mutual inductance problem

Thread Starter

JoyAm

Joined Aug 21, 2014
126
Hello there i am having a hard time solving a practice problem from the alexander sadiku book and when i looked up at the solution i saw something that is contrary to the method that i use . Can someone confirm that the given solution is correct ? According to my method the underlined should be +2jI1
Screenshot_7.png
 
Last edited by a moderator:

The Electrician

Joined Oct 9, 2007
2,848
You need to know, and take into account, the mutual inductance between the two inductances. The mutual inductance could be given by the problem, or you could calculate it if you are given the coefficient of coupling. Does the problem give either of these?
 

Thread Starter

JoyAm

Joined Aug 21, 2014
126
Oh i didnt see it was missing, i am sorry .The mutual inductance is 1 henry and the omega is 2 so we have that 2j factor. Is it clear now ?
 

DGElder

Joined Apr 3, 2016
351
So I assume omega is the frequency. The sign is correct as the book shows it. If the dots were not on the same side of the coils then you would be correct. But you would have to change the sign in the mesh 1 equation as well.

If I1 produces a negative voltage at the dot on the left side of the Xfer, then the dot on the right side will see an induced negative voltage.
 
Last edited:

Thread Starter

JoyAm

Joined Aug 21, 2014
126
So I assume omega is the frequency. The sign is correct as the book shows it. If the dots were not on the same side of the coils then you would be correct. But you would have to change mesh 1 sign as well. Or is I2 were counter clockwise your sign would be correct.
So this method is not right ?
Screenshot_9.png
 

DGElder

Joined Apr 3, 2016
351
"So this method is not right ?"


No. The signs on the dependent sources can not be oriented in the same direction because the current from I1 goes into the top of the Xfer and I2 goes into the bottom.

Go look at where ever you copied this equivalent circuit model and look at the chosen current directions and the polarity dot indicators on the Xfer coils.

Watch your signs.
 
Last edited:

WBahn

Joined Mar 31, 2012
26,398
The thing to remember is that an increasing current (positive di/dt) going into the dot on one coil produces a voltage that is positive at the dot of the other coil (and, in fact, this applies in the case of a single coil, as well).
 
Top