Mechanical part of the locomotive was designed in the Rolling-stock Industry Central Bureau of Construction (Pl.

: Centralne Biuro Konstrukcyjne Przemysłu Taboru Kolejowego) in Poznań, basing on instructions from the Railway Electrification Office (Pl.

[1] At the beginning those locomotives bore E600 designation, which was changed to E06 and after introducing RN-58/MK0001 norm to present ET21.

After producing 70 first units and gaining significant experience Pafawag decided to introduce several technical changes into locomotives.

3E and 3E/1 machines were and are still used for transporting filling sand, power plant waste and coal, especially in Upper Silesia region.

In the years 1973-1980 PKP borrowed several items of 3E/1 locomotives and changed their designation to ET21 with adding additional 9 figure to serial number.

The locomotive is equipped with six LKb-635 (though some sources give LKa-635) engines which are mounted on the bogies with a tram system.

Four-step reduction of engine excitation (up to 50%) is made by bypassing parts of main pole's windings with shunts and resistors.

[2] A Co′Co′ type of the locomotive means it is equipped with two bogies, each of them having three axles with separate propulsion for each wheel set.

Moving between crew cabins is enabled by 570 mm wide corridor running along the box wall.

Air manometers and Hasler type train event recorder are mounted in front of the engineer's place.

Additional, 300W heater is located under the dashboard in order to heat crew's legs, but this was installed only in modernised 3E/1 version.

There are compressors and blowers on the floors of machine compartments and two air tanks, 220 liters each, are mounted under the roof.