"Rick" wrote in message
oups.com...
wrote:
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

Or if he has a dynamometer, hold speed constant and record a change
in
torque vs a change in armature current. For the EMF constant spin
the
motor with field current applied and measure the generated open
circuit
voltage at the armature terminals. He should post all of the
information on the namplate as a start.


Of course, he could calculate counter emf at various points too, by
running the motor (if he knew the armature resistance) by measuring
armature current and applied voltage


As you say, a lot depends on the equipment available...


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