AN OVERVIEW OF ELECTRIC MACHINES
Tomáš Prachař
Fakulta Elektrotechnická, Západočeská univerzita v Plzni
Alešova 817, Klatovy, e-mail: Prachar.Tomas@gmail.com
Abstract:
This presentation describes electric machines. The subject of it is to show average educated
people this problematic and give them some basic information about it. Presentation contains
few basic things about transformers, DC machines, AC synchronous and asynchronous
machines. In each type of machine, there is basic principal, main usage and another
important information. There is also a picture to show how does it work. It could be use as at
high school focused on technical education, as at the university focused on the same type of
education. It's not recommended to use it as an educational material at primary school.
Keywords: electric machines, dynamos, generators, motors, transformers, synchronous
machines, asynchronous machines
INTRODUCTION
Electric machines are important devices in our lives. They are everywhere, from a little toy to an
electric power plant. They changed our lives a lot. Before them, everything has to be made by human
or horse power. There were no light bulbs, that can turn night into day, no big factories with a lot of
employees. When electric machines came, everything has changed. People could get an electric energy
from kinetic energy of flowing water. They could work on shifts during the night. It was a revolution.
So that's why electric machines are important part of our life.
1. DEFINITION OF AN ELECTRIC MACHINE
The electric machine is a device, which can change electric energy to mechanical or
conversely. Device, which changes electric energy to mechanical is called motor. Device
which makes it conversely is called generator. That's what the definition says. We can divide
this devices to three types. It's DC machine, AC asynchronous machine and AC synchronous
machine. But that's not everything. Electric machine is also a device for change only
parameters of electrical energy. That includes four main devices It's rectifier for changing AC
voltage to DC, pulse converter which change parameters of DC energy, an inverter, that's a
device for changing DC to AC, and finally the transformer, it can change the parameters of
AC voltage.
2. TRANSFORMERS
Transformers are non-rotary machines, which means they don't have any rotary parts. They
are composed of three parts: primary and secondary coil and magnetic circuit. The electric
current flows into primary coil and it forms there a magnetic field. Magnetic field inside the
coil makes the magnetic flux flow through the magnetic circuit. When the magnetic flux is
flowing through the secondary coil, it induces there an electric current.
Parameters of an induced current depends on transformer ratio. It's a ratio between threads of
primary and secondary coil. It's also a ratio between primary and secondary voltage and
current. It's important to know, that transformers works only with AC voltage. Another
important thing is, that they can't change a frequency of current and in ideal case, when there
are no losses, apparent power transferred is same.
They are divided in two types. Oil and dry transformers. The difference is in the cooling
medium. Oil transformers are filled in with an oil and they are mainly used outdoors for every
power. You can see them in every village as distribution transformers. Dry transformers are
surrounded and cooled by an air. They are made for an indoor usage. It's not easy to see any
dry transformer today.
3. DC MACHINES
The oldest electric machines, that are the DC machines. They work, as the title says, with DC
current and DC voltage. They've got three parts. Stator, rotor and commutator. Through the
commutator flows the DC current to the rotor and it makes there a static magnetic field. This
field reacts with another one made in stator, usually by a permanent magnet, and it starts to
spin.
This machines works also as a generator, but the usage of them is not big in now days. It's
because of commutator. When a big current flows through it, it starts to sparkle. Main usage
of them is in little toys. But you can see it also on old bicycles like small dynamos.
4. AC ASYNCHRONOUS MACHINES
Asynchronous machines are composed from two parts: stator and rotor. There is a three-phase
winding in a stator, which makes there a rotary magnetic field. This field induces a current
into the rotor and this current makes its own magnetic field, which reacts with a magnetic
field of a rotor and rotor starts to spin. When the rotor would reach the synchronous speed of
stator magnetic field, there would be no induced currents in it, no magnetic field of rotor and
it would start to slow down. So there's always any difference between speed of stator
magnetic field and rotor speed. This difference is called a slip. As I mentioned before, without
a slip it wouldn't work. Asynchronous machines are divided in two types: cage and wound
type. Main difference is in a construction of rotor. In case of wound type, there's a wounded
rotor and in case of cage type, there's a rotor looking like a cage. Asynchronous machines are
mainly used as motors. But you can find it everywhere, in a saw for a wood or in air
compressors, it doesn't matter. They are everywhere, where the motors of the mid-power are
needed.
5. AC SYNCHRONOUS MACHINES
Synchronous machines are mainly used as generators in power plants, but we can also find
them in electric cars as engines. They've got a similar construction as asynchronous machines,
they also have a stator with a three-phase winding, but the rotor is completely different. There
is DC current flowing into the rotor and it makes its own static magnetic field, which reacts
with the field of stator. So there's no slip needed, no induced currents. This makes the
synchronous machine rotate with constant spins. This speed depends on the type of machine.
There are two types. Turbo generator and hydro generator. Turbo machine is long (it can be
15 meters), narrow (about 1,5 meter) and it's placed horizontally. They are mainly used in
coal and nuclear power plants. Hydro generators are wide, short and placed vertically. They
are slow, spins are for example 187,5 rpm in contrast with turbos. They are used in hydro
power plants.
CONCLUSION
Electric machines changed our lives a lot. I think it's important to know about this devices at
least a few of information. And I'm glad I could offer you this information by this work. Now
you know some necessity about it.
REFERENCES:
1. Electrical machine. Retrieved November 17, 2014, from
http://en.wikipedia.org/wiki/Electrical_machine
2. Elektrické stroje. Retrieved November 17, 2014, from
http://www.kdejinde.cz/edee/content/file/static/encyklopedie/encyklopedieenergetiky/05/stroje_2.html