All questions of DC Machines for Electrical Engineering (EE) Exam

The voltage drop at brush-commutator contact is variable (1-2V) and independent of armature current.

Introduction:
In a DC shunt-generator, the electromagnetic torque is generated due to the interaction between the magnetic field produced by the field winding and the armature current. This torque is responsible for converting electrical energy into mechanical energy.

Explanation:
When the electromagnetic torque in a DC shunt-generator is opposite, it means that the direction of the torque is opposite to the direction of rotation. This situation indicates that the mechanical power is being absorbed by the machine rather than being delivered.

Reasoning:
To understand why the correct answer is option 'A' (Mechanical power is absorbed by the machine), let's consider the basic principle of operation of a DC shunt-generator.

- A DC shunt-generator consists of a field winding and an armature winding.
- When the field winding is excited with a direct current, it creates a magnetic field in the machine.
- The armature winding is connected to an external mechanical prime mover, such as a turbine or an engine.
- When the armature winding is supplied with a direct current, it produces an armature flux.
- The interaction between the field flux and the armature flux generates an electromagnetic torque.
- This torque causes the armature to rotate, converting electrical energy into mechanical energy.

Now, if the electromagnetic torque is opposite, it means that the torque is acting against the rotation of the machine. This implies that the prime mover has to exert more mechanical power to overcome the opposing torque and keep the machine running.

Since the prime mover is exerting more mechanical power, it indicates that the machine is absorbing mechanical power instead of delivering it. In other words, the machine is acting as a load or a consumer of mechanical power rather than a source of mechanical power.

Conclusion:
Therefore, when the electromagnetic torque in a DC shunt-generator is opposite, it can be concluded that the machine is absorbing mechanical power. This implies that the machine is acting as a load rather than a generator, and the prime mover has to supply additional power to keep the machine running.

Commutation is the process which reduces the induced emf and so the torque.

Assertion (A): Armature resistance control method of speed control of DC motors is wasteful and inefficient.

Reason (R): Large amount of power is wasted in the control resistances, specially in case of shunt motors.

Explanation:

The given assertion and reason state that the armature resistance control method of speed control of DC motors is wasteful and inefficient because a large amount of power is wasted in the control resistances, especially in the case of shunt motors. Let's analyze this statement in detail.

Armature Resistance Control Method:
In the armature resistance control method, the speed of a DC motor is controlled by varying the armature resistance. This is achieved by inserting a variable resistance in series with the armature winding. By changing the resistance, the voltage drop across the armature winding is varied, which in turn affects the motor speed.

Power Loss in Control Resistances:
The control resistances used in the armature resistance control method cause a significant power loss. This power loss happens due to the voltage drop across the resistance and the current flowing through it. The power loss can be calculated using the formula: Power Loss = Voltage Drop * Current.

Shunt Motors:
Shunt motors are a type of DC motors where the field winding is connected in parallel (shunt) with the armature winding. In shunt motors, the armature resistance control method is commonly used for speed control.

Explanation of Assertion and Reason:
The assertion states that the armature resistance control method is wasteful and inefficient. This is because a large amount of power is wasted in the control resistances. The reason provided further explains that this wastage of power is particularly significant in the case of shunt motors.

The armature resistance control method causes power loss due to the voltage drop across the control resistances. This power loss is undesirable as it reduces the overall efficiency of the motor and leads to wastage of energy. In the case of shunt motors, where the armature resistance control method is commonly used, the power loss can be even higher.

Conclusion:
Based on the above explanation, both the assertion and reason are true, and the reason correctly explains the assertion. The armature resistance control method of speed control of DC motors is indeed wasteful and inefficient due to the power loss in the control resistances, especially in the case of shunt motors.

Introduction:
The given circuit consists of a rotating loop between pole faces that is connected to a battery and resistor through a switch. The specifications of the machine are as follows: radius = 0.5m, length = 1m, resistance = 0.3 ohms, and magnetic field strength = 0.25T. The machine is supplied with 120V. The switch is closed at t=0 and we need to determine what is observed in the circuit.

Explanation:
When the switch is closed, the circuit is completed and a current starts to flow through the loop. Let's analyze the different aspects of the circuit to understand the observed behavior.

Magnetic Field and Loop:
The magnetic field strength is given as 0.25T. When the loop rotates, it cuts through the magnetic field lines and induces an electromotive force (EMF) in the loop. This phenomenon is known as electromagnetic induction.

EMF Induced:
As the loop rotates, the flux through the loop changes, resulting in an induced EMF. According to Faraday's Law of electromagnetic induction, the magnitude of the induced EMF is given by the equation:

EMF = -N * dΦ/dt

Where EMF is the induced electromotive force, N is the number of turns in the loop, and dΦ/dt is the rate of change of magnetic flux through the loop. In this case, since the loop has only one turn, the equation simplifies to:

EMF = -dΦ/dt

As the switch is closed at t=0, the rate of change of magnetic flux is initially zero. Therefore, the induced EMF at t=0 is also zero.

Current Flow:
The battery connected to the circuit provides a potential difference of 120V. As per Ohm's Law, the current flowing through the circuit is given by the equation:

I = V/R

Where I is the current, V is the potential difference, and R is the resistance. Substituting the given values, we get:

I = 120V / 0.3Ω
I = 400A

Therefore, a current of 400A flows through the circuit.

Conclusion:
Based on the analysis, we can conclude that when the switch is closed at t=0, a current will flow through the circuit but there will be zero induced EMF at that instant. Hence, the correct answer is option 'A': Current will flow but zero induced EMF.

 In the short shunt configuration, the winding interchanging does not change the configuration of the set up so none of the winding get affected.