Just a few vague comments. I don't really know the answer to your question.
When you reduced the frequency to 1hz, did you also reduce the voltage proportionally? If not, then you have severe saturation at 1hz.
Another change would be current distribution in large condcutors. Effects like skin effect and current interaction with other fluxes can cause nonuniform current distribution changes with frequency.
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the wire with D[mm] = 200 [mm]/ square_root(f[Hz] )
diameter will have 110 % of the dc resistance .
From this a wire has to be 1" or thicker to increase
the resistance with 10 % at 60 Hz.
Not to many motors have 1 inch thick wires and 10 %
higher resistance would increase the losses by about
5% ( assuming 50% resistive and 50% core loss).
The 'deep bar effect' is what I was searching for.
Here is a quote from Nailen's "Managing Motors" (I added the parts in [square brackets]).
"In most motors [this book applies to large motors], rotor bar temperature [during starting] is increased by another factor: the deep bar effect. The rotor bar currents during acceleration are at a fairly high frequency, because thte rotor sees the relative speed of the stator magnetic field compared to the physical rpm of the motor. When the actual rpm is low, rotor frequency is high and a t locked rotor will be full ine frequency. Under this condition, the effective current-carrying depth of the rotor bar becomes only about 3/8 inch for copper and half-an-inch for aluminum or brass, no matter how deep the bars actually are. Thus, the apparent a-c resistance of the bars becomes much higher at low speed than at full speed."
A similar effect is exploited in double-cage designs.
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One more point to mention - if current distribution within the bar were unimportant, rotor bar shape would not be important (only the area). However, rotor bar shapes are very important to the torque speed and current speed characteristics of the motor and are the subject of detailed study by manufacturers.
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Well, let's just say the skin effect is negligeable
in the case of smaller motors -- and in the last 30 years
the biggest motor I worked with was less than HP.
<nbucska@pc33peripherals.com> omit 33 Use subj: ENG-TIPS
Plesae read FAQ240-1032
Hi nbucska
I think the point is not so much skin effect but more of a "Typo" in a Teapot.
Please reread your original post carefully. You may want to reread aolalde's post also.
Cheers
I think that it is more about what nbucska said. He hasn't seen any large (hundreds to thousands of HP) for many years.
What he says is true for FHP motors. And I think that most of us assumed larger motors. The OP actually said nothing about the size of the motor. It is perhaps an FHP?
The OP question was about the validity of the motor model below 1 Hz. The simple answer is: It depends.
If the model is accurate enough and takes heating effects into account, it is valid all the way down to zero Hz. There will not be any transformer effect between s and r at DC - and that is also what you get in a real motor, no coupling between stator and rotor. So the model is accurate enough - but the motor does a lousy job at 0 Hz.
Many practical drives start to show problems already below 5 Hz and there are several ways of dealing with the problem. One manufacturer changes from voltage model to current model at about 5 Hz. But that doesn't solve the problem completely. An encoder or resolver is needed in most cases if complete control through zero is needed.
I'm mentioning about the motors which are below 25HP. Why i ask the question is mainly, i'm trying to investigate the parameters with different methods. In all cases, motor isn't revolving. If there is negligible effects, i'll also evaluate by injecting low frequency sinusoidal signals like 1Hz.
abfer, there are two effects that become more and more significant at very low Rpm/Hz. The first is motor slip rate. At higher Rpm motor slip is only really a nuisance from the efficiency and power loss point of view. At 1 Hz the slip rate will be significantly higher than the actual motor shaft speed, so the motor will stop (under load) long before you reach zero Hz.
The other effect is motor winding resistance. At high voltage and frequency there will be a voltage drop across the internal resistance of the windings. That is also just a nuisance and efficiency issue at higher speeds.
At very low drive Hz, the ac drive voltage will also be very low, but maybe with still up to full load current. The resistive voltage drop becomes larger and larger as a proportion of the available motor drive voltage.
The result being, that at very low frequencies you are going to require more Hz and more drive volts in order to sustain sufficient torque to do the job. In other words speed/load characteristics may be very poor.
The more sophisticated variable frequency drives allow compensation for these two effects, but that may still not be sufficient.
Some sort of active feedback control may be the answer, because the performance of an open loop system could leave rather a lot to be desired. A lot depends on what level of performance you require.
