Dear all,

I have some problems in understanding dc shunt motor. Can anyone help me ?

Why is it when no load, the armature current is zero ? I cant get to understand this. And from the diagram , where is the rotating part actually ? The armature part , Ea ?and V(T) is actually the power supply ?

Thanks !

 

Why is it when no load, the armature current is zero ? I cant get to understand this. And from the diagram , where is the rotating part actually ? The armature part , Ea ?and V(T) is actually the power supply ?

The armature by definition is the rotating component. Even a dictionary will confirm this. First, what do you think the subscript 'a' means under Ea? Second, how do you calculate the armature power, i.e. the load? Hint: Electric Power = what x what?

 

Ea means armature voltage which means internal generated voltage ? Armature power = Ea x Ia.

Correct me if I am wrong

Thanks

 

Yes! That's right.

 

So, Why is it when no load, the armature current is zero ?

 

Old post but just in case someone looks...

In a motor,

F = B I l

Where F = force, B = magnetic field, I = current and l = length of armature

If we solve this for I:

I = F/Bl

If there's no load, then there's no torque required for it to spin, so F = 0, making current I.

Slightly more intuitively, I think an unloaded motor increases its speed until the back-emf equals the supply voltage - resulting in no potential difference in the armature circuit, and therefore no current.

 

Yikes, "so F = 0, making current 0" is what I meant.

 

Slightly more intuitively, I think an unloaded motor increases its speed until the back-emf equals the supply voltage - resulting in no potential difference in the armature circuit, and therefore no current.

In practice the generated voltage can never usually equal the opposed voltage, (zero current) it requires some to overcome the inertia of the armature and overcome any friction and windage etc, the exact difference will depend on each motor characteristics..
Max.