Question

81. The low voltage winding of a 400/230 volt, 1-phase, 50 Hz transformer is to be
connected to a 25Hz supply in order to keep the magnetization current at the same
level as that for normal 50Hz
supply at 25 Hz the voltage should be
A) 230V
B) 460V
C) 115V
D) 65V
Ans: C​

Answer 1

Given

$V_1$ = Low voltage = 230 V

$f_1$ = Normal frequency = 50 Hz

$f_2$ = New frequency = 25 Hz

To find

$V_2$ = New voltage

Solution

We know that the magnetization current is at the same level so

$\dfrac{V_1}{f_1}=\dfrac{V_2}{f_2}$

$\Rightarrow V_2=\dfrac{V_1f_2}{f_1}$]

$\Rightarrow V_2=\dfrac{230\times 25}{50}$

$\Rightarrow V_2=115\ \text{V}$

The voltage should be C) 115 V.

Answer 2

Given:

Voltage Winding = 400/230

Flux = 50hz

Supply = 25Hz

To Find:

Voltage when the supply is 25Hz

Solution:

The flux should be the same to hold the magnetization current at the same amount.

This means that the voltage to frequency ratio or V/F should be the same.

Thus, Vf ratio -

V2/f2 = V1/f1

V2 = 230 x 25/50

V2 = 23 x 25/5

v2 = 575/5

= 115

Answer: The voltage is 115V.