Stuck on an assignment for college

[LotusBiscuit]

The question is:
If the supply voltage is 100v ac with a period of 20ms, and the supply current (Is) taken by the coil is 6A, calculate the frequency of the supply, overall impedance, Inductive reactance and the inductance of the coil.

I've worked out the frequency to be 50Hz and in order to get the Inductive reactance I need to work out the inductance of the coil.

f = 1/t
1/20ms

1/20x10^-3 = 50hz
Z = v/I
Z= 100/6
Z =17.6

Xl = sqroot z^2 - r^2
17.66^2 - 100 = 13.24

2pifl
L =xl ÷ 2pif
13.24 ÷ 2 x pi x 50=
0.04H

Just want to double check any help??

 

[pc1966]

Note you do not have a resistance in the question, or did it give a DC resistance as well?

Also 100/6 = 16.666... = 16.67

Without knowing any R, assume it is all inductive reactance Xl = 2 * PI * f * L

L = Xl / (2 * PI * f) = 16.67 / (2 * 3.141592 * 50) = 0.053 H

 

[LotusBiscuit]

Note you do not have a resistance in the question, or did it give a DC resistance as well?

Also 100/6 = 16.666... = 16.67

Without knowing any R, assume it is all inductive reactance Xl = 2 * PI * f * L

L = Xl / (2 * PI * f) = 16.67 / (2 * 3.141592 * 50) = 0.053 H

No it didn't mention the resistance in the question, however I used ohms law to work it out.

I did V/I = R
So 100/6 = 16.67 ohms

Thanks a lot for the help, looks like I've made a couple mistakes ?

 

[Julie.]

No it didn't mention the resistance in the question, however I used ohms law to work it out.

I did V/I = R
So 100/6 = 16.67 ohms

Thanks a lot for the help, looks like I've made a couple mistakes ?

If you say

Z = V/I = 16.7 ohm
AND
R =V/I = 16.7 ohm

Then XL must be zero! (Try XL^2 = Z^2 - R^2)

You have no idea what the resistance is unless it is given in the question, with ac circuits if you only have the voltage and current, all you can calculate is the Impedance Z.

You then decide how this is split between the resistance, and reactance(s) based on the question, in this case it doesn't give resistance and only highlights inductance, so you have to assume all the Z is from the inductance.

So given XL = 16.7 ohm at 50Hz the inductance will be 16.7/(100 pi)

 

[LotusBiscuit]

If you say

Z = V/I = 16.7 ohm
AND
R =V/I = 16.7 ohm

Then XL must be zero! (Try XL^2 = Z^2 - R^2)

You have no idea what the resistance is unless it is given in the question, with ac circuits if you only have the voltage and current, all you can calculate is the Impedance Z.

You then decide how this is split between the resistance, and reactance(s) based on the question, in this case it doesn't give resistance and only highlights inductance, so you have to assume all the Z is from the inductance.

So given XL = 16.7 ohm at 50Hz the inductance will be 16.7/(100 pi)

Oh ok, so ohms law is only applicable for dc circuits?

 

[LotusBiscuit]

If you say

Z = V/I = 16.7 ohm
AND
R =V/I = 16.7 ohm

Then XL must be zero! (Try XL^2 = Z^2 - R^2)

You have no idea what the resistance is unless it is given in the question, with ac circuits if you only have the voltage and current, all you can calculate is the Impedance Z.

You then decide how this is split between the resistance, and reactance(s) based on the question, in this case it doesn't give resistance and only highlights inductance, so you have to assume all the Z is from the inductance.

So given XL = 16.7 ohm at 50Hz the inductance will be 16.7/(100 pi)

Ok so then how would i work out the power factor, I dont have the resistance given to me nor can I work it out? So rules out cos phi = R/Z

Would i do sin phi = Xl/Z??

 

[LotusBiscuit]

Ok so then how would i work out the power factor, I dont have the resistance given to me nor can I work it out? So rules out cos phi = R/Z

Would i do sin phi = Xl/Z??

I'm lost ??
For the power factor I'd do R/ V x I?

 

[Julie.]

Oh ok, so ohms law is only applicable for dc circuits?

Ohms law can be used for both ac and dc, however with ac it will give you the impedance of the circuit not resistance, and dc will give you the resistance.

 

[Julie.]

Ok so then how would i work out the power factor, I dont have the resistance given to me nor can I work it out? So rules out cos phi = R/Z

Would i do sin phi = Xl/Z??

It doesn't ask for the power factor, and for good reason - it doesn't provide enough information.
If it gave you resistance then you could.

The question could be:

With ac 50Hz you have 6A at 100V
With dc you have 20A at 100V

In which case you could work out the Z from the ac values and the R from the dc values. Given these you could then get the resistance, reactance, inductance, and power factor.

But as it only gives the ac without resistance you can't do it

 

[LotusBiscuit]

It doesn't ask for the power factor, and for good reason - it doesn't provide enough information.
If it gave you resistance then you could.

The question could be:

With ac 50Hz you have 6A at 100V
With dc you have 20A at 100V

In which case you could work out the Z from the ac values and the R from the dc values. Given these you could then get the resistance, reactance, inductance, and power factor.

But as it only gives the ac without resistance you can't do it

No it doesn't ask for the power factor in that question however on the next question it asks me to "calculate the power factor and the phase angle of the coil". So I assume its to do with the previous question being the one i mentioned.