DC Motor

wrote:


you can... meat grinders have 300 watt motors...

Surely I see your point. Physics is about the same, but emotional impact of a large motor can have some extra educational value.
i
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Yes students do need labs with big machines and yes they do leave an indelible impression .
Many years ago I was in a lab group that shut down the whole Physics school of a large college 3 times doing an experiment with a big machine where a careless technician (not a student) had reversed the wiring of a Synchroscope. but the lab was designed and built by experienced engineers and so no-one was hurt. The OP in this thread is asking questions that are obviously beyond his current (sic) level of expertise and this is not the place to learn that sort of stuff. He will get bits and pieces of advice and never know till after the fire that he didn't ask all the relevant questions and so did not get all the answers.
--
John G

Wot's Your Real Problem?
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Hi Thank you all I agree with after reading all the messages that i really did not think of all the measures and questions. Could some body educate me what more i need to know. before testing I think i need to prepare a test plan/procedure and what all will i calculate and how i will show my demonstration. If they agree my experience with dealing large machines is not sufficient atleast they would see my test plan and do the test with some expeerienced. Hope i get some help and appreciate all for the info provided. Regards smitha
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Smitha
Would it be OK for me to ask what kind of school you are attending?? It is difficult to consider that there is a school that would present you with a 25 HP DC motor without first acquainting you with the the fundamentals.
Perhaps English isnt your first language.
When you ask this News Group to educate you, it seems that you have no instructions available from that school.
I's suspecting this is a prank post. How about you supply a little more background on what you are doing and maybe where you are.
Jerry

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Jerry Martes writes:

NNTP-Posting-Host: 12.2.142.7
resolves to:
Caterpillar, Inc. CATERPILLAR-142-0 (NET-12-2-142-0-1) 12.2.142.0 - 12.2.142.255
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On Wed, 10 Aug 2005 20:57:52 -0500, Richard J Kinch

Bingo. Troll
Gunner
The two highest achievements of the human mind are the twin concepts of "loyalty" and "duty." Whenever these twin concepts fall into disrepute -- get out of there fast! You may possibly save yourself, but it is too late to save that society. It is doomed. " Lazarus Long
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really
what
As you have seen from the replies, there's various ways to determine motor constants.You need to tell everyone what you DO know about DC motors first. Are you familiar with the basic DC motor equations? What are the capabilities of the lab-any equipment to measure large torques and currents?
The electrical and mechanical input/output levels you will be dealing with can be lethal. If this is for a school lab project I'd suggest you consider a 12 volt permanent magnet motor from a car (engine cooling fan or blower motor) and determine the motor constants.
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Heh, heh. A man after my own heart!!
I'm currently developing various lab exercises for ME students in machine design. Trying to do 'real' stuff in this day of computer simulations is a challenge. And getting across the message that flaws in your simulation have nasty effects in the real world is best done with things like 20hp motors and 10:1 gear boxes. Only issue is how to get it impressive but still safe for udergrads who's idea of a big power supply is a 300 watt ATX box for their computer.
Don Foreman wrote:

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basic
with
real
bricks
toward
big
Have to agree-the only computer simulation I ever did was a second order system patch wired on a General Dynamics analog computer. Everything else was lab work...
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wrote:

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

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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snipped-for-privacy@yahoo.com wrote:

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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bench
permanent
in a

excited.
rated
later
motor
is a

Field
starting
manufacturers
carry
of
safer
in
the
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

see
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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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In all actuality this might be a drill press from Sears. You know "Maximum Developed HP" crap they label everything with.
Sears has really turned into a joke. Everyday I am on edge that they will go out of business and my hand tools will be about worthless. Should I just start buying the Chinese crap now?
I actually took a glance at a drill press at Sears. Looked at the back to find the plate on the motor in an attempt to find the true HP. Conveniently the plate was missing, but the 4 rivet holes where there. Nothing more insulting.

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