Re: RPM of variable speed DC Motors

On Sat, 29 Sep 2007 22:22:15 -0500, Louis Ohland wrote:

Leo Lichtman wrote:
"Louis Ohland" wrote: (clip) does this mean more kick per RPM by the lower
speed drive?
^^^^^^^^^^^^^^^^
I've never heard of anyone calculating amps per RPM, and I think that is
because it is meaningless. First question: How do you know that both
motors are developing 1 HP? Are the figures you listed measured or read off
the nameplates? Of course, you get more torque at lower speed for a given
horsepower. If the figures are accurate, then motor A is slightly more
efficient than motor B.



Well, some manufacturers do not make finding their torque graphs readily
available. The amps were off a manufacturer's website, but they are just
summaries. No tables or graphs at all.

A theoretically perfect DC motor has a 1:1 relationship between armature
current and shaft torque, and a 1:1 relationship between armature voltage
and speed. It is, in fact, the same relationship if you use the right
units (Newtons, meters, volts, amps, radians and seconds) to do your
calculation.

Real DC motors, unless they're horribly inefficient, don't stray too far
from the theoretical -- so you can pretty much calculate the torque vs.
current characteristic from the speed vs. voltage characteristic.

Motor A: 1HP, 90V, 2500 RPM, 10.7A 4.28A per 1,000 RPM Motor B: 1HP,
90V, 1750 RPM, 9.2A 5.26A per 1,000 RPM

Efficiency aside, neglecting heat loss, wouldn't using a motor rated to
deliver 1HP at a lower RPM be better?

Define "better". The motor rated to deliver 1HP at the lower RPM is
clearly capable of delivering more torque without overheating. Assuming
that it's bearings and armature construction will stand up to the stress,
you'll be able to get nearly that torque out of it at higher speeds (and
voltages), but it'll probably be bigger.

If you need more than 1750 RPM and you don't want to mess with giving the
motor higher than rated voltage, then the second motor is clearly worse.
If you need more torque than the first motor can give and you'll always be
operating at less than 1750 RPM, the second motor is clearly better. If
neither of these two conditions hold, then you haven't given enough
information.

If you're going to be gearing either of them down no matter what, and you
haven't selected a gear ratio yet, then the speed and torque don't matter,
so you need to look at efficiency (and heat generation), weight, and other
factors that you haven't mentioned.

--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html
.

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