Controlling an a.c. induction motor by the technique of sinewave-weighted pulse-width modulation (PWM) switching gives the benefits of smooth torque at low speeds, andalso complete speedcontrol from zero upto the nominal rated speed of the motor, with only small additional motor losses.
Traditional power switches such as thyristors need switching frequencies in the audible range, typically between 400 and 1500Hz. In industrial environments, the small amount of acoustic noise produced by the motor with this type of control can be regarded as insignificant. By contrast, however, the same amount of noise in a domestic or office application, such as speed control of a ventilation fan, might prove to be unacceptable.
Now, however, with the advent of power MOSFETs, three-phase PWM inverters operating at ultrasonic frequencies can be designed. A three-phase motor usually makes even less noise when being driven from such a system than when being run directly from the mains because the PWM synthesis generates a purer sinewave than is normally obtainable from the mains.
The carrier frequency is generally about 20kHz and so it is far removed from the modulation frequency, which is typically less than 50Hz, making it economic to use a low-pass filter between the inverter and the motor. By removing the carrier frequency and its sidebands and
harmonics, the waveform delivered via the motor leads can be made almost perfectly sinusoidal. RFI radiated by the motor leads, or conducted by the winding-to-frame capacitance of the motor, is therefore almost entirely eliminated. Furthermore, because of the high carrier frequency, it is possible to drive motors which are designed for frequencies higher than the mains, such as 400Hz aircraft motors.
This section describes a three-phase a.c. motor control system which is powered from the single-phase a.c. mains. It is capable of controlling a motor with up to 1kW of shaft output power. Before details are given, the general principles of PWM motor control are outlined.
Traditional power switches such as thyristors need switching frequencies in the audible range, typically between 400 and 1500Hz. In industrial environments, the small amount of acoustic noise produced by the motor with this type of control can be regarded as insignificant. By contrast, however, the same amount of noise in a domestic or office application, such as speed control of a ventilation fan, might prove to be unacceptable.
Now, however, with the advent of power MOSFETs, three-phase PWM inverters operating at ultrasonic frequencies can be designed. A three-phase motor usually makes even less noise when being driven from such a system than when being run directly from the mains because the PWM synthesis generates a purer sinewave than is normally obtainable from the mains.
The carrier frequency is generally about 20kHz and so it is far removed from the modulation frequency, which is typically less than 50Hz, making it economic to use a low-pass filter between the inverter and the motor. By removing the carrier frequency and its sidebands and
harmonics, the waveform delivered via the motor leads can be made almost perfectly sinusoidal. RFI radiated by the motor leads, or conducted by the winding-to-frame capacitance of the motor, is therefore almost entirely eliminated. Furthermore, because of the high carrier frequency, it is possible to drive motors which are designed for frequencies higher than the mains, such as 400Hz aircraft motors.
This section describes a three-phase a.c. motor control system which is powered from the single-phase a.c. mains. It is capable of controlling a motor with up to 1kW of shaft output power. Before details are given, the general principles of PWM motor control are outlined.
ليست هناك تعليقات:
إرسال تعليق