A DC motor converts electrical energy into mechanical energy through electromagnetic induction. When a current-carrying conductor is placed in a magnetic field, it experiences a mechanical force. In a DC motor, this force causes the armature conductors to rotate, producing torque. The motor's magnetic field is produced by a field winding and direct current is supplied by an external DC power source. A three-point starter is used to gradually reduce armature current and limit sparking during startup as motor speed increases and back EMF rises.

1.  A motor is a device which converts an electrical energy
into mechanical energy.
The energy conversion process is exactly opposite to that
involved in a dc generator.
DC motor input electrical energy is supplied by DC supply.

2.  When a current carrying conductor is placed in a magnetic field it
experiences a mechanical force.
 Practical DC motor field winding produces a required magnetic field
while armature conductors play a role of current carrying conductors
and hence armature conductors experiences a force.
 The individual force experienced by the conductors acts as a
twisting or turning force on the armature which is called a torque.
 The magnetic field is produced by a permanent magnet but in a
practical DC motor it is produced by the field winding when it carries
a current.

3.  The magnitude of the force experienced by the conductor in a motor
is given by
 F=BLI Newton (N)
 B= Flux density due to the flux produced by the field winding
 L= Length of the conductor
 I = Magnitude of the current passing through the conductor

5.  The rule states that ‘out stretch the three fingers of the left hand’
 Fore finger, middle finger, and thumb such that they are mutually
perpendicular to each other.
 Fore finger in the direction of magnetic field.
 Middle finger in the direction of the current.
 Thumb gives the direction of the force experienced by the conductor

6.  after motoring action there exists a generating action. there is an
induced e.m.f in the rotating armature conductors.
 according to faradays law of electromagnetic induction this induced
e.m.f in the armature always acts in the opposite direction of the
supply voltage, this is according to the lenz’s law.
 in a dc motor the supply voltage is the cause and hence this induced
emf opposes the supply voltage.
 this tries to set up a current through the armature which is in the
opposite direction to that, which supply voltage, is forcing through the
conductor.

8.  D.c motor supply voltage ‘v’ has to overcome back e.m.f
 Eb =
 Eb which is opposing ‘v’ and also various drops as armature
resistance drop Ia Ra bresh drop.
v=Eb +IaRa+bresh drop
 Bresh drop is generalized voltage eq is v=E+IaRa
 The back e.m.f is aiways less than supply voltage.
 Voltage equation we can write :
Ia=v-Eb
 Ra


10.  Pm = Gross mechanical power developed =
Eb Ia
 = VIa - Ia
2Ra
 For maximum Pm, dPm/ dIa = 0
 0 = V - 2IaRa
 Ia = V/2Ra
 IaRa = V/2
 Substituting in voltage equation,
 V = Eb + IaRa = Eb + (V/2)
 Eb = V/2

12.  Self excited dc motor
◦ Series dc motor
◦ Shunt dc motor
◦ Compound dc motor
 Separately excited dc motor

13.  Field winding is connected across the armature winding and
the combination is connected across the supply.
 Ia0 =V-Eb0/Ra

14.  In this type of motor series field winding is
connected in series with the armature and supply.
 It is made of small no. of turns having large cross
section area.

15. Compound motor consists of part the field winding, connected
in series and part of the field winding connected in parallel
with armature.
 Long shunt compound motor:
In this connected across the armature and series field winding.
 Short shunt compound motor:
Shunt field connected purely in parallel with armature and
series field is connected in series with the combination

17.  Torque armature current characteristics:
 Speed and armature current characteristics:
 Speed and torque characteristics:

20.  Shunt speed is the
fairly constant and
medium starting
torque
 Series motor high
starting torque and
no-load condition is
dangerous variable
speed.
 Blowers and fans
 Lathe machine
 Machine tools
 Milling machines
 Cranes,hoists,elevators
 Trolleys, conveyors

21.  Cumulative compound
motor, high starting
torque no-load
condition is allowed
 Differential compound
motor speed increases
as load increases
 Rolling
mill,shears,heavy
planers, elevators.
 Not suitable for any
practical application

22.  The machine operating as a generator is driven by
some external driving force and dc output is
obtained.
 Generation operation signifies that the machine is
actually operating as a generator whereas the term
generator action signifies that a voltage is
generated by E.M.I
 Vice versa.

23.  Dc generator for the same direction of current in
the armature winding and for the same polarity of
the brushes the effect of the armature reaction in
dc motors will be same.
 Effect the armature reaction was to reduce the
generated voltage due to reduction.
 The backward movement of the brushes in the
motor is accompanied by demagnetizing and cross
magnetising.

25.  This concept of speed control or adjustment
should not be taken to include the natural change
in speed.
 Which occurs due to change in the load on the
drive shaft.
 Different methods of speed control.
1. Base speed
2. Speed regulation
3. Speed range
4. Constant power drive
5. Constant torque drive

26. i. Flux control method:-
ii. Armature voltage control method:-
iii. Voltage control method:-
 The flux can be controlled by adding a rheostat in
series with the shunt field winding

27.  An external resistance is inserted in series with the
armature circuit.
 Speed is reduced is directly proportional to the
voltage drop.

29. Multiple voltage control:
 This method permanent connected to a fixed voltage
supply.
 Control of a motor by two different working voltages
which can be applied to it with the help of
switchgears.
Ward-leonard system:
 If no supply is available, the three phase motor can be
replaced by some prime-mover.
 Current Ia is maintained equal to its rated current
during its speed control.

30.  Speed above base speed are obtained by decreasing the
motor field flux with constant generator voltage.
 The armature current Ia is kept equal to its rated value.
under these conditions, EaIa remains constant and the
electromagnetic torque proportional to ɸIa.

31. I. Flux control method
II. Armature direct method
III. Tapped field control
IV. Series-parallel connection of field coils
 The series field winding shunted by a variable
resistance known as a field diverter.

32.  used for the motor which require constant load
torque and armature of the motor is shunted with
an external variable resistance.
 Flux increases then speed of the motor reduces.

33.  Flux changes is achieved by changing the number
of turns of the field winding.
 The field winding is provided with the taps
 This method is often used in electric.

34.  Same torque if the field coils arranged in parallel
mmf produced by the coil changes, hence flux
produced also changes, hence field can be
controlled
 Parallel grouping this method is generally used
incase of fan motors.

35.  In this method, a variable Rx is inserted in
series with the motor circuit.
 so that voltage drop across their IaRx arm,the
speed reduces.

36.  Series motor is excited by the voltage
obtained by a series generator.
 The voltage obtained from the controlled by
a field diverter resistant connected across
series field winding of the generator.

37.  Motor is at rest the speed of the motor is zero so
that back emf zero.
Result:
1.Heavy sparking at the commutator.
2.Damage to the arm winding.
3.Damage to the rotating parts of the motor, at large
starting torque quick acceleration

38.  Starters is to limit the current in the
armature circuit during starting accelerating
period
 Three or two standards types of starters for
shunt and compound motors
1.Three point starter
2.Four point starter

39.  It consists of a series starting resistance divided
into several sections and contact points.
 Starting a motor, the starter armature is then
turned clockwise to the first stud and brass arc.
 Spiral spring ‘s’ by the force of attraction between
holding coil magnet and soft iron attached to the
starter

40.  When the starter arm reaches “ON” position, the
resistance is completely out-off and motor starts
running at normal speed.
 Starter arm may reach the “OFF” position automatically
as soon as the supply is cut-off.
 Advantage is connecting the no-volt release in series
with the shunt field winding is that it prevents the
motor from running.
Overload reclose coil:
 This coil is connected in series to the motor so carrier
full load current. when the motor is overloaded which
draws the heavy current.
 When the arm is pulled upwards the triangular piece
touches to two points which are connected to the two
ends of NVC.

41.  The starter used for a dc shunt motor have a
n-section of resistances.
 At the instant the starting arm is brought on
stud1,the speed is zero and back emf also
zero.
I1=V/R1------------------1
I2= (V-Eb)/R1------------
---2
I2=(V-Eb)/R2--------------3