how to use symmetrical component method to calculate current in transformer

[matts0]

Hello.
I have a Dyn transformer serving a single phase resistive load(with the other 2 phases on the secondary open).The turns ratio of the transformer is 1.
How can I use symmetrical component method to calculate the primary line current?

The current in the secondary winding is 5A.I have used the symmetrical component method to work out that two of the coils in primary will not conduct any current.How ever from my calculation the current flowing in the only coil in the primary is also 5A,which is not consistant with the data I get from experimet(2.9A)

Could someone point out which on step I may make a mistake?

 

[mivey]

Could someone point out which on step I may make a mistake?

Maybe on trying to use symmetrical components to start with? Is there a reason you want to do that vs just using the delta-wye relationship?

The normal relationship for currents and voltages given optional winding connections:
amps_wye = sqrt(3) * amps_delta
volts_delta = sqrt(3) * volts_wye

Are you sure the transformer winding ratio is 1:1? Usually you would have a ratio that would account for the normal delta-wye voltage ratios.

If you do have a 1:1 ratio, you know the current in the secondary is the same as the current in the primary. Trace the currents using vectors and you should have no problem combining the currents to get the correct values.

 

[Smart $]

...The turns ratio of the transformer is 1.
...

Are you certain 1:1 is not the transformer's voltage ratio?

If your transformer's voltage ratio is 1:1 (i.e. Vp:Vs), the winding voltage ratio is 1.732?(Vp:Vs). For example, you have a 208V delta to 208Y/120 isolation transformer, the winding-to-winding voltage ratio is 208:120, or 1.732:1. If you then had a 5A line-to-neutral load on a secondary winding, the associated primary winding would have 2.9A current passing through it...

V_pw ? I_pw = V_sw ? I_sw
208 ? 2.9 = 120 ? 5​

 

[matts0]

Thanks very much for the answers from both of you.It seems the problem is the transformer's voltage raito.I just checked the transformer rating, it is 220/220V delta-star which means the line voltage ratio,right?I need to work a little harder.

 

[Smart $]

Thanks very much for the answers from both of you.It seems the problem is the transformer's voltage raito.I just checked the transformer rating, it is 220/220V delta-star which means the line voltage ratio,right?I need to work a little harder.

Yes, the line-to-line voltages.


Welcome to the forum :grin:

 

[skeshesh]

I'm still interested in knowing why the OP was using symmetrical components in this application...