Motor Saturation Curve 1

[rockman7892]

I have recently been reading up on motor saturation curves and trying to understand how they apply to induction motors. From what I understand about saturation curves, they are basically a curve relating amperes to flux density. On any of these given curves there is a "saturation region" where there is too much flux density in a particular area (core, windings, etc..) and there for there is a greater reluctance to magnetic flux produced by current. In this region it will take much more per unit current to create per unit flux than the linear relationship that happens outside of this region.

With that said (not sure if i'm correct) i'm trying to understand how this relates to motors. One of the ways I understand this applies is through a "no load losses test" or something similar where the voltage is varied and current is recorded. During a no loads loss test the voltage is varied and the current will change in proportion to the voltage change to establish a saturation curve. Am I on the right track here or am I way off??

Can someone help me understand this relationship?

 

[dpc]

The saturation curve is related to the iron in the stator of the motor. If the voltage applied to the motor is increased so that the motor is operating above the "knee point" of the saturation curve, the motor will start drawing excessive current since the reactance seen at the motor terminals will decrease. The flux will leave the core, efficiency goes down, the motor will get hot, and generally a lot of bad things happen.

It is analogous to saturation in a transformer.

The steel core can only hold so much flux. Once the core is saturated, increasing the voltage produces very little additional flux, but a lot of extra current.

 

[7anoter4]

hi rockman7892
You are right, the saturation is related to the magnetic flux density measured in Gauss [or Wb/sqr.m=Tesla].
[10000 gauss=1Wb/sqr.m]
If we neglect the field losses in adjacent air, the same magnetic flux [measured in Wb [Webers] circulates from rotor
through the air gap to the stator and again through the air gap to the rotor.
The flux density is roughly the ratio flux/area. The air gap area is the largest. The rotor teeth area is the minimal area.
The maximum B will be in the rotor teeth and then in the stator teeth.
The rotor is less solicited than the teeth and the stator lesser. The minimum B is in entrefer [air gap].
From the Attachment one can see how for 31% voltage rise the MMF rises 350%.
MMF=magnetomotive Force is total Ampere-turns of the motor [of both stator and rotor] direct proportional with Imiu [as per Van Wormer model]
BTW the noload current contains also an active component due to iron losses and mechanical losses [ventilation and bearing friction] but the most of
noload current is actually Imiu.
Regards

 

[7anoter4]

Sorry,it is not "Imiu as per Van Wormer model" but "as per Steinmetz model"
Regards

 

[electricpete]

That's a good discussion 7anoter4.

rockman7892 - the question is a little vague.

One aspect of performance vs voltage as mentioned by others - if you operate at too high a voltage, core losses will go through the roof.

Also certain aspects of motor performance may be affected by localized saturation. In a closed slot rotor, we count on saturation of the thin layer of iron on the airgap side of the rotor bar... otherwise rotor leakage reactance would be very high. Designers use localized saturation to their advantage to meet multiple competing requirements for running and starting performance.

You mention a no-load test where voltage is varied. Typically this is useful to separate the mechanical low-load losses from the electrical no-load losses. As you vary voltage within the linear range (far below saturation), electrical no-load losses vary linearly with voltage and mechanical no-load losses are approx constant. So if you extrapolate the slope of the loss vs voltage line back to 0 voltage, you have the mechanical no-load losses. Now subract the mechanical no-load losses from the full-voltage total no-load losses, and you have left the electrical no-load losses.


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[7anoter4]

Very good explanation electricpete, thank you!

 

[rockman7892]

What stemmed this question was the fact that I am trying to set up sensorless vector control for a VFD. As a part of this setup I was told that the VFD goes through some motor tuning tests in which it reads the stator resistance and looks at the saturation curve of the motor.

I was curious how the VFD could determine this saturation curve and how it was used in this application and others?