Image:High-Current Copper-Brush Commutated Dynamo.jpg

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[edit] Summary

Text from public domain illustration:

Figure 328: A large current low voltage bipolar dynamo built for electrolytic work and here shown to illustrate the large size commutator and brushes necessary to collect the large current.

• The machine here illustrated gives 310 amperes at 7 volts when running at a speed of 1400 RPM, corresponding to an output of 2.17 kilowatts.
• Carbon brushes would not be suitable for this class of machine because even with copper brushes, whose conductivity is much higher than carbon, the commutator must be of considerable size to give the required brush contact area. The contrast between the axial lengths of the armature and the commutator is very marked.
• The rocker construction is of the ordinary type, and heavy flexible cables conduct the current from the brush holders to the fixed terminals.

[edit] My Notes

• While the size of this device is not listed in the book, the brushes alone are likely to have been 1.5 to 2 inches wide, meaning the commutator is probably a foot wide. The overal machine was probably about three feet tall and about four feet wide, weighing in at several hundred pounds.
• The short rod on the left side of the copper brushes is most likely a handle, to allow the operator to rotate the position of the brushes. As the speed of the rotor varies, the ideal position of the brushes changes due to the spinning rotor dragging the field lines around with it.
• Yes indeed, carbon brushes were not good enough for high-current applications. This needs to be mentioned in an article somewhere.
• Each brush sits in a separate spring-tension carrier, so that as the brush wears away it is pushed down into the commutator to maintain contact.
• The screws at the base of the unit were used to adjust the flat-belt tension, by moving the entire frame towards or away from the driving engine.

Public Domain source:

• Hawkins Electrical Guide, Volume 1
• Chapter 20: Commutation and the Commutator, Page 301.
• Copyright 1917 by Theo. Audel & Co.
• Printed in the United States

Scanned by Dale Mahalko, Gilman, WI, USA

DMahalko (talk) 08:07, 14 January 2008 (UTC)

[edit] Licensing

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