Sheet (1)-Transformer
1. An ideal transformer with a primary of 200 turns and a secondary of 20 turns has its
primary connected to a 120-V, 60-Hz supply, and its secondary connected to a 100∟30o-Ω
load. Determine:
a) The secondary voltage,
b) The load current,
c) The input current to the primary, and
d) The input Impedance measured from the primary side.
2. A 60-Hz, 100-kVA, 2400/240-V transformer is used as a step-down transformer from a
transmission line to a distribution system. Consider the transformer to be ideal.
a) Determine the turn’s ratio.
b) What secondary load impedance will cause the transformer to be fully loaded, and
what is the corresponding primary current?
c) Find the value of the load impedance referred to the primary side of the
transformer.
3. A 200-turns winding on a laminated magnetic steel core is excited by a 60-Hz sinusoidal
source of 200-V.
a) Neglecting the resistance of the winding and any leakage flux, find the maximum
flux density in the core if the uniform cross-sectional area of the core is 25-cm2.
b) Corresponding to the maximum flux density of 1.5-T, the core loss and the exciting
rms Volt-Amperes for the core are given to be:
Pc = 50-W; (VI)rms = 500-VA
Determine the exciting current, the core-loss component, the magnetizing
component, and the corresponding power-factor angle.
c) Find Rc & Xm.
4. A not quite ideal transformer having 90-turns on the primary and 2250-turns on the
secondary is connected to a 120-V, 60-Hz source. The coupling between the primary and
secondary is perfect, but the magnetizing current is 4-A. Calculate:
a) The effective voltage across the secondary terminals.
b) The peak voltage across the secondary terminals.
c) The instantaneous voltage across the secondary when the instantaneous
voltage across the primary is 37-V.
d) The magnetizing reactance.
5. A large transformer operating at no-load draws an exciting current of 5-A when the
primary is connected to a 120-V, 60-Hz source. From a wattmeter test it is known that the
core losses are equal to 180-W. Calculate:
a) The reactive power absorbed by the core.
b) The value of Rc and Xm.
c) The value of Ih+e and Iφ. Draw the phasor diagram.