Engineering Studies $(8080074)$

parallel. A load of $27$ W which draws a current of $3$ A is connected in series with the two resistors in parallel.

QP

Calculate the value of the unknown resistor if the circuit is connected across a $48$ V $,$ DC supply.

$1\mathrm{.}4$ Distinguish between a positive and a negative temperature coefficient of resistance.

$1\mathrm{.}5$ Briefly explain, what is understood by Kirchhoff's first law?

$1\mathrm{.}6$ A coil having $50$ turns, of a conductor with a cross$-$sectional area of $0,2m{m}^2$ and a mean length per turn of $120$ mm $,$ has an inductance of $0,3$ henrys. Calculate the following:

$1\mathrm{.}6\mathrm{.}1$ The resistance of the windings if the specific resistance of the conductor is $0,025$ micro$-$ohm metres.

$1\mathrm{.}6\mathrm{.}2$ The average value of the EMF induced in the coil when a current of $7$ A is reversed in $0,5$ seconds.

$[30]$

QUESTION $2$: DC MACHINES

$2\mathrm{.}1$ What are the THREE types of brushes normally used on DC machines?

$2\mathrm{.}2$ Define armature reaction

$2\mathrm{.}3$ Calculate the speed, a $6-$pole series generator having a wave$-$wound armature with $320$ conductors and resistance of $0,3$ ohms supplying a load of $75$ kW at $750$ V $.$ The field windings brush contact resistance is $0,2$ ohms. The field sets up a magnetic flux per pole of $0,05Wb.$

$2\mathrm{.}4$ Calculate the total resistance of a starter for a $440$ V $,$ DC shunt motor requiring an input of $11$ kW and the maximum armature current is not to exceed $1,5$ times the full$-$load armature current. The armature resistance is $0,23$ ohms. Neglect the current flowing in the field circuit.

$2\mathrm{.}5$ Show by means of a neat, fully labelled circuit diagram, a series faceplate starter used to start a DC machine.

$[20]$I$\text{'}$m