3.

J. Holtz and S. F. Salama, "Megawatt GTO-inverter with three-level PWM control and regenerative snubber circuits" in PESC '88 Record., 19th Annual IEEE Power Electronics Specialists Conference, 1988, pp. 1263-1270 vol.2.

Abstract:
Regenerative turn-on and turn-of snubbers for a three-level PWM inverter using gate turn-off thyristors (GTOs) are analyzed and discussed. Problems encountered in high-power GTO inverters, namely, trapped energy in the snubbers and switching losses, are discussed. The circuit configuration and operation are described and a comparison with a two-level inverter is made. Experimental results from the operation at reduced voltage of a 4 MVA phase-leg circuit are presented.{\textless}{\textgreater}

2.

J. Holtz, P. Lammert and W. Lotzkat, "High-Speed Drive System with Ultrasonic MOSFET PWM Inverter and Single-Chip Microprocessor Control", IEEE Transactions on Industry Applications, vol. IA-23, no. 6, pp. 1010--1015, 1987.

Abstract:
The design of high-current high-voltage MOSFET inverters requires measures to reduce the switching losses of the flyback diodes. Saturable reactances have been used in the case of a 4.5-kVA MOSFET inverter to limit the di/dt during commutations. The switching overvoltages of the reactances are absorbed by one common clamping circuit per bridge leg. The drive control system and the space vector modulator for the generation of the PWM switching sequences are implemented in a single-chip microcomputer.

1.

R. Venkataraman, B. Ramaswami and J. Holtz, "Electronic Analog Slip Calculator for Induction Motor Drives", IEEE Transactions on Industrial Electronics and Control Instrumentation, vol. IECI-27, no. 2, pp. 110--116, 1980.

Abstract:
For some schemes of variable speed control of squirrel-cage induction motor fed by a current-source inverter, accurate evaluation of motor slip frequency is essential for obtaining optimum torque output. For evaluating slip accurately, a digital speed transducer coupled to the shaft is generally required. Such a transducer and the associated digital circuits make the system complex and expensive. An analog speed transducer which is relatively cheap could be used only if the slip could be obtained accurately by some other means. This paper presents an inexpensive and accurate method of obtaining an analog signal proportional to the slip by using a simple calculator circuit that uses the dc link current, dc link voltage, and inverter frequency as its inputs. The slip calculator described here is capable of giving the correct output only under steady-state conditions. The design of the slip calculator is illustrated in the Appendix with the aid of a numerical example.