N2 Electrical Trade Theory: Module 1 – Alternating current circuit theory

Q1 – A conductor 25cm long moves through a magnetic field at a velocity of 10m/s.

Calculate the value of the induced emf in the conductor when it is at an angle of 80° to the plane of the magnetic field and the flux density of the field is 0.412 tesla.

Q2 – Define dynamically induced emf

Q3 – The current equation for a circuit is i = 200 sin 628/. What are the values of the following?

a) The RMS current

b) The frequency

c) 190,2 A

Q4 – When is maximum emf induced in a coil rotating at a constant speed in a uniform magnetic field?

Q5 – How can the angular velocity of an alternating emf be determined?

Q6 – Define the following terms of AC waves: instantaneous value, RMS value and form factor

Q7 – A conductor 100 mm long rotates through a magnetic field at a velocity of 45 mls. The flow density of the field is 0,12 tesla.

Calculate:

a) The value of the maximum induced emf in the conductor

b) The instantaneous emf at an angle of 40° to the plane of the magnetic field

Q8 – Explain the term self-induction

Q9 – Define statically induced emf.

Q10 – Define the term power-factor.

Q11 – Explain the meaning of the term impedance with regard to an AC circuit.

Q12 – Name TWO methods applied in practice to improve a poor power factor.

Q13 – A series circuit consists of a resistor of 40 ohms, a capacitor with a capacitive reactance of 60 ohms and an inductance with an inductive reactance of 40 ohms. If the supply voltage is 220V at a frequency of 50 Hz

Calculate:

a) The impedance of the circuit

b) The current flowing

c) The power factor

d) The inductance of the circuit

Q14 – A single-phase circuit consumes 4,2 KW at a power factor of 0,6 lagging. Find the value of the apparent input power and the reactive power under these conditions.

Q15 – Make two stretches to illustrate star and delta connections. Indicate the line, and phase voltages and currents on the diagrams.

Q16 – A 10 KW, Three -phase, 380-V motor is started by means of a star-delta starter. What is the phase voltage when the starter is:

a) In the star position?

b) In the delta position?

Q17 – A three-phase delta-connected motor draws a current of 8 A from a 525-V supply at a power factor of 0,9 lagging.

Determine:

a) The apparent input power

b) The actual input power

c) The phase currents of the motor’s windings