Modern industry requires more and more fully automatic electrical drives, and the actual control equipment is digital and/or analogue. Usually d.c. motors are used, and for this purpose a.c. to variable d.c. power conversion is required, but recently squirrel-cage motor drives have sometimes been favoured and, for this purpose, a.c. to variable-frequency a.c. power conversion has been developed. In order to improve efficiency and reliability of automatic drive systems, their planning and construction demands extensive knowledge of control theory, and, quite often, more than one feedback loop will be applied. The direct mathematical treatment of the transient behaviour of the closed-loop system and application of the root locus approach is discussed in the present paper, using various types of compensation and smoothing devices.