Segmented motor drive - with multi-phase induction motor
Publikation: Forskning › PhD. afhandling
This PhD project commences in modulation of motor drives, i.e. having the advantage of reducing the number of variants and improves the system reliability at error situations. Four different motor drive topologies with modular construction as common denominator are compared on a general level. The multi-phase motor is selected for further analysis. The project is limited to examine if increasing the number of phases can improve the characteristics for induction motor drives.
In the literature it is demonstrated that torque production in a six-phase motor can be increased, if a 3rd harmonic current with 1/6 amplitude is added to the 1st harmonic current. This claim is verified and the optimization of the motor design is extended to, beyond the stator tooth width, also to include the inner diameter of the stator. This means that the lamination sheet is optimized according to two geometrical dimensions. The possible torque increase proves to be strongly dependent on the physical dimensions in the initial three-phase motor. The torque increase according to the optimization is listed for a range of Grundfos motors, but in most cases the increase is only a few percent. In a single example with a six-pole motor a torque increase of 28.5% is however found. A plausible tendency is that when the number of poles is high the torque increase is also high, which is based on the trend that the stator yoke in these cases is small compared to the inner stator diameter.
A general multi winding model of the induction motor is set up. The model is able to calculate dynamical electric, magnetic and mechanic state variables, but initially it is used to calculate static characteristics in motors with different number of phases and different voltage supply shapes. This analysis show i.e. that the efficiency of the motor is not influence much by increasing the number of phases, regardless if the supply voltage is sinusoidal, sinusoidal with a 3rd harmonic component or square. The only exception is that the efficiency is lower if the number of phases is low (three) and at the same time the supply is sinusoidal with 3rd harmonic or square. Another tendency is that the torque ripple is decreased as the number of phases is increased, regardless of the supply type used. Torque ripple can be a source of acoustic noise generation, in this context a multi-phase motor can therefore be an advantage. According to various sources bearing currents, which in worst case conditions can destroy the bearings, on a multi-phase motor can be reduced considerably by using special pulse wide modulation strategies. This topic is however not analyzed further in this thesis.
A six-phase motor drive system is built in the laboratory, and a simple scalar control algorithm is implemented as a test example, to demonstrate the multi winding model. A good agreement between model and measurement is shown.
A more advanced control algorithm based on standard vector control is build for the six-phase motor. This control strategy enable, together with a return wire from the star-point of the motor back to the mid-point of the DC-link capacitors, a 3rd harmonic current component in the motor. Controllers for the speed loop and current loops are designed. In addition to this a controller for stabilization of the mid-point potential in the DC-link, which is influenced by the third harmonic current, is developed. The control strategy is demonstrated on the build test system and a good agreement between simulation and measurements is found. Hereafter reliability in an error situation, where three phases are disabled online, is tested. This only yields a small speed disturbance, which is compensated by the speed controller. The consequence of the 3rd harmonic current, which in fact triples the number of poles on the motor, is demonstrated by driving the motor and a small load entirely with a 3rd harmonic current.
It is concluded that multi-phase motors are more redundant, have lower torque ripple, have possibility for low bearing current and can divide the power in more power-components than ordinary three-phase motor drives. They are therefore in particular qualified for big motor drives. Dependent on the lamination dimensions the torque production from a multi-phase motor can be increased by adding higher harmonic currents. This is in particular an advantage for multi-phase motors.
|Udgiver||Institut for Energiteknik, Aalborg Universitet|