Its a Dynamo

When we read about the so-called “unidentified apparatus” in the Summer 2009 issue of the EMC Newsletter, we were impressed and positively surprised. The reason is that we pass close to this old-fashioned device every morning on our way to work. In fact, a specimen of this kind of electrical apparatus is placed, among other antique instruments and devices, in the corridor leading to the labs of the Department of Electrical Engineering (EE) at the Politecnico di Milano, Italy. Actually, our Department comes from one of the oldest EE institutions in Italy, the Istituzione Elettrotecnica Carlo Erba, which was founded in 1886 – two years after the foundation of the AIEE in US, which became the IEEE – to foster the development of that new promising science, Electrical Engineering, in northern Italy. As a consequence of this historical heritage, nowadays the Department owns quite a few antique electrical instruments and devices, which are permanently exhibited in a small museum.
Well, the “unidentified apparatus” is a dynamo, i.e., the DC machine used to convert mechanical to electrical power, as patented by Thomas A. Edison. At the dawn of the second industrial revolution this apparatus, moved by a steam engine, was used in DC power-distribution systems to generate electrical energy. Among other kinds of dynamos developed at that time, the Edison design is unmistakable, with its long coils raising from two pole shoes, and transversal iron bars on the top to close the magnetic core, thus ensuring efficient generation of magnetic flux density. Due to the long columns of the core, the generator was nicknamed “long-legged Mary-Ann” (a joke among the all-male staff of Edison laboratory in Menlo Park, New Jersey).


Fig. 1. Comparison between the old drawing and the Edison dynamo.	Fig. 2. Simplified scheme of the Edison dynamo.

     Fig. 1 shows the image published in the Summer EMC Newsletter (Issue No. 222) with a picture of our dynamo, whose dimensions are 520 x 800 x 680 mm, and it is made of iron, copper, wood, and glass. The sole difference is the number of coils: Two in our machine, four in the old drawing, which represents a more powerful version.
     The working principle is the same of any two-pole DC machine, and a simplified scheme is illustrated in Fig. 2. In short, the field winding (shown in blue) is represented by coils wound onto the columns, whereas the armature winding (shown in red) is wound onto the periphery of a cylindrical rotor placed between the two pole shoes. The current I flowing in the field winding generates a magnetic flux density B which crosses the rotor, exiting from the north pole (N) and entering into the south pole (S). Due to Faraday’s law, if the cylindrical rotor is rotated on its own axis with angular speed v, an electromotive force (i.e., a voltage) e = BL ωR (where R is the radius and L is the axial length of the cylinder) is induced on each turn of wire of the armature winding. The direction of e is shown in Fig. 2 with red crosses (entering) and dots (exiting). Thanks to the arrangement of the armature winding, the electromotive forces associated with the individual turns of wire add to give the total generated voltage, available at two terminals, whose connection to the moving winding is made possible by a sliding-contact commutator (not shown). It is worth noting that this machine is reversible, i.e., it could work as a DC motor, even though it was specifically designed to operate as a generator.
     Other interesting technical details can be inferred from the picture in Fig. 1. On the top of the apparatus one can see an elegant switch with a wooden handle, used to connect the field winding in parallel to the armature winding (in this fashion the machine is self-excited, i.e., the field winding is supplied by the dynamo itself). The bearings of the rotor shaft are lubricated by oil, retained in small glass-phials with level indicator. The commutator is shown in Fig. 3; instead of carbon brushes used in modern DC machines, here thin and flexible metallic plates are used.


Fig. 3. The glass-phial and the commutator.

Fig. 4. The Department of Electrical Engineering at Politecnico di Milano.	Fig. 5. Close to the Edison dynamo (from left) are Giordano Spadacini, Flavia Grassi, and Sergio Pignari.

Our dynamo has a fascinating history, as it was adopted in one of the first electric installations in Italy, used for lighting. Imported in 1890 from the US by Società Generale Italiana di Elettricità (the Italian partner of the Edison General Electric Corp.), it cost 200 Italian Lire, a small fortune in that time, and was used for the lighting of Biblioteca Nazionale Braidense, that is, the library annexed to the Brera Institution, a famous gallery in Milan which holds masterpieces by Bramante, Mantegna, Piero della Francesca, and Raffaello et Alii. The dynamo generated a DC voltage of 110 V, with a current up to 20 A, and was used to feed 25 bulbs.
Undoubtedly, electric arcing at the rough commutator creates significant radiated emissions in a wide frequency range, but EMC issues were far from the interests of electrical engineers in the XIX century. EMC