Say a motor is rotating at a constant speed... (1 Viewer)

[RishBonjour]

and it has no friction what so ever, just rotating.
The net force on it is zero right?

makes sense to think that if it wasn't it would keep increasing its speed/velocity

getting confused

 

[barbernator]

Yep, because at the point when a motor is rotating at its fastest, the back emf cancels out the voltage applied in the coil, so there is no net current and hence no net force.

 

[RishBonjour]

Yep, because at the point when a motor is rotating at its fastest, the back emf cancels out the voltage applied in the coil, so there is no net current and hence no net force.

I'm going to lol at the kid who lol'ed that me for asking that question.

thanks!

 

[john-doe]

Yep, because at the point when a motor is rotating at its fastest, the back emf cancels out the voltage applied in the coil, so there is no net current and hence no net force.

what keeps the motor rotating???

 

[RishBonjour]

Think about Newton's first law

no external force acting on it. Net force is zero - it keeps moving in its intial motion

 

[john-doe]

Think about Newton's first law

no external force acting on it. Net force is zero - it keeps moving in its intial motion

okay...i get it now!....hey but how would u account for friction (practically)

 

[RishBonjour]

my question says no friction

Well, think about it this way:
- if there is friction, it will slow the motor down right? - the back emf will be SLIGHTLY smaller due to the slower rate of change in flux. If you put a load on - friction + back emf should still equate to force exerted by supply emf


could be wrong.

 

[someth1ng]

Think about Newton's first law

no external force acting on it. Net force is zero - it keeps moving in its intial motion

In a motor, there is always a force acting on the coil except when it is disconnected as the commutator changes direction and the net force is always zero.

 

[RishBonjour]

I was thinking about external forces as in someone shoving their hand and stopping the motor lulz

One more thing

Torque is always positive for AC and DC (with split ring) right?

 

[someth1ng]

I was thinking about external forces as in someone shoving their hand and stopping the motor lulz

One more thing

Torque is always positive for AC and DC (with split ring) right?

I'm not sure about the torque graph but I tend to make it sinusoidal with positive/negative.

 

[RishBonjour]

Shouldn't it always be positive since it rotates in the same direction? (considering an AC - or DC with split ring)

 

[someth1ng]

Shouldn't it always be positive since it rotates in the same direction? (considering an AC - or DC with split ring)

But they thing is, the direction changes...you need it to be negative to show that the direction changes?