On Thu, 22 Jul 1999, Zuoyi Wang wrote:

> Dear Members:
>
>
>     I am measuring  the electric current of a rotating body, and tried
> using carbon brush.  Unfortunately, the brush did not work well and the
> current shifted tooo much.   Does anybody know any technique for measuring
> current of a rotating body?  Any suggestion will be highly appreciated.
>
> Zuoyi Wang
>

Zuoyi,

The voltage induced in a coil by a moving magnetic field is
the basis of dc motors, generators, and tachometers. These
employ carbon brushes to make a connection to the moving
coil (Armature). Because they are made of carbon, brushes
are resistive. Attempting to pass a current through brushes
will produce an ohmic voltage drop across the brushes, which
may determine the maximum current flow. Connecting an
external ammeter (low impedance device) to a rotating coil
through sliprings and two brushes will measure the current
as limited by the series resistance of the brushes.

Usually in tachometers you measure the voltage induced in
the coil by using an external voltmeter (high impedance
device). Due to the high internal resistance of the
voltmeter, the current in the brushes is very small, so the
drop across the brushes is a tiny fraction of the
open-circuit voltage in the coil.

Are you sure that you need to measure the short-circuit
current in the rotating coil? If so, imbed a Hall-effect
current sensor in series with the rotating coil.  The
Hall-effect sensor will convert the armature current to a
voltage signal. Then you can pass the signal voltage thru
sliprings and brushes to an external high-impedance
measuring device.

Michael Mladejovsky, Phd
Center for Engineering Design
University of Utah