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?

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?

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

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....

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....

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

Thanks,
Shaun

On Sat, 26 May 2007 00:36:00 GMT, "Shaun Van Poecke"
wrote:

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?

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?

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

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....

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....

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

To make a long story short, there are several different kinds of DC
motors, the two most common being series-wound and parallell wound.
The series-wound motor has the armature and field windings connected
in series with each other, and develops its highest torque at low RPMs
since that is when current draw and magnetic fields are the greatest.
Generally speaking, the rotational speed of a DC motor is proportional
to the voltage applied to it, and torque is proportional to the
current.

Wikipedia has a fairly lucid discussion of the details:

http://en.wikipedia.org/wiki/Electric_motor

"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.

On May 26, 12:36 am, "Shaun Van Poecke"
wrote:
just some general questions here to try to get my head around DC motors....

see:
http://jacquesricher.com/NEETS/14177.pdf
It's 1.5Meg. DC motors is chapter 2

Shawn,
You have asked way many more questions than you realize. As I don't
expect you would understand compete and accurate answers, I will post
short answers in line (I hate doing this). The only other solution
would be to lend you to you the text books from the electric machinery
courses that I took.

Shaun Van Poecke wrote:
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?
No, not maximum, but there is for rated load.

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?
If the 300 & 1000 are rated load speed then the one will produce three
times the torque at its rated speed. (speed*torque/5252=hp)

is maximum pull really available from zero RPM?
That is motor design dependent. A series motor can provide a torque
curve that is this way. Traction motors used on locomotives are this
built that way.
many people say this, but my experience with small DC motors
(cordless drills etc) doesn't point at this being true.
Does the nature of the controller have an effect?
No, the battery stack falls on its face and lets the motor power off.
Don't try this with a good cordless drill, you will hurt your hand.

can all DC motors be wired up in reverse to go backwards?
By and large - yes, but because miniature motors are missing lots of
windings, most are designed to run in one direction

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....
DC motor design gives little consideration to operating voltage. The
internal design is all based on ampere-turns. If you run a 12v motor on
24v it may run faster and have a higher power output, but it may not be
able to accommodate the thermal load and over heat. If you have seen a
motor with a rating you mention, it is a variable speed motor and
actually designed for the maximum source voltage listed.

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?
That is motor design dependent.

Will the 48V motor have more power than the 24V one, even though they
consume the same amount of power?
NO

Does current draw remain the same as the voltage goes up, or does it drop?
That is completely dependent on the load that the motor is driving

Some manufacturers say that as the voltage goes up,
so does the HP... ie. 8hp@48V, 12hp@60V etc etc....
Remember the ampere-turns thing??

is one specific type of design more efficient than any other?
Permanent magnet field motors are generally more efficient as they do
not use supplied power to create the stationary field.


Thanks,
Shaun

I don't know what you are planning to try to do, but I suggest that you
learn a lot more about electric machines before you spend much money on
anything. If you get a motor that has a different rating than you
believe you need, do not count on being able to push the curve and make
it work.
Good Luck Guy
Matt Colie

Shaun Van Poecke wrote:
just some general questions here to try to get my head around DC motors....

snip

Most of your questions have been answered about as well as they can be
here. Places like the following are invaluable for the guy who is low
on the learning curve:

http://www.thelearningpit.com/

http://www.patchn.com/

http://www.ind-info.com/

There is a lot of fun and benefit to be taken from changing from single
speed to DC motors. Example, I have a 13" drill press in the garage and
it only offered a choice of five speeds from 500-3760 RPM. For bigger
drills and things like chamfering, 500 RPM is simply way too fast.
About 10 or 15 times too fast. And changing speeds on it requires a
minute or so (moving a belt on two step pulleys).

So I spend about $80 to buy a used DC motor (90V, Permanent Magnet type,
1/3HP, 1725 RPM) and a used DC motor controller on eBay. Figuring out
the wiring for the controller was mildly technical. Sort of a case of
deciphering some of the hieroglyphics on the terminal strip (I did not
get any docs with it and could not find the older model I bought on line).

But I asked the question he

http://www.plctalk.net/qanda/forumdisplay.php?f=2

and those folks sorted it right out for me.

So now I have a drill press that will run (and pull chips in steel) from
just barely turning (30 RPM or so) up to about 2,500 RPM and I never
have to mess with the belts for almost anything I would ever want to do.
I can get higher speeds by playing with the belt and step pulleys if I
want.

Playing with PLCs and inverters and controllers is a natural leap from
DC motors once you get started on making machines work better. My goal
has always been to do something better, I don't really understand a lot
of the theory and PLC programming and stuff but I can always fumble my
way through it and learn as I go.

But I can't think of very many of the metalworking and woodworking
machine I have ever owned that either were not better or would have been
better for having an single speed AC motor replaced with a DC motor and
a variable speed controller.

Your classic 10" table saw for woodworking for example, does *not*
really need a DC motor because the fixed blade diameter and tooth count
variable, along with enough HP and rigidity in the table, are the keys
to good work there. But planers or jointers can benefit from switch.

Now, if I want to add another switch to the controller, I can have
instantly adjustable spindle speeds *and* reversing too. That is
another thing that is much easier to do with DC motors.

Jack