Electric motors with permanent magnets with two-zone rotational speed control

Abstract

An outstanding feature of electric motors with permanent magnets is their high efficiency. It is highest among all the motors used in electric drive applications. Permanent magnet synchronous motors (PMSM) dedicated for use in electric traction drives or in automotive vehicles should be additionally characterized by a high maximal to nominal torque coefficient and by a wide range of speed control. Extension of rotational speed control range can be achieved by a weakening of flux inside the motor thanks to appropriately controlled negative armature reaction in direct axis of rotor. In such drives, rotational speed control is divided in two zones. In the first zone with rotational speeds from zero up to the so called base speed n <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sub> , PMSM operates with constant voltage to frequency ratio U <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> /f, it means with the constant magnetic flux Phi and electromagnetic torque T developed by PMSM is proportional to the supply current I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> . At the base speed n <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sub> the voltage U <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> at motor terminals reaches the maximal value U <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1max</sub> which can be delivered by the inverter supplied for example from the automotive battery bank. Over the base speed n <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sub> PMSM operates in the second zone of speed control, in which the increasing of rotational speed is possible only with the appropriate weakening of flux in the airgap of motor. Flux weakening is increased proportionally to increasing of rotational speed, so the voltage at motor terminals is kept constant at its maximal allowable value U <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1max</sub> . In the second zone of speed control PMSM operates with constant supply voltage U <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> , with frequency f increased proportionally to increasing of speed n and with magnetic flux Phi proportional to 1/n. Assuming constant value of supply current I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> , the mechanical power Pm developed by motor in the second zone of speed control is approximately also constant. Because of the fact, that significant part of supply current I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> is used for producing a negative armature reaction to achieve appropriate flux weakening, the electromagnetic torque T is no longer in linear relation with supply current I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> in the second zone, but it is approximately proportional to 1/n. In the paper the calculated electromechanical characteristics of PMSM designed for use in electric vehicle are presented. This motor operates with two-zone speed control. The motor was designed in two versions: with magnets mounted on the rotor surface and with magnets mounted inside the rotor magnetic core. The static characteristics obtained from laboratory tests carried out on prototype of PMSM are also presented. The motor is predicted as main drive of small electric car.