Re: [CAD_CAM_EDM_DRO] measuring motor current

On Fri, Jan 12, 2001 at 10:47:23AM -0500, Joe Vicars wrote:

> I need someone to tell me exactly how to measure dynamic current
> rise in a step motor using an oscilloscope. Do I put like a 1/10th OHM
> resistor in series with one of the coils and measure the voltage across
> it? How is this related to the current?
> The Bridgeport maintenance manual has scope pictures of current in
> the motor windings at different speeds. I would like to be able to

Does the Bridgeport manual mention what equipment they used to measure
the current? There are current probes that clamp over a conductor to
measure the current. If they are AC current probes, they connect to an
oscilloscope through a termination box, which tell you the mA per mV
conversion factor. If they are DC current probes, then a knob on an
accessory box is just set to Amps/div. The bandwidth on a DC current
probe goes from DC up to 50 or 100 Mhz. AC current probes are limited
at low frequencies because they are transformer coupled. That means
that the tops of the steps will not be horizontal, but will tilt. Using
a current probe will not disturb the circuit under test nearly as much
as putting a resistor in series. However, if you want to compare with
the Bridgeport manual, then the measurements should be made the same way
they made the ones for the photographs. Perhaps they are actually
voltage measurements?

--
Ray Menke