2. Introduction to Converter Family
• In industrial applications, two forms of electrical energy are
used: direct current (dc) and alternating current (ac).
• However, for different applications, different forms,
magnitudes and/or frequencies are required.
• There are four different conversions between dc and ac power
sources.
• These conversions are done by circuits called power
converters
3. Classification of Converters
• The converters are classified as:
• Rectifiers: from single-phase or three-phase ac to variable
voltage dc
• Choppers: from dc to variable voltage dc
• Inverters: from dc to variable magnitude and variable
frequency, single-phase or three phase ac
• Cycloconverters : from single-phase or three-phase ac to
variable magnitude and variable frequency, single-phase or
three-phase ac
4. Introduction to Cycloconverter
• Traditionally, ac-ac conversion
using semiconductor switches is
done in two different ways:
• 1- In two stages (ac-dc and then
dc-ac) as in dc link converters.
• 2- in one stage (ac-ac)
cycloconverters.
• Cycloconverters are used in high
power applications driving
induction and synchronous
motors. They are usually phase-
controlled and they traditionally
use thyristors due to their ease of
phase commutation.
5. Operating principle for
cycloconverter
• Continuous modulation of
the firing angle α of the
individual converter.
• The voltage produced by
two converters have same
magnitude, angle and
phase.+ve and –ve
converter produce
respective half cycles.
6. 1-ф to 1-ф Cycloconverter Mid-point configuration
• Also known as centre tapped
transformer cycloconerter
• Assume purely resistive load
• SCRs 1,2,3 & 4 connected
antiparallel.
8. Care to be taken
• By mistake if from two pairs are turned on simultaneously(e.g.
S1 and S4 or S2 and S3) then the secondary winding of
transformer will be short circuited.
• To avoid this, we should connect L at the other end of load.
This will stop sudden change of current and will stop
secondary from getting damaged.
• In general, if o/p freq. is Nth part of i/p frequency than N
positive and N negative half cycles should be produced across
load by triggering SCRs in proper sequence.
• o/p waveforms tends to be square waves.
• For producing sine wave, the triggering of SCR should be
delayed by varying degrees.
9. Care to be taken
• If inductive load is used than portion of –ve half cycle will
appear in positive half cycle of load voltage waveform and
vice versa.
• The wave forms are as follows:
10. Care to be taken
• The o/p freq. should be always a sub multiple of i/p freq.
Why??
• Yes, because if not, than incoming SCR would turn on before
outgoing SCR and it will result in zero o/p voltage.
• This condition continues for less than half cycle period and
occurs at every half cycle.
• This periodic short ckt will distort o/p voltage.
• And due to asymmetrical waveforms the o/p transformer may
get saturated.(if used.)
12. Operation of bridge configuration
• 1.Non-Circulating Current mode:
• In this mode,SCRs only from one side are triggered at
any given instant.
• The firing pulses to the SCRs from the other bridge
are ‘’blocked’’.i.e. not given.
13. 2.Circulating mode
• Both bridges conduct simultaneously.
• Inductor L can be placed at point L in figure.
• Freewheeling diode can’t be connected across load as it wont
allow one cycle of ac o/p appear across the load.
• Semiconverters can’t be used in bridge because they do not
allow load voltage to be negative due to internal
freewheeling.
• Therefore inverting mode cant be achieved, which is actually
essential while feeding inductive loads.
14. Notes
• Cycloconverters are naturally commutated
and the maximum output frequency is a
fraction of the source frequency.
• Therefore, cycloconverters are low speed ac
motor drives in ranges up to 15 MW with
frequencies from 0 to 20 Hz.
17. Applications of Cycloconverters
• There are other newer
forms of cycloconversion
such as ac-ac matrix
converters and high
frequency ac-ac (hfac-ac)
converters and these use
self-controlled switches.
• Cement mill drives
• Ship propulsion drives
• Rolling mill drives
• Scherbius drives
• Ore grinding mills
• Mine winders
18. Advantages
• No “DC LINK” required as in case of inverters to get sine
waveforms
• Bidirectional power flow possible.
• Variable firing angle control possible so, we can obtain high
quality Sine wave at low frequencies.So filters also gets
eliminated and It will reduce losses and will increase
efficiency.
• Line commutated.Seperate circuit for commutation not
required.
• Why step down more used than step up??
• Due to addition of FORCED COMMUTATION circuit in step up
cycloconverter.Also, inverter are more better than cyclo for
high frequency operation than supply freq.
19. Disadvantages
• Smooth stepless control of o/p frequency is
not possible.
• More distortion at low frequencies.
• Control circuit is difficult to design and
complex.
• i/p power factor is poor at large values of α.
20. References
• P.E.-2 by Dr.P.S.Bimbhra
• P.E.-2 by J.S.Katre
• Burak Ozpineci, Leon M. Tolbert Department of
Electrical and Computer Engineering, University of
Tennessee-Knoxville, Knoxville
• Google Images
• HITACHI Electronics division
• Santosh Grampurohit,VGEC Electrical Student.
• PANTECH Solutions
