1). What is the function of a Commutator?
The commutator facilitates the gathering of current from armature conductors. It converts the alternating current from armature conductors to unidirectional current in external load circuit.
2). What is the function of Brushes?
Brushes transfer current from a revolving loop (or) winding to an external stationary load.
3). What are the different types of Generators?
Generators are classified by their field excitation methods:
- Separately Excited DC Generators and
- Self-Excited DC Generators.
4). Define Separately Excited Generators
A separately excited generator is one whose field magnet winding is powered by an external direct current source, such as a battery.
5). Define Self Excited Generators
A self-excited generator is a direct current generator with a field magnet winding powered by the generator’s output.
6). What are the different types of Self Excited Generators?
There are 3 types of self-excited generators, based on how the field winding is coupled to the armature.
- Series Generator – In a series generator, the field winding is linked in series with the armature winding, allowing the armature current to pass through both the field winding and the load.
- Shunt Generator – the field winding is linked in parallel with the armature winding to ensure that terminal voltage of generator’s output is applied across it.
- Compound Generator – Compound generators have 2 sets of field windings on every pole, one in series and the other parallel to the armature. A compound wound generator can be:
- Short Shunt: Short Shunt, where the shunt field winding is parallel to the armature winding.
- Long Shunt: The shunt field winding is parallel to both the series field and the armature winding.
7). What are Constant and Variable Losses?
- Constant Losses: Constant losses includes Iron, mechanical, and shunt field losses
- Variable Losses: Variable losses include copper loss.
8). What is Armature Reaction?
Armature reaction refers to how the magnetic field created by armature current affects the distribution of the flux under a generator’s main poles. The armature magnetic field has 2 effects:
- It demagnetizes and weakens the main flux.
- It induces cross-magnetism or distortion.
The first effect results in reduced produced voltage, while the second causes sparking at brushes.
9). Why do we utilize Compensation Windings?
Compensation windings are utilized to neutralize cross magnetizing effect of the armature response.
10). What if there is No Compensation Windings?
Without compensation windings, the flux will shift backward and forward with load changes, causing an EMF in armature coils.
The EMF may be strong enough to create an arc between commutator segments. This could lead to a flash-over around commutator, short-circuiting the entire armature.
11). What is Commutation?
Coil currents attached to a brush are either positive or negative. As a result, current in a coil reverses as it passes through a brush. The commutation refers to the reversal of current in a coil when it crosses the brush axis.
12). How to improve Commutation?
Improving commutation implies making current reversal in short-circuited coil as spark-free as possible.
This can be accomplished through two methods:
- Resistance commutation and
- EMF commutation.
13). What are the advantages of generators operating in parallel?
- The continuity of service
- Efficiency.
- Maintenance and Repair.
- Increased plant capacity.
- No availability of a single huge unit
14). What is the operating principle of a DC motor?
A direct current motor operates on the basis that once a current carrying conductor is put in a magnetic field, it experiences mechanical force.
The construction of a DC motor & DC generator is same. The same direct current machine can function as both a generator and a motor.
15). What is back EMF (or) counter EMF?
When a DC motor’s armature rotates with the driving torque, the conductors travel across the magnetic field, causing EMF to be induced, similar to a generator.
The induced EMF acts in the opposite direction of the applied voltage (Lenz’s law) and is referred to as back EMF (or) counter EMF (Eb).
When the motor runs normally, the difference between it and the applied voltage (V) is minimal.
16). What is Speed Regulation?
Speed regulation is change in speed caused by reducing the motor’s load from its rated value to zero. It is calculated as a percentage of the rated load speed.
17). Why can’t a series motor start without any load?
Without a load, a series motor cannot start due to its high starting torque. Series motors are used in many different applications, such as cranes and trains.
18). Which type of motor is used in trains, what is the rating of supply used?
The Dc series is utilized in trains to provide high starting torque, with a working voltage of 1500V DC.
19). Explain the various methods for controlling the speed of DC shunt motors.
To control the speed of a direct current shunt motor, use the
- Flux Control Method (N∝1/∅)
- Armature Control Method
- Voltage Control Method
20). Can you explain the various methods for controlling the speed of DC series motors?
To control the speed of a DC series motor, use
- Flux Control Method
- Armature-Resistance Control Method
21). How to reverse direction of rotation of a DC motor?
You can reverse the rotation direction of a direct current (DC) motor by reversing either the field current or the armature current.
If both currents are reversed, the motor will run in the original direction.
22). Why the field of a DC shunt motor should not be open?
The shunt motor will reach dangerously high speeds and may destroy itself.
23). What happens if the current at terminals of a DC motor is reversed?
The motor’s rotational direction remains constant, as does the current in the armature and field.
24). What occurs when a DC motor is coupled to an AC power supply?
- The motor will run, but it will not carry the same load as it would with a direct current supply.
- More sparks from the brushes
- Eddy currents will be significant, causing overheating and maybe burning the AC supply.
25). What are the methods for starting an induction motor?
- DOL (Direct Online Starter)
- Star Delta Starter
- Auto Transformer Starter
- Resistance Starter
- Series Reactor Starter.
