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DC machine speed/torque characteristic 
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#1
Jun2114, 07:17 AM

P: 18

Hello everyone!
In the following speedtorque characteristic of a DC shunt motor, we see that if the speed exceeds a certain value, then the machine becomes a generator. I don't understand something. How is it possible for the machine to have a negative torque and a positive speed? Shouldn't they always be of the same sign? For example if the machine (motor or generator) rotates clockwise, then speed and torque should be positive. So what does torque and speed of different sign mean? 


#2
Jun2114, 07:31 AM

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If the speed is positive but the torque is negative then the acceleration is in the opposite direction to the velocity.
Remember  torque causes acceleration, not speed. 


#3
Jun2114, 07:43 AM

P: 18

Thanks a lot! So the kind of rotation (clockwise or not) is defined by the sign of speed and not by the sign of toque. Is that right?



#4
Jun2114, 09:08 AM

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DC machine speed/torque characteristic
That is correct  just like the direction of linear motion is not determined by the sign of the force.
A clockwise applied torque produces a clockwise acceleration. If the initial velocity was anticlockwise, it will take the rotation a while to change direction. Usually an applied torque will be opposed by something  a load say and always friction. The friction (or other losses) is often speed dependent, so here will be some speed where the friction+load is the same as the applied torque and the acceleration will be zero. This is the concept behind torquespeed relations. You have to look carefully at the description to see where the "torque" mentioned comes from. 


#5
Jun2114, 11:23 AM

P: 18

Thanks a lot again. However it still bugs me: If speed is positive, that means that the rotation is clockwise and so the Laplace force is positive and the torque is positive. Similarly if speed is negative then the torque is negative. So I come again to my first question.



#6
Jun2114, 12:27 PM

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Arithmetically maybe this'll help ? Positive torque X Positive speed = positive power , a motor Negative torque X Negative speed = positive power , a motor wired to run the other way Negative torque X Positive speed = negative power , a generator sorry to butt in  just i sensed a need for some abject simplicity, and i'm as simple as they come.. 


#7
Jun2114, 01:23 PM

P: 18

jim hardy thanks. I get that about power, but the exact thing I don't get is this: How is it possible to have negative torque and positive speed? Because it seems to me that both speed and torque must have the same sign as the "sign" the rotation.



#8
Jun2114, 01:37 PM

P: 115

they are different things. Torque is a force, speed is a change in position. There is nothing that stops them from having different signs.
When a car is braking, its applied torque to the wheels is negative but it is still moving forward. 


#9
Jun2114, 02:26 PM

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Have you ever started a lawnmower or outboard motor? You must supply significant torque to spin it, but once the engine is started it produces plenty of torque. Viewed from the top my lawnmower rotates clockwise and i must supply clockwise torque to get it started. Once it has started i could apply counterclockwise torque to stop it, or to extract power from its shaft. Note reversal of torque.. Perhaps confusion stems from this fine point about actionreaction pairs. They act on different objects. So when you swap from motor to generator keep in mind who's doing the twisting and who's feeling that twist. To start the mower i apply clockwise torque from the top, and the motor's inertia produces equal counterclockwise torque. When mowing with it the motor produces clockwise torque and grass applies equal counterclockwise torque from the bottom. Observe torque felt by mower engine  clockwise to put power in, counterclockwise to take power out. These guys express it more succinctly : http://hyperphysics.phyastr.gsu.edu/hbase/newt.html 


#10
Jun2214, 04:15 AM

P: 18

Thanks a lot!



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