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.

Any of the above mentioned methods will produce EMF in the armature windings.

Copper Losses or Electrical Losses in DC Machine or Winding Loss. The copper losses are the winding losses taking place during the current flowing through the winding. These losses occur due to the resistance in the winding. In DC machine, there are only two winding, armature and field winding As we know “Energy neither can be created nor it can be destroyed, it can only be transferred from one form to another”. In DC machine, mechanical energy is converted into the electrical energy. During this process, the total input power is not transformed into output power.The copper losses are the winding losses taking place during the current flowing through the winding. These losses occur due to the resistance in the winding. In DC machine, there are only two winding, armature and field winding.To reduce eddy current as well as hysteresis losses

Commutation ideally does not reduce the terminal voltage of a dc machine. If it takes place properly at desired timings, the losses can be avoided. Armature reaction, ohmic losses contribute to the losses in the terminal voltage.

High Power Gain and Favorable Transient Conditions

The key factor that makes rotating amplifiers applicable in large power levels is the combination of high power gain and favorable transient conditions. Let's break down why this is the case:

High Power Gain:
- Rotating amplifiers are known for their ability to provide high power gain, which allows them to efficiently amplify signals to large power levels. This high power gain is crucial in applications where large power levels are required, such as in industrial settings or power systems.

Favorable Transient Conditions:
- In addition to high power gain, rotating amplifiers also offer favorable transient conditions. This means that they are able to handle sudden changes in input signals or load conditions without significant distortion or instability. This is important in applications where rapid changes in power levels occur, as rotating amplifiers can adapt quickly and maintain stability.

By combining high power gain with favorable transient conditions, rotating amplifiers become a reliable and efficient choice for large power level applications. Their ability to amplify signals effectively and handle dynamic changes in conditions makes them well-suited for demanding power systems and industrial settings.

Both 4 point and 3 point starters are almost similar and are also used for the similar purpose i.e. for limiting the starting current and protecting the DC shunt wound and DC compound wound motors. 

The correct option that affects the terminal voltage of a DC shunt motor is c) Compensating winding.

A DC shunt motor consists of a field winding connected in parallel with the armature winding. The field winding is connected to a constant source of voltage, typically a DC power supply. The armature winding is connected to the load and produces the mechanical power output.

The terminal voltage of a DC shunt motor is the voltage measured across the armature terminals. It is influenced by various factors, and one of these factors is the compensating winding.

1. **Armature reaction:** Armature reaction is the magnetic field produced by the armature current that interacts with the field flux produced by the field winding. This interaction leads to a distortion of the field flux and can affect the terminal voltage of the motor. However, armature reaction does not directly affect the terminal voltage.

2. **Source voltage variations:** Variations in the source voltage can affect the terminal voltage of the motor. If the source voltage decreases, the terminal voltage will also decrease, and vice versa. However, source voltage variations are not the correct option in this scenario.

3. **Compensating winding:** A compensating winding is an additional winding connected in series with the armature winding. It is wound in the opposite direction to the armature winding and is placed in the pole shoes of the motor. The purpose of the compensating winding is to counteract the armature reaction and maintain a more stable field flux. By counteracting the armature reaction, the compensating winding helps to regulate the terminal voltage of the motor. When the armature reaction causes a drop in voltage, the compensating winding produces a voltage in the opposite direction to compensate for the drop. Conversely, when the armature reaction causes an increase in voltage, the compensating winding produces a voltage in the same direction to limit the increase. Thus, the compensating winding plays a crucial role in maintaining a stable terminal voltage in a DC shunt motor.

Therefore, the correct option that affects the terminal voltage of a DC shunt motor is c) Compensating winding.

In order to achieve the sparkless commutation, the brushes on the commutator should be placed at the points known as neutral point where no voltage exists between adjacent segments. The conductors connected to these segments lie between the poles in position of zero magnetic flux which is termed as magnetic neutral axis (MNA).

Explanation:

To understand why the emf induced in the armature of a separately excited DC machine reduces to 1/4th of the original value when the speed of rotation is halved and the field strength is reduced to half, let's break down the process step by step.

1. Emf Induced in the Armature:
The emf induced in the armature of a DC machine is given by the equation:

E = kΦNZ/60A

Where:
- E is the induced emf
- k is a constant
- Φ is the flux per pole
- N is the speed of rotation in RPM
- Z is the total number of armature conductors
- A is the number of parallel paths

2. Effect of Halving the Speed of Rotation:
When the speed of rotation is halved, the value of N in the equation is reduced to half. Therefore, the induced emf is also reduced to half of its original value.

3. Effect of Reducing the Field Strength to Half:
The field strength is determined by the flux per pole (Φ). When the field strength is reduced to half, the value of Φ is also reduced to half.

4. Combined Effect:
Now, let's consider the combined effect of halving the speed of rotation and reducing the field strength to half.
- The induced emf is reduced to half due to the halving of the speed of rotation.
- Additionally, the induced emf is reduced to half again due to the reduction in field strength.

Therefore, the combined effect is such that the induced emf reduces to 1/4th (half * half) of its original value.

Conclusion:
Hence, the correct answer is option 'A' - the emf induced in the armature reduces to 1/4th of the original when the speed of rotation is halved and the field strength is reduced to half.