A. Greater than secondary flux.
B. Smaller than secondary flux.
C. Either 1 & 2
D. Equal to secondary flux.
Answer: D. Equal to secondary flux.
Explanation:
The transformer is said to be a constant main flux device. It is due to the high permeability and greater mutual flux which maintain a constant value.
When a load resistance is connected to the secondary winding the voltage induced into the secondary causes a flow of secondary current.
This current produces a secondary flux field which is in opposition to the primary field flux (Lenz’s law). Thus, the secondary flux cancels some of the primary flux.
With less flux surrounding the primary, the primary EMF is reduced and more current is drawn from the source.
The additional primary current generates more lines of flux nearly reestablishing the original number of total flux lines.
Similarly, for reducing the load, the secondary current decreases, and hence the primary current also reduces.
To summarize, as current is drawn from the secondary, total flux momentarily decreases, and primary current increases restoring the flux lines to almost their original number.
Thus, over the normal operating range of load § current, the total core flux does not change more than two or three percent.