DC Shunt Motor

Thread Starter

vincent19-mas

Joined Dec 27, 2012
83
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 !
 

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mlog

Joined Feb 11, 2012
276
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?
 

gbell12

Joined May 29, 2010
4
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.
 

MaxHeadRoom

Joined Jul 18, 2013
19,408
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.
 
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