UNIT – I INTRODUCTION
1).What is an electrical drives?
A drive consists of various system combined together for the purpose of motion control or movement control. Especially the drives which employ electric motors for motion control are known as Electrical drives.
2).List the elements of an electric drive system.
· Group drive
· Individual drive
· Multimotor drive
3).State the some of the advantage of an electric drive system
· Control characteristic can be manipulated as per requirements
· Availability of simple and easy speed control methods
· Electric braking can be employed in easy manner
· The operation is pollution free
· The variety of electric drives with wide range of speed, power and torque ratings are available.
· The efficiency is higher as no load losses are less.
· They have short time overload capacity.
4).List the factors affecting the selection of electric drives.
Efficiency, Braking, Limits of speed range, starting requirements, power factor, load factor, availability of supply, effects of supply varations, economical aspects, reliability of operation, environmental effects.
5).State the selection of motor based on load variation
· Continuous or constant loads
· Continuous variable loads
· Pulsating loads
· Impact loads
· Short time loads
6).State the various classes of duty
· Continuous duty
· Continuous duty, variable loads
· Short time loads
· Intermittent periodic duty
· Intermittent periodic duty with starting
· Intermittent periodic duty with starting and braking
7).What are the elements of an electric drive system?
- Electrical motors and load
- Power modulator
- Source
- Control unit
- Sensing unit
8).List the types of electrical drives?
- DC drives
- AC drives
9).Mention the application of electrical drives?
- Paper mills
- Electric traction
- Cement mills
- Steel mills
10).Define cooling time constant?
It is defined as the ratio between C and A. cooling time constant is denoted as ‘ ’
= C/A
Where C= amount of heat required to raise the temperature of the motor boby by 1 degree Celsius in / c
A= amount of heat dissipated by the motor per unit time per degree Celsius in J/S c
11).What are the three methods of operation for electrics drive?
- Steady state
- Acceleration including starting
- Deceleration including stopping
12).Define four – quadrant operation?
A motor operate in two modes and braking. In motoring, it converts electrical energy into mechanical energy, which support its motion. In braking it works as a generator converting mathematical energy into electrical energy and thus, opposes the motion. Motor can provide motoring and braking operations for both forward and reverse directions.
13).Compare a.c. and d.c. drives.
S.No | DC Drives | AC Drives |
1 | The commutator makes the motor bulky, costly and heavy | Motors are inexpensive, particular squirrel cage motor |
2 | The converter technology is well established. The power converter is simple and inexpensive | The inverter technology is still being developed. The power circuit of the converter and its control are simple |
3 | Line commutation of the converter is used | Forced commutation is used with induction motors. Sometimes machine commutation may be used with synchronous motors |
4 | Fast response and wide speed range smooth control | With solid state converter the speed range is wide. With conventional methods it is stepped and limited |
5 | Small power/weight ratio | Large power /weight ratio |
6 | Cost does not depend on the solid state converter | Solid state converter employed also decides the cost |
14) Mention the necessity of power rating?
Power rating of electric drives for particular operation is important since, following reasons.
1.To get economy with realiability
2.To obtain the maximum efficiency on their full load without any damaging.
15).what is duty factor?
The ratio of ON time(Ton) of the drive to total time period(Ton +Toff) is called duty factor.
16) What is cooling curve?
When a machine is switched off from the mains or when the load on the motor is reduced, the machine cools. The curve obtained temperature drop Vs time when the drive is switched off or load on the drive is removed.
BIG QUESTIONS
1).What is electric drive? Explain the basic elements of an electric drive system.
2).Explain the classification of electric drives.
3).Explain the various classes of duty. How it affect the selection of rating of a motor for the drive?
4).Draw a typical temperature rise –time curve and derive equation for temperature rise in an electric drive
5).Write a note on cooling curve of an electric derives, stating its expression.
UNIT II DRIVE MOTOR CHARACTERISTICS
1).What is back e.m.f in a D.C. Motor? State its expression.
Armature starts rotating, the main flux gets cut by the armature winding and an e.m.f gets induced in the armature. This e.m.f opposes the applied d.c voltage and is called back e.m.f denoted as Eb.
Eb= PNZ
60A
= Flux per pole
P= Number of poles
N= Speed in rpm
Z=Total armature conductors
A= Number of parallel paths
2) Write the voltage equation of D.C. Motor
V=Eb+IaRa. The back e.m.f is always less than supply voltage (Eb<V). But Ra is very small hence under normal running conditions, the difference between back e.m.f and supply voltage is very small.
3) State the various types of D.C. Motors
- Separately excited d.c. Motor
- D.C. Shunt motor
- D.C series motor
- D.C. Compound motor
4) What are the important characteristics of a d.c.motor?
- Torque – Armature current characteristics
- Speed – Armature current characteristics
- Speed- Torque Characteristics
5) Why series motor is never started on No load?
Under light load or no load as flux is very small, the motor tries to run at dangerously high speed which may damage the motor mechanically. This can be seen from the speed – armature current and the speed- torque characteristics that on low armature current and low torque condition motor shows a tendency to rotate with dangerously high speed.
6) List some application of d.c.shunt motor?
- Blowers and fans
- Centrifugal and reciprocating pumps
- Lathe machine
- Machine tools
- Milling machines
- Drilling machines
7) List some application of d.c.series motor?
- Cranes
- Hoists, Elevators
- Trolleys
- Conveyors
- Electric Locomotives
8) List some application of d.c.compound motors?
- Rolling mills
- Punches
- Shears
- Heavy Planers
- Elevators
9) What is synchronous speed?
The speed depends on the supply frequency (f) and the number of poles for which stator winding is wound (P).It is called synchronous speed denoted as Ns and given by
Ns= 120f/P in r.p.m
10) What is rotor conductor and end ring?
The rotor core is cylindrical and slotted on its periphery. The rotor consists of uninsulated copper or aluminum bars called rotor conductors. The bars are placed in the slots. These bars are permananently shorted at each end with the help of conducting copper ring is called end ring.
11) Compare Slip ring and squirrel cage motor
S.No | Slip Ring Rotor | Squirrel cage Rotor |
1. | Rotor consists of a three phase winding similar to the stator winding | Rotor consists of bars which are shorted at the ends with the help of end rings |
2. | Construction is complex | Construction is very simple |
3. | Resistance can be added externally | As Permanently shorted, external resistance cannot be added |
4. | High starting torque can be added | Moderate staring torque which cannot be controlled |
5. | Speed control by rotor resistance is possible | Speed control by rotor resistance is not possible |
6. | Slip rings and brushes are presented to add external resistance | Slip rings and brushes are absent |
7. | Rotor copper losses are high hence efficiency is less | Rotor copper losses are less hence have higher efficiency |
12).What is slip?
The difference between the synchronous speed(Ns) and actual speed(N) of the rotor is known as slip speed. The percentage of slip is given by
% slip s =[(Ns -N) / Ns] *100
13) Induction motor as a transformer?
Transformer is a device in which two windings are magnetically coupled and when one winding is excited by a.c. supply of certain frequency, the e.m.f gets induced in the second winding having same frequency as that of supply given given to the first winding. The winding of which supply is given is called primary winding while winding in which e.m.f gets induced is called secondary winding.
14) What is transformation ratio?
K=E2/E1 (or) k=N2/N1
Were E1= Stator e.m.f per phase in volts
E2=Rotor induced e.m.f per phase in volts at start when motor is at standstill.
15).what are the types of Single phase induction motors?
- Split phase induction motor
- Capacitor start induction motor
- Capacitor start capacitor run induction motor
- Shaded pole induction motor
16) What are the types of electric braking in D.C.Motors?
- Rheostatic or dynamic braking
- Plugging
- Regenerative braking
17) What is meant by Rheostatic or dynamic braking?
Dynamic braking of electric motors 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.
18) What is meant by Plugging?
It is one method of braking of induction motor. when phase sequence of supply of the motor running at a speed is reversed, by interchanging connections of any two phases of stator with respect to supply terminals, operation shifts from motoring to plugging region.
19) What is meant by Regenerative Braking?
Regenerative braking occurs when the motor speed exceeds the synchronous speed. In this case, the induction motor would runs 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.
20) What are the types of electric braking in induction Motors?
- Rheostatic or dynamic braking
- Plugging
- Regenerative braking
- D.C.Dynamic braking
21) What are the three regions in the speed –torque characteristics of induction motor?
I.Motoring region (0< s<1)
II.Generating region(s<0)
III.Plugging region (1<s<2)
22) List the advantage of squirrel cage I.M?
- Cheaper
- Light in weight
- Rugged in construction
- More efficient
- Require less maintenance
- Can be operated in dirty and explosive environments
BIG QUESTION
1).Draw and explain the torque –speed characteristics for the DC motors.
2).What is meant by braking. Explain in details.
3).Draw and explain the speed- torque characteristics curve of 3-phase induction motor.
4) Explain single-phase induction motors speed torque curve in details.
5).What are the braking methods available for induction motors and explain in details.
UNIT III STARTING METHODS
1) What is the Necessity of starter?
Both d.c motors as well as three phase induction motors are self starting but these motors show the tendency to draw very high current at the time of starting. Such a current is very high and can cause demage to the motor windings. Hence there is a need of a certain device which can limit such a high stating current. Such a device which limits the high starting current is called a starter.
2) What is meant by starting resistance?
To restrict this high starting armature current, a variable resistance is connected in series with the armature at start. This resistance is called starter or a starting resistance.
3) What are the two types of starters used for D.C shunt motors?
a). Two Point starter b)three point starter c)Four Point starter
4) What are the main parts of three point starter?
L = line terminal to be connected to positive of supply
A = to be connected to the armature winding
F= to be connected to the field winding
5) What are the disadvantages of three point starter?
Here NVC and the field winding are in series. so while controlling the speed of the motor above rated, field current is reduced by adding an extra resistance in series with the field winding. To avoid the dependency of NVC and the field winding, four point starter is used in which NVC and the field winding are connected in parallel.
6) What are the main parts of four point starter?
L = line terminal to be connected to positive of supply
A = to be connected to the armature winding
F= to be connected to the field winding
N=Neutral
7) What is automatic starter?
Upon pressing ON-push button(start button),current limiting stating resistors get connected in series with armature circuit in DC motor.Then,some form of automatic control progressively disconnects these resistors until full-line voltage is available to the armature circuit. On pressing an OFF push button the system should get back to its original position.
8).Why starts are used for DC motors?
In DC motors starters are used to limit the starting current within about 2 to 3 times the rated current by adding resistance in series with the armature circuit. Other than this starting resistances starters are variable fitted with protective devices like no –voltage protection and over-load protection.
9).Why stator resistance rarely used?
Due to addition of resistance in the stator side cause the voltage available to the motor X times the normal voltage i.e. The starting current drawn by the motor as well as the current drawn from the supply get reduced by X times where as the starting torque developed gets reduced by X2 times.
10) What are the effects of increasing rotor resistance in the rotor circuit of a 3-phase induction motor as starting?
Due to addition of resistance in rotor circuit by the stator not only reduces the staring current, in addition to that the starting torque developed than those given by DOL starting.
11) What are the advantages of Electronic starter?
- The moving parts and contacts get completely eliminated.
- The arcing problem gets eliminated.
- Minimum maintainenance is required as there are no moving parts.
- The operation is reliable
- Starting time also gets reduced.
12) What are the various types of reducing starting current?
- Stator resistance starter
- Autotransformer starter
- Star-delta starter
- Rotor resistance starter
- Direct on line starter
13).what is autotransformer starter?
A three phase star connected autotransformer can be used to reduce the voltage applied to the stator. Such a starter is called as autotransformer starter.
14).what is star-delta starter?
This is the cheapest starter of all and hence used very commonly for the induction motors. It uses triple pole double throw (TPDT) switch. The switch connects the stator winding in star at start. Hence per phase voltage get reduced by the factor 1/sq.3.Due to this reduced voltage, the staring current is limited.
15) What is the function of starters?
- For large capacity induction motors is to reduce the starting current
- Having necessary control devices to limit the starting current
- All starters are provided devices to protect the motor against overload and loss of supply voltage.
BIG QUESTION
1) Draw and explain a three point starter for a dc motor?
2) What are the different types of AC motor starter?
3) Explain the different types of starters used in cage induction motor with a neat sketch?
4) Why starter is necessary for the induction motor?
5) Draw and explain rotor resistance starter
UNIT IV CONVENTIONAL & SOLID STATE SPEED CONTROL OF D.C. DRIVES
1) What are the factors on which the speed of a dc motor depends?
- Flux in the air gap
- Resistance in the armature circuit
- Voltage applied to the armature circuit
2) What are the advantages of field control?
- The regulating resistance, which has to carry only a small current
- Power wasted in regulating resistance is very small
3) What are the methods on which speed of a dc motor be controlled?
- Flux control
- Armature resistance
- Armature voltage
4) What will be the effect of change in supply voltage on the speed of dc shunt motor?
The reduction of supply voltage to the armature of dc shunt motor causes reduction of back-emf of the motor which inturn reduces the speed as the speed is directly proportional to the back emf
5) List the different methods of speed control to 3 phase squirrel cage induction motor?
- Speed control by changing supply frequency
- Speed control by changing no. of poles
- Speed control by changing slip
6) What is meant by speed control?
The initial change of drive speed to a value required for performing the specific work process is called as a speed control.
7) What are the advantages of field control method?
- Conventional and easy method
- Since, the shunt field current Ish is small the power wasted in the field rheostat also small
- Independent of load on the motor
- Economical and efficient method
8) What are the disadvantages of field control method?
There is a maximum limit of speed that can be obtained with this method. It is due to fact that flux per pole is too much weakened commutation becomes poorer.
9) What is the application of ward- Leonard system speed control?
This method normally adopted in very sensitive speed control like electric excavators, elevators, coillery winders, main drives in steel mills and paper mills.
10) What are the advantages of Ward-Leonard speed control?
- Wide range of speed control is possible
- Full forward and reverse speed can be achieved
- Power is automatically regenerated to the ac line through the motor generator set which speed is reduced.
- Short time over load capacity is large
- The armature current of the motor is smooth
11) Define slip
The difference between the synchronous speed and the actual speed of the motor is called slip.
12) What is Slip-Power recovery system?
The slip power can be recovered to the supply source can be used to supply an additional motor which is mechanically coupled to the main motor.This type of drive is known as slip-power recovery system
13) In which type of control the field current and armature current control?
i).For armature control method (or) voltage control method the field current is kept constant
ii).For field control (or) flux control the armature current kept constant
14) What are the advantages of slip-power recovery system?
i).The slip power from the slip-rings can be recorded and fed back to the supply.
ii).The overall efficiency also improved
15) What is meant by frequency control?
The speed of the induction motor can be controlled by varying the supply frequency, because the speed is directly proportional to frequency.
16) What is meant by flux control (or) field control method?
By varying the field flux the speed can be controlled is called flux control. This method can be used for increasing the speed of the motor is inversely proportional to the field flux
17) Write the advantage of flux control method
- Convenient and easy method
- In this method is independent of load on the motor
- Economical and efficient method
18) What is meant by armature control?
The armature having controller resistance in series during the speed control. By varying the controller resistance R, the potential drop across the armature is varied. Hence the speed of the motor also varied. This method of speed control is applicable for speed less than no load speed.
19).What is meant by voltage control in induction motor? and where it is applicable?
In Induction motor speed can be controlled by varying the stator voltage. This can be done by using transformer. This method is called voltage control.
This is suitable only for controlling the speed below rated value.
20).What is static Ward – Leonard drive?
Controlled rectifiers are used to get variable dc voltage from an ac source of fixed voltage. Controlled rectifiers fed dc drives are known as “static Ward – Leonard drive”.
21) 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 behaviour is similar to the working of dc series motors.
BIG QUESTION
1) What is meant by armature control method?
2) List out the methods of speed control in Dc motors?
3) Explain the working of following methods with neat circuit diagram
i).Kramer system ii).Scherbius system
4) Explain in details rotor resistance method of speed control of a slip ring induction motor.
5) Explain any one of the slip power recovery method of speed control
UNIT - V CONVENTIONAL & SOLID STATE SPEED CONTROL OF A.C. DRIVES
1) What is meant by solid state speed control?
A process which is used to control the speed by using purely semi conducting material ie power semiconductor devices like thyristors, power Transisitors, MOSFET and etc.,is called solid state speed control.
2) What are the arrangements are available using power semi conducting materials?
- Controlled converters
- AC voltage regulator
- Switch (or)chopper
- Inverter
- Cycle converter
3) What are the advantages of solid state drive methods?
- Very fast response
- Less losses
- Higher efficiency
- Pollution free method
- Small size maintenance easy and controlled
4) What are disadvantage of solid state drive methods?
- Very difficult to regeneration of power
- It contains low power overload capacity
- Causes the interference in commutation in communication system due to harmonics in supply
5) What is meant by dc chopper?
It is a derive which is used to converter fixed dc voltage into variable dc voltage. this is designed with the help of power semiconducting devices like power transistors, MOSFETS thyristors.
6) What are the different types of choppers?
- First quadrant chopper –motoring control
- Second quadrant chopper – braking control
- Two quadrant chopper
- Four quadrant chopper
7) Write down the main feature of v/f control?
1. The starting current is constant
2. Maximum torque should be constant
8) What is meant by stator frequency control?
The three phase induction motor speed can be controlled by varying the stator frequency. The variable stator frequency can be obtained by inverters circuit.
9) What is meant by ac voltage controller?
AC voltage controller is nothing but, which is used to converters fixed ac voltage into variable ac voltage without changing supply frequency.
10) What is stator voltage control?
Three phase induction motor speed can be controlled by varying the stator voltage. This stator voltage can be varied by using ac voltage controller. This method of speed control of induction motor is called as stator voltage control.
11) What are the advantages of stator voltage control?
1. very simple control circuit
2. More compact and less weight
3. Quick response and economical method.
12) What are the disadvantages of stator voltage control?
1.Input Power factor is low
2. Operating efficiency is low
3. Voltage and current waveform are highly distorted due to harmonics, which affects the efficiency of the machine
13) What is the function of freewheeling diode?
Free wheeling diodes are introduced to maintain the continuous current flow to the load when ever all thyistors are turned off condition.
14) Write the output voltage equation for single phase full converter and half converter.
Half converter
Vo = Vm(1+cos A)/2*3.14
Full converter
Vo = 2Vm (cos A)/3.14
15).What are the possible methods of speed control available by using inverters?
- Variable voltage input(VVI) inverter control
- Variable voltage output(VVO) inverter control
- Pulse width modulated (PWM) inverter control
- Current controlled inverter.
16) Compare static Kramer and scherbius system
S.No | Kramer | Scherbius |
1. | The system consists or SRIM,diode bridge rectifier and line commutated inverter | This system consists of SRIM,two SCR bridge (or) cyclo converter |
2. | The slip power can flow in one direction | The slip power can flow in both direction |
3. | This is applicable for below synchronous speed operation | Applicable for both below and above synchronous speed operation |
17) Why do we for PWM inverter control?
The output from inverter is square with some harmonic contents so we have to remove or reducing the harmonic contents by using some voltage control technique called PWM
18) What are the classifications of PWM technique?
1. Single pulse width modulation
2. Multiple pulse PWM modulation
3. Sinuodal Pulse PWM
19) What are the limitations of cyclo converter method of speed control?
1. It requires more semiconductor devices like thyristors, MOSFETs compared with inverters
2. Harmonic contents more with low power factor.
BIG QUESTION
1) What are the different methods of speed control of induction motors?
2) Explain in detail about V/F control?
3) Explain in detail about voltage control method?
4) Explain in details about slip power recovery scheme?
5) Explain in details about single phase half wave converter drive speed control for DC drive
6) Explain in detail the static Kramer methods for controlling speed using solid state devices.
http://eduframe.net/KSRGI/Texts%5CDrives%20&%20Controls.htm
