This presentation is for Polytechnic M-Scheme, IInd Year 4th semester Mechanical Engineering students.

1. ELECTRICAL DRIVES AND CONTROL
32044
M – SCHEME
2019-2020
𝐈𝐈 𝐧𝐝 Year – B section
DEPARTMENT OF MECHANICAL ENGINEERING
Prepared by,
Mr.B.Don Dougles.,M.E.,
Lecturer., D.M.E

2. UNIT – I
DC CIRCUITS AND DC MACHINES
Questions and topics to be focussed on,
 Definition on Ohm’s law, Kirchhoff’s laws, Faraday’s laws of electromagnetic
induction.
 Resistances in series and parallel.
 Construction and working of DC generator.
 Construction and working of DC motor.
 Starters in DC motor-3 Point starter.
 Starters in DC motor-4 Point starter.
Slide – 01EDC – Unit I

3. Ohm’s Law
• Ohm’s Law states that at constant temperature, the current flowing through a
conductor is directly proportional to the potential difference across the conductor.
V α I
V = IR
Kirchhoff’s Laws
Kirchhoff’s Current law:
• The algebraic sum of current flowing towards a junction in an electric circuit is
zero.
• Sum of the currents entering the junction = Sum of the currents leaving the junction
Slide – 02EDC – Unit I

4. • The algebraic sum of all voltages around a closed path at any instant is zero.
i,e., Algebric sum of emf’s + Algebraic sum of voltage drops = 0
Kirchhoff’s Voltage law:
Resistances Series and Parallel
Resistances in Series
A circuit is said to be in series when the current flowing through the resistances are same.
V
Slide – 03EDC – Unit I
The total equivalent resistance of the circuit, 𝑅 𝑆 = 𝑅1 + 𝑅2 + 𝑅3 + 𝑅4 Ω

5. • In parallel connection the potential difference across all the resistor is the same, but the
current in each is different.
• In this circuit the resistors R1 and R2 are connected in parallel.
• The total equivalent resistances in parallel is,
1
RP
=
1
R1
+
1
R2
Ω
Resistances in Parallel
Slide – 04EDC – Unit I

6. • There are two principle laws of electromagnetic induction.
First Law:
Whenever a conductor cuts a magnetic flux an emf is induced in that conductor.
Second Law:
The magnitude of emf induced in the coil is equal to the rate of change of flux
that linkages with the coil. The flux linkage of the coil is the product of number
of turns in the coil and flux associated with the coil.
e =N
𝐝∅
𝐝𝐭
volt
Faraday’s Law of Electromagnetic Induction
Slide – 05EDC – Unit I

7. Construction and Working of DC Generator
 A dc generator is an electrical machine which converts mechanical energy into direct current
electricity.
Principle:
 According to Faraday’s laws of electromagnetic induction, whenever a conductor is placed in
a varying magnetic field an emf (electromotive force) is induced in the conductor.
Construction:
Slide – 06EDC – Unit I

8. • Basic constructional parts of a DC machine are described below.
1. Yoke: The outer frame of a dc machine is called as yoke. It is made up of cast iron or steel. It
provides mechanical strength to the machine and also carries the magnetic flux produced by
the field winding.
2. Poles and pole shoes: Poles are joined to the yoke with the help of bolts or welding. They
carry field winding and pole shoes are fastened to them. Pole shoes serve two purposes; (i)
They support field coils (ii) Spread out the flux in air gap uniformly.
3. Field winding: They are usually made of copper. Field coils are former wound and placed on
each pole and are connected in series. They are wound in such a way that, when energized,
they form alternate North and South poles.
Slide – 07EDC – Unit I

9. 4. Armature winding: It is usually a former wound copper coil which rests in
armature slots. The armature conductors are insulated from each other and
also from the armature core. Armature winding can be wound by one of the
two methods; lap winding or wave winding.
5. Commutator and brushes: A commutator consists of a set of copper segments which are
insulated from each other. The number of segments is equal to the number of armature coils.
Each segment is connected to an armature coil and the commutator is keyed to the shaft.
Working of DC Generator:
• In a DC generator, field coils produce an electromagnetic field and the armature conductors are
rotated into the field. Thus, an electromagnetically induced emf is generated in the armature
conductors. It’s given by Flemming’s right hand rule.
Slide – 08EDC – Unit I

10. • According to Fleming’s right hand rule, the direction of induced current changes whenever the direction of motion
of the conductor changes.
• Let’s consider an armature rotating clockwise and a conductor at the left is moving upward. When the armature
completes a half rotation, the direction of motion of that particular conductor will be reversed to downward.
• Hence, the direction of current in every armature conductor will be alternating.
• If you look at the above figure, you will know how the direction of the induced current is
alternating in an armature conductor.
Slide – 09EDC – Unit I

11. • But with a split ring commutator, connections of the armature conductors also get reversed
when the current reversal occurs. And therefore, we get unidirectional current at the terminals as
shown above.
Applications: (i) Used as supply source of DC motors; (ii) Used to charge the battery; (iii) Used
as booster in distribution networks.
Slide – 10EDC – Unit I

12. Construction and Working of DC Motor
• A DC motor like we all know is a device that deals in the conversion of electrical energy to
mechanical energy.
Construction:
• Two major parts in a DC motor are stator and rotor.
1. Stator – The static part that houses the field windings and receives the supply.
2. Rotor – The rotating part that brings about the mechanical rotations.
Slide – 11EDC – Unit I

13. 3. Yoke: The magnetic frame or the yoke of DC motor made up of cast iron or steel and forms an
integral part of the stator or the static part of the motor. Its main function is to form a protective
covering over the inner sophisticated parts of the motor and provide support to the armature.
4. Armature Winding: The armature winding of DC motor is attached to the rotor, and as a result
is subjected to altering magnetic field in the path of its rotation which directly results in
magnetic losses. Armature core made with several low-hysteresis silicon steel lamination, to
reduce the magnetic losses like hysteresis and eddy current loss respectively.
5. Commutator: Its main function as far as the DC motor is concerned is to commute or relay the
supply current from the mains to the armature winding housed over a rotating structure through
the brushes of DC motor.
Slide – 12EDC – Unit I

14. 6. Brushes: The brushes of DC motor are made with carbon or graphite structures, making sliding
contact over the rotating commutator. The brushes are used to relay the current from external
circuit to the rotating commutator form where it flows into the armature winding.
Working of DC Motor:
Principle: When a current carrying conductor is placed in a magnetic field, it experiences a force.
Slide – 13EDC – Unit I

15. • The very basic construction of a DC motor contains a current carrying armature which is
connected to the supply end through commutator segments and brushes it is placed within the
north south poles of a permanent or an electro-magnet as shown in the diagram above.
• Now to go into the details of the operating principle of DC motor its important that we have a
clear understanding of Fleming’s left hand rule to determine the direction of force acting on the
armature conductors of DC motor.
• Fleming’s left hand rule says that if we extend the index finger, middle finger and thumb of our
left hand in such a way that the current carrying conductor is placed in a magnetic field
(represented by the index finger) is perpendicular to the direction of current (represented by the
middle finger), then the conductor experiences a force in the direction (represented by the
thumb) mutually perpendicular to both the direction of field and the current in the conductor.
Slide – 14EDC – Unit I

16. Applications of DC motors:
• DC Shunt motor: (i) Used for driving centrifugal pumps; (ii) Light machine tools; (iii) Wood
working macines.
• DC Series motor: (i) Used in electric trains; (ii) Cranes, hoists, fans, blowers, conveyors,
lifts etc.
• DC Compound motor: (i) Punching machines; (ii) Driving heavy machine tools.
Slide – 15EDC – Unit I
Starters in DC Motor
Necessity of starters:
• At the instant of starting DC motors, the back emf Eb = 0. So, if normal voltage is applied to the
armature the starting current would be high.
• This high current produces high stress and heat and damage the winding. Therefore to limit the starting
current, reduced voltage should be applied to the motor at the time of starting through series of
resistances. This is achieved by starters.

17. THREE POINT STARTER
• 3 Point Starter is a device whose main function is starting and maintaining the speed of the DC
shunt motor. Mainly there are three main points or terminals in 3 point starter of DC motor.
They are follows,
(i) L – Line terminal-connected to positive supply.
(ii) A – Armature terminal-connected to armature windings.
(iii)F – Field terminal-connected to field terminal windings.
• It consists of a graded resistance R to limit the starting current. The handle H is kept in the OFF
position by a spring S.
Slide – 16EDC – Unit I

18. • The handle H is manually moved, for starting the motor and when it makes contact with
resistance stud 1 the motor is said to be in the START position.
• In this initial start position, the field winding
receives full supply voltage, and the armature
current is limited by the resistance value.
Working:
• The starter handle is now moved from stud to
stud, and this builds up the speed of the motor
until it reaches the RUN position. The Studs are
the contact point of the resistance
Slide – 17EDC – Unit I

19. • The handle H is held in RUN position by an electromagnet energized by a no volt trip coil
(NVC). This no volt trip coil is connected in series with the field winding of the motor.
• In the event of switching OFF, or when the supply voltage falls below a predetermined value, or
the complete failure of supply while the motor is running, NVC is de-energized.
• The handle is released and pulled back to the OFF position by the action of the spring. The
current to the motor is cut off, and the motor is not restarted without a resistance R in the
armature circuit. The no voltage coil also provides protection against an open circuit in the field
windings.
No Volt Coil (NVC): The No Voltage Coil (NVC) is called No-Volt or UNDERVOLTAGE
protection of the motor. Without this protection, the supply voltage might be restored with
the handle in the RUN position. The full line voltage is directly applied to the armature.
Slide – 18EDC – Unit I

20. Over Load Release: The Over Load Release (OLR) provide the overload protection of the motor.
The overload coil is made up of a small electromagnet, which carries the armature current.
• The magnetic pull of the Overload trip release is insufficient to attract the strip P, for the normal
values of the armature current When the motor is overloaded, that is the armature current
exceeds the normal rated value, P is attracted by the electromagnet of the OLR and closes the
contact aa thus, the No Voltage Coil is short-circuited
Disadvantages of 3 point starter:
• Serious drawbacks for the motors with large variation of speed.
• The field current may become very low because of addition of high resistance.
• These drawbacks are overcome by 4 point starter.
Slide – 19EDC – Unit I

21. FOUR POINT STARTER
• A 4 Point Starter is almost similar in functional characteristics like 3 Point Starter. In the absence of back
EMF, the 4 Point Starter acts as a limiting current device while starting of the DC motor. 4 Point Starter
also acts a protecting device.
• The basic difference in 4 Point Starter as compared to 3 Point Starter is that, in this a holding coil is
removed from the shunt field circuit. This coil after removing is connected across the line in series with a
current limiting resistance R.
Construction:
• The schematic diagram of 4 point starter is shown below. This forms three parallel circuits.
(i) Armature, starting resistance and the shunt field winding.
(ii) A variable resistance and shunt field winding.
(iii) Holding coil and the current limiting resistance.
Slide – 20EDC – Unit I

22. • Four point starter uses four terminals for speeding the motor.
(i) Armature terminal; (ii) Field terminal (F and N); (iii) Line terminal.
• Currently, regular push-button starters are also utilized. In these starters, the ON-switch is
pushed to link the current limiting beginning resistors in series through the armature circuit,
then the complete line voltage is obtainable to the circuit. The beginning resistor is slowly
detached with an automatic controlling plan.
Slide – 21EDC – Unit I

23. • The armature circuit is detached once the OFF switch is pressed. The usual starter circuits have
been designed with time delay relays and electromagnetic contactors.
No Volt Coil (NVC): The No Voltage Coil (NVC) is called No-Volt or UNDERVOLTAGE
protection of the motor. Without this protection, the supply voltage might be restored with the
handle in the RUN position. The full line voltage is directly applied to the armature.
Over Load Release: The Over Load Release (OLR) provide the overload protection of the motor.
The overload coil is made up of a small electromagnet, which carries the armature current.
• The magnetic pull of the Overload trip release is insufficient to attract the strip P, for the normal
values of the armature current When the motor is overloaded, that is the armature current
exceeds the normal rated value, P is attracted by the electromagnet of the OLR and closes the
contact aa thus, the No Voltage Coil is short-circuited
Slide – 22EDC – Unit I

24. Disadvantages:
• The only drawback or limitation of the four-point starter is that it cannot control the speed of the
high current in the motor.
• Thus, the motor’s speed enhances thoroughly, which is unsafe and therefore safety is not
feasible. This unexpected rise in the motor’s speed is known as “high-speed act of the motor”.
Slide – 23EDC – Unit I