Three Phase Drives: Single phase drives discussed in the previous section are employed for low and medium powers. When the power of the drive is very large the three-phase converter is preferred for supplying the load. As has been described early that increase in the number of pulses of the output voltage improves the performance. […]
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Current Source Inverter with Forced Commutation: Current Source Inverter with Forced Commutation – As has already been discussed, the disadvantages of machine commutation are (a) limitation on the speed range (b) the machine size is large and due to overexcitation it is underutilised. CSI with individual commutation for feeding synchronous motor. 1.used for providing commutation
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Synchronous Speed on Variable Frequency Supply: As has already been discussed, the speed of a synchronous motor can be varied by supply frequency variation similar to an induction motor. The applied voltage must be varied in this case also to have the maximum torque capability and to avoid saturation at all frequencies. A Synchronous Speed
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Static Scherbius Drive: The main limitation of the Kraemer drive discussed is the operation at sub-synchronous range due to diode rectifier. Super synchronous speeds can be achieved if the power is fed to the rotor from the ac mains. This can be made possible by replacing the converter cascade by a Static Scherbius Drive system using
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Braking and 4 Quadrants Operation: Subsynchronous converter cascades have been used, till now, in applications requiring one quadrant operation. These can be employed for drives where at least one electrical braking is required. A 4 Quadrants Operation can also be made possible in these cascades, using suitable switching. The braking is obtained by connecting the
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Speed Control Using Slip Energy Recovery Schemes: The foregoing discussion makes it very clear that the methods of voltage control and rotor resistance control have poor efficiency, particularly at low speeds, and find limited application. The Slip Energy Recovery Schemes is wasted in the rotor resistance, either inherent in the rotor (high resistance rotors for
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Chopper Resistance in the Rotor Circuit: Speed control by means of slip variation can be achieved by employing a variable resistance Chopper Resistance in the Rotor Circuit. From the equations it is clear that the maximum value of torque does not depend upon the value of rotor resistance. However, the rotor resistance influences the slip
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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.
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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: 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
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