John L. Oldenkamp

Selection and Design of an Inverter-Driven Induction Motor for a Traction Drive System

The considerations and trade-offs involved in the selection and design of an inverter-driven induction motor for a traction drive system are described. The inverter is transistorized and operates in a pulsewidth modulation (PWM) mode in the constant torque region and in a six-step square wave mode in the constant power region. Drive system requirements establish the 50-hp peak motor rating and the motor voltage. The aspects of an induction motor designed to be driven from an inverter are presented. These include the number of poles and rotor bar shape of the motor. Expressions relating the motor performance to motor design parameters are derived. The aspects of the inverter power source are presented. These include the constant volt-ampere characteristics of the inverter and the relationship of motor inductance to the inverter design. A final motor design is reached, Contrary to previous expectations, the best design is a two-pole motor.

A Study of System Losses in a Transistorized Inverter-Induction Motor Drive System

A study is described of the system losses in a transistorized pulsewidth modulated (PWM) inverter and induction motor in a traction drive system and the development of a strategy for minimizing them. The losses include inverter losses, conventional fundamental frequency motor losses, and motor harmonic losses. The losses in both PWM and six-step square wave (SSSW) inverter operation are studied. The loss models are introduced into a motor performance analysis program allowing the computation of the drive system performance over the entire speedtorque range. The study shows that a strategy for minimizing drive system losses is a function of torque. The air gap flux density should be held at its maximum level until the torque requirement is reduced to a low value, and then the air gap flux level is reduced.

A Single-Phase Induction Motor with One Distributed Winding

A single-phase induction motor with one winding develops starting torque if the stator iron is not symmetrical about the winding axis. Four methods of producing the dissymmetry are described. The most significant design variables are rotor resistance, the location of the dissymmetry, and the difference of the magnetizing reactances perpendicular to and along the dissymmetry. Test and calculated data are shown. The motor can be used where low starting torque is required and, under certain conditions, it is superior to other motors.

Capacitive Regenerative Braking in Single-Phase Motors

Capacitive regenerative braking may occur in capacitor start motors when power is removed. A series resonant circuit is formed with the main winding and start winding connected in series through a relay or centrifugal switch to the start capacitor. The circuit will resonate over a range of speeds. Slip (which is negative) and saturation are determined by the conditions that the sum of the real impedances in the circuit must be zero and the sum of the imaginary impedances must also be zero. The rotational energy of the motor is converted into stator and rotor heating losses. Formulas are derived for the range of speed over which regeneration may take place, and a method of predicting braking torques is given. Braking torques several times larger than the rated maximum motor torque can be developed.