"Shaun Van Poecke" wrote in
:

just some general questions here to try to get my head around DC
motors....

is there a direct relationship between max RPM and torque for motors
of the same wattage? ie. if you have 2 ungeared motors that both draw
500watts and one has a max 300RPM the other has a max 1000RPM, will
the 300RPM motor have more torque?

Yes, but I forget the formula at the moment.


is maximum pull really available from zero RPM? many people say this,
but my experience with small DC motors (cordless drills etc) doesnt
point at this being true. Does the nature of the controller have an
effect?

Maximum torque occurs in DC motors that are stalled because that is the
speed at which counter EMF created by the spinning windings in the
magnetic field is ZERO...generating maximum current. There's so much
torque available that a starter motor trying to turn over a hydrolocked
diesel may pull its own armature apart, destroying it, or the massive
torque will break injector holders because the water in the cylinders
will not compress on the compression stroke. Something's gotta give!

The controller on cordless appliances has a current limiter to prevent
you destroying the motor and battery with overcurrent on stalls.


can all DC motors be wired up in reverse to go backwards?

Yes. In permanent magnet motors just reverse the connections to the
commutator. In field coil motors just reverse the connections to the
commutator.


Are they really a 'free voltage' device, can i use a 12VDC motor with
24VDC, or a 36VDC motor with 48VDC? am i shortening the life of the
motor by doing so? some motors that i have seen specify operating
voltage from say, 48-144VDC....

When a motor is spinning at a certain RPM, a counter EMF is generated in
reverse to the source power polarity that is exactly equal to the source
voltage. Counter EMF generated is directly related to motor speed and is
not linear. As applied voltage increases, motor speed increases until
counter EMF increases due to speed up to this new balance point. As
applied voltage increases further, there's a point at which the armature
can no longer oppose the centrifugal force trying to pull it apart and
the motor explodes, destructively, as it armature crams into the stator.
If you are nearby a large industrial motor when this happens, you will
never forget the sound or experience....sort of like remembering the 16"
guns on the USS New Jersey going off when you were nearby. I'll never
forget either! Life in overpowered motors may be measured more
accurately in milliseconds than hours. Being a former slot car addict
and racer of highly modified slot car motors known to melt track
conductors, I been there, done that...got several T-shirts!


For two motors of the same given rating say, 500W, if i have a 24VDC
and a 48V motor, will they actually have the same operating
characteristics? Will the 48V motor have more power than the 24V one,
even though they consume the same ammount of power? Does current draw
remain the same as the voltage goes up, or does it drop? Some
manufacturers say that as the votage goes up, so does the HP... ie.
8hp@48V, 12hp@60V etc etc....

1hp = 746 watts....an electrical to mechanical conversion factor. It's
not quite that high because the damned windings heat up and the bearings
drag on it, reducing the actual output power, but it's close...ideal.

A 500W 24V motor draws half as much current as a 48V 500W motor putting
out the same torque and speed to power a load. They'll both put out
500/746 or about 2/3 of a HP at whatever design speed they were built
for. P = I x E. 500 = I x 24. I = 20.83A 500 = I x 48. I = 10.48A
Ohm's law applies... If there's no output "power"...torque spinning a
load...I = the sum total of all the losses in the motor...friction,
leaking magnetism from the core, resistive heat heating up the wires and
core, etc. Unloaded motors still draw current because they are not 100%
efficient machines, but it's very little as they are very efficient.


is one sepcific type of design more efficient than any other?


That depends on what you are driving. For instance, a series wound motor
is better for vacuum cleaners, egg mixers, etc., where we want the motor
to try harder when the load changes. But, to power a washer at a
relatively constant speed, a parallel-wound motor is superior because it
has better speed regulation. Listen to your vacuum cleaner when you put
your hand over the end of the hose. It goes berserk with RPM as the fan
unloads. You wouldn't want the washer to spin out of control just
because it's not full, would you?? "Efficient" has a touch of reality in
it, you see.

Larry
--
My politicians lied to me and said they support Net Neutrality.
They must pay at the voting machine.....well, once we figure
out how to stop Diebold from changing my votes.