Electrical Machines and Energy Conversion (EENG 202) – Assignment 1
Due: 27/02/2023
Question 1
The secondary winding of a transformer has a terminal voltage of vs(t) = 141.4 sin (377t )V. The
turns ratio of the transformer is 100:200 (a = 0.50).
The impedances of this transformer referred to the primary side are:
Req = 0.10Ω
RC = 150Ω
Xeq = 0.375 Ω
XM = 40Ω
If the secondary current of the transformer is is(t) = 3.54 sin (377t - 36.87˚) A,
(a) Show the approximate equivalent circuit for this transformer referred to the primary side [3]
(b) Calculate the following values for the approximate equivalent circuit developed in (a):
i) The secondary voltage referred to the primary [2]
ii) The primary circuit voltage [2]
iii) The total excitation current [3]
iv) What is the total primary current of this transformer? [2]
v) What is its voltage regulation? [3]
vi) What is the input power for this transformer? [2]
vii) What is its efficiency? [2]
(c) Explain why hysteresis losses occur in ferromagnetic materials? [3]
(d) Give 3 types of transformers? [3]
Question 2
A magnetic circuit is shown in Figure 1. The cross sectional area of the core is 20 mm X 25 mm.
The length of the flux path a-b-e-f-a is 160 mm, and the length of flux path a-c-d-f-a is 250 mm.
The B-H curve for the core steel shows that at a flux density of 0.04 T, H = 8 A/m. The length of
the air gap is 0.5 mm. A current of 2A is flowing in the coil.
1
a) Draw a schematic of the magnetic circuit showing all the reluctances and magnetomotive forces
making up the machine [2]
b) Calculate the values of all the reluctances in your circuit in (a). [5]
c) What is the value of the flux flowing in the centre leg b-e? [5]
d) What causes eddy currents? [2]
e) How can the effects of eddy currents be overcome in electrical machines? [1]
Total [40 marks]
2