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SIXTH SEMESTER DIPLOMA EXAMINATION IN ENGINEERING

/TECHNOLOGY APRIL 2019

Subject: ELECTRIC DRIVES AND CONTROLS.

Subject code: 6034

Branch: ELECTRICAL AND ELECTRONICS ENGINEERING

Prepared By

Name: RAGESH P

Designation: LECTURER

Department : Electrical and electronics engineering

Mobile No. : 9567299837

Solved question paper (Revision 2015)

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I)

II)

1

1. The electric drive has very large range of torque, speed and power. Their working is independent of the environmental condition

2. Using DOL Starter

Using STAR DELTA Starter

3. Plugging dynamic

breaking

Regenerative breaking

4. Two point starter Three

point starter

5. A solar powered pump is a pump running on electricity generated by photo voltaic panels

POWER SOURCE The power source in the above block diagram offers the necessary energy for the system

POWER MODULATOR This modulator can be used to control the o/p power of the supply. The power modulator limits the motor current as well as the source.The power modulator can change the energy based on the motor requirement.Power modulator changes the direct current into alternating current eg ;controlled rectifiers inverters,choppers etc LOAD The mechanical load can be decided by the environment of the industrial process we can choose the other electric components namely electric motor, controller CONTROL UNIT The control This unit produces the rules for the safety of the motor as well as power modulator. l unit is mainly used to control the power modulator

SENSING UNIT The sensing unit in the block diagram is used to sense the particular drive factor such as speed, motor current. This unit is mainly used for the operation closed loop otherwise protection.

Motor

The electric motor intended for the specific application can be chosen by believing various features

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2)

Classification of Electric Drive

Based on operation

1 ac drives

2 dc drives

Based on application

I. Group drive

II. Individual Drive

III. Multi motor Drive

Group Drive :

If several group of mechanisms or machines are organized on one shaft and driven or actuated by one motor, the system is called a group drive or shaft drive. Individual Drive:

1. If a single motor is used to drive or actuate a given mechanism and it does all the jobs connected with this load , the drive is called individual drive.

2. All the operations connected with operating a lathe may be performed by a single motor.

Multi Motor Drive:

Each operation of the mechanism is taken care of by a separate drive motor. The System contains several individual drives each of which is used to operate its own mechanism.

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· 3)

Speed control is obtained by varying the firing angle of the Triac. These controllers are known as Solid State fan regulators. As the solid state regulators are more compact and efficient as compared to the conventional variable regulator. Thus, they are preferred over the normal regulator. In case of a three phase induction, motor three pairs of Thyristor are required which are connected back to back. Each pair consists of two Thyristor. The diagram below shows the Stator Voltage Control of the three phase induction motors by Thyristor Voltage Controller.

4)

VARIABLE FREQUENCY CONTROL

The figure shows variable frequency control from voltage source

Operator controls the frequency through a keyboard

A controller convet or control the frequency

Due to this voltage controlled and also the seed controlled

5)

A transistor Chopper Control of Separately Excited DC Motor drive is shown in Fig. Transistor Tr is operated periodically with period T and remains on for

a duration ton.Present day choppers operate at a frequency which is high enough to ensure continuous conduction. Waveforms of motor terminal voltage

va and armature current ia for continuous conduction are shown in Fig. During on-period of the transistor, 0 ≤ t ≤ ton, the motor terminal voltage is V

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.

6) Speed Control Of Shunt Motor

1. Flux Control Method

a) speed of a dc motor is inversely proportional to the flux per pole.

b) Thus by decreasing the flux, speed can be increased and vice versa.

2. Armature Control Method

a) Speed of a dc motor is directly proportional to the back emf Eb and Eb = V - IaRa.

b) That means, when supply voltage V and the armature resistance Ra are kept constant, then the speed is directly proportional to armature current

Ia.

3. Voltage Control Method

1) Multiple voltage control:

In this method, the shunt field is connected to a fixed exciting voltage and armature is supplied with different voltages.

a) Voltage across armature is changed with the help of suitable switchgear.

b) The speed is approximately proportional to the voltage across the armature.

2) Ward-Leonard System:

This system is used where very sensitive speed control of motor is required (e.g electric excavators, elevators etc.).

7)

In a paper industry, the drives are required for

(i) Pulp making, synchronous motors are used for these drives. These motors normally run at 1000-1500 rpm and gears are used to reduce the speed to 200-300 rpm.

(ii) Paper making. slip ring induction motors with gears are used to drive these beaters at about 150-200 rpm.

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III)

a)

Electric drive Mechanical drive

Electric drive is dependent of electric supply, electric traction system is tried up to electrified routes only.

It is self-contained unit and therefore can be suitable for any track.

In case of failure of supply the whole system will come to rest as it is dependent of electric supply.

The system is self contained there is no such condition.

electric drive is more clean as there are burning of any fuel and no fumes etc.

mechanical drive requires burning of therefore it may contains ash or fumes etc.

Electric drive is flexible as it can be brought anywhere and can be control from any location.

due to mechanical components this drive is not flexible.

It occupies less space as compared to other forms of drives and is , therefore very compact source of drive.

due to mechanical components this drive required more space.

Motor operating characteristics can easily be modified as per the requirement of the drive.

It is very difficult to modify the characteristics of the mechanical drive.

Maintenance cost is less due to less no. of mechanical parts. Maintenance cost is more due to more no. of mechanical parts.

Electric braking much superior and economical.

Mechanical breaking system has few disadvantages like were & tear of parts etc.

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b)

IV)

Nature of electric supply

Nature of the drive

Maintenance requirement

Space ad weight restrictions

Environment and location

Nature of load

Electrical characteristics of motor

Size, rating and duty cycle of motors

a) The following are the advantages of electrical drive. The electric drive has very large range of torque, speed and power.

Their working is independent of the environmental condition.

The electric drives are free from pollution.

The electric drives operate on all the quadrants of speed torque plane.

The drive can easily be started and it does not require any refuelling.

The efficiency of the drives is high because fewer losses occur on it.

It is more reliable

Maintenance is less and easy

Noiseless operation

Can be remote controlled

b)

POWER SOURCE The power source in the above block diagram offers the necessary energy for the system

POWER MODULATOR This modulator can be used to control the o/p power of the supply. The power modulator limits the motor current as well as the source.The power modulator can change the energy based on the motor requirement.Power modulator changes the direct current into alternating current eg ;controlled rectifiers inverters,choppers etc LOAD The mechanical load can be decided by the environment of the industrial process we can choose the other electric components namely electric motor, controller CONTROL UNIT The control This unit produces the rules for the safety of the motor as well as power modulator. l unit is mainly used to control the power modulator

SENSING UNIT The sensing unit in the block diagram is used to sense the particular drive factor such as speed, motor current. This unit is mainly used for the operation closed loop otherwise protection.

Motor

The electric motor intended for the specific application can be chosen by believing various features

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V)

a)

Control From Stator Side:

Control From Rotor Side:

Speed Control by frequency variation:

Speed Control By Pole Changing:

Speed control by varying Supply voltage:

Speed control by varying Rotor Resistance:

Speed control by injected EMF:

Speed control by Cascade Connection:

In this method of speed control, two motors are used.Both are mounted on a same shaft so that both run at same speed. One motor is fed from a 3phase supply and the other motor is fed from the induced emf in first motor via slip- rings. The arrangement is as shown in following figure.

b) Current-source inverter (CSI) drive

In a CSI drive, the DC output of the SCR-bridge converter

stores energy in series-Inductor connection to supply stiff current input to the inverter.

CSI drives can be operated with either PWM or six-step waveform output.

VI)

a)

Regenerative Braking of Induction Motor

When the speed of the motor is more than the synchronous speed

Relative speed between the motor conductors and air gap rotating field reverses,

Plugging Braking of Induction Motor

Plugging induction motor braking is done by reversing the phase sequence of the motor.

Plugging braking of induction motor is done by interchanging connections of any two phases of stator with respect of supply terminals.

And with that the operation of motoring shifts to plugging braking.

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Dynamic Braking of Induction Motor

AC Dynamic Braking-

This type of induction motor braking is obtained when the motor is made to run on a single phase supply by disconnecting any one of the

three phase from the source, and it is either left open or it is connected with another phase.

DC Dynamic Braking

To obtain this type of braking the stator of a running induction motor is connected to a DC supply.

Zero Sequence Braking

In this type of braking all the three stator phases are connected in series and single phase AC or DC is connected across them

b)

dol starter

auto transformer starter

star delta stater

rotor resistsnce starter

DOL STARTER

Used up to 5 HP

Start button NO and stop button NC

When start button pressed coil energies and supply to the motor Stop button pressed then coil de energies and open contacts

olc used for over load protection

VII)

a)

Voltage Control Method

Multiple voltage control

In this method, the shunt field is connected to a fixed exciting voltage and armature is supplied with different voltages.

Voltage across armature is changed with the help of suitable switchgear.

The speed is approximately proportional to the voltage across the armature.

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Ward-Leonard System:

This system is used where very sensitive speed control of motor is required (e.g electric excavators, elevators etc.).

The arrangement of this system is as shown in the figure at right.

M2 is the motor to which speed control is required.

M1 may be any AC motor or DC motor with constant speed.

G is a generator directly coupled to M1.

In this method, the output from generator G is fed to the armature of the motor M2 whose speed is to be controlled.

The output voltage of generator G can be varied from zero to its maximum value by means of its field regulator and, hence, the armature

voltage of the motor M2 is varied very smoothly.

Hence, very smooth speed control of the dc motor can be obtained by this method. Speed Control Of

Series Motor

b)

4 POINT STARTER

For compound motors

Also used in shunt motors

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4 points name as L1,L2 F,A

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VIII)

a)

Basically, there are three types of electrical braking done in a DC Motor

1. Regenerative Braking

2. Dynamic Braking

3. Plugging

1. Regenerative Braking

It is a form of braking in which the kinetic energy of the motor is returned to the power supply system.

This type of braking is possible when the driven load forces the motor to run at a speed higher than its no- load speed with a constant excitation.

The motor back emf Eb is greater than the supply voltage V,

which reverses the direction of the motor armature current.

The motor begins to operate as an electric generator.It is very interesting to note that regenerative braking cannot be used to stop a motor but to control its speed above the no-load speed of the motor driving the descending loads.

2. Dynamic Braking

It is also known as Rheostatic braking.

In this type of braking, the DC motor is disconnected from the supply and a braking resistor Rb is immediately connected across the armature.

The motor will now work as a generator and produces the braking torque.

During electric braking when the motor works as a generator,

Dynamic Braking is an inefficient method of braking as all the generated energy is dissipated as heat in resistances.

3.Plugging

It is also known as reverse current braking.

The armature terminals or supply polarity of a separately excited DC motor or shunt DC motor when running are reversed.

Therefore, the supply voltage V and the induced voltage Eb i.e. back emf will act in the same direction.

The effective voltage across the armature will be V + Eb which is almost twice the supply voltage.

Thus, the armature current is reversed and a high braking torque is produced.

Plugging is a highly inefficient method of braking because, in addition to the power supplied by the load, the power supplied by the source is wasted in resistances.

It is used in elevators, printing press etc.

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b) UNCONTROLLED RCTIFIER CONTROL

IX)

a

variable voltage for dc motor can also be obtained by either using auto transformer or transformer with tapings it also contain uncontrolled rectifier a reactor is used to improve armature current autotransformer used in low power ratings ,in high power ratings transformer with tapings are used

(i) DC motors,

(ii) AC slip ring motors with speed control.

(i) Slip ring induction motor.

(ii) Three phase shunt wound commutator motor.

(iii) Cascade controlled AC motor.

(iv) Ward Leonard controlled DC motor.

(v) DC motor with transformer step switch control

IX b)

The solar pump block diagram mainly includes a solar panel, water pump, electric motor, and controller. This pump is basically an electrical

pump, and this pump uses the electricity which is received from the solar panels to work..These panels store the energy from the solar. The

electric motor manages the alternating current or direct current. The controller used in this system adjusts the output power as well as speed.

X) a)

In mines, induction motors are mostly used in a flameproof enclosure. Besides the enclosure, performance of the induction motors is the same as that of the other motors, as per the particular design. squirrel cage-types are the most simple of all electric motors. The squirrel cage motors can be designed to operate from single or three phase supplies. A three phase Induction motor will start under load as soon as the supply is switched on. Starters are used only if it is necessary to reduce the starting current. Because of their simplicity, squirrel cage motors are widely used in mines and also in other industries. They are used underground to drive drills, coal cutters; loaders, conveyors and haulages, and they may also be found to be used extensively in pumps, auxiliary fans and small compressors.:

b)

Every stage has its own drive. Several drives in Cement Mill Process are raw mill drives, cement mill drives, kiln drives, crusher drives, waste gas fan drives and compressor drives.three-phase slip ring induction motor for raw mill drives The requirements of a kiln motor are the following: 1. Power requirement is very high. 2. Speed control ratio is 1:10. Very low creeping speeds of 1 rpm may be required 3. Starting torque should be in the range 200 to 250% of full load torque.

The Requirements of a crusher drive are as follows:

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1. The starting torque is of the order of 160% of full load torque.

2. The breakdown t

orque is of the order of 200-250% of full load torque.

Slip ring induction motors with rotor resistance control are suitable in fan blower system

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