DC motor torque characteristics??
Jeff O'Malley
Has any one a chart or graph for the torque vs rpm for dc motors (pm as
well as wound field) ?? I have heard or read somewhere that the low rpm
torque on pm type motors is supposedly very high.
What size pm dc motor would I need to get to replace the 85 lb. cast
iron 2HP capacitor type motor on my Tiwanese import 13 x 36 lathe??
The low rpm torque is of more concern than mid to high (5000rpm) figures
as I almost never run my machine at high speeds. Thanks for any
references, url's, etc. that will help give me this info.
TIA Jeff in Houston, TX
Fitch R. Williams
You will still want the full 2hp - metal removal is horse power related.
One way worth considering would be to build a jack shaft and belt it up so that
the lathes top speed is achieved with the DC motor at about 4700 rpm or so.
This is probably easier on your lathe than running it in a lower gear to
accomplish the same thing. You will get a very wide speed range variation and
won't be overspeeding the shafts in your lathes gear head.
Note that the torque delivered to the chuck will be noticably less than when you
gear down your existing 2hp induction motor to the same speed. This is because
the induction motor receives the benefit of the gear reduction to multiply the
torque. The DC motor doesn't unless you perform the same gear shift.
Fitch
Doug VL
For a DC motor, torque is proportional to current (or vice versa,
increasing the motor's load increases the current) and the speed is
proportional to the applied voltage. Torque isn't related to rpm, but
horsepower is. A dc motor can be considered to have the same torque at any
speed up to its rated speed, but if it's running slow, its fan won't cool
the motor. And as we all know, motors run on smoke which is installed at
the factory. Once you let the factory-installed smoke out of the motor,
you just can't get it back in.
If you keep a dc motor's speed constant, meaning the voltage is constant,
and increase the load (and therefore the current), the heat generated by
the current increases by the *square* of the increase in current. For
example, if you double the current, you multiply the heat by a factor of
four. Watts equal current squared times voltage.
DC motors are used industrially where high torque is needed over a wide
speed range.
Doug VanderLaan
Electrical machine controls engineer
Jeff O'Malley <jbom...@phoenix.net> wrote in article
<3411B8...@phoenix.net>...
> Has any one a chart or graph for the torque vs rpm for dc motors (pm as
> well as wound field) ?? I have heard or read somewhere that the low rpm
> torque on pm type motors is supposedly very high.
> What size pm dc motor would I need to get to replace the 85 lb. cast
> iron 2HP capacitor type motor on my Tiwanese import 13 x 36 lathe??
> The low rpm torque is of more concern than mid to high (5000rpm) figures
> as I almost never run my machine at high speeds. Thanks for any
> references, url's, etc. that will help give me this info.
>
DGoncz
> A dc motor can be considered to have the same torque at any
>speed up to its rated speed, b
Not for PM motors, which is what the poster asked for. PM motors have
maximum torque at stall. Any movement of the rotor will induce counter EMF.
Their power output is max at half speed, along a parabolic curve. Their
efficiency is a hyperbolic function, simply power out divided by power in.
I recommend the Maxon PM motor catalog for a good tutorial. They are Swiss
and make fine small motors. Efficiency is not normally a concern in machine
tool power.
>What size pm dc motor would I need to get to replace the 85 lb. cast
>> iron 2HP capacitor type motor on my Tiwanese import 13 x 36 lathe??
I get the Surplus Center catalog and they have some big motors and a tiny
tutorial on hydraulic, gas, and electric. Aircraft starter motors are not
too dear, but not made for continuous use at full load. The entire surplus
market is loaded with motors and plans to build your own electric automobile.
Yours,
Doug Goncz
My Soc. Sec. card says right on it:
"FOR SOCIAL SECURITY AND TAX PURPOSES--NOT FOR IDENTIFICATION" !
(Form OA-702 Rev. 9-61)
http://users.aol.com/DGoncz (sonoluminescence)
http://users.aol.com/ReplikonVA (machine tools)
Micheal Cranford
Jeff O'Malley <jbom...@phoenix.net> wrote:
"Has any one a chart or graph for the torque vs rpm for dc motors
(pm as well as wound field) ?? I have heard or read somewhere
that the low rpm torque on pm type motors is supposedly very high."
PM DC motors exhibit a linear torque vs current relationship. Torque will be
at a maximum when stalled (minimum RPM) and at a minimum when unloaded (maximum
RPM). An excellent reference covering DC motor characteristics (among several
other things) was published by Electro-Craft Corp. "DC Motors, Speed Controls,
Servo Systems". I'm pretty sure that these are still available in reprint form.
Wound-field DC motors usually exhibit torque fall-off as they approach stall
due to the high armature current causing partial field de-magnetization. For
high torque at low RPM applications it will be necessary to provide additional
cooling for either type of motor since low RPM reduces the effectiveness of a
built-in fan (assuming it even has one).
I wrote a computer program (several years ago) that plots the characteristics
of any PM DC motor given a few parameters. It solves the system of simultaneous
equations to determine the motor's Torque, Output Power, and Efficiency vs RPM.
It will automatically prompt the user for additional measurement data (i.e. RPM
at given supply voltages) if insufficient motor data is entered. In addition to
plotting the performance curves the program also prints the motor's voltage and
torque constants, static and viscous frictional losses and armature resistance.
The complete results can also be saved into a file or sent to a printer. This
runs on an IBM PC with at least a 286 and a VGA.
If there is sufficient interest, the first 10 people requesting a copy of the
program can have it for free - the remainder will have to pay 10 times as much.
--
******************************************************************************
Micheal Cranford Resident Skeptic Intel P6 Architecture Labs
Martin A. Escarcega
Micheal Cranford <mcra...@ichips.intel.com> wrote:
: Jeff O'Malley <jbom...@phoenix.net> wrote:
Given the below information, does that mean the permanent magnet DC motors
are better in variable speed applications? I'm wanting to set one up for a
WOOD Lathe....controller on the way. Controller is heavy duty and rated
for 3/4HP, will it run a 1hp? (It was the controller recently advertised
here)
Thanks for the insight
Marty
: PM DC motors exhibit a linear torque vs current relationship. Torque will be
altavoz
altavoz: "T avail' at half the unloaded RPM "
Thats a contradiction is it not ? If it's unloaded , it can't
make any torque .
______________thread below_________________________
You must have been thinking of physics in an alternate universe since
the
2nd quoted statement clearly contradicts the 1st.
The torque available at 1/2 the unloaded RPM is about 1/2 of the
torque available at stall (this is for a PM DC motor, wound-field
motors will differ). Likewise, at near the maximum
unloaded
RPM the available torque will be quite small.
Micheal Cranford
Micheal Cranford
altavoz <alta...@mail.idt.net> wrote:
"T avail' at half the unloaded RPM
Thats a contradiction is it not ? If it's unloaded , it can't
make any torque ."
No, I didn't make the contradiction, you did. I said "The torque
available at 1/2 the unloaded RPM ..." and 1/2 of the unloaded RPM
is not the same as the unloaded RPM. It is, in fact, half as fast
so about 1/2 of the stall torque would be available (not zero).
Micheal Cranford
Doug VanderLaan wrote:
"[DC motor] Torque isn't related to rpm, but horsepower is."
At a constant supply voltage, the Torque and RPM are definitely related since
the RPM is primarily determined by the supply voltage and the torque load. The
no-load RPM will be determined by the Tachometer Constant of the motor (Ke, it
is usually expressed in Volts / KRPM), resistance (both commutater and wiring)
and all of the frictional losses (dynamic and viscous). If the torque load on
the motor increases, the armature current increases to match (unless stall is
reached) - this current will be determined by the Torque Constant of the motor
(Kt, the units might be in Inch-Ounce / Amp, Newton Meters / Amp, etc). (As an
aside Ke = Kt in the metric system but this is not true with inch-ounce units).
The increase in current causes a voltage drop across the total resistance which
in turn causes a drop in RPM so that the Tachometer Constant times RPM equals
the power supply voltage minus the voltage drop: Ke * RPM = Vs - I * R. This is
only correct to a first order - second order affects would include the decrease
in viscous frictional losses which increases the available torque thus reducing
the current in the above equation, etc.
"For a DC motor, torque is proportional to current (or vice versa,
increasing the motor's load increases the current) and the speed is
proportional to the applied voltage. ... A dc motor can be considered
to have the same torque at any speed up to its rated speed, ..."
You must have been thinking of physics in an alternate universe since the 2nd
quoted statement clearly contradicts the 1st. The torque available at 1/2 the
unloaded RPM is about 1/2 of the torque available at stall (this is for a PM DC
motor, wound-field motors will differ). Likewise, at near the maximum unloaded
RPM the available torque will be quite small.
"Watts equal current squared times voltage."
That is surely a typo (remove "squared", or else substitute "resistance" for
"voltage").
David Scott
mcra...@ichips.intel.com (Micheal Cranford) wrote:
>Jeff O'Malley <jbom...@phoenix.net> wrote:
> "Has any one a chart or graph for the torque vs rpm for dc motors
> (pm as well as wound field) ?? I have heard or read somewhere
> that the low rpm torque on pm type motors is supposedly very high."
Wound field DC motors can have any of a number of charateristics
depending on wether the feild is connected in series or parallel with
the armature or powered from a seperate feild supply. In the case of
compound wound motors (haveing both series and parallel feilds wound
on the same feild pole) They can be wound for any charateristic you
want. I have even seen dc motors that when loaded heavily enough will
actually reverse by them selves and run in the oppsite direction (the
series feild is wound in oppsition to the parallel feild and when the
armature current gets high enough the series feild takes over and the
motor reverses). You can imagine what a funky rpm vs torque curve a
compound wound motor could have. The motors are wound to match their
application. For instance a series feild motor has very high starting
torque and no speed requlation so they are used in locomotives, hand
drills, gasoline engine starters, etc. While a shunt wound feild has
better speed requlation and higher torque at high speed and are used
in applications where speed control is important. A seperately excited
feild motor is just a pm feild that can be changed at will. At any
given armature voltage increasing the feild voltage will slow the
motor down and torque will go up while decreaseing the feild voltage
will increase speed and torque will go down. If fact if the feild
voltage is removed completely and the load is removed the motor will
run away and destroy it self. If you want more info let me know. Sorry
I got so wordy but I work on dc motors every day and I can warm to the
subject very quickly.
Yours
Scotty
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Garrett Fulton
> For a DC motor, torque is proportional to current (or vice versa,
> increasing the motor's load increases the current) and the speed is
> horsepower is. A dc motor can be considered to have the same torque at any
> the motor. And as we all know, motors run on smoke which is installed at
> the factory. Once you let the factory-installed smoke out of the motor,
> you just can't get it back in.
>
> If you keep a dc motor's speed constant, meaning the voltage is constant,
> and increase the load (and therefore the current), the heat generated by
> the current increases by the *square* of the increase in current. For
> example, if you double the current, you multiply the heat by a factor of
> DC motors are used industrially where high torque is needed over a wide
> speed range.
>
> Doug VanderLaan
> Electrical machine controls engineer
>
> <3411B8...@phoenix.net>...
> > well as wound field) ?? I have heard or read somewhere that the low rpm
> > torque on pm type motors is supposedly very high.
> > iron 2HP capacitor type motor on my Tiwanese import 13 x 36 lathe??
> > as I almost never run my machine at high speeds. Thanks for any
> > references, url's, etc. that will help give me this info.
> >
> > TIA Jeff in Houston, TX
> >
Tim Wescott
Yes, power is equal to current squared times resistance (or current times
voltage, from which you can derive the current^2 * resistance).
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