- primary voltage: secondary voltage = primary turns: secondary turns
- primary current: secondary current = secondary turns: primary turns
From the above equations it is seen that the watts of the primary circuit equal the watts of the secondary circuit.
Ques. Are the above relations strictly true, and why?
Ans. No, they are only approximate, because of transformer losses.
In the above example, the total wattage in the primary circuit is 1,000 × 10 = 10 kw., and that in the secondary circuit is 100 × 100 = 10 kw. Hence, while both volts and amperes are widely different in the two circuits, the watts for each are the same in the ideal case, that is, assuming perfect transformer action or 100% efficiency. Now, the usual loss in commercial transformers is about 10%, so that the actual watts delivered in the secondary circuit is (100 × 100) × 90% = 9 kw.
Fig. 1,919.—Wagner transformer coil formed, ready for taping. These are known as "pan cake" coils. They are wound with flat cotton covered copper strip. In heavy coils, several strips in parallel are used per turn in order to facilitate the winding and produce a more compact coil.
The No Load Current.—When the secondary winding of a transformer is open or disconnected from the secondary circuit no current will flow in the winding, but a very small current called the no load current will flow in the primary circuit.