Variable Frequency AC Motor Drive Systems
AC motors. Power switching devices. The six-step voltage source inverter for induction motors. The pulse width modulated voltage source inverter for induction motors. The six-step current source inverter drive. The six-step synchro-convertor system for synchronous motors. The current source inverter for the capacitor self-excited induction motor. The cycloconvertor. The slip energy recovery system for wound rotor induction motors.
Inspec keywords: synchronous motors; induction motors; PWM invertors; AC motor drives
Other keywords: synchronous motors; power switching devices; self excited induction motor; six step synchroconverter system; variable frequency AC motor drive systems; cycloconvertor; power switching circuits; pulse width modulated voltage source inverter; six step current source inverter drive; six step voltage source inverter; induction motors
Subjects: Asynchronous machines; Drives; Synchronous machines
- Book DOI: 10.1049/PBPO008E
- Chapter DOI: 10.1049/PBPO008E
- ISBN: 9780863411144
- e-ISBN: 9781849194266
- Page count: 404
- Format: PDF
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Front Matter
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1 AC motors
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This chapter deals with conventional AC machines with three-phase windings, machines which have been designed for use on standard power frequency supply networks or which are derived from such machines. This means cage or wound-rotor induction motors and synchronous machines of the salient-pole, cylindrical, slip-ring or brushless types.
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2 Power switching devices
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There is a very wide range and variety of semiconductor switching devices which can be considered for use in AC variable speed drives, and this chapter explains the principles, the capabilities and the performance characteristics of those devices which are most likely to be used for this purpose. The chapter deals with semiconductor devices specifically from an AC variable speed drive point of view; it is not in any way an exhaustive study of all semiconductor devices available. As such, the devices have been split into three broad classifications, namely thyristors, transistors and the gate turn-off thyristor (GTO).
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3 Power switching circuits for variable speed drives
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In this, the last preliminary chapter before getting into the drive systems themselves, we consider the circuits in which the switches already discussed will be used. The three-phase bridge or double way circuit is now almost universally used in variable-speed drive systems, but its operation varies with the type of switches being used and the overall characteristics of the remainder of the system. In its naturally commutated form, it can operate in its rectifying or regenerative mode, depending on whether the power flow is from the AC to the DC or vice versa. The bridge circuit is also used for motor convertors to direct the DC link power to the correct motor windings. In this case, its operation depends on whether the circuit in total has a high or a low impedance, i.e. whether it is a current source or voltage source system.
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4 The six-step voltage source inverter for induction motors
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This system was one of the earliest DC link induction motor drive systems to be developed and it came into use soon after the principles of forced commutation of thyristors became established in the 1960s. This system consists of a convertor to change a fixed frequency, fixed voltage mains supply into variable voltage DC, followed by a forced commutated inverter to convert the DC to a variable frequency AC output. In this case, the output is a quasi-square voltage waveform which is very well suited to supply the most reliable and robust of motors, the induction motor.
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5 The pulse width modulated voltage source inverter system for induction motors
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This pulse-width modulated system is the most widely used method of improving the low-speed performance of DC link inverter systems. The principle is to use high-speed switching to enable the motor current waveshapes at low speed to be more sinusoidal, and hence lead to a smoothly rotating magnetic field in the motor.
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6 The six-step current source inverter drive
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This chapter discusses the design of six-step current source inverter drives. In this system, the inverter switches operate to alter the path the current takes through the circuit and the motor, directing it to those motor windings which will cause the appropriate level and direction of torque to be produced. Whereas in the voltage source design, the convertor produces a voltage to the motor and the current drawn by the motor then takes up whatever value is needed, in this circuit the current is applied to the motor and the circuit voltages take up the value and waveform they need to ensure the correct operating conditions.
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7 The six-step synchro-convertor system for synchronous motors
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In this chapter, this current source drive is naturally commutated and specifically for use with synchronous motors. Because it uses convertor-grade thyristors, it is possible to produce the larger power ratings economically and systems operating at up to 11 kV for high-power applications. This drive has been variously called the synchro-drive, the load commutated inverter, the brushless DC motor, etc., and it is used under various trade names such as SyncDrive (G.E.C.), LCI (G.E.) and Synchrosil (Brush). The principle used in this drive was first used as a means of electrically starting large pumped-storage motors. The convertor was applied to the machine stator terminals and the frequency was slowly increased to accelerate the large generator with its turbine. When it was up to speed, the generator could be synchronised on to the supply system busbars and the convertor disconnected. It is still used for this and similar purposes, but it now has a much wider role as a fully controlled variable-speed drive suitable for many industrial applications.
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8 The current source inverter for the capacitor self-excited induction motor
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This system is a combination of the two previous current source systems explained in Chapters 6 and 7. It is a system which enables induction motors to be controlled by a naturally commutated convertor, hence making it possible for large power and high-voltage drives to be produced. It can be considered as a synchro-convertor for use with induction motors or as a Ioad-commutated inverter for induction motors.
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9 The cycloconvertor
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The cycloconvertor is a direct frequency convertor without an intermediate DC link which can convert power from one fixed frequency to a lower variable frequency. The cycloconvertor is a mains commutated system which has been known about since the advent of grid controlled mercury arc rectifiers in the 1930s. The system was then used extensively to produce 162 3 hertz power for traction applications, from the 50 hertz mains supply. Since that time, it has been used for induction heating and for low-frequency arc furnaces for slag refining. It has also been used for motor drives since the late 1940s, when it was used to supply low-frequency roller table drives in steel mills, using this time steel tank mercury arc rectifiers. It has been used more widely since the advent of thyristor switches and its most important application is for supplying large synchronous motors driving low speed cement mill furnaces where units up to 8,000 kW have been built operating at up to 10 hertz. It has been and still is being used, though less extensively, for a variety of low frequency applications from steel rolling mills and tables, for mine hoist drives and for ship propulsion drives.
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10 The slip energy recovery system for wound rotor induction motors
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The overall principle of slip energy recovery systems (or 'static Kramer systems') is to insert a variable back-EMF into the rotor circuit in such a way that the resultant energy can be recovered and fed back into the AC mains network which is feeding the stator of the induction motor. The result can then be an efficient method of reducing the speed of the motor. These principles were initially established using motor generator sets to achieve the energy recovery and feedback, with a DC motor to absorb the energy and an AC generator to return the power to the mains network. However, due to high cost and the relatively high power losses in such schemes, the principles of slip energy recovery are now universally applied using static convertors. This system requires the use of a wound rotor induction motor with slip rings to connect into the rotor circuit. It therefore tends to be used in custom designed systems where the motors and convertors are specifically chosen for the application. It is used for drives in the hundreds or thousands of kilowatts ratings where the cost of a specially designed system can be justified.
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Back Matter
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