Why does the torque output of a DC motor decrease linearly with RPM?

[Sharkbiteattack]

I know this applies to brushed DC motors and I think it applies to brushless as well?

 

[itsmoked]

Because the EMF increases reducing the current available thru the windings. The current is the torque motivator.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[itsmoked]

Often called the "Back EMF".
It represents a counter voltage developed by the spinning armature.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[cswilson]

The basic motor voltage equation (ignoring short term inducive effects) is:

V = Ke*w + R*I

where:

V is the voltage across the leads (constant applied in your case);
Ke is the back-EMF constant of the motor (volts per unit speed);
w (omega) is the angular velocity;
R is the resistance;
I is the current

The higher the motor speed, the more of the supply voltage is "used up" by the back-EMF term, and the less is available to drive current through the motor. Since torque is proportional to current, as your speed increases, your torque generation capability decreases linearly.

 

[djs]

Depends on the type of motor. For a series motor the torque is related to the square of the current. For a shunt type motor, the torque is linear to the motor armature current. This is with a fixed field current. For the other posters, the heading is torque not voltage.

 

[mikekilroy]

and to add to Curt's voltage makes speed equation is the other one, current makes torque:

T=Kt*I where Kt is same thing as Ke but in different units (torque/amp)

So studying them together one can see as the voltage is used up to make speed, less is left over to make I (V/R).

So as Curt showed, the faster you go with a constant voltage applied, the less is left over to make current in the motor resistance, hence less torque as u go faster.

So you see you stated it backwards by now; torque does NOT go down as speed goes down, but rather as it goes up.

 

[mikekilroy]

oops; you did not say which way rpm goes so I probably misread it when i assumed you meant as rpm goes down rather than up. if so, sorry.

 

[Sharkbiteattack]

Yea your right mike I did my question wrong, I meant why does torque decrease as rpm increases. I guess the correct way of stating the relationship is that torque and rpm of a DC motor are inversely proportional?

So how about brushless DC or PM AC motors? Is it the same deal?

 

[itsmoked]

Yes and Yep and yep. You seem to have in now.

Just to toss in a little more the Horsepower is a direct function of the speed times the torque.

Keith Cress
kcress -

http://www.flaminsystems.com