# Multiple secondary windings, primary current?

#### AngryGecko

Joined Jul 7, 2017
44
What will happen to the primary current if you have multiple secondary windings drawing current from the same load. Will the instantaneous current on the primary be the sum of all the instantaneous secondary currents, even if there were some reactive components connected across some of the secondary windings, assuming all windings have the same mutual inductance? (I'm completely ignoring the exciting current on the primary)

#### crutschow

Joined Mar 14, 2008
24,922
First you say the same load and then you say a different load on some of the secondaries.
Which is it?

But yes, the primary current is the insantaneous sum of all the secondary currents times the square of the turns-ratio between the primary and each secondary.

#### RichardO

Joined May 4, 2013
2,271
I prefer to think of it as the primary power is equal to the sum of the seconday powers. Of course, this is more complicated with reactive loads.

#### AngryGecko

Joined Jul 7, 2017
44
(Sorry, in the first sentance i meant source not load.) Thanks for the explanation!

#### MrAl

Joined Jun 17, 2014
7,585
First you say the same load and then you say a different load on some of the secondaries.
Which is it?

But yes, the primary current is the insantaneous sum of all the secondary currents times the square of the turns-ratio between the primary and each secondary.
Hi there,

I think you mean not to say square of the turns ratio, just the turns ratio.
The resistance reflects as the square of the turns ratio, but i am sure you know that the secondary current reflects as the turns ratio or sometimes because we consider the turns ratio to be input to output turns ratio, the output secondary current reflects to the primary as the inverse turns ratio.

So for example, if we have a turns ratio input to output of 1/10 and secondary current of 10 amps, then the primary current is of course 1 amp. That's in the ideal transformer as we often quote to start.

Im sure you know this and so i assume you just made a small typo, i do that too from time to time Last edited:

#### MrAl

Joined Jun 17, 2014
7,585
What will happen to the primary current if you have multiple secondary windings drawing current from the same load. Will the instantaneous current on the primary be the sum of all the instantaneous secondary currents, even if there were some reactive components connected across some of the secondary windings, assuming all windings have the same mutual inductance? (I'm completely ignoring the exciting current on the primary)

Hi,

If you have windings with input to output turns ratios as follows:
Primary to Secondary A: 10 to 1
Primary to Secondary B: 2 to 1

and the current in secondary A is 20 amps and in secondary B is 8 amps, then the primary currents are as follows:
Prim due to A: 20/10=2 amps
Prim due to B: 8/2 =4 amps

and the total current in the primary is the sum of those two:
Total Primary Current=2+4=6 amps.

But since you talked about mutual inductance, if there is a mutual inductance that does not provide perfect coupling, then we must introduce these into the equations using a transformer model that includes inductances of each winding.
For example, for two windings with high mutual inductance we might consider thinking in terms of perfect coupling just for a quick idea what is going on, but if they are separated then the mutual inductance goes down and so we no longer get that ideal coupling. This results in reduced values from that calculated above.

If you can quote the inductance of three windings as well as the mutual inductances between windings we can then calculate the values more accurately.

Also, if there is a phase shift in one or more outputs then we have to include the effects of that too. In general though, once we know all the parameters and source and loads we can just do a circuit analysis on the entire circuit and get all the quantities.

#### crutschow

Joined Mar 14, 2008
24,922
I think you mean not to say square of the turns ratio, just the turns ratio.
Yup, I was thinking of impedance. 