# DC Motor

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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

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What's the voltage that's listed on the plate?

i
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1) hope the motor powers from some multiple of 6 or 12 volts. 2) lacking the above, build one MAN of a DC supply. 3) take it to a good motor shop

It would cost you more than it's worth to figure out if the torque, speed, and stall current are within spec, if you had to build or buy the equipment.

LLoyd

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its 12 V thanks

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I believe i can bulid in my school lab. Aprreciate if you can expalin it in detail Thanks Rgards smitha

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A 12 volt, 20 kW motor???

That would mean that it consumes approximately 20000/12 = 1,600 amperes!

Can you look at the nameplate of the motor and copy some information?

How big is the motor?

i
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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.

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sorry it rated for 250-360 volts any steps for the test regards smitha

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it needs 80 amps at 250 V DC to run at rated load. Probably much less without load, except at startup.

If you have a suitably rated rectifier, and a good electrical subpanel, you can rectify your 220V AC.

If your motor is a permanent magnet design, it can function as a generator. You can try spinning the shaft with anything and measuring voltage across leads. If it spins and produces voltage, it is probably good.

There are much better experts than me, so let's see what they say.

i
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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

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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.

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

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This is not a job to be undertaken by someone who has to ask such basic questions in a METALworking group.

It is proper engineering job for an experienced consulting engineer with experience in this type of indstallation.

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That will deliver more than 300 volts.

Best not to listen to casual advisors with machinery of this size.

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Not sure what you mean by bench testing. Do you want to determine the characteristics or just see if it runs?

Don explained some of the methods for determining motor constants. Depending on your use, and since it's a big motor, you may also want to measure friction and the armature inertia

Other parameters would be armature resistance and inductance to complete the full set of parameters.

Then you can model the DC motor...

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his motor is rated for 360 volts max, so I am not sure why you use this number to make an objection.

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I thought he said it was rated for 250 volts.

Looking back I see he said "it [sic] rated for 250-360 volts".

So, who knows. The OP does not project competence for this kind of tinkering.

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measuring

probably

Drives for DC motors of this size typically use a 3 phase SCR bridge, but motor-generator sets (Ward Leonard system) were quite popular years ago...

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exactly.

If I got a 20 kW DC motor (shudder), I would not try to run it. It it was a permanent magnet motor, I would try to spin it up and see if there is DC voltage generated by the motor.

i
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permanent

excited.

later

starting

manufacturers

circuit

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

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How do you suppose engineers become trained to do such consulting? That's why schools have labs.

I'm glad there are still students working with real machines in a real laboratory rather than with 'puter sims. You can't simulate the experience of a 50 HP DC motor drawing a gazillion amps of armature current at stall, or if the field comes loose. Ya gotta see it in person, after duly noting the motor bits still embedded in the bricks from previous oopsies.

Unscheduled experiment: how fast can a twit lab TA dive under a bench? Open field on big DC motor running with rated armature voltage, time the twit when he hears it accelerating rapidly toward kaboom. (Do close the field pretty quick. Forget about killing armature current at that point; opening the switch results in a big green arc)

The show is well worth the ensuing loud lecture endured with simulated contrite humility. Your grade may partly depend on the quality of your simulation....

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