3 pulse cycloconverter

The out frequency of a Cycloconverters can be reduced upto to zero which helps us to start very large motors with full load at minimum speed and then gradually increase the speed of the motor by increasing the output Frequency. Before the invention of Cycloconverters, these large motors has to be unloaded completely and then after starting the motor it has to be loaded gradually which results in time and man power consumption.

Based on the output frequency and number of phase in the input AC power source the Cycloconverters can be classified as below.

But it is not widely used since it not have much particle application. Most application will require a frequency less than 50Hz which is the default frequency here in India. These are most commonly used and work with help of natural commutation hence comparatively easy to build and operate.

The Step-Down CCV is further classified into three types as shown below we will look into each of these types in detail in this article. Although there are three different types of Cycloconverters, the working of them are very similar except for the number of power electronic switches present in the circuit.

The bare minimum for a Cycloconverter is shown above. It will have a Switching circuit on either side of the Load, one circuit will function during the positive half cycle of the AC power source and the other circuit will function during the negative half cycle.

The switching circuits will also need a control circuit, which instructs the Power electronic device when to conduct and when to turn off. This control circuit will normally be a Microcontroller and might also have a feedback from the output to form a closed loop system. The user can control the value of output frequency by adjusting the parameters in the control circuit. The diodes in the above diagram are used to represent the direction of flow of current.

The positive switching circuit always sources current into the load and the negative switching circuit always sinks current from the load. One set is placed straight while the other is placed in anti-parallel direction as shown on the picture below. This control circuit will be responsible for triggering the SCRs. Let us analyse the wave form below to understand how frequency is stepped down using a CCV.

Three-Phase to Three-Phase 3f-3f Cycloconverter. The resulting cycloconverters are shown in Figs. 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. On the other hand, the 3f-3f bridge cycloconverter is also called a 6-pulse cycloconverter or a thyristor cycloconverter. The operation of each phase is explained in the previous section. The three-phase cycloconverters are mainly used in ac machine drive systems running three- phase synchronous and induction machines.

They are more advantageous when used with a synchronous machine due to their output power factor characteristics. A cycloconverter can supply lagging, leading, or unity power factor loads while its input is always lagging. A synchronous machine can draw any power factor current from the converter.

This characteristic operation matches the cycloconverter to the synchronous machine. On the other hand, induction machines can only draw lagging current, so the cycloconverter does not have an edge compared to the other converters in this aspect for running an induction machine.

However, cycloconverters are used in Scherbius drives for speed control purposes driving wound rotor induction motors. Cycloconverters produce harmonic rich output voltages, which will be discussed in the following sections.

When cycloconverters are used to run an ac machine, the leakage inductance of the machine filters most of the higher frequency harmonics and reduces the magnitudes of the lower order harmonics.

The operation of the cycloconverters is explained above in ideal terms. When the load current is positive, the positive converter supplies the required voltage and the negative converter is disabled. On the other hand, when the load current is negative, then the negative converter supplies the required voltage and the positive converter is blocked.

This operation is called the blocked mode operation, and the cycloconverters using this approach are called blocking mode cycloconverters. However, if by any chance both of the converters are enabled, then the supply is short-circuited. To avoid this short circuit, an intergroup reactor IGR can be connected between the converters as shown in Fig.

Instead of blocking the converters during current reversal, if they are both enabled, then a circulating current is produced. This current is called the circulating current. It is unidirectional because the thyristors allow the current to flow in only one direction. Some cycloconverters allow this circulating current at all times.

These are called circulating current cycloconverters. Therefore, the cyclo-converter can operate in four quadrants, i. The above figure shows the conversion of three phase supply at one frequency to single phase supply of lower frequency. In this, the firing angle to a positive group of thyristors is varied progressively to produce single phase output voltage.

At point M, the firing angle is 90 degrees and it is reduced till point S where it is zero. Again from point T to Y, the delay angle is progressively increased. This varied triggering signals to the thyristors, varies its conduction time periods and hence the frequency of the output voltage. Three-phase to Three-phase Cyclo-converters These are obtained by connecting 3 three-phase to single-phase cyclo-converters to the load.

These converters can be connected in star or delta. Three phase cyclo-converter of both half-wave and bridge types are shown in figure below. Three-phase to three-phase cyclo-converter is also called as thyristor cyclo-converter or 3-pulse cyclo-converter and three-phase to three-phase bridge type cyclo-converter is called as 6-pulse cyclo-converter or thyristor cyclo-converter.

This converter consists of six groups of converter circuits where three groups are called as positive group while other three are negative group. During each positive half cycle, positive group carries the current and during negative half cycle, negative group carries the current.

The duration for conduction of each group of thyristor determines the desired output frequency. Here average value of output voltage is varied by varying the firing or delay angle of SCRs conduction whereas the output frequency can be varied by changing the sequence of firing the SCRs.

The neutral connection is no longer necessary with a balanced load and hence this connection can be omitted.

Three-phase cyclo converters are more popular than single-phase type as these can handle very large currents and produce smooth output waveform. It is a highly efficient variable frequency drive, because the pulse number is increased due to the large number of thyristors, which causes small ripple content in the output voltage waveform.

Cyclo-converters are mainly used for producing low frequency AC voltage. In such systems, three-phase squirrel cage induction motor is controlled by a suitable cyclo-converter circuit. Other applications of Cyclo-converter include. The major advantage of cyclo-converter is that it uses a line commutation technique for turning OFF the thyristors, thus it eliminates the need of forced commutation. And these are also more efficient than DC link converters which require two power conversion stages.

But, the major problem with cyclo-converter is that it produces much distorted waveforms for higher frequencies. Also the power factor of the system gets affected when controlling the speed of AC motor using cyclo-converter.

It requires a large number of thyristors. In addition, these converters are more costly. Due to these reasons, cyclo-converters have limited applications. Nowadays, most of the cyclo-converters are being replaced with variable frequency drives. Your email address will not be published. Cyclo-Converters — Principle,Applications.