# DC Shunt Motor

#### 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?

#### vincent19-mas

Joined Dec 27, 2012
83
Ea means armature voltage which means internal generated voltage ? Armature power = Ea x Ia.

Correct me if I am wrong

Thanks

#### mlog

Joined Feb 11, 2012
276
Yes! That's right.

#### vincent19-mas

Joined Dec 27, 2012
83
So, Why is it when no load, the armature current is zero ?

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#### 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.

#### gbell12

Joined May 29, 2010
4
Yikes, "so F = 0, making current 0" is what I meant.