presentation on dc and ac electrical motors

1. DC & AC MOTORS

2. MOTOR
 an electric motor converts electrical energy into mechanical energy
 electric motors operate through the interaction between the motor’s
magnetic field and electric current in a wire winding to generate force in
the form of rotation of shaft.
 Electric motor can be powered by direct current(D.C.) sources or by
alternating current(A.C.) sources

3. DC MOTOR
 converts direct current electrical energy into mechanical energy.

4. PARTS OF DC MOTORS
 Yoke:
1. acts as the outer support of a DC motor. 2. provides mechanical support for the poles.
 Poles:
1.pole of a dc motor is an electromagnet. 2. The field winding is wound over the poles.
3. Poles produces magnetic flux when the filed winding is excited.
 Field winding:
1.The coils wound around the pole are called field coils and they are connected in series with each other to form field winding.
2.When current passing through the field winding, magnetic flux produced in the air gap between pole and armature.
 Armature:
1.Armature is a cylindrical drum mounted on shaft in which number of slots are provided.
2.Armature conductors are placed in these slots. 3. armature conductors are interconnected to create armature winding.
 Brushes:
1.Commutator is rotating. So it is not possible to connect the load directly to it.
2.Hence current is conducted from the armature to the external load by the carbon brushes which are held against the surface of
commutator by springs.
 Commutator:
1.A commutator is a cylindrical drum mounted on the shaft along with the armature core.
2.collects the current from the armature conductors and passed it to the external load via brushes.

5. TYPES OF DC MOTORS
 Depending on the way of connecting the armature and field windings of D.C. motors are
classified as follows:
DC Motor
DC series
motor
Separately
excited motor
Compound
motor
Shunt motor
Short shunt
compound
Long shunt
compound

6. APPLICATION OF DC MOTORS
 Series motor- (high starting torque, variation in speed)
traction system, cranes, air compressors
 Shunt motor- (constant speed)
Fans, lathe machine, conveyors, lifts,
 Compound motors- (higher starting torque, constant speed)
Conveyors, presses, rolling mills
 Separately excited motor- (speed variations)
Rolling mills paper machines

7. AC MOTOR
 An AC motor Is an electric motor driven by an alternating current (AC).
 AC motor commonly consists of two basic parts, an outside stator having coils
supplied with alternating current to produce a rotating magnetic field, and an
inside rotor attached to the output shaft producing a second rotating magnetic
field.
 The two main types of AC motors are induction motors and synchronous motors.
 Induction motor uses electromagnetic induction from the magnetic field of the
stator winding to produce an electric current in the rotor and hence Torque.
 synchronous motor, the rotation of the rotor is synchronized with the frequency
of the supply current and the speed remains constant under varying loads, so is
ideal for driving equipment at a constant speed and are used in high precision
positioning devices

8. CONSTRUCTION OF AC INDUCTION MOTOR
 three phase motors consist of a stator and a rotor and between which no electrical
connection exists.
 These stator and rotors are constructed with the use of high-magnetic core materials
in order to reduce hysteresis and eddy current losses.
 Stator frame provides necessary mechanical protection and support for stator
laminated core, windings and other arrangements for ventilation.
 The rotor of three phase AC induction motor is different for the slip-ring and squirrel-
cage induction motors.

9. WORKING OF INDUCTION MOTOR
 When the motor is excited with three-phase supply, three-phase stator winding produce a rotating
magnetic field with 120 displacements at constant magnitude which rotates at synchronous speed. This
changing magnetic field cuts the rotor conductors and induces a current in them according to the
principle of Faraday’s laws of electromagnetic induction. As these rotor conductors are shorted, the
current starts to flow through these conductors.
 In the presence of magnetic field of stator, rotor conductors are placed, and therefore, according to the
Lorenz force principle, a mechanical force acts on the rotor conductor. Thus, all the rotor conductors
force, i.e., the sum of the mechanical forces produces torque in the rotor which tends to move it in the
same direction of rotating magnetic field.
 the rotor starts rotating in the same direction of the stator rotating magnetic field. If the rotor speed more
than stator speed, then no current will induce in the rotor because the reason for rotor rotation is the
relative speed of the rotor and stator magnetic fields.
 This stator and the rotor fields difference is called as slip.
 the relative speed between the stator field and the rotor conductors causes to rotate the rotor in a
particular direction. Hence, for producing the rotation, the rotor speed Nr must always be less than the
stator field speed Ns, and the difference between these two parameters depends on the load on the
motor.

10. CONSTRUCTION OF A SYNCHRONOUS MOTOR
 It consists of a stator and a rotor.
 The stator core is constructed with thin silicon lamination and insulated by a surface coating,
to minimize the eddy current and hysteresis losses.
 The stator has axial slots inside, in which three phase stator winding is placed.
 The stator is wound with a three phase winding for a specific number of poles equal to the
rotor poles.
 The rotor in synchronous motors is mostly of salient pole type. DC supply is given to the
rotor winding via slip-rings.
 The direct current excites the rotor winding and creates electromagnetic poles.

11. WORKING OF SYNCHRONOUS MOTOR
 The stator is wound for the similar number of poles as that of rotor, and fed with three phase AC
supply.
 The 3 phase AC supply produces rotating magnetic field in stator. The rotor winding is fed with DC
supply which magnetizes the rotor. Consider a two pole synchronous machine.
 the stator poles are revolving with synchronous speed . If the rotor position is such that, N pole of
the rotor is near the N pole of the stator ,then the poles of the stator and rotor will repel each other,
and the torque produced will be anticlockwise.
 The stator poles are rotating with synchronous speed, and they rotate around very fast and
interchange their position. But at this very soon, rotor can not rotate with the same angle (due to
inertia). In this case, poles of the stator will attract the poles of rotor, and the torque produced will
be clockwise.
 the rotor will undergo to a rapidly reversing torque, and the motor will not start.
 But, if the rotor is rotated up to the synchronous speed of the stator by means of an external force,
and the rotor field is excited near the synchronous speed, the poles of stator will keep attracting the
opposite poles of the rotor Now, the rotor will undergo unidirectional torque. The opposite poles of
the stator and rotor will get locked with each other, and the rotor will rotate at the synchronous
speed.

12. APPLICATION OF AC MOTORS
 Induction motor- (load capacity)
Pumps, conveyor, lifting gear etc.
 Synchronous motor- (speed remains constant under varying loads)
Instrumentation, machines, process control etc.