• Document: ELECTRONIC CONTROL OF A.C. MOTORS
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CONTENTS Lear ning Objectiv Learning + 0 ) 2 6 - 4 es Objectives "$ Ï Classes of Electronic AC Drives ELECTRONIC Ï Variable Frequency Speed Control of a SCIM Ï Variable Voltage Speed CONTROL OF Control of a SCIM Ï Chopper Speed Control of a WRIM A.C. MOTORS Ï Electronic Speed Control of Synchronous Motors Ï Speed Control by Current- fed D.C. Link Ï Synchronous Motor and Cycloconverter Efficient control of motors becomes critical Ç where high precision, accuracy, flexibility, reliability and faster response are of para- mount importance. Electronic and digital controls are employed in such conditions CONTENTS 1824 Electrical Technology 46.1. Classes of Electronic A.C. Drives AC motors, particularly, the squirrel-cage and wound-rotor induction motors as well as synchronous motors lend themselves well to electronic control of their speed and torque. Such a control is usually exercised by varying voltage and frequency. Majority of the electronic a.c. drives can be grouped under the following broad classes : 1. static frequency changers like cyclo-converters which convert incoming high line frequency directly into the desired low load frequency. Cyclo-converters are used both for synchronous and squirrel-cage induction motors. 2. variable-voltage controllers which control the speed and torque by varying the a.c. voltage with the help of SCRs and gate turn-off thyristors (GTOs). 3. rectifier-inverter systems with natural commutation. 4. rectifier-inverter systems with self-commutation. Fig. 46.1 46.2. Variable-frequency Speed Control of a SCIM Fig. 46.1 shows a 3-phase SCIM connected to the outputs of three 3-phase cycloconverters. As seen, each cyclo-converter consists of two 3-phase thyristor bridges, each fed by the same 3-phase, 50-Hz line. The +R bridge generates the positive half-cycle for R-phase whereas −R generates the negative half. The frequency of the cycloconverter output can be reduced to any value (even upto zero) by controlling the application of firing pulses to the thyristor gates. This low frequency permits excellent speed control. For example, the speed of a 4-pole induction motor can be varied from zero to 1200 rpm on a 50-Hz line by varying the output frequency of the cycloconverter from zero to 40 Hz. The stator voltage is adjusted in proportion to the frequency in order to maintain a constant flux in the motor. Electronic Control of A.C. Motors 1825 This arrangement provides excellent torque/speed characteristics in all 4-quadrants including regenerative braking. However, such cycloconverter-fed motors run about 10°C hotter than normal and hence require adequate cooling. A small part of the reactive power required by SCIM is provided by the cycloconverter, the rest being supplied by the 3-phase line. Consequently, power factor is poor which makes cycloconverter drives feasible only on small and medium power induction motors. 46.3. Variable Voltage Speed Control of a SCIM In this method, the speed of a SCIM is varied by varying the stator voltage with the help of three sets of SCRs connected back-to back (Fig. 46.2). The stator voltage is reduced by delaying the firing (or triggering) of the thyristors. If we delay the firing pulses by 100°, the voltage obtained is about 50% of the rated voltage which decreases the motor speed considerably. Fig. 46.2 Unfortunately, I2R losses are considerable due to distortion in voltage. Moreover, p.f. is also low due to large lag between the current and voltage. Hence, this electronic speed control method is feasible for motors rated below 15 kW but is quite suitable for small hoists which get enough time to cool off because of intermit- tent working. Of course, p.f. can be improved by using special thy- ristors called gate turn-off thyristors (GTOs) which force the current to flow almost in phase with the voltage (or even lead it). 46.4. Speed Control of a SCIM with Rectifier- Inverter System A rectifier-inverter system with a d.c. link is used to control the speed of a SCIM. The inverter used is a self-commuted type (differ- ent from a naturally commutated type) which converts d.c. power A commonly used electronic into a.c. power at a frequency determined by the frequency of the power inverter 1826 Electrical Technology pulses applied to the thyristor gates. The rectifier is connected to the 3-phase supply line whereas the inverter is connected

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