Operation of Transformer

Question 1

Transformation Ratio?

Answer 1

k = N2/N1
k = E2/E1

Question 2

Relation between transformation ratio (k) and turns ratio?

Answer 2

Turns ratio = 1/k

The transformation ratio is defined as the ratio of the secondary voltage to the primary voltage.
And
Turns Ratio is the number of turns in primary winding to the secondary winding

Question 3

In 5KVA, 400V/200V Transformer, what is E1 and E2?

Answer 3

E1 = 400V
E2 = 200V

Question 4

For the same rating transformer, relation between frequency and specific weight?

Answer 4

Frequency ∝ 1/ Specific Weight

Question 5

Components of no-load current?

Answer 5

Io = Iw + Iμ

Iw - Active component or wattfull component of current
Iμ - Reactive component or wattless component of current

Question 6

What is the iron loss component of NL current?

Answer 6

Iw - It is drawn from the source to supply the power required for iron loss.

Question 7

What is the magnetizing component of NL current?

Answer 7

Iμ - Responsible to create the flux in core

Question 8

Iw and Iμ formulas?

Answer 8

Iw = Io cos Φo
It is always in phase with applied voltage.

Iμ = Io sin Φo
It is always in quadrature with applied voltage.

Question 9

Percentage of Iw and Iμ in FL current? Io?

Answer 9

Iw - 1-2% of FL current
Iμ - 4-5% of FL current

Io - 5-6% of FL current

Question 10

No-load current formula?

Answer 10

Io = √Iw2 + Iμ2

Question 11

No-load power is also called? formula?

Answer 11

No-load power = Iron losses = Core losses = V1 x Iw

Question 12

Where is the no-load branch placed?

Answer 12

The no-load current is never transferred from primary to secondary, hence the shunt branch/no-load branch is parallel to the primary winding.

Question 13

No-load branch resistance and reactance formulas?

Answer 13

Ro = V1/ Iw
Xo = V1/Iμ

Question 14

Parameters in the operation of the transformer?

Answer 14

Io - NL current
I1 - Primary current
I2 - Secondary current - Load current
I1’ - Load component of secondary current

Φo - No load flux - Main flux
Φ1 - Primary flux
Φ2 - Load component of secondary flux - Load angle
Φ1’ - Load component of primary flux

V1 - Supply voltage
V2 - Secondary terminal voltage - Load voltage

Question 14

Operation of transformer?

Answer 14

Iμ produces flux (Φmain) that links with the secondary winding producing emf.
The emf in the secondary winding sets up a current (I2) which in turn produces its own flux (Φ2) opposing the main flux.
The resultant flux, Φr = Φmain - Φ2
leading to a drop in (V1-E1) to compensate for this, an additional current I1’ is drawn.
I1’ produces flux Φ1’ equal to Φ2.
Now, the resultant flux is Φr = Φmain - Φ2 + Φ1’
Φ1’ cancels out Φ2 ; so Φr = Φmain
Hence, the transformer is a constant flux machine.

Question 15

Equivalent parameters referred to the primary side?

Answer 15

V2' = V2/k
I2'= I2 x k

R01 = R1 + R2'
X01 = X1 + X2'
Z01 = Z1 +Z2'

R2' = R2/sq(K)
X2' = X2/sq(K)
Z2' = Z2/sq(K)

Question 16

Equivalent parameters referred to the secondary side?

Answer 16

V2' = V2 x k 
I2'= I2/k

R02 = R1' + R2
X02= X1' + X2
Z02 = Z1' +Z2

R1' = R1 x sq(K)
X1' = X1 x sq(K)
Z1' = Z1 x sq(K)

Question 17

Total Cu losses formula?

Answer 17

1. (I1)2xR1+ (I2)2 xR2

Question 18

Total Pu resistance drop?

Answer 18

R01 pu = I1R01/E1

&

R02 pu = I2R02/E2

where:
I1 = KVA / E1
I2 =KVA / E2

Question 19

Relation of pu resistances and % resistances in a trasformer?

Answer 19

R01 ≠ R02

But,

R01 pu = R02 pu
% R01 pu = % R02 pu

Question 20

What is useful flux? and leakage flux?

Answer 20

In a transformer, both primary and secondary currents produce flux. The flux Ф which links both the windings is the useful flux and is called mutual flux. It is independent of load and remains constant.

However, the primary current would produce some flux Фl1 which would not link the secondary winding. Similarly, secondary winding would produce Фl2 that would not link the primary winding. The flux such as Фl1 or Фl2 which links only one winding is called leakage flux. They do not participate in the energy conversion process.

Question 21

What is the effect of leakage flux in the operation of the transformer?

Answer 21

Фl1 produces Ex1 and Фl2 produces Ex2.

These emf generated do not participate in the energy conversion process and can be considered as voltage drop.

This drop is given by: I1 X1 and I2X2

The reason to choose reactance over resistance is that flux causes a drop in voltage not any losses.