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Summary

If the inductance value is not very large, the energy stored in the inductance is
able to maintain the load current only upto ωt = β, where π < β < 2π , well before the
next gate pulse and the load current tends to become discontinuous....

Description

ID: 1602100

Definition: A cycloconverter is a direct frequency changer that converts ac power at one frequency to ac power at another frequency by ac-ac conversion.

Classification: The cycloconverters can be broadly divided into two categories a. Naturally (Line Commutated) Cycloconverters and b.Forced-Commutated Cycloconverters. Depending on output-frequency: i. Step-up cycloconverter. ii. Step-down cycloconverter. Based on firing angle controlling strategy: i. Phase-controlled cycloconverter. ii. Envelope cycloconverter. Depending on way of operation: i. Blocking-mode Cycloconverters. ii. Circulating-current Cycloconverters.

Topology: i. Single-phase (1φ) to single-phase (1φ). ii. Three-phase (3φ) to single-phase (1φ). iii. Three-phase (3φ) to three-phase (3φ).

Operation principle:

(a) Single Phase Converters: The principle of operation of a single-phase cycloconverter is explained using the following diagram:

Dept. of EEE, CUET

ID: 1602100

Figure: Single-phase cycloconverter

The two single-phase controlled converters are operated as bridge rectifiers. The output voltage of one converter is however equal and opposite to that of the other converter V1 = - V2. If converter P is operating alone, the average output voltage is Positive and if converter N is operating, the output voltage is negative. The waveforms are shown for the output voltage and gating signals of +ve and –ve converter, with +ve converter ON for time T/2 and the negative converter for time. When one converter is operating the other is disabled, so that there is no circulating current between the two rectifiers.

Performance analysis: Output RMS voltage ( Vo(RMS) ) : Input voltage ,

V o (RMS)=

√

v s=V m sin ωt d (ωt)

1 V 2 2 ∫ m sin ωt d (ωt ) π

2 ωt 1−cos ¿ d (ωt) ¿ ¿ Or, V m2 π ∫¿ 2π α V o (RMS)=√ ¿

Dept. of EEE, CUET

ID: 1602100

V m2 sin 2 ωt π [ωt − ] 2π 2 α

√ √ √

Or,

V o (RMS)=

Or,

V m2 sin 2 π sin 2 α − [ ( π−α )− V o (RMS)= ] 2 2 2π

Or,

V o (RMS)=

Or,

V o (RMS)=

(

)

V m2 sin2 α [ ( π−α )+ ] 2 2π

(

√

)

(

)

Vm 1 sin 2 α [ ( π−α ) + ] 2 √2 π

Output RMS Current: Vo , RMS R

Io(RMS)=

Vm √2

Io(RMS)=

√

sin 2 α 1 ( [ π−α ) + ] π 2 R

√

sin2 α ℑ 1 ] [ ( π −α ) + √2 π 2

Io(RMS)=

Output Power: Po= Io(RMS)2 R Output Performance: PF =

P o (RMS) Vs Is

PF =

Io( RMS )2 R Vin ( RMS ) Io ( RMS )

PF =

Io ( RMS) ∗R Vin ( RMS )

PF =

Vo ( RMS) Vin ( RMS )

PF =

√

1 ( sin 2 α ] [ π −α )+ 2 π

Dept. of EEE, CUET

ID: 1602100 (b) Three Phase Converters: The principle of operation of a single-phase cycloconverter is explained using the following diagram:

Figure: three-phase cycloconverter

The output of a three-phase/single phase converter is One phase output. In order to have three phase output two more three phase/single phase converter are employed. Thus, a total of 36 switches will be used. The two ac-dc converters shown in the circuit diagram are three phase controlled rectifiers. The positive converter operates for half the period of output frequency and the negative converter for the other half the period. The analysis is the same (similar) to that of a single phase converter.

(c) Single-phase to Single-phase (1f-1f) Cycloconverter: A single phase to single phase cycloconverter is a type of cycloconverter whose input & output are single phase AC. The basic function remains the same i.e. frequency changing. A single phase to single phase cycloconverter may either be mid-point type or bridge type step-up or step-down cycloconverter.

Dept. of EEE, CUET

ID: 1602100

Figure: Single-phase to single-phase cycloconverter

Figure: Wave form of Single-phase to single-phase cycloconverter

(d) Three-Phase to Single-Phase (3f-1f) Cycloconverter: There are two kinds of threephase to single-phase (3f-1f) cycloconverters: 3f-1f half-wave cycloconverter and 3f-1f bridge cycloconverter. Like the 1f-1f case, the 3f-1f cycloconverter applies rectified voltage to the load. Both positive and negative converters can generate voltages at either polarity, but the positive converter can only supply positive current and the negative converter can only supply negative current. Thus, the cycloconverter can operate in four quadrants: (+v, +i) and (-v, -i) rectification modes and (+v, -i) and (-v, +i) inversion modes. The principle of operation of a three-phase to single-phase cycloconverter is explained using the following diagram:

Dept. of EEE, CUET

ID: 1602100

Figure: 3f-1f half-wave cycloconverter

Figure: 3f-1f bridge cycloconverter

Figure: 3f-1f bridge cycloconverter waveform

Dept. of EEE, CUET

ID: 1602100 (e) Three-Phase to Three-Phase (3f-3f) Cycloconverter: If the outputs of 3 3f-1f converters of the same kind are connected in wye or delta and if the output voltages are 2pi/3 radians phase shifted from each other, the resulting converter is a three phase to three-phase (3f-3f) cycloconverter. If the three converters connected are half-wave converters, then the new converter is called a 3f-3f half-wave cycloconverter. If instead, bridge converters are used, then the result is a 3f-3f bridge cycloconverter. 3f-3f halfwave cycloconverter is also called a 3-pulse cycloconverter or an 18-thyristor cycloconverter. On the other hand, the 3f-3f bridge cycloconverter is also called a 6-pulse cycloconverter or a 36-thyristor cycloconverter. The principle of operation of a threephase to three- phase cycloconverter is explained using the following diagram:

Figure: 3f-3f half-wave cycloconverte

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ID: 1602100 Figure: 3f-3f bridge cycloconverter

Figure: Waveform of Three-Phase to Three-Phase (3f-3f) Cycloconverter

Applications: 1. The applications of cycloconverters include: Cement mill drives 2. Rolling mills 3. Ship propulsion drivers 4.

Water pumps

5.

Washing machines

6. Mine winders Industries

Advantages:

i.

Efficiency is very high compared to other converters

ii.

Four quadrant operation is possible because cyclo-converter is capable of power transfer in both the directions

iii.

AC power at one frequency is directly converted to a lower frequency in a single conversion

iv.

If one of the SCR fails, the cyclo-converter operates with a distorted output

v.

In this converter, power transfer is possible from supply load and vice versa at any power factor Dept. of EEE, CUET

ID: 1602100 vi.

Dynamic response is good

vii.

Smooth low-speed operation

Disadvantages:

i.

Control circuit become complex so difficult to design

ii.

The output frequency of the cyclo-converter is 1/3th or 1/2th for responsible power output and efficiency

iii.

Power factor is too much poor at large values of α

iv.

More distortion at low frequencies

Recent improvement: (a) Matrix Converter: A matrix converter is defined as a converter with a single stage of conversion. It utilizes bidirectional controlled switch to achieve automatic conversion of power from AC to AC. Matrix converters are characterized by sinusoidal waveforms that show the input and output switching frequencies. The principle of operation of a matrix converter is explained using the following diagram:

Figure: Matrix converter

(b) Integral Pulse Modulated (1f-3f) Cycloconverters: The input to these cycloconverters is single-phase high frequency sinusoidal or square waveforms with or without zero voltage gaps. Every half-cycle of the input signal, the control for each phase decides if it needs a positive pulse or a negative pulse using integral pulse modulation. For integral pulse modulation, the command signal and the output phase voltage are integrated and the latter result is subtracted from the former. For a positive difference, a negative pulse is required,

Dept. of EEE, CUET

ID: 1602100 and vice versa for the negative difference. For the positive (negative) input half-cycle, if a positive pulse is required, the upper (lower) switch is turned on; otherwise, the lower (upper) switch is turned on. The principle of operation of a high frequency ac link converter is explained using the following diagram:

Figure: High frequency ac link converter (1f hf inverter + (1f-3f) Cycloconverter)

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