EE6801- ELECTRICAL ENERGY GENERATION UTILISATION AND CONSERVATION TWO MARKS QUESTIONS AND ANSWERS
UNIT 1. ELECTRIC DRIVES TRACTION
1) What are the requirements of an ideal traction system?
- The starting tractive effort should be high so as to have rapid acceleration.
- The wear on the track should be minimum.
- Pollution free
- Speed control should be easy.
- The equipment should be capable of withstanding large temporary loads.
- Low initial and maintenance cost.
- There should be no interference to the communication lines running along the lines.
- Braking should be such that minimum wear is caused on the brake shoes.
2) Name the various systems of traction.
1. Direct steam engine drive
2. Direct Internal Combustion Engine Drive
3. Steam Electric Drive
4. Internal Combustion Engine Electric Drive
5. Petrol Electric traction
6. Battery Electric Drive
7. Electric Drive
3) Classify the supply system for electric traction.
A. D.C system
B. A.C system i) Single phase ii) Three phase
C. Composite system i) Single phase AC-DC ii) single phase-Three phase
4) What are the advantages of electric traction?
• High starting torque Less maintenance cost
• Cheapest method of traction Rapid acceleration and braking
• Less vibration Coefficient of adhesion is better
• It has great passenger carrying capacity at higher speed.
5) What are the disadvantages of electric traction?
- High capital cost
- Problem of supply failure
- Additional equipment is required for achieving electric braking and control
- The leakage of current from the distribution mains and drop of volts in the track are to be kept within the prescribed limits.
- The electrically operated vehicles have to move on guided track only.
6) Define average speed and scheduled speed.
Average speed is the ratio of distance between two consecutive stations to time taken to travel the distance, Scheduled speed is the ratio of distance between two consecutive stations to total time taken for moving including the time for stops.
7) Name the different stages of train movement
2.constant speed or free running
3. coasting , running with power switched off and brake not applied
4.retardation with braking
8)What are the essential features (electrical )of an ideal traction motor
1.high starting torque.
2.series speed torque characteristics
3.Simple speed control
4.Possibility of regenerative braking
9)What are the need for traction motor control?
1.To limit starting current
2.Smooth acceleration without jerk
3.Both manual and automatic control should be possible.
10) What is meant by speed-time curve? Why it is used?
The curve drawn between speed in Kw/hr along Y-axis and time in seconds along X-axis is called speed=time curve. The speedtime curve gives the complete information about the motion of the train.
This curve gives the speed at various time instants after the start of run directly. Slope of the curve at any point gives the speed at that instant. The area under the curve gives the total distance traveled by the train.
11) What do you mean by average speed in electric traction?
The mean of the speeds from the start to stop i.e the distance between two stops divided by the actual time of run is known as average speed.
Average speed= Distance between stops in km/ Actual time of run in hours
12) What do you mean by schedule speed in electric traction?
The ratio of distance covered between two stops and total time of run including time of stop is known as schedule speed.
Schedule speed = Distance between stops in km/(Actual time of run in hours+ Stop time in hours)The schedule speed is always smaller than the average speed.
The difference is large in case of urban and suburban services and is negligibly small in case of main line service.
13) What is tractive effort?
The effective force necessary to propel the train at the wheels of the locomotive to which the motor is geared is called the geared effort. It is measured in Newtons and is tangential to the driving wheels.
Total tractive effort required to run a train on track = Tractive effort to produce acceleration + Tractive effort to overcome effect of gravity + Tractive effort to overcome train resistance.
14) What are the factors affecting energy consumption?
The various factors affecting energy consumption are
(i) Distance between the stops
The greater the distance between the stops, the lesser will be the specific energy consumption for suburban service is 50 to 75 watts-hour/ ton-km and for main line service it is between 18 to 32 watt-hour/ton-km.
(ii) Train resistance
The train resistance depends upon the nature of track, speed of the train and shape of the rolling stock, particularly the front and rear portions of the train. If the train resistance is greater, the specific energy consumption is more.
(iii) Acceleration and retardation
If the acceleration and retardation increases, the specific energy consumption is increased.
The steep gradients will involve more energy consumption though regenerative braking is applied.
(v) Train equipment
More efficient train equipment will reduce the specific energy consumption.
15) Define dead weight, adhesive weight.
(i) Dead weight
The total weight of locomotive and train to be pulled by the locomotive is known as dead weight.
(ii) Adhesive weight
The total weight to be carried on the driving wheels is known as the adhesive weight.
16) Name the various methods of traction motor control.
There are various methods for controlling the speed of d.c series motors. They are
- Rheostatic control
- Series parallel control
- Field control
- Buck and Boost method
- Metadyne control
- Thyristor control
17) What are the basic requirements of braking system?
The basic requirements of a braking system are given below It should be simple, robust, quick and reliable in action.
Easy to use for driver to operate. Maintenance should be minimum.
The braking system should be inexhaustible.
In case of emergency braking, safety consideration is taken into account.
Kinetic energy of the train must be storable during braking which could be used
subsequently during acceleration of the train.
18) What are the various methods of applying electric braking?
There are three methods of applying electric braking are
• Plugging or Reverse current braking
• Rheostatic braking
• Regenerative braking.
19) Name the advanced methods of speed control of traction motors.
The latest methods of speed control of traction motors are
Tap changer control Thyristor control
Chopper control Microprocessor control
20) What are the advantages of microprocessor based control of traction motors?
The advantages of microprocessor based drives are
High speed of response High accuracy
Over voltage and over speed protection. Electronic interlocking
Less sensitive to temperature variations and drift. Numbers of components used are less.
21) What is meant by drives?
Systems employed for motion control are called “DRIVES” and drives employ any of the prime movers such as, diesel or petrol engines, gas or steam turbines, hydraulic motors and electric motors for supplying mechanical energy for motion control. Drives employing electric motion known as “Electric Drives”.
22) Define an electric drive.
The combination of an electric motor, the energy transmitting shaft and the controlling devices for controlling the performance of the motor is called an electric drive
23)Name the factors governing the selection of a motor for a particular purpose.
1. Mechanical output required
2. Electrical input required
5. Electrical characteristics,
6. Mechanical characteristics,
7. Size and rating
24) How the types of loads of a drive can be classified?
1. Continuous loads (pumps)
2. Intermittent loads(mixy)
3. Fluctuating loads(traction)
25) What are the essential requirements of braking in an electrical drive?
1. Fast reliable and controllable
2. Stored energy should be dissipated efficiently
3. Failure jn any part should result in braking only.
26) Name the 2 types of braking
2 . Electrical(electro dynamic)
27) State the merits and de merits of electrical braking.
1 less maintenance
2 no dirt 3regenerative braking possible
1 motor should have suitable braking characteristics .
2 no holding torque
3 during failure of supply mechanical braking needed
28) Mention the parts of electrical drives?
• Electric motors and load
• Power modulator
• Control unit
• Sensing unit
29) Mention the application of electric drives?
Paper mills Electric traction
Cement mills Steel mills.
30) Mention the different types of classes of duty?
• Continuous duty
• Discontinuous duty
• Short time duty
• Intermittent duty
31) Define equivalent current method.
The motor selected should have a current rating more than or equal to the current. It is also necessary to check the overload capacity of the motor. This method of determining the power rating of motor is known as the equivalent current method.
32) What are the three methods of operation of electric drive?
Steady state Acceleration including starting
Decceleration including stopping.
33) Define four-quadrant operation?
A motor operate in two modes, motoring and braking. In motoring, it converts electrical energy into mechanical energy, which supports its motion. In braking it works as a generator converting mechanical energy into electrical energy and thus, opposes the motion. Motor can provide motoring and braking operations for both forward and reverse directions.
34) Mention the types of braking?
• Regenerative braking
• Dynamic braking
35) Define and mention different types of braking in a dc motor?
In braking, the motor works as a generator developing a negative torque which opposes the motion. Types of regenerative braking are Dynamic (or) Rheostat braking; and plugging (or) reverse voltage braking.
36) List the drawbacks of armature resistance control?
In armature resistance control, speed is varied by wasting power in external resistors that are connected in series with armature.
Since it is an inefficient method of speed control, it was used in intermittent load application where the duration of low speed operation forms only a small proportion of total running time.
37) Mention the methods of armature voltage control dc motor?
When the supply voltage is ac
i) Ward-Leonard schemes
ii) Transformer with taps and uncontrolled rectifier bridge
iii) Static Ward-Leonard scheme (or) controlled rectifiers When the supply is dc
i) Chopper control
38) What are the disadvantages of conventional ward-Leonard schemes?
• Higher initial cost due to use of two additional machines.
• Large weight and size.
• Needs more floor space and proper foundation.
• Required frequent maintenance.
• Higher noise and higher loss.
39) Mention the drawbacks of rectifier fed dc drives?
Distortion of supply Low power factor
Ripple in motor current
40) What are the different methods of speed control of induction motors?
• Stator voltage control
• Supply frequency control
• Rotor resistance control
• Slip power recovery control
41) What is meant by stator voltage control?
The speed of the induction motor can be changed by changing the stator voltage. Because the torque is proportional to square of the voltage.
42) Mention the applications of stator voltage control?
The stator voltage control method is suitable for applications where torque demand reduced with speed, which points towards its suitability for i) Fan ii) Pump drives
43) Mention the applications of AC drives?
AC drives are used in a number of applications such as fans, blowers, mill run-out tables, cranes, conveyors, traction etc.
44) What are the advantages of stator voltage control method?
- The control circuitry is simple
- Compact size
- Quick response time
- There is considerable savings in energy and thus it is economical method as compared to other methods of speed control.
45) What are the features of variable frequency control?
- Speed control and braking operation are available from zero speed to above base speed.
- Drop in speed from no load to full load is small.
- Copper losses are low. Hence efficiency and power factor are high as the operation is
- restricted between synchronous speed and maximum torque point at all frequencies.
- During transient (starting, braking and speed reversal) operation can be carried out at the maximum torque with reduced current giving good dynamic response.
46) What is meant by frequency control of induction motor?
The speed of the induction motor can be controlled by changing the supply frequency, because the speed is directly proportional to supply frequency. This method of speed control is called frequency control.
47) What is meant by V/F control?
When the frequency is reduced, the input voltage must be reduced proportionally so as to maintain constant flux. Otherwise the core will get saturated resulting in excessive iron loss and magnetizing current. This type of induction motor behavior is similar to the working of dc series motor.
48) What is meant by regenerative braking?
Regenerative braking occurs when the motor speed exceedsthe synchronous speed. In this case, the induction motor wouldruns as the induction machine is converting the mechanical power into electrical power, which is delivered back to the electrical system. This method of braking is known as regenerative braking.
49) What is meant by dynamic braking?
Dynamic braking of electric motor occurs when the energy stored in the rotating mass is dissipated in an electrical resistance. This requires the motor to operate as a generator to convert this stored energy into electrical.
50) What is meant by plugging?
It is one method of braking of induction motor. When phasesequence of supply of the motor running at a speed is reversed,by interchanging connections of any two phases of stator withrespect to supply terminals, operation shifts from motoring toplugging region.