John S. Hsu

Monitoring of defects in induction motors through air-gap torque observation

This paper suggests a method to monitor defects such as cracked rotor bars and the shorted stator coils in induction motors. Air-gap torque can be calculated while the motor is running. No special down time for measurement is required. Data of the air-gap torque for a motor should be periodically kept for comparison purposes. Since more data than just a line current are taken, this method offers other potential possibilities that cannot be handled by examining only a line current. The theoretical foundation for this proposed method is presented. Experiments conducted on a 5-hp motor show the validity and potential of this approach. Further studies are planned to extend the proposed method in detail and to monitor defects developed in other types of rotating machines. >

Direct control of air-gap flux in permanent-magnet machines

A new field weakening and adjustment technology for PM machines by direct control of air-gap fluxes is introduced. This new technology requires no current decomposition by the inverter eliminating the need for a position sensor. Demagnetization, which normally occurs during field weakening, does not occur with this new method. The field-weakening ratio can reach 10:1 or higher. This technology is robust and particularly useful for, but not limited to, electric vehicle drives and PM generators.

Field test of motor efficiency and load changes through air-gap torque

Although air-gap torque equations have been known for several decades, this is the first known paper suggesting the use of air-gap torque to measure efficiency and load changes of electric motors in the field. Very few assumed values are required, and the method is applicable for induction motors as well as brushless-DC adjustable-speed and synchronous motors. Theoretical foundation for this proposed method is presented. Experiments conducted on an adjustable-speed motor show the validity and potential of this approach. >

A five-leg inverter for driving a traction motor and a compressor motor

This paper presents an integrated inverter for speed control of a traction motor and a compressor motor to reduce the compressor drive cost in electric vehicle/hybrid electric vehicle applications. The inverter comprises five phase-legs; three of which are for control of a three-phase traction motor and the remaining two for a two-phase compressor motor with three terminals. The common terminal of the two-phase motor is tied to the neutral point of the three-phase traction motor to eliminate the requirement of a third phase leg. Further component reduction is made possible by sharing the switching devices, dc bus filter capacitors, gate drive power supplies, and control circuit. Simulation and experimental results are included to verify that speed control of the two motors is independent from each other

Field assessment of induction motor efficiency through air-gap torque

Induction motors are the most popular motors used in industry. This paper further suggests the use of the air-gap torque method (see J.S. Hsu et al., ibid., vol.10, no.3, p.471-7, 1995) to evaluate their efficiency and load changes. The fundamental difference between Method E and the air-gap torque method is discussed. Efficiency assessments conducted on induction motors under various conditions show the accuracy and potential of the air-gap torque method.

An integrated traction and compressor drive system for EV/HEV applications

This paper presents an integrated traction and compressor drive to reduce the HVAC compressor drive cost in electric and hybrid electric vehicle (EV/HEV) applications. The drive system employs a five-leg inverter to drive a three-phase traction motor and a two-phase compressor motor. The common terminal of the two-phase motor is tied to the neutral point of the three-phase traction motor to eliminate the requirement of a third phase leg. The cost of the compressor drive can be significantly lowered due to the elimination of one phase leg and additional part count reduction made possible by sharing the switching devices, DC bus filter capacitors, gate drive power supplies, and control circuit. Simulation and experimental results are included to verify that speed control of the two motors is independent from each other.

Soft commutated direct current motor

A novel soft commutated direct current (DC) motor is introduced. The current of the commutated coil is intentionally drained before the brush disconnects the coil. This prevents the spark generation that normally occurs in conventional DC motors. A similar principle can be applied for DC generators.

Nature and assessments of torque ripples of permanent-magnet adjustable-speed motors

Torque ripple of permanent magnet motors can be classified into four types depending on the nature of their origin. The four types are pulsating torque, fluctuating torque, reluctance cogging torque, and inertia and mechanical system torque. Pulsating torques are inherently produced by the trapezoidal back-EMFs and trapezoidal currents used in certain permanent magnet adjustable-speed motors. The torque ripples caused by pulsating torques may be reduced by purposely produced fluctuating counter torques. Air-gap torque assessments are conducted on a sample motor. Experimental results agree with theoretical expectations.

Peaked-MMF smooth-torque reluctance motors

The authors investigate the steady-state torque characteristics of reluctance motors with nonsalient stator punchings, but with peaked rotating magnetomotive forces (MMFs). The torque calculation includes the effects of saturation and fringing and groove fluxes. The peaked rotating MMF is produced by properly coordinated current waveforms and winding. Peaked-MMF reluctance motors have tow major advantages: the torque is smooth and the flux per pole required to produce a given torque is lower than that of conventional reluctance motors. This property is most beneficial to two-pole reluctance motors, for a given frame whose bore diameters and slot areas can be increased significantly for higher ratings or better performance. Unlike switched reluctance motors, shaft encoders are not required for peaked-MMF motors. >

Comparison of the nature of torque production in reluctance and induction motors

The nature of torque production is different in reluctance and inductance motors. One significant difference occurs in a reluctance motor that has nonsalient stator punching and a salient motor. When the flux per pole is small in such a motor, the torque can still be high, as long as the rate of energy change with respect to the rotor angular displacement at the rotor pole fronts and pole ends is high. A theoretical foundation to improve the torque capability of reluctance motors is provided. Effects of saturation and stray-load loss are also studied. Experimental results show agreement with theoretical conclusions. >