Help with C&G 2330-203 level 2 question - just can't figure it out!

[chris.rigby]

Hi,

I'm revising for my level 2 203 exam which I'm due to sit on Wednesday. I'm struggling a bit trying to get my head round power factors and lagging and leading..

One of the practice questions I have is:

What is the power factor of a circuit with a 4 ohms resistor and a 10 mH inductor?

The answer is 0.8 lagging but I don't know how this was calculated.

I know that PF = R/Z so if I divide 4 by 10 this gives me 0.4 and obviously if I multiply that by 2 then I get the answer of 0.8..but don't understand it. I'm also confused as to when it's lagging and when it's leading.

I'd appreciate any help you more experienced guys out there can give me.

Thanks in advance!
Chris

 

[telectrix]

look here:

Impedance

 

[chris.rigby]

Cheers for that. I will print it off and read through it and hopefully that will help me get to grips with it!

If there's still something I don't understand - no doubt I will pop up here again!


Thanks again..

 

[Inteificio]

That is a bit of a hard question for lvl 2.

Are you sure you are not looking at 302 not 203 ;-)

This is off memory, but if wrong someone will come and slap me down in a sec.

The impedance of an inductor is 2 x pi x frequency x inductance.

so

Xl = 2 x 3.14 x 50 x 0.01.

And always remember the units of Ohms.

so doing a bit of sums you get 3.14 Ohms.

Pf= R/Z

Pf= 4 / 3.14

Pf ~= 0.8

And remember the units (Pf doesn't have any as a ratio).


Still think I might have this horribly wrong (bad nights sleep last night) so if wrong please correct me!


regarding leading and lagging a simple mnemonic always helps me.

CIVIL

Capacitance Current (symbol I) Voltage Current L (inductance)

So you either read right to left or left to right depending on whether cap or ind.

eg.

Cap so left current then voltage
Inductance so right hand side Voltage leads current


Hope this helps!

 

[telectrix]

but are the resistor and coil in series or parallel? and 4/3.17 ain't 0.8.

 

[Inteificio]

heh, got it upside down. Said I was sleepy

Back to the drawing board

 

[Inteificio]

That was it

Ok you have resistance of 4Ohm

Reactance of 3.14 Ohm

But formula says impedance.

Impedance = ROOT (r^2+L^2)

Impedance = ROOT (4^2+3.14^2)

Impedance = 5Ohm

Pf = R / Z
Pf = 4 / 5

Pf = 0.8


Hows it this time? =-)

 

[Inteificio]

Still a bitch of a question for LVL 2.

I didn't do my lvl 2, but jumped straight in to lvl3.
When I started lvl 3 most people in the class were struggling with Ohms law, we did not even cover powerfactor until late lvl3.

 

[chris.rigby]

At college we didn't really cover this, yet it came up in my exam...which I failed, so now have the resit on Wednesday. Thanks for all the help on here - it's been more helpful than college has been!