bought I got they with a higher rating than needed. For 2 x 50w birdies we are now using mainly 150w transformers. So far they seem to be OK.

 

From college the lecturer always recommended Mode transformers. I have got a few in stock and yes they seem to be very good but they are at the pricey end of the market. Although if they do last much longer they may be worth the investment in the first place.