# DC Shunt Motor

Discussion in 'Homework Help' started by vincent19-mas, Jun 5, 2013.

1. ### vincent19-mas Thread Starter Member

Dec 27, 2012
83
1
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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2. ### mlog Member

Feb 11, 2012
276
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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?

3. ### vincent19-mas Thread Starter Member

Dec 27, 2012
83
1
Ea means armature voltage which means internal generated voltage ? Armature power = Ea x Ia.

Correct me if I am wrong

Thanks

4. ### mlog Member

Feb 11, 2012
276
36
Yes! That's right.

5. ### vincent19-mas Thread Starter Member

Dec 27, 2012
83
1
So, Why is it when no load, the armature current is zero ?

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6. ### gbell12 New Member

May 29, 2010
4
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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.

7. ### gbell12 New Member

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

Jul 18, 2013
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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.