CONVERTERS FOR ELECTRIC MOTORS

Field Oriented Control of Induction Motor

Field Oriented Control of Induction Motor: In the slip controlled drives using VSI or CSI discussed in the foregoing sec­tions, the stator voltage or stator current is controlled using slip frequency. They are controlled in magnitude only. The stator current control does not take care of its phase position with respect to flux. The control […]

Three Phase AC Voltage Controller

Three Phase AC Voltage Controller: To control the current and voltage of three phase loads, Three Phase AC Voltage Controller are required. The single phase controller described previously can be introduced singly in each phase or line, to form a three phase controller. There exist a variety of connections for Three Phase AC Voltage Controller. […]

Cycloconverter

Cycloconverter: A variable frequency supply for feeding a three phase motor can be obtained from a cycloconverter which operates on a 50 Hz supply and provides a variable frequency supply at the output (Fig. 3.105). These are single stage frequency conversion devices which make use of both line commutation as well as forced commutation. The […]

Harmonic Reduction

Harmonic Reduction: The output voltage waveform of an inverter is non-sinusoidal. It contains a rich harmonic content. The Harmonic Reduction cause additional losses and torque pulsations if a three phase motor is used as a load. These torque pulsations pose a problem at low speeds. Therefore it is necessary to improve the voltage waveform of […]

Inverter Control for Variable Link Voltage

Inverter Control for Variable Link Voltage: It has already been mentioned that Inverter Control providing a variable frequency supply to three phase motors should be capable of providing a variable voltage. This is required to avoid saturation and ensure operation at constant flux density. The voltage control can be affected either external to the Inverter […]

Current Source Inverter for Feeding Three Phase Motors

Current Source Inverter for Feeding Three Phase Motors: In these type of Current Source Inverter, the controlled quantity is the current in the dc link. The current from the dc source remains constant at the controlled value, irrespective of the load and events in the inverter. The voltage across the load adjusts itself. The link […]

Features of Variable Voltage Inverters

Features of Variable Voltage Inverters: The inverter has an impressed dc voltage. The output voltage of the inverter is decided by the firing and duration of the thyristors. The conduction of the thyristors can be either 180° or 120°, depending upon the control employed. In the case of 180° conduction three thyristors conduct at any […]

Three Phase Inverter

Three Phase Inverter: The variable frequency required for the speed control of three phase ac motors is obtained from a Three Phase Inverter. To avoid magnetic saturation and to obtain constant flux conditions in the machine, the voltage fed to the motor must also be varied. Therefore an inverter feeding a three phase motor must […]

Charging a Capacitor in Inverter

Charging a Capacitor in Inverter: Charging a Capacitor in Inverter – Inverters are static power converters for converting dc to ac. By controlling the conducting periods of the thyristors it is possible to obtain variable frequency at the output terminals of the inverter. This variable frequency supply can be used to feed an ac motor, […]

Chopper Controlled Resistance

Chopper Controlled Resistance: By interrupting the flow of current through a resistance R in a periodic fashion using a switch, the value of resistance can be effectively varied. The interrup­tion of the current can be accomplished by means of a dc chopper connected across the resistance, working as a switching device. When the chopper is […]

Selection of Ratings of Devices of a Chopper Converter

Selection of Ratings of Devices of a Chopper Converter: Chopper Converter – The waveforms of currents and voltages are useful in deciding the ratings of devices. The main thyristor carries the load current. Superimposed on this there is a current pulse during the recharging process of the capacitor through the resonating circuit. Therefore the peak […]

Two Quadrant Chopper

Two Quadrant Chopper: Sometimes a chopper may be required to provide a two quadrant operation by retaining the current direction in both motoring and braking modes. Such a Two Quadrant Chopper is shown in Fig. 3.86, and its waveforms in Fig. 3.87. The chopper permits a change in the polarity of terminal voltage keeping the […]

Regenerative Braking

Regenerative Braking: Sometimes the energy of the load may have to be fed to the supply system. A Chopper in this mode is called regenerative. The chopper working as a switch is connected across the load, which is normally a dc machine and the diode is connected in the line. The dc machine works as […]

Methods of Controlling Chopper Circuit

Methods of Controlling Chopper Circuit: The basic principle of control of a Chopper Circuit is the effective change of the value of the time ratio. This is done in two ways: 1.Time ratio control 2.Current limit control In the former, the ratio TON/T is varied. This can be achieved by varying TON, keeping the chopper […]

Performance of Chopper in Steady State Condition

Performance of Chopper in Steady State Condition: Assuming Steady State Condition of the chopper, we have during TON The solution of Eq. 3.89 is where Io is the current at the starting of TON. At TON = aT, the current is when the thyristor is OFF, free wheeling takes place and the operation is described […]

Chopper Circuit with Input Inductance

Chopper Circuit with Input Inductance: The preceding discussion of the chopper assumes ideal conditions and does not consider the effects of inductances in series with the thyristors. With choppers of large power, the leads connecting the Chopper Circuit to the supply have a definite amount of inductance (Le). Also, reactors are placed in series with […]

DC Chopper

DC Chopper: DC Chopper are mainly dc to dc single stage conversion devices which provide a variable voltage on the load side when fed from a constant dc voltage source. The commutation of the current from the thyristors cannot be achieved by means of supply voltage. The necessary reactive power for the converter must be […]

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