enter image description here

Just a simple question, usually we can determine the sign of the mutual inductance M by seeing the position of the dots (as the current enters through it). But if you see the upper picture, we don't have any dots. How to determine if it's -M or +M from the arrows?


1 Answer 1


It doesn't matter. As long as you consistently choose to calculate as if the dot is either at the head or the tail of the arrow, the results of your analysis won't change.

That is: Either always put the dot at the head of the arrow or always put the dot at the tail of the arrow.

  • \$\begingroup\$ Suppose that I put the dot at the head of the arrow, I got \$ M_{lm}, -M_{lr}, M_{ml}, -M_{mr}, M_{rl}, -M_{rm}\$ but the correct answer, everything is positive. Is this a wrong concept of my understanding? \$\endgroup\$
    – hello
    Aug 15, 2016 at 16:00
  • \$\begingroup\$ Sorry, you get \$M_{lm}\$ etc for what? And "the correct answer" to what question? \$\endgroup\$
    – The Photon
    Aug 15, 2016 at 16:09
  • \$\begingroup\$ Based on the circuit from the first picture \$\endgroup\$
    – hello
    Aug 15, 2016 at 16:20
  • \$\begingroup\$ What is the question about the circuit in the first picture that is supposed to have the answer \$M_{lm}\$, etc.? \$\endgroup\$
    – The Photon
    Aug 15, 2016 at 16:21
  • \$\begingroup\$ I asked to calculate the total inductance, and this picture has the correct answer as \$ L = L_l+L_m+Lr+2M_{lm}+2M_{mr}+2M_{lr}\$. But in other cases, I also found the equation like this \$ L = L_l+L_m+Lr-2M_{lm}-2M_{mr}-2M_{lr}\$. So, I'm confused how to determine the sign of M, which will affects my calculations later... \$\endgroup\$
    – hello
    Aug 15, 2016 at 16:28

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