On Sunday, September 7, 1997 at 3:00:00 AM UTC-4, Doug VL wrote:
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 wrote in article
...
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


"Watts equal current squared times voltage." Shouldn't that be current squared times resistance?

On Tue, 24 Nov 2015 11:31:01 -0800, Garrett Fulton wrote:

On Sunday, September 7, 1997 at 3:00:00 AM UTC-4, Doug VL wrote:
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 wrote in article
...
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


"Watts equal current squared times voltage." Shouldn't that be current
squared times resistance?

Wow, that's an old post to revive!!

Yes, power is equal to current squared times resistance (or current times
voltage, from which you can derive the current^2 * resistance).

--
www.wescottdesign.com