Engineering Answers

A coil is wound with 300 turns over a stee ring of relative permeability 900 having a mean circumference of 400 mm and a cross-sectional area of 500 mm2 . If the coil has a resistance of 8Ω and is connected across a 20V dc supply, using a neat sketch, calculate:

a).The coil magneto-motive force

b).The magnetic field strength

c).The total flux

d).The reluctance of the ring

e).The permeance of the ring and

f).The flux density in the ring

The equation for the instantaneous voltage across a

discharging capacitor is given by v = Voe

-l-

, where Vo is

the initial voltage and r is the time constant of the circuit.

The tasks are to:

a)

b)

C)

Draw a graph of voltage against time for Vo=12V

and r = 2s, between t = Os and t = 10s.

Calculate the gradient at t = 2s and t = 4s.

Differentiate v = 12e 2 and calculate the value of

du

dt

at t= 2s and t= 4s.

d)

e)

Compare your answers for part b and part c.

Calculate the second derivative of the instantaneous

¿2.

di2

3).

A 162-MHz carrier is deviated by 12 KHz by a 2-KHz modulating signal. What is the modulation index?

An 8-ohm resistor is in series with an inductive coil and the combination is connected

across a 120 V, 60 Hz source. A voltmeter connected across the resistor reads 32 V and 104 when connected across the coil. Determine the resistance of the coil.

The ohmic resistance of a large magnetic contractor is measured to be 20 ohms. A 230 V is

impressed on the contractor and the current is taken as 3.2 A. Neglecting core loss, determine

the inductance of the contractor in mH?