Electric Motor
Definition: Electric motor is the electro-mechanical machine which converts the electrical
energy into mechanical energy. In other words, the devices which produce rotational force is
known as the motor. The working of the electric motor mainly depends on the interaction of
magnetic and electric field
Working Principle:-
They operate using principles of electromagnetism, which shows that a force is applied when
an electric current is present in a magnetic field. This force creates a torque on a loop of wire
present in the magnetic field, which causes the motor to spin and perform useful work. Motors
are used in a wide range of applications, such as fans, power tools, appliances, electric vehicles,
and hybrid cars.
Types of motors:-
○Ac Motors:-
The motor that converts the alternating current into mechanical power by using an
electromagnetic induction phenomenon is called an AC motor. This motor is driven by an
alternating current. The stator and the rotor are the two most important parts of the AC motors.
The stator is the stationary part of the motor, and the rotor is the rotating part of the motor. The
AC motor may be single phase or three phase.
Applications:-The three phase AC motors are mostly applied in the industry for bulk power
conversion from electrical to mechanical. For small power conversion, the single phase AC
motors are mostly used. The single phase AC motor is nearly small in size, and it provides a
variety of services in the home, office, business concerns, factories, etc. Almost all the domestic
appliances such as refrigerators, fans, washing machine, hair dryers, mixers, etc., use single
phase AC motor.
Types of ac Motors:-
Synchronous Motor
The motor that converts the AC electrical power into mechanical power and is operated only at
the synchronous speed is known as a synchronous motor.
Working Principle of a Synchronous Motor:-
When supply is given to synchronous motor, a revolving field is set up. This field tries to drag
the rotor with it, but could not do so because of rotor inertia. Hence, no starting torque is
produced. Thus, inherently synchronous motor is not a self-starting the motor.
Main features of synchronous machine:
A synchronous machine is an ac machine whose speed under steady-state conditions is
proportional to the frequency of the current in its armature.
▪︎Armature winding: on the stator, alternating current.
▪︎Field winding: on the rotor, dc power supplied to built a rotating magnetic field.
▪︎Cylindrical rotor: for two- and four-pole turbine generators.
▪︎Salient-pole rotor: for multi-polar, slow-speed, hydroelectric generators and for most
synchronous motors.
The rotor, along with the magnetic field created by the dc field current on the rotor, rotates at the
same speed as, or in synchronism with, the rotating magnetic field produced by the armature
currents, and a steady torque results.
Applications :-
Synchronous motors find applications in all industrial applications where constant speed
is necessary.
▪︎Improving the power factor as Synchronous condensers.
▪︎Electrical power plants almost always use synchronous generators because it is
important to keep the frequency constant at which the generator is connected.
▪︎Low power applications include positioning machines, where high precision is required,
and robot actuators.
▪︎Mains synchronous motors are used for electric clocks.
Induction Motor or Asynchronous Motor:-
The machine which converts the AC electric power into mechanical power by using an
electromagnetic induction phenomenon in called an induction motor. The induction motor is
mainly classified into two types., i.e., the single phase induction motor and the three phase
induction motors.
Working Principle of an Induction Motor:-
In an induction machine the armature winding serve as both the armature winding and field
winding. When the stator windings are connected to an AC supply flux is produced in the air
gap. The flux rotates at a fixed speed called synchronous speed. This rotating flux induces
voltages in the stator and rotor winding.
If the rotor circuit is closed, the current flows through the rotor winding and react with the
rotating flux and a torque is produced. In the steady state, the rotor rotates at speed very close to
synchronous speed.
Main parts:- The A.C. Induction Motor has three main parts, rotor, stator and enclosure. The
stator and rotor do the work and the enclosure protects the rotor and stator.
Applications:-
Wound rotor motors are suitable for loads requiring high starting torque and where a
lower starting current is required.
The Wound rotor induction motors are also used for loads having high inertia, which
results in higher energy losses.
Used for the loads which require a gradual build up of torque.
Used for the loads that require speed control.
The wound rotor induction motors are used in conveyors, cranes, pumps, elevators and
compressors.
The maximum torque is above 200 percent of the full load value while the full
load slip may be as low as 3 percent. The efficiency is about 90 %.
Single phase induction motor:-
A single phase induction motor consists of a single phase winding on the stator and a cage
winding on the rotor. When a 1 phase supply is connected to the stator winding, a pulsating
magnetic field is produced. In the pulsating field, the rotor does not rotate due to inertia.
Therefore a single phase induction motor is not self-starting and requires some particular
starting means. Two theories have been suggested to find the performance of a single phase
induction motor.
1. Double revolving field theory.
2. Cross-field theory.
Three phase induction motor:-
A three phase induction motor runs on a three phase AC supply. 3 phase induction
motors are extensively used for various industrial applications because of their following
advantages -
1. They have very simple and rugged (almost unbreakable) construction,
they are very reliable and having low cost, they have high efficiency and good
power factor
2. minimum maintenance required,
3. 3 phase induction motor is self starting hence extra starting motor or any special
starting arrangement is not required.
○dc Motors:-
DC motor is one type of motor that uses the DC current to convert electrical energy into
mechanical energy. When the electric current passes through a coil in a magnetic field,
a magnetic force will be generated, which produces a torque in the DC motor.
dc Motors parts:-
Stator :
The stator consists of either a permanent magnet or electromagnetic windings, and
generates a stationary magnetic field around the rotor which occupies the central part
of the motor.
Rotor ( Armature ) :
The rotor is made up of one or more electric windings around armature arms. These
electric windings generate a magnetic field when energized by the external current. The
magnetic poles thus generated by this rotor field are attracted to the opposite poles
generated by the stator field and repelled by the similar poles, which causes the rotor
to rotate.
Commutator :
The DC motor doesn’t use an external current switching device, instead it uses a
mechanical connector called the commutator which is a segmented sleeve usually made
of copper, mounted on the rotating shaft. The current +/- is supplied to this
commutator segments with the help of brushes.
Brushes :
As the motor turns the brushes slide over the commutator segments hence creating the
variable magnetic field in different arms through the commutator segments attached to
the windings. Hence a dynamic magnetic field is generated in the motor when a voltage
is applied across the brushes.
Types of dc Motors:-
□Stepper dc Motors:-
Stepper motors use an internal rotor, electronically manipulated by external magnets.
The rotor can be made with permanent magnets or a soft metal. As windings are
energized, the rotor teeth align with the magnetic field. This allows them to move from
point to point in fixed increments.
Before work begins on any new system, think carefully about the competing properties
of the different motors. The selection of the right motor gets any project off to a better
start.
□Dc brushless Motors:-
DC brushless motors were first developed to achieve higher performance in a smaller
space than DC brushed motors, and they are smaller than comparable AC models. An
embedded controller is used to facilitate operation in the absence of a slip ring or
commutator.
□Dc brushed Motors:-
In a DC brushed motor, brush orientation on the stator determines current flow. In
some models, the brush’s orientation relative to the rotor bar segments is decisive
instead. The commutator is especially important in any DC brushed motor design.
□parmanent magnet Dc Motors:-
The permanent magnet motor uses a permanent magnet to create field flux. This type
of DC motor provides great starting torque and has good speed regulation, but torque
is limited so they are typically found on low horsepower applications.
□ Series dc Motors:-
In a series DC motor, the field is wound with a few turns of a large wire carrying the full
armature current. Typically, series DC motors create a large amount of starting torque,
but cannot regulate speed and can even be damaged by running with no load. These
limitations mean that they are not a good option for variable speed drive applications .
□ shunt dc Motors:-
In shunt DC motors the field is connected in parallel (shunt) with the armature
windings. These motors offer great speed regulation due to the fact that the shunt field
can be excited separately from the armature windings, which also offers simplified
reversing controls.
□ compound dc Motors:-
Compound DC motors, like shunt DC motors, have a separately excited shunt field.
Compound DC motors have good starting torque but may experience control problems
in variable speed drive applications.
Applications of dc Motors:-
Separately excited DC Generators are used in laboratories for testing as they
have a wide range of voltage output.
Used as a supply source of DC motors.
DC shunt wound generators are used for lighting purposes.
Used to charge the battery.
Providing excitation to the alternators.
