Load current and No load current in AC induction motors

Thread Starter

jinksung

Joined Jul 20, 2012
22
Thanks tinkerman for your kind explanation.
I also studied on this subject a lot.
In materials scientist's view, what happen during loading in induction motors can be described like below;
- current flow in primary windings induces magnetic field (H, A/m)
- Because of the field, stator tooth will be magnetized (actually, magnetic domain will arrange according to the given external field)
- This magnetic field will magnetize the rotors. (Align the magnetic domains in the rotor)
- The induced magnetic field in rotor lamination will induce EMF at rotor bars.
- Current flows(electron motion) through rotor bars in the given magnetic field will make a torque in the rotor.

And the torque is proportional to BxIxl, magntization, current and length of rotor bar.
So if we have a high permeability materials, we will have higher torque.
This is because we have higer B and therefore higher I.
So with higher permeability materials, we can make a small and strong motors.

Now, what will happen to the motor efficiency with high permeability materials?

It is still puzzling.

This is because
with no load, it will comsume electric power to rotate.
This means that the electirc power is needed to flow current in stator coils, magnetize stator cores, air gap, and rotor lamination.
Now, if there is a load, then, we need load current.

Then, my question is in no load case, we have low power factor. So although we have very high current consumption, about 33% of full load current, during no load operation, actully we are not using current? Or actually are we using 33% of full load current? In other words, although we are passing 33% of full load current during no load case, we are actually using much low electric power? Or during loading, this 33% of FLC at no load case will be converted to load current?

Or 33% of FLC means actual energy consumption for magnetization? So load current is only for loading and magnetizing current is only for magnetization?

If this is the case, how could we obtain efficiency of 70~90% in induction motors?
This is because we already consumes 33% of FLC by magnetization. This component is not for loading. So we are actually starting from 67% efficiency.

It is still confusing.

If the magnetizing current is reactive component, so that we are not consuming energy at all, assuming no copper and iron loss and no mechanical loss, what is the no load current?
 
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