Question 1

Decide whether each of these statements is True (T) or False
With a d.c. generator, a split-ring commutator is used to:
(i) Reverse the connections to the rotor winding every half-cycle.
(ii) Via brushes give the output voltage.

A) (i) T (ii) T
B) (i) T (ii) F
C) (i) F (ii) T
D) (i) F (ii) F

فتح الحزمة

افتح القفل للوصول البطاقات البالغ عددها 6 في هذه المجموعة.

فتح الحزمة

k this deck

Accepted Answer

(i) True because according to the capacitor discharge formula, the current in the circuit at any time $t$ is given by $I = \frac{V}{R}e^{-\frac{t}{RC}}$, where $V$ is the initial voltage, $R$ is the resistance, and $C$ is the capacitance. This shows that the current is indeed proportional to the rate of change of voltage across the capacitor. (ii) False because the time constant $\tau$ of an RC circuit is given by $\tau = RC$. For a 1 MΩ resistor and an 8μF capacitor, $\tau = 1 \times 10^6 \Omega \times 8 \times 10^{-6} F = 8$ seconds. The current (and voltage) across a capacitor drops to about 36.8% of its initial value after one time constant, not half.

Question 2

Decide whether each of these statements is True (T) or False
When a capacitor of 8μF, which has been charged to 12 V, is connected across a 1 MΩ resistor:
(i) The circuit current is proportional to the rate of change of voltage across the capacitor.
(ii) 8 s after the connection, the circuit current has dropped to half its initial value.

A) (i) T (ii) T
B) (i) T (ii) F
C) (i) F (ii) T
D) (i) F (ii) F

Accepted Answer

(i) The initial circuit current is given by I = V/R = 24 V / 100 Ω = 0.24 A.(ii) The time constant of the circuit is given by τ = L/R = 50 mH / 100 Ω = 0.5 ms. After a time equal to the time constant, the circuit current has decreased to 36.8% of its initial value, which is 0.24 A * 0.368 = 0.088 A.

Question 3

Decide whether each of these statements is True (T) or False
With a d.c. generator, a split-ring commutator is used to:
(i) Reverse the connections to the rotor winding every half-cycle.
(ii) Via brushes give the output voltage.

A) (i) T (ii) T
B) (i) T (ii) F
C) (i) F (ii) T
D) (i) F (ii) F

Accepted Answer

In a series wound DC motor, the armature and field coils are connected in series, meaning they do indeed carry the same current. This configuration contributes to the motor's high starting torque, as the current through the field winding increases with the load, strengthening the magnetic field and thus increasing torque.

Question 4

Decide whether each of these statements is True (T) or False
With a d.c. generator, a split-ring commutator is used to:
(i) Reverse the connections to the rotor winding every half-cycle.
(ii) Via brushes give the output voltage.

A) (i) T (ii) T
B) (i) T (ii) F
C) (i) F (ii) T
D) (i) F (ii) F

Accepted Answer

(i) is true for a shunt wound DC motor as the armature and field coils are connected in parallel and hence carry the same current.
(ii) is false for a shunt wound DC motor as it has a moderate starting torque compared to other types of DC motors. Universal motors and series wound DC motors have high starting torque.

Question 5

Decide whether each of these statements is True (T) or False
With a d.c. generator, a split-ring commutator is used to:
(i) Reverse the connections to the rotor winding every half-cycle.
(ii) Via brushes give the output voltage.

A) (i) T (ii) T
B) (i) T (ii) F
C) (i) F (ii) T
D) (i) F (ii) F

Accepted Answer

(i) True because the split-ring commutator reverses the connection to the rotor winding every half-cycle to maintain the same direction of current flow in the external circuit. (ii) True because the brushes, in contact with the commutator, provide the path for the output voltage to be taken from the generator.

Question 6

The Th&#233;venin equivalent circuit for the circuit to the left of the terminals X and Y in the following circuit is:&#10;A) A voltage source of 4 V with series resistance 25&#937;&#10;B) A voltage source of 4 V with series resistance 8&#937;&#10;C) A voltage source of 5 V with series resistance 25&#937;&#10;D) A voltage source of 5 V with series resistance 8&#937;

Accepted Answer

To find the Thévenin equivalent circuit, we need to first find the equivalent resistance of the circuit looking into terminals X and Y.

To do this, we can combine the 2Ω and 3Ω resistors in parallel to get:

1/R = 1/2 + 1/3 = 5/6

R = 6/5 Ω = 1.2 Ω

Next, we need to find the open circuit voltage between terminals X and Y. Since no current will flow through the circuit when there is an open circuit, we can simply find the voltage across the 4Ω resistor using voltage division:

V_oc = 20V * (4Ω / (2Ω + 4Ω)) = 8V

Now we can put the circuit back together with the Thévenin equivalent circuit, which consists of a voltage source with voltage V_oc in series with a resistor with resistance R. Thus, the Thévenin equivalent circuit for the circuit to the left of the terminals X and Y is:

Answer: B
 The Thévenin equivalent circuit for the given circuit is a voltage source of 4 V with series resistance 8Ω. This is found by calculating the open-circuit voltage across terminals X and Y, which is 8 V, and finding the equivalent resistance looking into those terminals, which is 1.2 Ω. The voltage source is then placed in series with this equivalent resistance to form the Thévenin equivalent circuit.

Deck 8: Transients, D.C Circuit Analysis, and D.C Machines