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Many authors reported the decrease of performances when electric machines and electromagnetic devices were supplied by pulse width modulated (PWM) voltages. However, these statements are rarely supported by measurements performed under fair conditions. The aim of this paper is to compare the performances of a single‐phase transformer and a three‐phase permanent magnet synchronous motor (PMSM) supplied by sinusoidal and PWM voltages and to find a way to evaluate the decrease of performances when PWM voltages are applied.

In order to perform a fair comparison between performances of the tested objects supplied by sinusoidal and PWM voltages, an experimental system was built. It contains a single‐phase and a three‐phase linear rectifier for supply with sinusoidal voltages and an H‐bridge inverter and a three‐phase inverter for supply with PWM voltages. The tests and measurements were performed on a single‐phase transformer and three‐phase PMSM, where different constant loads and different modulation frequencies were used. The test conditions were identical for the supply by sinusoidal and PWM voltages. The measured data, used for the evaluation of performances, were the input and output power and the time behaviours of currents and voltages together with their THDs.

The results presented in the paper clearly show that the efficiency of the singe‐phase transformer and three‐phase PMSM decreases with the increasing level of voltage THD. To properly determine the THD of PWM voltage, the sampling frequencies above 1 MHz and special equipment are normally required. However, if the modulation frequency is not too high, also the current THD, which can be easily determined, can be used to evaluate the decrease of efficiency in the case of supply by PWM voltages.

The results presented in the paper clearly show that the efficiency of the singe‐phase transformer and three‐phase PMSM decreases with the increasing level of voltage THD.

ELSEWHERE we deal with the inaugural meeting of the Association of Assistant Librarians. Those who were privileged to attend tell us that the meeting was quite without precedent in many ways. There is no doubt that this Association is virile, enthusiastic and alive to the best issues in literature. A better foundation for young librarians cannot be conceived. We doubt whether the Society is as strong on the technical side, but that may be because so much of library technique is now standard, and, therefore, only very inventive librarians can bring new views and methods. One of the obvious things in the assembly was the number of girls. At one time a girl at a L.A.A. meeting (to use its old and much better initials) was so rare an appearance that everybody's curiosity was aroused. Now the girls are in the majority. We hope that it will not always be a majority, but we do think that half‐and‐half would be very much better than too many girls or too many boys. In any case those who were present at the meeting are optimistic as to the future of the library profession.