Detecting Regenerated Voltages 11

[buzzp]

I would like to discuss the details of how a motor acts when a phase is lost and if votlage imbalance and single phase protection devices actually work. Here is what I know about the behavior of the motor:
If you lose a phase on a 3 phase motor, you are losing a pole. The speed of the motor drops, meanwhile the other pole is acting like a generator. What is the amplitude of the voltage generated? I think the voltage of the generated leg will be in phase as the 'lost'.
The voltage protection devices I am familiar with protect against single phase, low voltage, and voltage unbalance conditions. They assume that the generated voltage will not be the same amplitude as the line voltage. Is this a correct statement? What affect, if any, does multiple motors on the same line have on the ability of voltage protection device?
Thank you.

 

[buzzp]

Thanks for the last post Marke, you get a star. This has been my thinking it is just nice to have someone explain why.
So for normally loaded motors, the regenerated voltage will be picked up as a voltage imbalance. This is the case for installations I am worried about.
How could you detect regenerated voltages with a voltage monitoring device on a lightly loaded motor? Is it even possible?

 

[rhatcher]

thanks for the quick answer marke, i havent had time to really absorb what you have said but am working on it.

 

[gjones33]

Hi Guys,
Yes I agree with Marke. If the motor is running the rotor must be producing an induced magnetic field, just like a transformer. (All electric motors operate on the reactions between two magnetic fields). As Marke stated, the physical position of the phase windinngs determines the phase difference. The voltage induced in the open circuit stator winding is proportional to the magnitude of the rotor magnetic field which is the same for all the windings and to the speed which is again equal to all the windings.

Best wishes,
G

 

[cbarn24050]

Hi, Ive been reading this post with interest for a while now and it seems to me that people are posting opinion and not much in the way of facts.Most of what has been written here is just plain wrong if the contributors took the trouble to try and prove there assersions they would see that they are wrong.The important facts are 1 the regenerated voltage is dependent on the rotor speed. 2 a fully loaded motor stops if you lose a phase.

 

[SteveKW]

Not all motors will stall when fully loaded with a loss of a leg. Different applications have different effects on a single phased motor. Centrifugal pumps, blowers and fans can keep running until they burn up.

 

[Marke]

Buzzp, from my experience, voltage monitoring protection is adequate for loaded motors but ineffective for unloaded motors. Current monitoring is adequate for unloaded and loaded motors provided the phase loss isolates the phase to the motor. Current imbalance protecion also should have a sliding scale of acceptable imbalance. Generally, motors operating at very light load will experience a moderately high current imbalance and this is acceptable, but under high load conditions, the imbalance most be small. The light load imbalance is very much influenced by the supply imbalance and is much higher in magnitude.
Sorry, don't have a magic answer, as usual there are compromises and you need to ascertain the level of prtection you wnat and susceptability to supply variations that you can tolerate. Mark Empson

http://www.lmphotonics.com

 

[buzzp]

Summary of all of the above:
- The amplitude of regenerated voltages vary depending on
load
- The 120 deg difference stays the same (obviously)
- Voltage monitoring will protect against single phase conditions if the motor is loaded as it was designed for or is heavily loaded
Questions:
- What we don't know is where to distinguish between lightly loaded and loaded
- We also don't know the exact affect of the momentum or inertia of the load when a single phase condition exists except that it could potentially add to the amplitude of the regenerated voltage (this can not go on forever and will decay with time, we don't know how much time, assuming the motor OL doesn't trip first or the motor stalls/dies)
- The data in the above two statements will, presumably, vary from mfg to mfg under the same conditions
Thanks to all and a star to cbarn for his the last sentence of his last post.

 

[electricpete]

It's a great discussion touching on a lot of areas.

It appears that several people have real-world experience that suggests that the missing 3rd phase will have a generated voltage almost identical to what it would be if you had 3-phase voltage applied (120 degrees apart and same magnitude as other 3 phases). I cannot dispute that, but I still do not understand the explanation proposed above.

The explanation proposed is that the induced voltage on the 3rd phase arises from the rotor field. I agree that the rotor field is rotating at sync speed but I don't see that it should be important in this problem.

Let's go back to the basic model of a [balanced] induction motor, neglecting leakage reactance. The air-gap flux is established by the stator exciting current (which lags stator voltage by 90 degrees in time). That air-gap flux is NOT affected by the rotor field. How can that be? Very simply that any current flowing in the rotor is balanced by an equal-opposite (on an amp-turn basis) load current in the stator.

If you don't believe me look at the model. Istator=Im+Irotor. The stator current contains two components: one Im which established the magnetizing field and one Irotor (referred to the stator) which is the load current which exactly cancels the effect of the rotor field.

If that much doesn't make sense then look at the load dependence. As load increases Irotor increases and Brotor increases, but airgap flux does not change (again neglecting leakage reactance). The reason airgap flux does not change is because the increase in Irotor and Brotor is exactly cancelled by a load-component of the stator current and associated field.

I'll admit that the above is based on balanced conditions. Which parts of it do not apply during the serverly-unbalanced single-phase conditions requires much careful thought (more than I am capable of at the moment).

But the bottom line is that simple analysis suggests that rotor field has nothing to do with the airgap flux which links the stator. What establishes the airgap flux is the exciting component of the stator current.

Looking at stator exciting flux alone I see no reason why any significant voltage would be created as discussed above. Above I analysed 2-pole scenario with pole-phase groups a, b', c, a', b, c', with C phase missing. To add clarity to that analysis, consider that b must be equal an opposite to a (any current flowing in b must flow in opposite direciton in a... assuming wye connection). On that basis substitute a' for ever b and a for every b'. We will see the sequence of pole phase groups is a, a, c, a', a', c'. and if we add additional poles beyond two the sequence will repeat.

These letters represent the physical location of the coils as we move around the stator. It should be obvious from the sequence a, a, c, a', a', c', a, a, c, a', a', c' etc that there is a symmetry which would tend to prevent inducing any voltage in c since it is in equal proximity to a and a'. The same applies to c'.

I don't dispute the contention that the missing 3rd phase voltage is "regenerated" but I certainly don't understand why it should be so.

 

[rhatcher]

Hey Pete...I have been trying to resolve how this could be using the same reasoning as yourself but I don't see it either. By the way, a star for your good explanation.

While researching this I have seen some references to 'home made' rotary phase converters which are essentially 3-phase motors operated from a single phase supply with no load for the purpose of generating the third leg, but these all have capacitors connected between one (or both) of the hot legs and the third generated leg. Presumably this is to provide excitation for the generated leg. The articles did not get into details about theory of operation, fluxes, etc. but did suggest that the voltages were not balanced and that the magnitude and phase of the voltages generated varied greatly with load. Some of the commercial units I checked into did claim true balanced 3-phase output at any load, but no info was provided to suggest how this was accomplished. I suspect a LC/RLC network and/or electronics in place of the capacitors.

I have only found one reference to support that a motor running single phase would generate voltage in the third leg without capacitors or other external devices, but that was not from a technical source and it did not provide any more explanation of the phenomenon than any of the posts in this thread.

Anyway, I am still trying to think this through but in the meantime I am going to take the old fashioned approach to finding the answer. In a little while one of our techs is going to test a small 3 phase motor that was just rewound (I didn't want to do this on a 500HP unit...). When he is done I will run the motor single phased and measure the voltages...I am limited right now to mulitmeter measurements, but believe that phase-phase and phase-neutral checks of the voltage at the two hot legs and the one generated leg will give me a good idea of whats going on.

So, now it is time for "Famous Redneck Last Words" from your friend in South Carolina...

- "Hey guys, watch this."
+ "What's gonna happen?"
- "Ummm, I'm not sure but we'll know in a minute."

 

[gjones33]

Hi Guys,

Perhaps the bottom line on this post is that three-phase induction motors are used as three-phase generators in several well known wind generation schemes. In this case the rotor flux is produced from the load current in the stator coils.

Best wishes
G

 

[GordS]

In my plant, we use a blown fuse indicator intended to trip when we lose a fuse on a 575V motor. This device fits in the starter and has three wires terminating on the load side of the motor fuses. The company that sells this claims regenerated voltages can reach 90% of supplied voltages. The relay trips when it senses a loss of about 8% voltage.

The motor model of a stator branch, magnetizing branch, and rotor branch is a simplification. A more accurate model has a stator branch feeding a magnetizing branch and an ideal transformer primary. The ideal transformer secondary feeds a voltage source and rotor branch. (The transformer represents the air gap). The magnitude of the rotor voltage source is related to the slip.The rotor current generates a magnetic field that cuts through the stationary stator winding.Since the rotor current is AC, the rotor flux is AC. This results in a changing magnetic flux being seen within the coils of the open winding. This generates a voltage proportional to the rate at which flux is changing (E= - d(flux)/dt).

The reason the voltage on the "good" phases is slightly higher is to supply the energy to supply power to the magnetizing branch.

 

[electricpete]

Thanks gord and gary for responding to my question. I know that it appears an academic discussion. But I am interested in understanding where my model fails.

gjones - I am not terribly familiar with induction generators. To the best of my knowledge they require to be connected to the grid. Ifit is true that an induction generator must be connected to an external voltage source, then the excitation which establishes the air-gap flux would comes from the grid voltage applied at the stator terminals (the same as a motor). The air gap flux would have nothing to do with the rotor field. If an induction motor can operate without being connected to a grid or external voltage source, then I would be interested to hear more.

gords - it sounds like your blown-fuse-indicator manufacturer has presented some relevant and unambiguous info. I'm still at a loss to understand the theory. Your explanation does not ring a chord with me. You focus on rotor flux without regard to the other fluxes. I focus on the fact that the "airgap flux" = "magnetizing flux" = "resultant flux" is the difference between the stator field and the rotor field. This is a mirror of the fact that
Imagnetizing=(Istator_total) - (Irotor)
which can be readily observed from applying KCL to the equivalent circuit.
ie the stator current is comprised of a magnetizing component and a torque-producing load component which cancels the rotor current. Likewise in a transformer a primary load current I1 will flow which is approx equal to N2I2/N1 and therefore "cancels" the flux contribution of the secondary current. The flux in the core will remain on it's excitation level which is directly dependent on the primary voltage, but is INDEPEDENT of the load currents (neglecting leakage reactances).

So once again I present to an apparent contradiction: IF the rotor flux is responsible for the induced voltage in the open winding, then why doesn't the induced voltage increase in direct proportion to motor load? After all:
s~ P (load). Vrotor~s. Irotor~Vrotor. Brotor~Irotor.

And if the induced voltage varies with load, what load corresponds to full induced voltage? Is there any reason to suspect that full nameplate load would just happen to be the load at which the induced voltage matches the missing voltage (I can see no reason whatsoever). I don't believe that rotor field is the whole story.

One possible flaw in my original argument is that I have assumed that stator current can flow to completely cancel the rotor field (as is the normal model). It may be a little more complicated when stator current can only flow in 2 phases.

 

[SteveKW]

If I can oversimplify it I will. The three phase motor, single phased, now has a set of windings with NO power flowing into it. The other two phases are driving the motor and inducing a rotating magnetic field (North and South poles) into the rotor and dragging the rotor along with it. The rotor is now a rotating magnetic field spinning past the OPEN phase. It just a simple as moving a magnetic field past a wire to generate a current in the wire. It all falls back to Motion, Flux and Current. Its just a generator.

As for a three phase motor generating power when on a grid ... the motor is DRIVEN faster than is synchronous speed. Just as any generator underpowered on a grid will not produce an output...A three phase motor DRIVEN faster will pick up the load and have an output of electricity! You induce a field in a rotor and spin it...it is now a generator. Not being an engineer my technical explanation would be the CEMF of the rotor is now GREATER than the applied power.

 

[electricpete]

Thanks steve for your discussion. Thank rhatcher for your kind words.

It is certainly possible that you all are right that the rotor field is responsible.

But let me clearly explain that during NORMAL operation the rotor field has no effect on the air-gap flux (flux linking the stator coils) whatsoever. I believe the reason that rotor field may come into more importance in this single-phase condition is that the assumption of amp-turn balance which holds during normal breaks down during single-phase operation. More details follow:

In general magnetic theory there is a premise that an amp-turn balance will always hold. That is that for a transformer N1I1=N2I2+N1Im. Im is a magnetizing component of I1 that is small and can sometimes be neglected for simplicity. The magnitude of Im is dependent on V1 and geometry factors, but not upon the load.

For motors the equivalent amp-turn balance is I1=I2+Im where all quantities are referred to the stator. For motors Im is not as small but is in quadrature to the load currents I1 and I2 so for high loads its effect becomes small.

Inherenent in all of the above formulations is that the core has such high permeability and low reluctance that any small mismatch between N1I1 and N2I2 creates a huge voltage which will drive that mismatch down. As a result, the mismatch between N1I1 and N2I2 (N1Imag=N1I1-N2I2) always remains relatively small.

If the above model is true, then Irotor does create a field but that field is irrelevant to determining air gap flux because the load component of Istator (I1-Im) will exactly cancel I2=Irotor. This gives rise to the well-known fact that transformer core flux and motor air gap flux do not vary with load, except for the effect of series leakage reactances.

The above model is good for normal situations but I think that perhaps the breakdown does occur when I assume that the primary currents will continue to cancel the rotor mmf even with one phase open. The amp-turn balance holds in most situations but I'm not sure if it holds here.

If we artificially considered the stator windings to be non-overlapping, then it would be clear that when the max rate-of-change of rotor field is adjacent to the open-circuited phase C, then that entire voltage must appear in the open-circuited phase.

But in the actual case, there is substantial overlap between phases. There is no portion of c phase that is not overlapped by a "b" phase or an "a" phase coil in some manner. Whether or not those other stator phases are capable conducting current to cancel the rotor field, given the phase relationship of the voltage is tricky. My initial gut feel was that amp-turn balance could not be substantially violated under any circumstances.

It is certainly plausible that my assumption of amp-turn balance is incorrect in this scenario. If that is the case, then rotor field does provide the explanation.

 

[Marke]

electricpete, I think that perhaps this is one of those cases that we come accross so often that models we apply to things to describe them are based on certain conditions and when the conditions change, then the model does not hold up. I suspect that you are correct, applying the model that you wold normally apply to a motor where all three windings are driven by the supply does not cover this particular situation. Unfortuately, a fully comprehensive model to describe behaviour under all conditions gets too complicated. Under normal circumstances, the flux in the gap is related to the magnetising current in the stator and that is constant and independant of the load. In this case, there is no magnetising current in the stator and so there is no flux in the gap, well thats not actually true, the model falls over because there is flux coming from the rotor and I am not sure how much of the gap flux is directly dependant on the rotor current and how much is effectively from the residual magnetism of the rotor poles. If the flux was due to the rotor current, then I would expect the output voltage to increase with shaft load whereas the regenerated voltage actually reduces with shaft load. I suspect that the flux at the open circuited winding is related more to either the residual magnetism, or the current through the inductive component of the rotor winding or a combination of both.
If we consider the rotor, effectively we will have a voltage induced in this winding byt the stator flux field. The rotor winding has both an inductive component and a resistive component. As the shaft load is increased, the rotor current increases. I would expect that the effective rotor voltage will then reduce due to the effective series impedances and therefore the rotor magnetisng current will also reduce. This would explain the reducing voltage with load or speed but I am not sure that this is a correct model. Someone may be able to shed more light on this interesting subject. Mark Empson

http://www.lmphotonics.com

 

[rhatcher]

For gjones, induction or asynchronous generators are common for wind generation and small hydro applications. This is not the same case as 'regenerative voltages' since the asynchronous generator must be excited from a 3 phase line and is driven at greater than synchronous speeds by a prime mover.

------
About my 'experiment', the test conditions were not ideal. The motor was a 7.5 hp, 2 pole, 1 delta, concentric winding rated for 460V. No problem there. The power supply was a hipotronics motor test set, 500kVA, 0-4160V...essentially a 'variable autotransformer'. The part that is not ideal is the base condition of the voltage output for the test set with no load:

Vac= 246 Vab= 246 Vbc=245
Va= 151 Vb= 139 Vc= 75 (phase-ground)

Similarly

Vac= 444 Vab= 443 Vbc=442
Va= 253 Vb= 247 Vc= 158 (phase-ground)

The first difficulty was that once the motor was at speed on the 3 phase source that I could not kill the power, disconnect a phase, and then reset the test set before the motor stopped. It was a small motor with little inertia. In the end I found the minimum voltage at which I could 'pull the plug' on one phase and the motor would remain rotating (200V). Do not try this at home.

Three test were done. First, single phase applied to motor at stop. Obviously no rotation, but I wanted to establish the case where there was no potential for rotor flux contribution. Next, the motor was started on 3-phase power, 'switched' to single phase once at speed, and then operated at voltages of approximately 240V and 440V. In all tests phase B was the dead phase, Vac represents the single phase input voltage, and the autotranformer tap was set for 480V. The phase to ground volts are included but are of little value due to the fact that my system 'neutral' is obviously different from ground. Of course, if any of you can draw conclusions from the phase-ground data please do.

No rotation: Vac=63 Vab=31 Vbc=31 Va=56 Vb=31 Vc=10

240V: Vac=246 Vab=226 Vbc=209 Va=145 Vb=124 Vc=101

440V: Vac=440 Vab=399 Vbc=401 Va=248 Vb=224 Vc=206

I haven't really formed any conclusions yet. Clearly the 'no rotation' case shows straight voltage division (Vac parallel with Vab and Vbc). Equally as clear is the fact that for the 240V and 440V cases where the rotor was at speed something else is going on. Although the voltages are not balanced and therefore not 120' out of phase, they also do have some phase difference from the single phase supply

I have to admit that I went into this as a 'non-believer' and that I was surprised by the results. However, I will state now so there is no confusion that until I can quantify this based on motor theory that I will not draw any conclusions one way or the other.

I have some ideas on the theory of all of this but it is late. I will throw this idea out to pete (and marke)though. I have not thought this through yet but here it is...you are right on with your analysis but it would seem that in the case of the 3 phase motor operating single phase that the only winding contributing to the flux is the single phase that is fully energized and the transformer action takes place between the rotor and that phase. The rotor mmf wave will be ideally 90' (electrical) behind the voltage of the active phase at low slip. Although this does not correspond with the physical position of any of the adjacent windings or the 'electrical' position in terms of the 120' model, that is not to say that a voltage may not be induced in the dead stator windings. Of course, the frequency would be less than synchronous based on slip and the apparent difference in phase at any moment different than 120'. With greater load, although the rotor mmf would increase, the slip would increase leading to a greater difference in frequency and greater difference in apparent phase. Of course, this does neglect the fact that in the case of a delta motor the dead legs do assume some contribution as shown in your previous post so I am not sure that it is a useful line of reasoning.

 

[SteveKW]

Pete...looking at the last part of your argument... I suspect you are saying (at least I hope I understand where your coming from) that since the windings overlap each other they cancel each other out ... as in the paragraph just before where you say if they where NON OVERLAPPING then you could believe what we say.

If that were true, how can the motor run at all with everything overlapping in the stator!

Even if you were to separate each winding on its own pole piece (like the poles on a DC motor) the poles field flux would cross over each winding in the same manner as a lap wound stator. The flux traveling from north to south to make a pole will have to have the same path no matter how you mechanically set the windings or the motor would not work.

Let me try a different tack...just as you take the vector sums to plot a voltage you also take the vector sums of the flux. At a point of the flux a vector is SINGLING out the opened winding (C) and imparting a voltage on it! At that particular vector it is different on phase C than phases A & B!

 

[electricpete]

Steve - yes, the span of windings in phase A will DEFINITELY overlap the span of windings in phases B and C.

Anyone who has traced the wiring of a standard induction motor can confirm this for you.

Although I am usually a man of many words (to the dismay of my peers on this forum!), at this late hour I can come up with no simple way to explain it in a reasonable manner without diagrams.

Do you have access to a textbook which shows a lap-wound winding? Maybe rhatcher can pitch in some explanation.

 

[electricpete]

A follow up discussion for Steve. I think that parts of the reason for using overlapping winding have to do with minimizing harmonics (the same reason we use distributed windings). Maybe someone else can provide more insight.

It's also worth pointing out that the overlapping does not decrease the peak flux, but actually increases it above that which would come from a single pole phase group alone. At the moment when pole phase group A is at it's peak flux, adjacent pole phase groups B' and C' (60 electrical degrees from A) will have magnitude of 1/2 in the same polarity as A. If we also consider the 60 degree spatial (vector) separation and compute vector sum of the contributions from B', A and C', we get 3/2 of the peak. See Fitzgerald page 149 if you have it.

 

[electricpete]

rhatcher - Thanks for some good data regarding actual performance of a delta-connected motor. I vote you a star.

Clearly the induced voltage in the open phase is much closer to full line voltage (as others have said) than it is to zero (as I had predicted).