Control in Power Electronics : selected problems

The machine in accordance with the present disclosure is an AC machine whose pole numbers can be switched (from pole p 1 to pole p 2 ), and whose number of series turns per phase N can be switched say from N 0 =N rated to N 1 =N 0 /2. Furthermore, it employs an inverter so that the frequency can be changed from a low value (e.g., 5 Hz) to a high value (e.g., 200 Hz). Due to the combination of pole number and number of series turns switching/reconfiguration, a high torque at low speed (e.g., 0 rpm) and a high torque at high speed (e.g., 5,000 rpm) can be achieved, making mechanical gears obsolete. In addition, the output power of the motor can be increased at high speed in direct proportion to the speed increase.

Alternating current machine with increased torque above and below rated speed for hybrid electric propulsion systems

Automotive starter-generator applications require a high peak torque for cranking and a wide speed flux weakening region for generation. In such applications, the pole-changing winding induction machine offers a wide speed flux weakening region with a low pole number and a high peak torque capability when operated with a higher number of poles. In this article, a 6-phase, electronic pole-changing winding induction machine with improved performance over the existing 3-phase pole-changing is discussed. Compared to the 3-phase 120o phase belt winding, the proposed 6-phase pole-changing winding induction machine shows a 15 % increase in peak torque due to improved winding factor. Finite element analysis is used to predict the torque capability, losses and efficiency of the 6-phase and 3-phase pole-changing winding induction machines. A 1kW induction machine is prototyped and the improved torque capability of the 6-phase pole-changing machines over 3-phase counterparts is experimentally validated under various operating conditions.

Six-Phase Pole-Changing Winding Induction Machines With Improved Performance

The concept of online electronic switching of windings will be applied to a single-phase induction motor with two separate windings with different pole numbers p1 = 4 and p2 = 6 and an auxiliary winding required for starting. During switchover from the low-speed winding to the high-speed winding, and vice versa, lasting less than a few milliseconds, small electrical torque and speed transients occur. Relying on closed-loop speed control, a smooth speed response is measured. This concept is, therefore, applicable to variable-speed drives, where a large speed range is desirable. Test data indicate that a speed-control range from 600 to 4000 r/min is obtainable with a single-phase induction motor.

Experimental Verification of Critical-Speed Increase of Single-Phase Induction Machines via Winding Reconfiguration With Solid-State Switches

Electric drives for automobiles require at low speed for starting a torque which is a multiple of the rated torque, and for above rated speed a torque which is about the rated torque. Known (V/f) control results in a decreasing torque above rated operation while the (V · p)/(f · N) control -where the terminal voltage V=3DV t is less or equal to V rated , the number of series turns of the stator winding N is less or equal to N rated - increases torque, speed operation, and power rating. This new control is accomplished by employing pole-changing techniques and/or changing/switching the number of series turns per phase N of an alternating current/voltage machine combined with the use of an inverter/rectifier supplying/absorbing power to/from the machine when operated as a motor/generator. The switching techniques have about the same effect as a mechanical gear.

Speed and torque range increases of electric drives through compensation of flux weakening

Commonly used speed-control concepts permit speed ranges up to two-three times the base speed via voltage to frequency (V/f), that is, field-weakening control. This paper teaches the extension of the speed-control range up to nine times the base speed through online reconfiguration of the motor windings via electronic switches. The switchover of the windings, either from a winding with p1 poles to p2 poles, or from series to parallel connection of the number of turns per phase-so-called (V/Nldrf) control-lasts less than a 60-Hz cycle. Such fast switchover causes small transients only, and therefore, this concept is applicable to most variable-speed drives.

J. Schraud

Papers

Analysis of Critical-Speed Increase of Induction Machines via Winding Reconfiguration With Solid-State Switches