When a transformer is loaded, three things occur: 1) The secondary current I2 sets up its own flux Φ2 opposing the primary flux Φ. 2) This causes the primary voltage V1 to increase slightly and induce an additional primary current I2' equal in magnitude and opposite in phase to I2. 3) The flux Φ2' set up by I2' exactly cancels out the flux Φ2, such that the net flux through the transformer core remains approximately the same as under no-load conditions.

1. Lecture 4

2. Transformer on Load
Fig. 32. 17(a) Transformer without load
It has already seen that at no load condition only current I0 flows in
the circuit as shown in Fig. 32.17(a). This current sets up the flux .
When the secondary is loaded, the secondary current I2 is set up.
The magnitude and phase of I2 with respect to V2 determined by the
characteristics of the load.

3. • The secondary current sets
up its own mmf (=N2I2) and
hence its on flux 2 which is
in opposition to the main
primary flux  which is due
to I0 as shown in Fig.
32.17(b).
• The secondary ampere-
turns N2I2 are known as
demagnetizing amp-turns.
• The opposing secondary
flux 2 weakens the primary
flux , hence primary back
emf E1 tends to be reduced.
Fig. 32.17 (b) Transformer on load

4. For a moment V1 gains upper hand over E1 and hence causes more
current to flow in primary.
Let the additional primary current be I2
’ as shown in Fig. 32.17(c).
Fig. 32.17(c)
It is known as load component of primary current. This current is anti-
phase with I2 .
The additional primary mmf N1I2’ sets up its own flux 2
’, which is in
opposition to 2 (but is in the same direction as ) and is equal to it in
magnitude.
Hence the two cancel each other out as shown in Fig. 32.17(d).

5. • So, we find that the magnetic effects of secondary current I2
are immediately neutralized by the additional primary current
I2
’, which is brought into existence exactly at the same instant
as I2.
• Hence, whatever the load conditions, the net flux passing
through the core is approximately the same as at no-load.
Fig. 32.17(d)

6. Fig. 32.17
An important deduction is that
due to the constancy of core
flux at all loads; the core loss is
also practically the same under
all load conditions.
When transformer is on load,
the primary winding has two
currents in it; one is I0 and the
other I2
’ is which is anti-phase
with I2 and K times in
magnitude (i.e. I2
’ = I2K).
The total primary current is the
vector sum of I0 and I2
’.

7. Quick Quiz
1.

8. 2. In a transformer
(a) All turns are equally insulated
(b) The end turns are more strongly insulated
(c) The end turns are closely wound
(d) The end turns are widely separated
3. Select the correct statement:
(a) emf per turn of both the windings are equal
(b) emf per turn in HV winding is more than the emf per turn in LV
winding
(c) emf per turn in HV winding is less than the emf per turn in LV
winding
4. The flux involved in the emf equation of a transformer has
(a) rms value
(b) Average value
(c) Total value
(d) Maximum value

9. 5. Transformer changes the value of
(a) Power
(b) Frequency
(c) Voltage
(d) Current
6. Draw the vector diagram of no load condition.

10. In Fig. 32.8 are shown the vector diagrams for a load transformer when load is non-
inductive and when it is inductive.
Voltage transformation ratio of unity is assumed so that primary vectors are equal to
the secondary vectors.
With reference to Fig. 30.18(a), I2 is secondary current in phase with E2.
It causes primary current I2
’ which is anti-phase with it and equal to it in magnitude.
Total primary current I1 is the vector sum of I0 and I2
’ and lags behind V1 by an angle
1.
Here, I2 lags E2 (actually V2) by 2.
Current I2
’ is again in anti-phase with I2
and equal to it in magnitude.
As before, I1 is the vector sum of I2
’ and
I0 and lags behind V1 by 1.
In Fig. 30.18(b) vectors are drawn for an
inductive load.

11. In Fig. 32.8 are shown the vector diagrams for a load transformer when load is non-
inductive and when it is inductive.
Voltage transformation ratio of unity is assumed so that primary vectors are equal to
the secondary vectors.
With reference to Fig. 30.18(a), I2 is secondary current in phase with E2.
It causes primary current I2
’ which is anti-phase with it and equal to it in magnitude.
Total primary current I1 is the vector sum of I0 and I2
’ and lags behind V1 by an angle
1.
Here, I2 lags E2 (actually V2) by 2.
Current I2
’ is again in anti-phase with I2
and equal to it in magnitude.
As before, I1 is the vector sum of I2
’ and
I0 and lags behind V1 by 1.
In Fig. 30.18(b) vectors are drawn for an
inductive load.