Re: [CAD_CAM_EDM_DRO] servo motors

"M. SHABBIR" wrote:

> hi all
> i have found yesterday a dc permanent magnet brush motor. it is just like
> who type is using in edm. it is similar to edm servo. but no written any
> voltage and amps. it has to permanent magnet in motor and to armatures one
> heavy duty and other small.

The large armature is the motor, itself. The small one is the DC tachometer.

You can derive simple readings of the important motor characteristics with
pretty simple tools. A stroboscope and a fish scale are the lowest form, but

they work! You paint one or several stripes on the shaft and apply some
modest known DC voltage. Using a neon light bulb with a rectifier diode and
current limiting resistor all wired in series, you get 50 or 60 flashes per
second (depends on your local power line frequency). It takes a little
practice to get consistent results with this, but if you can vary the DC
supply
voltage, then you can pretty easily find the point where the shaft is
rotating
exactly at 3000 or 3600 RPM (for 50 or 60 Hz), or 1500/1800 for lower
speed motors. Divide the voltage by RPM and multiply by 1000, and you have
the Kv motor constant (volts/1000 RPM).

If you have a constant current power supply, or a large resistor, you can
deliver a steady current to the motor, and using the fish scale (or a weight
on a string), you can measure the torque per amp. the motor constant
Kt is some torque unit per Amp.

You can also try to measure the DC resistance of the armature, but the
brushes make this a difficult measurement, as the brushes provide a VERY
inconsistent reading. Measuring the deviation from a Kv under no load
is a more accurate measure, but not as easy to do.

One remaining figure that is VERY hard to esablish is the safe peak current
the motor can handle. The amount of armature reaction that it requires to
demagnetize the permanent field magnets is difficult to calculate even if
you know the magnet material well. If you had a warehouse of the same
motors, it might make sense to destroy one to determine the safe rating,
but if you've only got one motor, you are pretty much left to common sense
to make a guess at it, unless, of course, the manufacturer has that
information
available.

Jon