Cycloconverter
Encyclopedia
A cycloconverter or a cycloinverter converts an AC
waveform
, such as the mains supply
, to another AC waveform of a lower frequency
, synthesizing the output waveform from segments of the AC supply without an intermediate direct-current link ( and ). They are most commonly used in three-phase applications.
Usually silicon-controlled rectifier
s (SCRs) are used in these circuits. By contrast, low cost, low-power cycloconverters for low-power AC motors tend to use TRIAC
s in place of SCRs. Unlike an SCR which conducts in only one direction, a TRIAC is capable of conducting in either direction, but it is also a three-terminal device. Cycloconverters are not common in low power range of variable frequency drive
s, which is dominated by transistor-based inverters
.
In most power systems, the amplitude
and the frequency of input voltage
to a cycloconverter are fixed values, whereas both the amplitude and the frequency of output voltage of a cycloconverter are variable. The output frequency of a three-phase cycloconverter must be less than about one-third to one-half the input frequency . The quality of the output waveform improves if more switching devices are used (a higher pulse number of the thyristor bridges).
Cycloconverters are used in very large variable frequency drives with ratings from few megawatts up to many tens of megawatts. Typical applications of cycloconverters are rolling mill drives, ball mill
s for ore processing, cement kilns and ship propulsion.
Cycloconverters have also been used to produce single phase 16 2/3 Hz or 25 Hz power for electric railroads from the 50 Hz or 60 Hz AC grid.
The switching of the AC waveform creates current harmonics in the power system that depend mostly on the frequency of the input waveform. These harmonics can disturb or overload sensitive electronic equipment in the AC grid. Cycloconverters produce subharmonics and interharmonics in addition to harmonics. Subharmonic noise occurs at a frequency below the output frequency. The interharmonics are at frequencies that are between harmonic frequencies. Subharmonics and interharmonics are often problematic for the AC grid that is feeding the cycloconverter and their mitigation may require filters with considerable size. One of the subharmonic components will pass through zero frequency when the output frequency of the cycloconverter reaches half of the input frequency. Because DC component in the input current will saturate the transformer feeding the cycloconverter badly this limits the output frequency relative to the input. Another reason to limit the maximum output frequency below half of the input frequency is the deterioration of the output voltage waveform with increasing output frequency. These limitations make cycloconverters often inferior to a DC
link converter system for most applications.
, invented and perfected between 1981 and 1984 by Sandler, Wrzesniewski, Wilner, and Fung in the United States
. The tap converter utilizes a Scott transformer connection and somewhat more sophisticated control logic (adding several more SCRs) to switch among a variety of transformer taps and thereby fabricate a far smoother signal.
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
waveform
Waveform
Waveform means the shape and form of a signal such as a wave moving in a physical medium or an abstract representation.In many cases the medium in which the wave is being propagated does not permit a direct visual image of the form. In these cases, the term 'waveform' refers to the shape of a graph...
, such as the mains supply
Mains electricity
Mains is the general-purpose alternating current electric power supply. In the US, electric power is referred to by several names including household power, household electricity, powerline, domestic power, wall power, line power, AC power, city power, street power, and grid power...
, to another AC waveform of a lower frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
, synthesizing the output waveform from segments of the AC supply without an intermediate direct-current link ( and ). They are most commonly used in three-phase applications.
Usually silicon-controlled rectifier
Silicon-controlled rectifier
A silicon-controlled rectifier is a four-layer solid state device that controls current. The name "silicon controlled rectifier" or SCR is General Electric's trade name for a type of thyristor. The SCR was developed by a team of power engineers led by Gordon Hall and commercialized by Frank W...
s (SCRs) are used in these circuits. By contrast, low cost, low-power cycloconverters for low-power AC motors tend to use TRIAC
TRIAC
TRIAC, from Triode for Alternating Current, is a genericized tradename for an electronic component that can conduct current in either direction when it is triggered , and is formally called a bidirectional triode thyristor or bilateral triode thyristor.TRIACs belong to the thyristor family and are...
s in place of SCRs. Unlike an SCR which conducts in only one direction, a TRIAC is capable of conducting in either direction, but it is also a three-terminal device. Cycloconverters are not common in low power range of variable frequency drive
Variable frequency drive
A variable-frequency drive is a system for controlling the rotational speed of an alternating current electric motor by controlling the frequency of the electrical power supplied to the motor. A variable frequency drive is a specific type of adjustable-speed drive...
s, which is dominated by transistor-based inverters
Inverter (electrical)
An inverter is an electrical device that converts direct current to alternating current ; the converted AC can be at any required voltage and frequency with the use of appropriate transformers, switching, and control circuits....
.
In most power systems, the amplitude
Amplitude
Amplitude is the magnitude of change in the oscillating variable with each oscillation within an oscillating system. For example, sound waves in air are oscillations in atmospheric pressure and their amplitudes are proportional to the change in pressure during one oscillation...
and the frequency of input voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
to a cycloconverter are fixed values, whereas both the amplitude and the frequency of output voltage of a cycloconverter are variable. The output frequency of a three-phase cycloconverter must be less than about one-third to one-half the input frequency . The quality of the output waveform improves if more switching devices are used (a higher pulse number of the thyristor bridges).
Cycloconverters are used in very large variable frequency drives with ratings from few megawatts up to many tens of megawatts. Typical applications of cycloconverters are rolling mill drives, ball mill
Ball mill
A ball mill is a type of grinder used to grind materials into extremely fine powder for use in mineral dressing processes, paints, pyrotechnics, and ceramics.-Description:...
s for ore processing, cement kilns and ship propulsion.
Cycloconverters have also been used to produce single phase 16 2/3 Hz or 25 Hz power for electric railroads from the 50 Hz or 60 Hz AC grid.
The switching of the AC waveform creates current harmonics in the power system that depend mostly on the frequency of the input waveform. These harmonics can disturb or overload sensitive electronic equipment in the AC grid. Cycloconverters produce subharmonics and interharmonics in addition to harmonics. Subharmonic noise occurs at a frequency below the output frequency. The interharmonics are at frequencies that are between harmonic frequencies. Subharmonics and interharmonics are often problematic for the AC grid that is feeding the cycloconverter and their mitigation may require filters with considerable size. One of the subharmonic components will pass through zero frequency when the output frequency of the cycloconverter reaches half of the input frequency. Because DC component in the input current will saturate the transformer feeding the cycloconverter badly this limits the output frequency relative to the input. Another reason to limit the maximum output frequency below half of the input frequency is the deterioration of the output voltage waveform with increasing output frequency. These limitations make cycloconverters often inferior to a DC
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
link converter system for most applications.
Tap converter
A dramatically improved output waveform can be achieved by employing the circuit known as the tap converterTap converter
The tap converter is a variation on the cycloconverter, invented in 1981 by New York City electrical engineer Melvin Sandler and perfected in 1982 through 1984 by graduate students Mariusz Wrzesniewski, Bruce David Wilner, and Eddie Fung....
, invented and perfected between 1981 and 1984 by Sandler, Wrzesniewski, Wilner, and Fung in the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
. The tap converter utilizes a Scott transformer connection and somewhat more sophisticated control logic (adding several more SCRs) to switch among a variety of transformer taps and thereby fabricate a far smoother signal.