FREQUENCY CONVERTERS
Jan Hentschel
Fakulta elektrotechnická, Západočeská univerzita v Plzni
Železničářská 1092/10, Ústí nad Labem, e-mail: hentsjan@students.zcu.cz
Abstract :
A frequency converter is an electrical device used for controlled changing of frequency of an
alternating current signal. At the beginning this article explains difference between the two types of a
frequency converter. The electromechanical motor-generator and an electronical frequency converter.
After that, the working principle of the electronical frequency converter also called the variablefrequency drive (VFD) is showed and explained. The third point of this project was to show and
demonstrate the usage of these devices in particular areas. At the end, this article focuses on
comparation of the pros and cons of a frequency converter. This device has been shown to spread
widely into more areas due to lowering price and its uniqueness.
Key words: frequency converter, alternating current, electromechanical, motor-generator, electronical,
variable-frequency drive
INTRODUCTION
The frequency converter is a relatively new device, used to control frequency of an alternating
current electrical signal. It can change input frequency to another certain output frequency due
to its working principle based on switching nonlinear semiconductor circuit elements. It has
not been used widely for a long period of time due to its complexity and price. However the
price is still lowering because of the discovering of new methods, materials and cheaper
manufacturing process. Today this device is being recognized all around the world as the
unique and also effective electrical frequency and drive control.
This article shows the two types of a frequency converter, explains the complex working
principle of an electronical frequency converter, describes possible areas of usage, and
compares the pros and cons.
1. TYPES OF FREQUENCY CONVERTERS
1.1 Motor-generator
A motor-generator or a rotary changer is basically a set of two electrical machines as a motor
and a generator connected together by a shaft. They are used for changing electrical energy as
shown on an image on the next page. It is possible to use motor-generator for changing
voltage, frequency and also for rectifying alternating current to direct and backwards.
In the case of alternating current generators there is also a smaller dynamo used to create
higher excitation current or the generator. This solution was mainly used in the past when
there was not enough of semiconductor rectifiers or when high power semiconductors were
not affordable yet.
Fig. 1 A motor-generator set
(http://www.electrical4u.com/wp-content/uploads/2013/05/motor-generator-set.gif)
Compared to transformers, the motor-generator has lower efficiency and also reliability. On
the other side, it can operate with direct current more easily than an electronical frequency
converter. Today the ability to control direct current is replaced by modern high power
semiconductor parts.
1.2 Variable-frequency drive
An electronical frequency converter or as it is also called, the variable-frequency drive (VFD)
is a complex device mainly used to control alternating current motors. It is consisting of 5
main parts. The first part of the VFD is a transformer, which transforms the input voltage. The
second part is a semiconductor rectifier which rectifies the alternating voltage to a DC value.
However this value may vary its amplitude. That’s why there is the third part – a DC filter.
The next part is an inverter, which inverts the DC voltage back to an alternating value. The
last part is a controller, which controls the inverter. An image of a common VFD is shown
here.
Fig. 2 A variable-frequency drive
(http://img.diytrade.com/cdimg/915914/17611162/0/1291356729/frequency_converter.jpg)
2. WORKING PRINCIPLE OF A VFD
The working principle of a VFD is more complicated. By filtering the rectified voltage to an
almost constant value by a semiconductor diode uncontrolled rectifier and a filtering circuit
consisting of capacitors, resistors and inductors, the DC voltage can be used as a reference
value for the inverter. The inverter than creates an alternating voltage with the frequency
required. However, the creation of the desired frequency is not that simple. The alternating
value of the voltage is created by semiconductor parts in the inverter called IGBT transistors.
These circuit elements are correctly designed, connected and switched on and off at the right
time by a controller to create something called a modified sine wave. This special kind of
transistor switching is called pulse-width modulation. The pulse-width modulation creates
short direct current pulses with variable duration and right polarity accordingly, so the
average value of voltage and the output waveform looks like a sine wave. A basic VFD
schematic and voltage waveforms are shown on images below.
Fig. 3 The VFD schematic and voltage waveforms
(http://www.ctiautomation.net/Images/Articles/VFD/Figure-5.gif)
Fig. 4 Actual VFD output waveform – modified sine wave
(http://www.imakenews.com/tmeic_news/word_images/7187859_image009.jpg)
3. FREQUENCY CONVERTER USAGE
Because of the lowering price of electronical frequency converters, they are becoming more
and more common to appear in public transport as speed control for AC motors in modern
trains, trams and trolleybuses. The trend was to use a motor-generator, to create a different
value of frequency in order to control the speed, starting or braking of an AC motor or
generator. However, motor-generators are big machine sets, so nowadays they tend to be in
use mainly in the industry, where the size does not matter. Another possible use of a
frequency converter is to create a local power network for an industrial factory where special
machines need a special value of voltage and frequency such as outsourced factories to
another countries.
4. PROS AND CONS
The plus sides of a frequency converter are simple. Because AC motor speed, starting or
braking cannot be controlled any other way than by changing the input frequency, their plus is
simply the ability to control such machines. Another pro of a VFD can be a relatively easy
way of user usage as the VFD user interface is consisting of few buttons which control the
output frequency and a simple display showing the desired frequency. The biggest con of an
electronical frequency converter is still the price. It is lowering but a VFD is not yet
affordable for a single person to use for some own interests. The second con is the unstability
of device’s power consumption. Its power consumption is not linear and it is not ideal for the
power network. The third con of a VFD is the output signal waveform. To big machines like
motors and generators, the output waveform of a modified sine wave makes no problem to
handle. However, considering other lower power devices and appliances using some elements
sensitive to frequency or waveform purity, the modified sine wave can be a problem.
CONCLUSION
Frequency converters both motor-generator and variable-frequency drive have their bright and
dark sides but since the best motor or generator to use today and in the near future is the AC
induction machine and the only way to control these machines is to use frequency converters,
motor-generators and VFDs will always have their place and usage. Because they are very
easy to use and are relatively affordable for industrial and public transportation use today,
they earned the right to begin to be used widely and start a new era of electrical drive control.
REFERENCES:
1.
Variable frequency drive. Retrieved November 17, 2014, from
http://en.wikipedia.org/wiki/Variable-frequency_drive#VFD_types_and_ratings
2.
Motorgnerátor. Retrieved November 17, 2014, from
http://cs.wikipedia.org/wiki/Motorgener%C3%A1tor
3.
Ondřej Došek – consultation (2014)
4.
Motor-generator set. Retrieved November 17, 2014, from
http://www.electrical4u.com/wp-content/uploads/2013/05/motor-generator-set.gif
5.
A VFD. Retrieved November 17, 2014, from
http://img.diytrade.com/cdimg/915914/17611162/0/1291356729/frequency_converter.jpg
6.
VFD schematic. Retrieved November 17, 2014, from
http://www.ctiautomation.net/Images/Articles/VFD/Figure-5.gif
7.
VFD output waveform. Retrieved October 1, 2014, from
http://www.imakenews.com/tmeic_news/word_images/7187859_image009.jpg