Lav-pris og lav-effekt Drivsystemer / Low-Cost and Low-Power Motor Drives

This project considers power electronic converters, that interfaces a single phase utility grid and a three phase induction motor, for variable speed control of a three phase induction motor. The converters subjected to analysis are able both to control the shaft speed of the induction motor and to control the current drawn from the utility grid. As the IEC 61000-3-2 standard claims for circuit topologies that reduce the harmonic content of the input current, emphasis is placed on the rectifier part as an element in a low-power and low-cost motor drive. Compliance with IEC 61000-3-2 is achieved by shaping the input current by a Power Factor Correction (PFC) circuit. Both active and passive solutions exist, but an active PFC is the used as this is advantageous in terms of size, price and performance. A specific single to three phase converter topology is selected and this converter topology can operate in different modes which makes it possible to compare different converter and motor drive topologies. The converter consists of a special rectifier topology (PFC circuitry) and a conventional three phase inverter connected in series. As the rectifier is able to control the DC-voltage that interfaces the PFC and the inverter this can be utilized in the inverter control in order to increase the converter (Rectifier+inverter) efficiency: usually the inverter controls the motor by adjusting both the motor voltage and the motor frequency (Pulse-Width-Modulation PWM control), but as the rectifier can vary the dc-voltage the inverter tasks can be reduced to control the frequency of the motor voltage (Pulse-Amplitude-Modulation PAM control). This provides lower switching losses in the inverter and the converter efficiency is thereby increased. The PFC circuitry has two tasks: To control the DC-voltage, and to shape the grid current to comply with the IEC 61000-3-2. The ideal PFC has a unity power factor which means that the PFC circuitry loads the grid as a pure resistor. Thus, for an ideal PFC the grid current is proportional to the grid voltage and for an ideal sinusoidal grid voltage the grid current is also sinusoidal. The challenge for making the PFC a resistor emulator is to make a controller that can track sinus-reference signal as this cannot be solved by a traditional PI controller. This project has put focus on modelling, controlling and designing the PFC part of the motor drive. Both advanced control algorithms for unity power factor control and simple control algorithms that just aim to comply with the regulation. Three (one advanced and two simple) new control algorithms have been proposed an realised and an advanced design tool for the simple control algorithms is developed. As the rectifier can operate both with a constant DC-voltage (boost rectifier) and with a variable DC-voltage (buck-boost) both PWM and PAM controlled motor drives are compared regarding efficiency and torque ripple. This project is a part of the Danfoss professor Program. (Gert K. Andersen, Frede Blaabjerg, John K. Pedersen)