Download General Information on electro-motors An electric motor converts

General Information on electro-motors
An electric motor converts electricity into mechanical motion. The opposite act, i.e. the
conversion of mechanical motion into electricity, is performed by the generator. These two
devices are similar to each other, except in the performance. Most electro-motors work by
electromagnetism, but there are motors that work based on other phenomena such as
electrostatic forces and the piezoelectric effect. The general idea is that when an electric
current- carrier substance is placed in a magnetic field, a force is applied to it by the magnetic
field. In a cylindrical motor, the rotor rotates due to the torque resulted from a force applied
to the rotor in a certain distance from the rotor axis.
Most electro-motors are rotary, but there are also linear motors. In a rotary motor, the moving
part is called rotor (which is usually inside the engine) and the stationary part is called stator.
The rotor consists of the electric magnets which are wire-wound on a frame. Although this
frame is often called the armature, this term is often incorrectly used. In fact, the armature is
that part of the motor to which the input voltage is applied or that part of generator where the
output voltage is generated. Due to the design of these machines, each of the parts of rotor or
stator can be an armature. Some kits are used in the schools to build very simple motors.
Different Types of electro-motors
DC motors
One of the first rotary motors was made by Michael Faraday in 1821. It was composed of a
free- hanging wire dipped into a container of mercury. A permanent magnet was placed in the
middle of the dish. When a current was passed through the wire, the wire around magnet
came into circulation and showed that the current leads to the increased circle magnetic field
around the wire. This motor is often shown in the physics classes of schools, but sometimes,
instead of toxic substance of mercury, the salt water is used.
The classic DC motor has an electromagnet armature. A rotating switch called commutator
reverses the direction of the electric current twice every cycle, to be flowed in the armature.
Electric magnets absorb and excrete the permanent magnet outside the motor. The speed of
DC motors depends on the voltage, the current passing through the motor windings and
motor load or braking torque. The speed of the motor is dependent on the voltage and its
torque is dependent on the current. The speed usually is controlled by varying voltage or the
passing current using pulses (a sort of keys that change the coil state) in the motor winding,
or through having a varying voltage source. Because this kind of motors can produce high
torque in low speeds; they are usually used in traction applications such as in locomotives.
However, there are several limitations in classic designing. Many of these limitations are due
to the necessity of using brushes for being connected to the commutator. The commutators’
and brushes’ wear causes the friction; and the higher the motor speed, the stronger brushes’
pressure should be to establish a proper connection. Not only this friction leads to the motor
noise, but also, it is a higher limitation on the speed. This means that the brushes will be
eventually destroyed and there will be a need to replace them.
Incomplete electric connection also causes electric noises in the connecting circuit.
The problems are removed through displacing the inside of motor with its outside. A brusherfree design can be achieved by placing permanent magnets inside the motor and windings
outside the motor.
Wire-wound field motors
Permanent magnets located in the external stators of a DC motor can be replaced with electric
magnets. With the current change (the electromagnet coil) the speed/torque of the motor is
changed. If the field winding is placed in series with the armature winding, we will have a
motor with high torque and low speed and if is placed in parallel, we will have a high-speed
motor with low torque. We can reduce the magnetic field currents in order to get even more
speed but less torque for the same amount. This technique is ideal for electric traction and
many other similar applications. Application of this technique can lead to removal of the
equipment of a mechanical variable gearbox.
Universal motors
Universal motors are one of field-winded DC motors. The name of these motors has derived
from the fact that these motors can be used with both DC and AC currents, although they
often feed with alternative currents. The working principle of these motors is when a fieldwinded DC motor is connected to the alternating current, the current will be changed
simultaneously both in the field winding and armature winding (and the resulting magnetic
fields); and hence the generated mechanical force will be always unchanged. In practice, the
motor must be specifically designed to be compatible with AC current (Impedance/reactance
must also be considered). The final motor is generally less efficient than a pure equivalent
DC motor. The advantage of these motors is that AC power can be used in the motors that
have some characteristics of DC motors. Specifically, these motors have a high startup torque
and a very compact design at high speeds. The negative aspect of these motors is their
maintenance and reliability problems, due to the presence of the commutator. As a result,
these motors are rarely seen in the industries. But the most common AC motors are used in
devices such as mixers and power tools that are sometimes applied.
AC Motors
Single-phase AC motors
Shaded-pole synchronous motor is the most common single-phase motor which is often used
in devices that require low torque such as electric fans, microwave ovens and other small
appliances. Induction motor is another type of single-phase AC motors which is often used in
the large appliances such as washing machines and clothes dryers. Generally, they can create
a larger running torque using a running winding with a running capacitor and a centrifugal
switch. During the startup, running capacitor and winding are connected to a power source
through a bunch of contacts under spring pressure on a centrifugal rotary switch. The
capacitor helps to increase motor running torque. When the motor reaches nominal speed, the
centrifugal switch and the bunch of contacts are activated and disconnect the series running
capacitor and winding from the power source. At this moment, motor only operates with the
main winding.
Three-phase AC motors
For applications requiring higher power, three-phase induction motors (AC or multiple
phases) are used. These motors use the difference between the power supply phases of
several electric phases to create a rotating electromagnetic field inside them.
The rotor often includes a number of copper conductors placed in the steel. Through
electromagnetic induction, the rotating magnetic field induces currents in these conductors
resulting in a balanced magnetic field and causes the motor to move in the direction of the
rotation of magnetic field.
This type of motor is known as induction motor. In order for the motor to get moving, it must
always rotate with a speed less than the frequency of the power applied to the motor. Because
otherwise the magnetic field of balancers of the rotor will not be created. The use of this type
motors in traction applications, such as locomotives, which are known as Asynchronous
Traction Motors, are increasing day by day. Separation field is applied to the rotor windings
to create a constant magnetic field which is in the synchronous motors. Synchronous motor
rotates simultaneously with rotating magnetic field induced by three-phase AC power. The
synchronous motors can also be used as the current generator.
AC motors' speed depends first on the frequency of feeding; and the amount of slip or
difference in the rotation speed between the rotor and stator determines the torque generated
by the motor. The speed changes in these types of motors can be possible by a bunch of
windings or poles in the motor, so that through switching them on or off, the speed of the
rotating magnetic field would be varied. However, with advances in power electronics, a
more uniformed control on the motors’ speed can be applied by changing the frequency of
power supply.
Stepper motors
A Stepper motor is another type of electro-motors, in which an internal rotor containing
permanent magnets is controlled by a set of external magnets that are switched on and off
electronically. A stepper motor is a combination of a DC electric motor and a solenoid.
Simple stepper motors are put in certain positional states by a part of the gear system. But
relatively controlled stepper motors can rotate very slowly. Computer controlled stepper
motors are one of the forms of position adjustment systems, especially when they are a part
of the digital system with control servo systems.
Linear motors
A linear motor is essentially an electric motor came out of the rotating mode to generate a
linear force, by creating an electromagnetic field traveling along itself, instead of generating a
rotating
torque.
Linear motors are often induction or stepper motors. You can find a linear motor in a high
speed Maglev, a train that flies off the ground. AC alternative current motor is an electric
motor fed with alternative current which converts electric power to the rotary or linear rotary
mechanical power. An AC alternative current motor consists of two main parts:
* Stator: an external and generally constant core that produces a rotary field using alternative
current.
* Rotor: an internal and removable core that is connected to the output shaft and produces the
torque consistent with the rotating field created by the stator. AC motors are classified into
two categories based on the type of rotor used in the motors.
* Synchronous motor in which the rotor rotates at exactly the speed of the rotating field. In
these motors, the electric field of the rotor is supplied by an external source.
* Asynchronous or induction motor, in which the electric field of rotor is produced through
the induction of the stator field.