Electronics and Communication Engineering - Measurements and Instrumentation - Discussion

3. 

The coil of a moving iron instrument has a resistance of 500 Ω and an inductance of 1 H. It reads 250 V when a 250 V dc is applied. If series resistance is 2000 Ω, its reading when fed by 250 V, 50 Hz ac will be

[A]. 260 V
[B]. 252 V
[C]. 250 V
[D]. 248 V

Answer: Option D

Explanation:


Suma said: (May 14, 2014)  
I can't understood the measuring of ac current. Why you put j314.16?

Annu Kumari said: (Jun 21, 2014)  
From where you put the value of j314.16?

Cheetha said: (Aug 25, 2014)  
Inductive reactance = 2*pi*f*L.

Khasim Chotu said: (Dec 14, 2014)  
In dc there will be no term of inductive reactance.

So that meter current reads for ac = (i) = v/r = 250/(2500+j 314.16) = 0.09922 A.

J 314.6 because Inductive reactance = 2*pi*f*L.

NOW voltage(v) = ir = 0.09922*2500 = 248 v.

Ritika said: (Jul 30, 2015)  
Measuring of alternate current sine wave form so imaginary axis will also take as i-axis.

Current = Voltage/Resistance.

Ravinder said: (Nov 6, 2015)  
Still not understood because if 2pifl = what its not j314.6 then how?

Zameer said: (Apr 3, 2016)  
Why didn't you consider the 500 ohms of moving iron at the calculation of ac?

Shyam Babu Yadav said: (May 20, 2016)  
I can't understand the measuring of AC current. Why you take j314.16? Please explain that.

Karthik said: (Aug 1, 2016)  
I didn't get the value of 2pifl, here f = 50hz, l = 1H, and how to find pi values can you elaborate this please.

Prabhas said: (Sep 12, 2016)  
The value of φ = (22/7).

Prabhas said: (Sep 12, 2016)  
At last why they have divided AC value and DC value? Please help me.

Rahul(Power Engineer) said: (Oct 3, 2016)  
In AC we have to consider the effect of both resistance and reactance (i.e Xl = 2 pi f l) f = 50, l = 1.
To calculate current I through meter I = voltag/ impedance.
Impedance in AC only = R + j Xl ohms.

Deepshikha said: (Dec 12, 2016)  
Why not considered the 2000 ohm resistance for dc current?

Shikha said: (Dec 12, 2016)  
Inductance value is added in dc it's right but why resistance value is different?

Surya said: (Dec 17, 2016)  
Ac impedance is Z = R + jXl, so here j314.16 = 2 * pi * 50 * 1 as given inductance is 1H.

Bijendra said: (Jan 1, 2017)  
I have doubt in Meter current of AC?

Please enplane more about this part.

Ester said: (Jan 4, 2017)  
I've calculated the 2 * pi * f * L and I got 0.8883.

I didn't get the 0.09922. why?

Pipo said: (Jan 6, 2017)  
Find first fall Idc.
Find Iac.
Reading = iac/idc * 250.

Sidhu said: (Jan 15, 2017)  
xl=j314.16=2πfl.
=2 * 3.14 * 50 * 1 = 314.
250/(2500+314) = 0.08884.

But all of you said 0.09922.
Can anyone explain?

Lou said: (Jan 19, 2017)  
It should be 250/(2500+j314) to get 0.09922.

Jitedra Kumar Gupta said: (Mar 7, 2017)  
I have a doubt.

As this is an moving iron type instrument so, T (torque)must be proportional to (current)^2.

So, if I1 and I2 are currents dor AC and DC.
THEN,
The answer would come as 246.

Can someone please explain?

Skm said: (Apr 3, 2017)  
How we calculate i taking j?

Please help me.

Praveen said: (Apr 20, 2017)  
Inductance only makes a contribution in AC circuits.Hence it is neglected in the DC calculation.

@Shikha, @Deepshikha: R value taken in both cases is same (2000+500=2500).

Sneha.V.K said: (Nov 14, 2017)  
V/(R+jXl).
250/(2500+j (2πfl)).
250/(2500+j 314.159).

Deepika said: (Aug 5, 2018)  
For 250 V DC current is 0.1 A.
But for the same voltage in AC i.e 250 V AC current is 0.9922 A.
Looking from the other side, for 0.1 A reading is 250 V.
for 0.9922 A we need to find the reading!!
so,
0.9922/0.1*250 = 248 V.

Avinash said: (Nov 23, 2018)  
Here, We need to find the Impedance. X=πr^2+XL^2.

Mahendra Prajapati said: (Feb 20, 2019)  
In AC we have to consider the effect of both resistance and reactance (i.e Xl = 2 pi f l) f = 50, l = 1.

To calculate current I through meter I = voltage/ impedance.
Impedance in AC only = R + j Xl ohms.

Here,
jxL = jwL = J*2*(22/7)*50*1 = J2200/7 = J314.2.
NOW PUT R+JJWL VALUE FOR CURRENT CALCULATION.

Lavanya said: (Sep 25, 2019)  
I am not understanding this, Please explain it clearly to get it.

Ykj said: (Nov 16, 2019)  
D.C = V/R = 250/(2000+500) = 0.1A.

A.C = V/Z = 250/sqrt(R^2+(XL)^2) = 250/sqrt(2500^2+(2*3.14*50*1)^2) = 0.09922A

Rakesh said: (May 28, 2020)  
Clearly explained.

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