Society of Robots  Robot Forum
Mechanics and Construction => Mechanics and Construction => Topic started by: vidam on June 04, 2008, 12:36:37 PM

don't believe I understand the graph for a DC motor.
(http://i262.photobucket.com/albums/ii102/melvida/SpeedvsTorque.jpg)
When there is no external load (i.e. torque) applied to the motor then the motor runs at maximum speed, zero current, and I assume max voltage. I don't understand how that is even possible since P=I*V. maybe it approximates a small current and are calling it neglible.
confused...
Melanie

The power of an ideal motor that experiences no torque is zero. The reason this is theoretically possible in an idea motor is that a motor spins when a voltage is applied to it and a motor generates a voltage when it spins. If you have no power losses, the voltage generated by the motor as it spins exactly cancels the voltage that is spinning the motor, and you have no current passing through the coils. Since it is never possible to have a lossless system (there will always be friction and the coils will always have some resistance), you won't ever see this in a realworld motor.
 Ben

Is there such a thing as a Speed and voltage vs torque curve?

the laplace force increases when current increases (according to laplace force theory)
dF=I .* (VECTOR (Dl) ^ VECTOR(B))
I is current
dl minimum length of wire
B magnetic field
you have also
power = force * velocity
V.*I = INTEGRAL OF (dF) * velocity
guess u can calculate velocity now ;)
this also might be usefull
http://www.ajdesigner.com/phppower/power_equation_velocity_force.php

Here is a simple linear equation that relates Power to torque and rotational speed. You don't have to do integral math of differential variables.
Power (kW) = ( torque (Nm) times 2pi times rotational speed (rpm) ) / 60000
Substituing Power = V * I
You can then solve for I (current) and you get how current varies with respect to torque, voltage, and rotational speed.
http://en.wikipedia.org/wiki/Torque

please no more double posting ;)
http://www.societyofrobots.com/robotforum/index.php?topic=4653.0

I have 2 words for this post:
STALL TORQUE
This cleared up everything for me. I no longer have questions for this post.
http://en.wikipedia.org/wiki/Stall_torque
http://www.faulhabergroup.com/n390432/n.html