Engineering Studies $(8080074)$

QUESTION $3$: AC THEORY

$-6-$

Qp

$3\mathrm{.}1$

A coil of $120$ turns covering a mean area of $100{10}^{-4}{m}^2$ is rotated at $3000\frac{r}{m}in$ in a magnetic field between two poles having a uniform flux density of $0,7$ tesla.

Calculate:

$3\mathrm{.}1\mathrm{.}1$ The frequency

$3\mathrm{.}1\mathrm{.}2$ The maximum value of the induced EMF

$(2)$

$3\mathrm{.}1\mathrm{.}3$ The RMS and average values of EMF

$(3)$

$(2)$

$3\mathrm{.}2$ In a certain circuit having three parallel branches, the instantaneous branch currents are presented by:

$i_1=25sinwt$

$i_2=15sin(wt-\frac{}{5})$

$i_3=30sin(wt+\frac{}{2})$

Calculate the total current and write it in the form:

$i=I_{\text{m}\text{a}\text{x}}sin(wt+)$

$3\mathrm{.}3$ Briefly explain, what do you understand about the following concepts:

$3\mathrm{.}3\mathrm{.}1$ Apparent power

$3\mathrm{.}3\mathrm{.}2$ Reactive power

$[20]$

QUESTION $4$: TRANSFORMERS

$4\mathrm{.}1$ The no$-$load current of a single$-$phase transformer is $8$ A at a power factor of $0,5$ lagging.

Calculate the value of the magnetising current and the value of the core loss current.

Draw an equivalent circuit diagram and a phasor diagram for these conditions

$(6)$

$4\mathrm{.}2$ State whether each of the following losses is a variable or a constant loss:

$4\mathrm{.}2\mathrm{.}1$ Electrical Losses

$4\mathrm{.}2\mathrm{.}2$ Dielectric losses

$(2\backslash$times $1)$

$(2)$