Cycloconverters MCQ Quiz in বাংলা - Objective Question with Answer for Cycloconverters - বিনামূল্যে ডাউনলোড করুন [PDF]

The correct  answer is option 2):Positive converter only

Concept:

• Cycloconverters are frequency changers that convert AC power of specific frequency and voltage to different frequencies and voltage of AC power without any intermediate DC link.
• A cycloconverter is a naturally commutated converter in which the output frequency and voltage can be controlled independently and continuously.
• It consists of back-to-back connected controlled rectifiers whose output voltage and frequency can be controlled by tuning the firing angles of rectifiers.
• A cycloconverter having positive half of load current is carried by the positive converter only
• Each two-quadrant converter (phase-controlled) is represented as an alternating voltage source, which corresponds to the fundamental voltage component obtained at its output terminals. The diodes connected in series with each voltage source, show the unidirectional conduction of each converter, whose output voltage can be either positive or negative, being a two-quadrant one, but the direction of current is in the direction as shown in the circuit, as only thyristors − unidirectional switching devices, are used in the two converters.
• Normally, the ripple content in the output voltage is neglected. The control principle used in an ideal cyclo-converter is to continuously modulate the firing angles of the individual converters. So, that each produces the same sinusoidal (ac) voltage at its output terminals. The output voltage and current waveforms of the cyclo-converter are shown in the fig

 

Explanation:

The basic  cyclo-converter is shown below:

Principle of operation

• Each two-quadrant converter (phase-controlled) is represented as an alternating voltage source, which corresponds to the fundamental voltage component obtained at its output terminals.
• The diodes connected in series with each voltage source, show the unidirectional conduction of each converter, whose output voltage can be either positive or negative, being a two-quadrant one, but the direction of current is in the direction as shown in the circuit, as only thyristors − unidirectional switching devices, are used in the two converters.
• Normally, the ripple content in the output voltage is neglected.

The control principle used in an ideal cyclo-converter is to continuously modulate the firing angles of the individual converters. So, that each produces the same sinusoidal (ac) voltage at its output terminals.

The output voltage and current waveforms of the cyclo-converter are shown in the fig

The displacement angle of the load (current) is zero in fig.

In this case, each converter carries the load current only, when it operates in its rectifying region, and it remains idle throughout the whole period in which its terminal voltage is in the inverting region of operation.

Conclusion:

Option 1 is correct.

A cycloconverter is a frequency changer from higher to lower frequency with one or two stage conversion. A load factor at the input terminals of a cycloconverter is generally low and lagging.

Only lagging reactive component is produced at the input because of the phase control mechanism.

The input displacement angle is greater than zero and lagging even if the load displacement angle is zero. Foe reactive loads the angle increases.

Concept:

• A cycloconverter converts a constant voltage, constant frequency AC waveform to another AC waveform of different frequency.
• It converts AC to AC only by changing the frequency.
• Normally, the output frequency is less than the input frequency.

Types of cycloconverter:

There are two types of cyclo-converters:

• Blocking mode cycloconverters.
• circulating current cyclo converters.

 

Blocking mode cyclo converters:

• In this case, the positive converter will provide the necessary voltage when there is a positive load current.
• At that time, the negative converter will be in a blocked condition.
• During the negative load current, the negative converter will provide the necessary voltage, and at that time, the negative converter will be in a blocked state.
• This operation is known as blocking mode operation, and the converter which is used in the operation is termed as a blocking mode cycloconverter.

 

Circulating current type:

• In this, both converters are in the operating state at the same time.
• Sometimes supply gets short when both are in ON STAGE.
• An intergrouping reactor is connected in between them to avoid short circuits.
• The short-circuiting current, which is unidirectional, is present here.

Further, we can classify them on the basis of output frequency. They are:

1. Step up the cycloconverter.
2. Step down the cycloconverter.

Step up type:

It provides an output frequency greater than the input frequency by using line commutation. (fo > fs)

Step-down type:

It provides an output frequency lower than that of its input frequency with the help of forced commutation. (fo < fs) 

Cyclo converters:

A Cyclo converter is a converter that converts the AC voltage of one frequency to another frequency without an intermediate DC link.

If the output frequency is greater than the input frequency, it is a step-up Cyclo converter.

If the output frequency is less than the input frequency, it is a step-down Cyclo converter.

1ϕ to 1ϕ step up midpoint type cycloconverter:

• The circuit consists of a single-phase transformer with a mid tap on the secondary winding and four thyristors. 
• Two of these thyristors P1 & P2 are for the positive group. Here positive group means when either P1 or P2 conducts, the load voltage is positive.
• The other two thyristors N1 & N2 are for the negative group.
• Load is connected between secondary winding mid-point O and terminal A.

Working of step up Cyclo converter:

Case 1: 0 < ωt < π :

Thyristors P1 & N2 are forward biased.

V_O = V_S

Case 2: π < ωt < 2π :

Thyristors P2 & N1 are forward biased.

VO = -V_S

For a single input cycle of time period 2π, two-cycle of time period π are produced.

Hence, f_O > f_S

Single-phase to single-phase cyclo-converter:

• This converter is having back to back connection of two full-wave rectifiers.
• Suppose for getting one-fourth of input voltage at the output, for the first two cycles of Vs, the positive converter operates supplying current to the load, and it rectifies the input voltage.
• In the next two cycles, the negative converter operates supplying current in the reverse direction.
• When one of the converters operates the other one is disabled so that there is no current circulating between rectifiers.
• In the below figure Vs represents input supply voltage and Vo is the required output voltage which is one-fourth of the supply voltage.
• The magnitudes of harmonic components are quite large and they can be reduced by using a three phase input supply.

 

1. Single-phase to single-phase cyclo-converter

2. Three-phase to single-phase cyclo-converter

3. Three-phase to three-phase cyclo-converter

A cycloconverter is a type of power electronic converter that directly converts AC power of one frequency to AC power of another frequency. The quality of the output AC voltage is generally improved by operating at higher frequencies because it reduces the harmonic content and results in a smoother waveform.

A cycloconverter is a one-stage frequency changer. It directly converts AC power at one frequency to AC power at a different frequency in a single conversion stage, without the need for intermediate DC conversion. This is achieved through the use of controlled semiconductor switches that modulate the output waveform, making cycloconverters efficient for applications requiring variable frequency and voltage.

Explanation:

Single Phase Cycloconverter

Definition: A single-phase cycloconverter is an AC-AC converter that directly converts an input AC voltage at a certain frequency to an output AC voltage at a different frequency without the need for an intermediate DC stage. Cycloconverters can be used to generate lower frequencies from a higher frequency supply voltage.

Working Principle: In a single-phase cycloconverter, the output frequency is determined by the control of the switching devices which are typically thyristors. The fundamental operation involves controlling the firing angle of the thyristors to shape the output waveform. To obtain a specific fraction of the supply frequency, a precise triggering sequence of the thyristors is required.

Advantages:

• Ability to directly convert AC power from one frequency to another without the need for intermediate stages.
• High efficiency due to the absence of an intermediate DC link.

Disadvantages:

• Complex control circuitry required to manage the firing angles of the thyristors.
• Generation of harmonics which may need filtering.

Applications: Single-phase cycloconverters are used in applications such as variable frequency drives for AC motors, induction heating, and power supplies for electrical testing where precise frequency control is necessary.

Correct Option Analysis:

The correct option is:

Option 3: P1P[2P1 and N1N[2N1

This option is correct because it provides the appropriate triggering sequence to obtain one-third of the supply frequency in a single-phase cycloconverter circuit. In this sequence, P1, P2, and N1, N2 are the thyristors that are triggered in a specific order to produce the desired output frequency. By firing the thyristors in this sequence, each thyristor conducts for a time period corresponding to one-third of the supply cycle, effectively reducing the frequency to one-third of the original.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 1: P1P1P2 and N1N1N2

This option is incorrect because it suggests a sequence where P1 and N1 are fired twice consecutively before P2 and N2. This sequence does not provide the correct timing for each thyristor to reduce the frequency to one-third of the supply frequency. The output waveform would not be correctly shaped to achieve the desired frequency reduction.

Option 2: P2P2P1 and N2N2N1

This option is incorrect for similar reasons as Option 1. Firing P2 and N2 twice consecutively followed by P1 and N1 does not result in the proper timing to achieve one-third of the supply frequency. The control of the thyristors must be precise and evenly distributed to obtain the desired frequency conversion.

Option 4: P2P1P2 and N2N1N2

This option is also incorrect because the sequence does not evenly distribute the firing periods across the supply cycle to achieve the correct frequency reduction. The timing of the thyristor firing must be such that each thyristor is on for a period corresponding to one-third of the supply cycle to produce the desired output frequency.

Conclusion:

Understanding the correct triggering sequence of thyristors in a single-phase cycloconverter is crucial for achieving the desired frequency conversion. The correct sequence ensures that the output frequency is precisely controlled and matches the required fraction of the supply frequency. In this case, the sequence P1P[2P1 and N1N[2N1 correctly achieves one-third of the supply frequency, making it the appropriate choice for the given problem.

Explanation:

Cycloconverter

Definition: A cycloconverter is an AC-AC power converter that directly converts an alternating current (AC) of one frequency to an alternating current of another frequency without an intermediate DC stage. The output frequency is typically lower than the input frequency and can be varied continuously within a certain range. Cycloconverters are widely used in applications requiring variable frequency drives for large induction motors, such as in the cement, mining, and ship propulsion industries.

Working Principle: Cycloconverters operate by segmenting the input AC waveform into smaller sections and then recombining these sections to form the desired output waveform. This is achieved using a network of controlled rectifiers or thyristors that can be switched on and off in a precise manner to generate the required output frequency. The process involves converting each phase of the input AC into a sequence of positive and negative half-cycles, which are then recombined to produce the lower frequency output.

Advantages:

• Direct conversion from AC to AC without the need for an intermediate DC stage, resulting in higher efficiency.
• Ability to produce variable output frequencies, making them suitable for applications requiring precise speed control.
• High power capability, making them ideal for large industrial applications.

Disadvantages:

• Complex control circuitry required to manage the switching of thyristors.
• Generation of harmonics and potential distortion in the output waveform, requiring additional filtering.
• Limited to lower output frequencies compared to the input frequency.

Applications: Cycloconverters are primarily used in applications that require low-frequency output and high power, such as driving large induction motors in heavy industries, rolling mills, mine winders, and ship propulsion systems. They are also used in power supplies for high-frequency induction heating.

Correct Option Analysis:

The correct option is:

Option 4: A one-stage frequency changer

This option correctly describes the function of a cycloconverter. Cycloconverters serve as one-stage frequency changers by converting an input AC frequency directly to a different output AC frequency without any intermediate conversion stages. This direct conversion process is what distinguishes cycloconverters from other types of converters that might require intermediate DC stages.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 1: A controlled rectifier

This option is incorrect because a controlled rectifier is a device that converts AC to DC using thyristors or other controlled switches. While cycloconverters use controlled rectifiers within their circuits, their primary function is not to convert AC to DC, but rather to convert one AC frequency to another AC frequency.

Option 2: A multilevel inverter

This option is incorrect as multilevel inverters are devices that convert DC to AC and can produce multiple levels of voltage output to improve the quality of the output waveform. Cycloconverters, on the other hand, are involved in AC to AC conversion and do not function as inverters.

Option 3: A DC-DC transformer

This option is incorrect because a DC-DC transformer, often referred to as a DC-DC converter, is used to convert one DC voltage level to another DC voltage level. Cycloconverters deal with AC frequencies and are not involved in DC-DC voltage conversion.

Conclusion:

Understanding the function and operation of cycloconverters is essential for correctly identifying their role as one-stage frequency changers. Cycloconverters directly convert an input AC frequency to a different output AC frequency, making them distinct from other types of converters such as controlled rectifiers, multilevel inverters, and DC-DC transformers. Their ability to handle high power and provide variable frequency output makes them valuable in heavy industrial applications where precise speed control and efficiency are critical.