AC AND DC MOTORS

2. Fleming’s left hand
rule

3. Fleming’s left hand
ruleUsed to determine the direction of force acting
on a current carrying conductor placed in a
magnetic field .
The middle finger , the fore finger and thumb of
the left hand are kept at right angles to one
another .
The middle finger represent the direction
of current
The fore finger represent the direction of
magnetic field
The thumb will indicate the direction of
force acting on the conductor .
This rule is used in motors.

4.  Conventional DC generators are being replaced by the
solid state rectifiers where ac supply is available.
 The same is not true for dc motors because of
 Constant mechanical power output or constant torque
 Rapid acceleration or deceleration
 Responsiveness to feedback signals
 1W to 10,000 hp
 Applications – in electric vehicles to extend their range
and reduce vehicle weight, in steel and aluminum
rolling mills, traction motors, electric trains, overhead
cranes, control devices, etc.

5.  Commutator along with the armature
on the rotor
 Salient-pole on the stator and, except
for a few smaller machines,
commutating poles between the main
poles.
 Field windings (as many as 4):
 Two fields that act in a corrective
capacity to combact the
detrimental effects of armature
reaction, called the commutating
(compole or interpole) and
compensating windings, which are
connected in series with the
armature.
 Two normal exciting field
windings, the shunt and series
windings

6. Sectional view of a DC
machine

7. Field
winding

8. Rotor and rotor
winding

9. Working principle of DC
motor

10. Working principle of DC
motor

11. Force in DC motor

12. Field system
It is for uniform magnetic field within
which the armature rotates.
Electromagnets are preferred in
comparison with permanent magnets
They are cheap , smaller in size ,
produce greater magnetic effect and
Field strength can be varied

13. Field system consists of the
following parts
Yoke
Pole cores
Pole shoes
Field coils

14. Armature core
The armature core is cylindrical
High permeability silicon steel
stampings
Impregnated
Lamination is to reduce the eddy
current loss

15. Commutator
Connect with external circuit
Converts ac into unidirectional current
Cylindrical in shape
Made of wedge shaped copper segments
Segments are insulated from each other
Each commutator segment is connected to
armature conductors by means of a cu strip called
riser.
No of segments equal to no of coils

16. Carbon brush
Carbon brushes are used in DC machines
because they are soft materials
It does not generate spikes when they contact
commutator
To deliver the current thro armature
Carbon is used for brushes because it has
negative temperature coefficient of resistance
Self lubricating , takes its shape , improving
area of contact

17. Brush rock and holder

18. Series wound Motor or series motor
Shunt wound Motor or shunt motor
Compound wound motor

19. dcmotor 19
Shunt Field Coil Armature
RA
Shunt Excited DC Machine

20. dcmotor 20
Series Field Coil
Armature
RA
Series Excited DC Machine

21. dcmotor 21
Shunt Field Coil Armature
RA
Compound Excited DC Machine
Series Field Coil
•If the shunt and series field aid each other it is called a cumulative
excited machine
•If the shunt and series field oppose each other it is called a differen
excited machine

24. •The induced emf in the rotating armature
conductors always acts in the opposite
direction of the supply voltage .
• According to the Lenz’s law, the direction
of the induced emf is always so as to
oppose the cause producing it .
• In a DC motor , the supply voltage is the
cause and hence this induced emf
opposes the supply voltage.

25. BACK EMF equation
Let,
Ø= flux per pole in weber
Z = Total number of conductor
P = Number of poles
A = Number of parallel paths
N =armature speed in rpm
Eg = emf generated in any on of the
parallel path

26. EMF equation
Flux cut by 1 conductor
in 1 revolution = P * φ
Flux cut by 1 conductor in
1 sec = P φ N /60
Avg emf generated in 1
conductor = PφN/60
Number of conductors in
each parallel path = Z /A
Back emf = PφNZ/60A

29. Series Motor:
Cranes
Hoists , Elevators
Trolleys
Conveyors
Electric locomotives

30. Shunt Motor:
Blowers and fans
Centrifugal and reciprocating pumps
Lathe machines
Machine tools
Milling machines
Drilling machines

31. Cumulative compound Motor:
Rolling mills
Punches
Shears
Heavy planers
Elevators