Stuck on an assignment for college

LotusBiscuit · Mar 13, 2021

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 · Mar 14, 2021

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 · Mar 14, 2021

pc1966 said:
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. · Mar 14, 2021

LotusBiscuit said:
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 · Mar 14, 2021

Julie. said:
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 · Mar 14, 2021

Julie. said:
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 · Mar 14, 2021

LotusBiscuit said:
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. · Mar 14, 2021

LotusBiscuit said:
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. · Mar 14, 2021

LotusBiscuit said:
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 · Mar 14, 2021

Julie. said:
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.

Julie. · Mar 14, 2021

LotusBiscuit said:
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.

Ah, OK

In that case, either they give the resistance somewhere, or you know the phase angle and power factor without calculation.

LotusBiscuit · Mar 14, 2021

Julie. said:
Ah, OK

In that case, either they give the resistance somewhere, or you know the phase angle and power factor without calculation.

Referring to question 2c and 2d, neither the resistance nor power factor and phase angle are given.

pc1966 · Mar 14, 2021

Question (2a) has your DC aspect, from that you can compute R, and from Z from (2c) you can then compute Xl separately to get L.

From Xl and R you can compute your power factor / phase angle for (2d)

#1 tip for examinations - read the questions carefully as often the examiner is leading you towards a solution by the order of things you are asked for.

#2 tip - sometimes you get too much information, so be prepared to locate the stuff you need.

LotusBiscuit · Mar 14, 2021

pc1966 said:
Question (2a) has your DC aspect, from that you can compute R, and from Z from (2c) you can then compute Xl separately to get L.

From Xl and R you can compute your power factor / phase angle for (2d)

#1 tip for examinations - read the questions carefully as often the examiner is leading you towards a solution by the order of things you are asked for.

#2 tip - sometimes you get too much information, so be prepared to locate the stuff you need.

Oh wow, I just realised that all those questions are about the same coil, it all is starting to make sense, thank you.

Julie. · Mar 14, 2021

LotusBiscuit said:
Referring to question 2c and 2d, neither the resistance nor power factor and phase angle are given.

Yes they are; the resistance can be calculated from the previous part of the question - exactly how I suggested previously.

You calculate the dc resistance from that portion of the question, the impedance Z from the ac part giving Z and R, therefore you can calculate pf and phase angle

You really need to look at the whole question - just selecting one aspect will not give the whole answer