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Producing constant torque in the simple electric motor. 
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#1
May1012, 09:29 AM

P: 68

In the simple electric motor, applying a constant current produces a pulsating torque. But if we apply a pulsating current will it produce a constant torque?



#2
May1012, 09:41 AM

Mentor
P: 11,629

Probably not constant (it depends on the current and the geometry), but at least in the same direction if the frequency is right. This is the concept of motors with alternating current.



#3
May1012, 09:50 AM

P: 834

With a constant current, shouldn't you have a constant torque with only torque ripple that is dependent on how well the commutation timing is done (commutation at the right time, when the magnetic fields are perpendicular)?



#4
May1012, 10:28 AM

P: 68

Producing constant torque in the simple electric motor.



#5
May1012, 10:36 AM

P: 834

In both AC and DC equations, torque is directly proportional to a DC current value, and so torque is constant at a constant current. I assumed you were talking about torque ripple, but maybe there's something that I'm unaware of though. 


#6
May1012, 10:44 AM

P: 4,663




#7
May1012, 11:00 AM

P: 68




#8
May1012, 11:17 AM

P: 834




#9
May1012, 12:08 PM

P: 68

The second animation is for DC generators, where constant torque produces pulsating DC.
I think here the back EMF remains constant. In the image attached, the right diagram represents electromagnetic phenomena, where applying a current in the conductor in the direction shown, in the magnetic field (where field lines are pointing into plane of paper), produces a force F and therefore a movement leftwards. Now if apply the same movement to a stationary conductor in the same magnetic field, it will produce a current in the opposite direction by electromagnetic induction. My reasoning is that, in a motor, if constant current in a particular direction, produces pulsating torque, then the pulsating torque will induce a current in the opposite direction, by electromagnetic induction. 


#10
May1012, 01:28 PM

P: 4,663

added There is back emf when the rotor is turning. 


#11
May1012, 02:07 PM

P: 834

A DC motor with the shaft mechanically rotated will generate a voltage on its open terminals, which can be used to power other things. This is how a generator works in a simple comparison, and you can consider a DC motor as a DC generator if it is connected as the 2nd animation shows. I must ask you, how can the back EMF remain constant if the B fields of the rotor and stator are not always in the same direction with respect to eachother? I think the back EMF is varying in this motor, and so the current must be varying too if the source is a DC voltage supply. 


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