THE PHYSICS OF... POWER

[Besoeker]

By spreadsheet, do you mean the almost unreadable mass of numbers you posted in #380? Or did you link to an actual spreadsheet at some point that I missed?

I have some problems with some of the values I think I see in #380 in any case. Specifically, how do you get such a high torque and such a high mechanical load figure with almost zero slip? That seems to contradict my understanding of the effects of slip.
I also do not see a term in that mass that directly represents the coefficient of j in Zinput.

It's for a slip ring induction motor with a static Kramer for speed control. One of many I did and at the lower end of the power range. As rule, you are given fixed speed values fo 4/4, 3/4 and 2/4 loading. I had to work back to get the Steinmetz equivalent circuit component values. It's just a wee bit more than a spreadsheet in that there are VB routines to underpin the individual circuit values.

I'll happily send it to you by email if you wish.

 

[mike_kilroy]

OK, Mike, let's also make a small tweak to what you are saying.
The inductances (and therefore component reactances) in the motor model do not change, and the amount of reactive current does not change significantly. That does not, however, mean that the reactive term of the overall input impedance, Reff + jZeff, does not change.
See the discussion of the effect of parallel resistance on the coefficient of j in a simplified situation.

Part of the apparent conflict between some of FZ's statements and those of Mike and others, including me, is that there is no agreement of what the simple word reactance refers to. It can either refer to the lumped component reactances in the model, which do not change, or the reactive term of the overall input impedance, Z, which does change.

Also, FWIW, getting back to FZ, I do not see that changing the "frequency" and therefore the relation between the inductances of the components and their inductive reactances, is a valid way to analyze the model or the real motor. Something is missing in the context of FZ's quote.

When I was a wee lad in school, I learned that if I asked for a milk shake in Boston, I would not get the thick delicious ice cream filled milk shake I was used to! They said I would get a glass of milk that was shook up then served! Boy funny how the same term could be interpreted so differently!

So when I get to college, I learned that resistance, R, was that thing caused a voltage drop due to current thru it - a current in phase with the voltage.

Then I learned reactance, Xm or Xc, was the similar term for a pure inductance or capacitive device that when that current or voltage went thru got offset from the other by 90 degrees.

To top it off, I learned that impedance, Z, was the combination of both! Damn, it all came together for me and seemed to stick to this day!

But when ZF wrote that last comment, my mind reeled and resisted. I actually QUESTIONED my long learned idea! So I went to the dictionary and looked it up:

re·ac·tance
ˌrēˈaktəns/
nounPHYSICS

[COLOR=#878787 !important][/COLOR]

• the nonresistive component of impedance in an AC circuit, arising from the effect of inductance or capacitance or both and causing the current to be out of phase with the electromotive force causing it.
  
  
  
  
  
  

So I wrote my reply. No morphing today!

 

[FionaZuppa]

then i guess all these guys are just wrong when they say "loading causes reactance to go down" ? but note he eludes to slip.
perhaps he means to explain in terms of I² ??
HP remains constant load or no load ??

has anyone done any Henry (L) measurements on the input of a live running motor, no load then load?

 

[mike_kilroy]

then i guess all these guys are just wrong when they say "loading causes reactance to go down" ? but note he eludes to slip.

I listened to the utube and only heard one guy, not "all these guys."

That said, I hope no one considers sending their children to Dunwoody College of Technology for an engineering degree!

This guy is SO WRONG on MOST of his comments!!!!!!!! Doesn't anyone check these prof's performances anymore?

Bottom line is he flat wrong on most comments. He only had it right when he said current goes up when the load is applied, and the rotor slows down. Everything else he says is way wrong.

Its the same old same old: I heard it on the internet so it must be true.

Sorry.

 

[FionaZuppa]

anyone ever take Henry (L) measurements on input of live motor no-load then load?
can i possibly write the jX term as an expression that involve S (slip) ??

 

[mike_kilroy]

anyone ever take Henry (L) measurements on input of live motor no-load then load?
can i possibly write the jX term as an expression that involve S (slip) ??

Sure, most everyone who has worked with high performance vector drives! current no load vs loaded and their associated phase angle (or PF). Then you got the inductance at the terminals from Z=r+jw. just calc it out!

 

[FionaZuppa]

Sure, most everyone who has worked with high performance vector drives! current no load vs loaded and their associated phase angle (or PF). Then you got the inductance at the terminals from Z=r+jw. just calc it out!

not asking about amps. asking about the component related to jX, namely Henries (L). just because Z goes down and amps goes up that does not mean jX changes. when you load a motor the effective R goes down. but how can R of the input winding go down, the copper is constant length ??

 

[mike_kilroy]

anyone ever take Henry (L) measurements on input of live motor no-load then load?
can i possibly write the jX term as an expression that involve S (slip) ??

FZ, would real world impirical data suffice to prove to you the Xm does not change significantly loaded vs unloaded? Just the resistance term r'/s? It sure would be a lot easier to convince you than getting out my old college books and refreshing how to calculate L from 2piFL and Xm... Try these:

75kw during accel then no load running:
1) You can see red current to make torque to accelerate is around 40% of motor rating
2) you can see red running current (no load) is around 1%
3) you can see blue magnetizing current is constant 25% AT ALL TIMES.

Although it is irrelevant, for reference, 100% speed on this 100HP motor is 12,000rpm, 100% current is 188 amps @ 8khz PWM. Bottom line is NO CHANGE in Xm (aka J) term even tho LOAD changed from 1 to 40%. PF (aka, phase angle) changed but not imaginary current.

200kw while machining in and out of part:
1) You can see purple current to make torque to machine @ 200HP is 100% of motor rating
2) you can see purple running current (no load) is around 0.5%
3) you can see blue magnetizing current is constant 20% AT ALL TIMES.

Although it is irrelevant, for reference, 100% speed on this 100HP motor is 18,000rpm, 100% current is 200 amps @ 12khz PWM. Bottom line is NO CHANGE in Xm (aka J) term even tho LOAD changed from 1 to 100%. PF (aka, phase angle) changed but not imaginary current.

75kw,10k during accel, run no load, then decel:
1) You can see green current to make torque to accelerate is around 15% of motor rating
2) you can see green running current (no load) is around 0.5%
3) you can see purple magnetizing current is constant 32% AT ALL TIMES.

Although it is irrelevant, for reference, 100% speed on this 75HP motor is 10,000rpm, 100% current is 100 amps @ 5khz PWM. Bottom line is NO CHANGE in Xm (aka J) term even tho LOAD changed from 1 to 15%. PF (aka, phase angle) changed but not imaginary current.

I hope these real scope pixs are enough proof FZ that reactance/X/J do NOT change with load - only the r'/s term that is the Isq IN PHASE current you pay for.

I have a zillion more scope pix of the imaginary current along with the real current I can share. From drives from 2kw to the 200kw sizes. But the bottom line is all show the same thing! Some motor designs DO lead themselves to 10-20% variations in Isq loaded vs unloaded, but NOT for the reasons of morphing! But since some designs have more leakage reactance than others. But that is irrelevant to these discussions, just like starting current is irrelevant. Although once THIS is understood, then the huge starting current pulse is very easy to understand too (all related again to R'/s during start).

If you need further proof, maybe someone who runs Z=R+Jw calcs daily can convert these values for you into L as you ask. I do not want to.

ADDED note: I am only showing you scope pictures BELOW BASE SPEED, or at a constant speed here. I did not want to introduce how the magnetizing current must go down when we get into the field weakening range... Brrhhh not yet anyway!

 

[mike_kilroy]

not asking about amps. asking about the component related to jX, namely Henries (L). just because Z goes down and amps goes up that does not mean jX changes. when you load a motor the effective R goes down. but how can R of the input winding go down, the copper is constant length ??

Now THAT is a great question for someone trying to understand the induction motor model!!

Bestest answer should be easy: the R'/S term is from the rotor. This term gets to the motor terminals (stator) just like a transformer! THAT is why some of the more complex motor equiv circuits show a transformer between the stator and rotor! That silly magnetic field from the reactance lets the rotor effective resistance (aka torque load on motor) get transformed right over to the stator wires!

Look at the equiv circuit again: the stator resistance is IN SERIES WITH THE ROTOR RESISTANCE.

Since we all showed you that the Xm (reactance to make magnetic field to make the hunk of metal into a motor) is CONSTANT, that term does not change. SO THE ONLY THING LEFT IN SERIES WITH THE L-L INPUT IS R'/s.

 

[FionaZuppa]

Now THAT is a great question for someone trying to understand the induction motor model!!

Bestest answer should be easy: the R'/S term is from the rotor. This term gets to the motor terminals (stator) just like a transformer! THAT is why some of the more complex motor equiv circuits show a transformer between the stator and rotor! That silly magnetic field from the reactance lets the rotor effective resistance (aka torque load on motor) get transformed right over to the stator wires!

Look at the equiv circuit again: the stator resistance is IN SERIES WITH THE ROTOR RESISTANCE.

Since we all showed you that the Xm (reactance to make magnetic field to make the hunk of metal into a motor) is CONSTANT, that term does not change. SO THE ONLY THING LEFT IN SERIES WITH THE L-L INPUT IS R'/s.

you mean parallel ??

 

[mike_kilroy]

you mean parallel ??

No, I mean series like I said. Look again. YOUR comment was how does STATOR R change with load? It does not of course. But stator R is in SERIES with rotor R where rotor R varies with load. Sure, the Isq/magnetizing/reactive/Xm current is in parallel with rotor R, but ignore that since it does not change with load PLUS it is 90 degrees out of phase so is not real! the ONLY term changing with load is the SERIES rotor R called R'/s.

It is called R' since it is REFLECTED to stator side of the 'transformer' created by the Isq/magnetizing/reactive/Xm magnetic field. A few post back had the transformer and non tranformer equiv circuit shown... R' is in SERIES with Rstator.

Heck, for this discussion, throw out ALL Jx terms! the ONLY in phase current is for R terms, so now look again: your stator R is in series with rotor R'.

 

[mike_kilroy]

...Heck, for this discussion, throw out ALL Jx terms! the ONLY in phase current is for R terms, so now look again: your stator R is in series with rotor R'.

Since you are getting this close, why is there a 5-600x inrush current when starting across the line? R' is 0! I am sure you have measured R stator of motors and know it is like .05 ohms - really low. So at startup, all you got is that stator R - until you get some rotation and bring R' in series... Why we sometimes have to set v/hz curve really way LOW from 0 to 2-5hz (typical slip freqs) - or this inrush will fault older drives in a heartbeat. set v/hz artificially 10x too low and that inrush current is manageable until the motor increases R' from 0 whilst making a magnetic field.. tricks of the trade...

 

[GoldDigger]

Since you are getting this close, why is there a 5-600x inrush current when starting across the line?

I hope that you mean 500-600%!

 

[mike_kilroy]

I hope that you mean 500-600%!

HA! Thanks for correction.

 

[FionaZuppa]

No, I mean series like I said. Look again.

R'/s is a parallel term to input Z. there is no real R that gets reflected, its a function of magnetic coupling as slip angle changes. why is there no real R change, because when your motor arrives in a box the physical real R of copper in that box is fixed and constant. the term R'/s is an equivalent to reactive power being transferred/absorbed from one side to the other. with magnetic fields you can only produce torque when the mag fields separate by some angle. HP remains constant at any load.

here, take two small 3/16"dia neodymium magnets, let them stick together, they align axially, now attempt to slip them apart, a forceis needed due the diff in magnetic field angle, work is done to the distance you stop, but in motor as the rotor rotates and loaded the work being done is continuously increasing.

 

[GoldDigger]

R'/s is a parallel term to input Z. there is no real R that gets reflected, its a function of magnetic coupling as slip angle changes. why is there no real R change, because when your motor arrives in a box the physical real R of copper in that box is fixed and constant. the term R'/s is an equivalent to reactive power being transferred/absorbed from one side to the other. with magnetic fields you can only produce torque when the mag fields separate by some angle. HP remains constant at any load.

Two totally false statements.

1. Reactive power is not being transferred from one side to the other to cause the R'/s term. Instead the magnetic fields are doing work on the rotor and through that to the shaft. The extra current drawn is what is necessary to maintain the magnetic field as the current through the coil does work and the rotor field moves with respect to a fixed frame of reference under the influence of the (rotating) stator field. It shows up as a current component in phase with the input voltage.

2. If by HP you mean the rated HP of the motor, nothing changes that. It is a capability not an operating parameter. As the load changes, by definition the work done by the motor (in units of HP) changes.

 

[Besoeker]

I can't believe this discussion is still doing on.
Power is power. That it. There are no different kinds.

 

[GoldDigger]

I can't believe this discussion is still doing on.
Power is power. That it. There are no different kinds.

Would you be as quick to say "Voltage is voltage, there are no different kinds."
Just wondering.

 

[Besoeker]

Would you be as quick to say "Voltage is voltage, there are no different kinds."
Just wondering.

The thread is about power. Let's try not to derail it.

 

[mike_kilroy]

The thread is about power. Let's try not to derail it.

The discussion morphed into discussion of reactive power vs real power and how the two interact in an induction motor. It is still very much a discussion about these two terms and there is still serious misunderstanding about the two and their relationship. If that is not a discussion about power, I do not know what it.

One other side track was whether one is allowed to call reactive power watts or not; that seemed to be settled to most everyone's satisfaction, so it slowed. Was it wrong to discuss here also?

If, as administrator, you command we stop discussing reactive vs real power using the induction motor as the vessel, then say so and I for one will cease and desist.

But I believe leaving the last FZ post unchallenged does this forum a great disservice as others may someday read it and not realize how awfully wrong it is.