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  #1  
Old 07-23-2012, 02:17 AM
jinksung jinksung is offline
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Default Load current and No load current in AC induction motors

I need help from you, electrical guyes.

I am a materials guy and I am working on inductor materials, soft magnetic materials.

I tired to understand how induction motor works. And I found lots of difficulties.

My questions are;
1. Is induction motors a constant power machine or not?
Since the resistance of stator winding is fixed and the applied voltage is constant to the stator, so it looks to me that current flow is fixed regardless of loading. But I found that this is not the case. Why is that?

If the rotor draws current by magnetization, still the resistance of the stator is same. Could you please explain it not by an electrical circuit but by reasoning? Say
No load case
- current in stator winding induce magnetic field,
- stator tooth are magnetized by the magnetic field,
- magnetic field in the stator magnetizes the rotor,
- EMF is induced in a rotor bar,
- magnetic field of opposite direction forms inthe rotor bar,
- rotation of rotor
Load case
- current in stator winding induce magnetic field,
- stator tooth are magnetized by the magnetic field,
- magnetic field in the stator magnetizes the rotor,
- EMF is induced in a rotor bar,
- magnetic field of opposite direction forms inthe rotor bar,
- slip in the rotor due to load
- rotation of rotor

The only difference is slip.
How does slip influence more current at stator winding?

2. No load current vs load current

What is the meaning of efficiency of induction motors?
People mention that the efficiency of induction motors is 75-90%.
And I found that the magnetizing current for inductor is about 20~60% (average 33% of FLC). Magnetizing current means current for no load case. (No mechanical work at all.)
If the magnetizing current is 33%, does this mean that sum of loading current and loss current is 67%? If this is the case, the efficiency of , say, 90% is actually 67%x0.9?

According to www.lmphotonics.com/energy.htm

The current flowing into an induction motor comprises three major components, magnetizing current, loss current and load current. The magnetizing current is essentially constant, being dependent only on the applied voltage....For a large motor, the magnetizing current can be as low as 20% of the rated full load current of the motor.


3. If high permeability materials were developed, how will it affect the efficiency of motor?
For example, to magnetze the stator core to 1.5T, an existing material needs 1000A/m. And the new material needs 500A/m.
Does the above case mean that with the new material, magnetizing current is reduce by half? So we can save lots of energy?
Is there any effect on load current?

Thanks in advance!
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Old 07-23-2012, 02:39 AM
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takao21203 takao21203 is offline
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There are many different types of AC motors.

For instance synchronuous, and asynchronous.

The different current is explained very simple.

If the motor must produce more torque, then this will result in higher back EMF.

The magnetic force to overcome it must be higher. And so the current!

AC electrical circuits, and DC, are not the same. If you do not fully understand the fundamental difference then I suggest to buy a book about this topic.

Motors are most likely very effective, not 100%, but often 70 to 95%.

For instance a hand drill having nominal rating of 700W will heat up after a while, but no way near to 700W. If it is heavily loaded, then it will heat up more.
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Old 07-23-2012, 05:10 AM
jinksung jinksung is offline
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Thanks takao21203. Let's say a squirrel-cage induction motor.
And also assume using 1.5T magnetic field for stator.
Then, what will happen?
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Old 07-23-2012, 05:14 AM
jinksung jinksung is offline
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Also, if we use the new material with higher permeability in a DC motor, then what will happen? How much energy will be saved?
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Old 07-23-2012, 05:49 AM
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I don't know motors enough to know what will happen (or not) at 1.5T, eventually using new high-perm materials.

I have read some related books, but this is more than 10 years ago.

Currently I only have one book here about motors, enough to understand different types of motors.

Small DC motors often use magnets for instance inside hard drives.

It is an example where a motor is used to rotate an object, but otherwise it is not loaded. But it is not exactly a classical DC motor. Brushless commutation...

The biggest loss will be caused by mechanical friction.

On the linked website there is a lot of explanation however no actual motors, or energy saving devices can be seen.
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Old 07-23-2012, 07:05 AM
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1. An induction motor power input is proportional to the mechanical power output (neglecting magnetizing current and other losses).

The rotor and stator act as a transformer. The frequency of the induced current in the rotor is determined by the slip frequency between the rotating field and the rotor speed. As the mechanical load increases, the rotor slows down, increasing the slip frequency and thus the amount of current induced in the rotor. By transformer action the stator current is also increased. Thus the stator current is proportional to the mechanical load (and amount of slip).

2. Magnetizing current is reactive current and requires no power (other than that dissipated by the current through the winding resistance). Since the magnetizing current is 90 degrees out of phase with the voltage, you add that to the load current using vectors (phasors). It does not add linearly.

3. A higher permeability material should lower the magnetizing current but that is only a small factor in the motor efficiency at full load. Most of the wasted power comes from load current through the winding resistance in the stator and rotor. A motor with superconducting windings, for example, would have very high efficiency.
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  #7  
Old 07-23-2012, 08:06 AM
jinksung jinksung is offline
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Thanks crutschow, but still I have a question on magnetizing current.
When people mention about magnetizing current, they are talking about no load current (almost no slip condition) and it is about average 33% of full load current. So, if a high permeability material is used(say 2000 vs 1000), can I save 50% of the magnetizing current to get the same magnetic flux density? In other words, is no load current reduced down to 16.5%?
And some people say that magnetizing current is almost constant during operation of the motors with any load. If this is the case, can we save 16.5% of electric power to get the same mechanical work?
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Old 07-23-2012, 01:56 PM
jinksung jinksung is offline
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Let's change my question as follows;
Generally materials scientists consider only loss component.
Iron loss, copper loss, windage loss, stray current loss.
And we believe the rest of energy is for mechannical work.
So we are trying to reduce Iron loss without sacrificing magnetic induction.

Let's assume we have materials with same level of iron loss.
However, its magnetization behavior is different.
For example, permeability of 1000 and 2000 for the same 1.5T magnetization.

Iron loss and windage loss appears to be same.
The major difference lies in copper loss since magnetizing current is different.

At this point, what I have noticed is that the reduction in copper loss is due to reduction in exciting power.

Now, how will the reduction in exciting power affect the electric power consumption? or efficiency?

If we only consider loss components, except for the copper loss everythig is same.
In full load condition, how much portion can be attributed to the magnetizing current?

It's really puzzling!
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Old 07-23-2012, 02:05 PM
jinksung jinksung is offline
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As crutschow said, if magnetizing current is reactive current and requires no power, then maybe what appears to be important is only loss components. Is it really so?
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Old 07-23-2012, 02:39 PM
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Small motors with high torque are built using high-permability materials, if I remember right.

A transformer is somehow analogy to motor even if not a very good one. Transformer does not have moving parts.

And single toroid inductor is not much different from a Transformer, indeed in principle, a full transformer can be built on a toroid. The physics for only a coil are the same.

Indeed permability, and magnetization losses are important properties for toroid cores. There are many different materials, even if appearance is similar. Granularity is different I think.

I might have a large PDF from Siemens Matsushita about this topic.
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