On Tue, 09 Aug 2005 12:06:45 -0500, Don Foreman
wrote:

On 9 Aug 2005 06:22:06 -0700, "smitha4u"
wrote:

Hi Guys

Can any one suggest what are the various methods of performing bench
test for a 20 Kw(25HP) DC electrical variable speed motor.

Thank you
Regards
smitha

Assuming that such a motor has a wound field rather than permanent
magnet:

1) estimate rated field current by seeing what current results in a
steadystate temperature rise of about 20C with armature not excited.

2) With that field current applied, energize the armature with rated
voltage. Measure no-load speed and stall torque. (do the later
test quickly!) Measure armature current while stalled.

These tests are sufficient to determine the constants for the DC motor
equations describing speed and torque as fns of armature voltage,
armature current and field current.


A bit difficult to second guess the field rating and this is a
problem because it has such a major effect on motor performance. Field
dissipation of 5% of rated power or 20 deg C rise is a good starting
point but a this assumption may be well removed from the manufacturers
intended rating. I would have expected a motor of this size to carry
more nameplate information.

At rated voltage, the stalled armature current of a motor of
this size is little short of spectacular! I think it would be safer
to settle for a measurement of stalled torque per amp at close to
rated full load amps together with a measurement of back EMF per
thousand RPM

Proper evaluation of a motor of this size needs pretty
substantial ancilliary equipment and a good understanding of it's
operation and limitations. For anything more than a rough test to see
if it's a dud motor it's not a task to be taken lightly. The light
hearted uncertainty on whether it's a 12 volt or 360 volt machine
doesn't engender a lot of confidence!

Jim