Motor Equivalent Circuit (Long & Detailed)

[rsdoran]

With most of what we have been saying so far, the basic justification has been 'Well, the mtor model says...'. Well the motor model also says that the stator resistance goes negative. I don't know about you but my meter doesn't go that way when I switch it to Ohms.

I see why now. I can not explain it in detail but maybe able to offer some ideas that will make it easier for YOU TO EXPLAIN.

I think the main problem may be terminology being used, technically resistance can not change but "impedance" (total resistance of an AC ckt) can, so I always thought of it with that in mind.

The rotor when it over_hauls changes it frequency which in turn changes its "impedance" which in turn should change its voltage, the greater voltage/frequency will be the one that supplys power...I know its more complicated but it should basically be a differential computation...ie the stator is at 480vac but the rotor is at 485vac and the impedance factor of the rotor is 5 ohms so the difference is 5v divided by 5 ohms which will put 1 amp of current from the rotor into the stator.

A magnetic field does not instataneously collapse so there should be residual magnetism which will be used for "excitation". It would be very complicated to "exactly" match the stator to 1800 rpm for a period long enough to collapse the magnetic field(s).

I know it is not as simple as I have stated but at this time I am not capable of nor qualified to explain it using all the equations. I do not claim my math above to be fully accurate, it is shown to give you an idea.

For years I worked with generators that were paralleled, to accomplish this you had to sychronize (match voltage and frequency) to couple them. Once synchronized then you would increase or decrease the speed (slightly) to develop the current supplied to/from the other set(s). The faster running genset would be at a slightly higher frequency and voltage. I guess I always used this as a basis for understanding re-generation in motors.

Again the genset info is in basic terms but maybe it offers some ideas so you can understand then maybe you will write a technical paper on the subject so more of us can understand it in "better" detail.

 

[DickDV]

You and me both, keith, reqarding that "little bit more involved" part.

It seems to me that using the analogy of a transformer to relate a motor's stator to its rotor is convenient but probably not very accurate. I say that because the rotor (transformer secondary) has almost no inductance.

There was one model in one of the links you posted that showed a transformer-like coupling but then it went on to say that it would only represent a special case. Hmmmmm!!

With typical rotor slip being only 2%, I wonder just how much coupling back into the stator windings could really occur. The model indicates pure inductance in the stator without any compensation for slip speed or rotor speed. Makes me wonder.

 

[kamenges]

Ron, what you are saying feathers in with what Dick posted a few posts back. When you talk about impedence you have a resistive and a reactive element. The currents associated with them are 90^o electrical apart. The resistive element is in phase with the fundamental voltage and the reactive element is 90^o behind in a purely resistive/inductive circuit. The combined current vectors produce a single sum current vector with it's own phase angle relative to the fundamental voltage. In a transformer and (presumably) an AC motor, no load condition produces all reactive current. This gives you a voltage and current that are 90^o out of phase. As you add resistive load this angle decreases toward 0^o.

But now, as Dick said, let's allow the motor model to produce negative resistive current. If what I say above is correct (which at this point is highly suspect) then the sum current vector will increase past 90^o relative to the fundamental voltage. I think this in and of itself is enough to start a net current flow back to the source without the need for an increased reverse voltage. As long as the magnetizing current exists, driven by the fundamental voltage, this phase shift can occur.

Ron, you make an interesting point about holding right on synchronous speed. That would be a pretty tough thing to do for any period of time. And, like you said, the reason an induction motor works at all is that the rotor magnetic field tends to hang around for a while. I recently did some autotunes on some motors and got rotor time constants in the area of 250msec to 350msec depending on size. Without this hysteresis I don't think you could develop enough of a coherent magnetic field on the rotor to allow consistent torque generation.

Ok, I may have been fully swung over to the Dark Side. Dick, as you said, modelling the stator and rotor purely as a transformer gets a little awkward. The problem is that the stator magnetic field is changing at the fundamental voltage frequency but the rotor magnetic field is changing at the slip frequency. Have fun modelling THAT purely as a transformer. It doesn't seem to make sense. So modelling it as a mutual inductance with a variable resistance makes it easier to handle that case. The thing with rotor to stator induction is we know that the rotor has to have a pretty good sized magnetic field in order to generate torque. We also know that this magnetic field is turning relative to the stator at near rotor speed ((rotor rotational frequency / voltage frequency * pole pairs) - slip frequency). So we have a strong magnetic field whipping around at high speed inside a coil of wire. I've given up on the whole back EMF as a current limiter thing. But there has to be an induced voltage and it has to do something.

Thanks for sticking with me on this. I find this highly entertaining. Yea, I know, I ain't quite right.

Keith

 

[Thomas Sullens]

KEITH you make my head hurt~!

 

[Leadfoot]

The transformer analogy is useful to help get the basics of a motor thru to those that have not yet or ever will understand them. I do consider myself somewhat knowledgeable on how motors work. I have to know more than the average electrican and plant engineer I run into while on a service call. I specialize in VSD's.

I also teach a Fundamentals of AC and DC motor controls course, primarily drive oriented. We touch on how motors function. I run a couple of tapes on basic motor function. 15 years ago, the typical student in the class was an electrician with the ability to use hand tools and a meter. Today most have tech school or higher levels of education. The questions I get asked these days are on a much higher level. I realized I needed to elevate the course a bit to keep up with the level of education and abilities of the student sitting in the class. I had some information from a couple of mfgr's on the motors and began to use the equivalent circuit and transformer analogy. It just makes teaching fundamentals easier. At the level we have been discussing and re-learning, the step to the next level becomes harder. Many of us have been at this a while and were taught the fundamentals of electricity and electronics differently. Anybody that has gotten an EE was taught conventional, aka ion flow, aka hole flow. Current moves with the arrow. I was taught electron flow, the current moves against the arrow. I see discussion flowing current both ways happening here.

Most things boil down to terminology and how we perceive things. So far no one has really contridicted anybody and everyone is building upon what was presented.

The concept of negative resistance is a hard pill to swallow. Typically when the term negative resistance comes about, I believe it is refering to the CEMF resistance during regen. Resistance is the opposition to current flow. I believe Negative resistance is related to the current flow caused by the CEMF being larger than the EMF.

I like most on this forum, came looking for information on PLC's. I have gotten a tremendous boost in my confidence to program them and a better understanding of their capabilities. Occasionally we get diverted on discussions like this one. I enjoy the information that others post as it is giving me a broader perspective of how others see what I think I know.

I am always looking for a deeper or better understanding of how things work and an easier way to explain them. This thread has been doing that. My thanks to all.

 

[DickDV]

Keith, I am thinking just the same as your last post. This discussion has pretty much destroyed my comfort level with the motor-transformer analogy and has increased my comfort level with the concept of the rotor phasing controlling which way the current flows.

Ron, I find it very useful what you said about synchronizing parallel alternators and controlling loading with phasing. Your comments on that have made me more comfortable with the concept of the motor rotor phasing controlling the direction of current flow.

Thomas Sullens, your head isn't the only one that hurts but, if that's what it takes to gain a new understanding into technical things, I'm willing to go thru it. Even at my age (60), I find it fun to push my understanding and it seems there are some others like that on this BBS.

Bottom line-------it's a good thing! Hopefully, I can help someone else understand motors better. Then it will have been really worthwhile.

Thanks again to everyone who has tried to add anything to this thread.

 

[Lancie1]

Quote:
Originally posted by DickCV:

Second, I have heard of one special condition where an overhauled induction motor can self-excite which involves the installation of capacitors on the motor leads.

Yes, the self-excitation of a 3-phase AC motor used-as-a-generator is very common in less-developed parts of the country, and is the method that many people around the world use, where low-cost and widely available AC motors generate electricity at small-scale microhydro projects. Here is an excerpt that shows how to calculate the value of the capacitors to turn a motor into a generator, from the Yahoo Microhydro Users Group:

http://groups.yahoo.com/group/microhydro/message/4469

I understand that it is common to put one capacitor, value C across Phase 1, a second value 2C, across Phase 2 leads, and then the Load across Phase 3 motor leads. I tried this once with a motor driven by a lawn mower engine, and it did spin the meter backwards.

Using the motor as generator is simple: the start-up is done by the associated turbine, so you don't have to use any auxilliary phase at all. You just need to make the motor to self-excite, using AC capacitors. This is the standard case of 3-phase induction motors as generators. You can calculate the capacity you need based on the motor plate data.

Let's say: 1-phase; U=220 Vac; I=1.8 A; P=0.25 kW; f=50 Hz; cosfi = 0.63;

Apparent power S=UxI=396 VA;
Active power P=250 W;
Reactive power Q=sqrt(S^2-P^2)=307 VAR;
Ireactive=Q/U=1.37 A;
Xr=U/It=160 ohm;

C=1/(2*pi*f*Xr)=20 uF

So, I guess you will need around 20 uF (check with your specific motor data) in parallel with this basic motor winding, so you could then have the motor self-excited.

More things could be found in Nigel Smith's book [url="http://www.microhydropower.net/literature.html#Smith"]http://www.microhydropower.net/literature.html#Smith[/url]