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.

 

[gjones33]

To rhatcher,

The point I was making about generators is that a rotating squirrel cage rotor will induce a voltage in a stationary stator winding. This is a fact of life whether the rotor is driven by a prime mover, or, in this case the two live phases. The only reason you need to drive at a higher than synchronous speed in the generator mode is to ensure power flows to the electrical load. If the speed is less than synchronous then power flows to the mechanical load. In a generator this would drive the prime mover.

How does the power know which way to flow? The only link between the mechanical and electrical system is the magnetic field in the airgap, which, in turn, is dependant on currents in the stator and rotor windings.

Cheers
G

 

[SteveKW2]

Pete - being in the motor repair business I have rewound hundreds of three phase motors and am quite intimate with how the phases are placed in a stator. The overlapping is very pronounced in a two pole machine. I was just trying to elaborate on your earlier post of ... "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".

 

[cbarn24050]

Hi, have you ever seen a little single phase indution motor? Ever woundered what that shorted turn(BIG CLUE HERE) on the pole does?

 

[electricpete]

Steve - my deepest apologies on your meaning. I thought you were disagreeing that there is any overlap at all. I remember being quite surprised that you would say that. Sorry to misinterpret you.

The reason I discussed overlap is that it would be the determining factor of induced voltage in the open coil IF we make the [questionable] assumption that rotor load current is still cancelled by the two remaining stator phases (wye winding). For example portions of C (open phase) which overlap with adjacent B' will cancel those portions of C which overlap with adjacent A' (because A and B currents are equal and opposite during single-phase condition). More details on my earlier messages this thread…although I am sympathetic to the fact that they are not very understandable.

gjones - If an induction generator is operating in parallel with an external voltage source (grid), then I believe the excitation to establish the airgap flux comes from the external voltage source (not necessarily the rotor). I believe the induction generator will not produce vars but absorb vars as a consequence of this excitation current drawn from the source. That is quite a different scenario from a motor with an open phase where the open phase cannot recieve any excitation from the external voltage source.

I'd be happy to reconsider those statement if you tell me that induction generators are capable of operating without being connected to any external grid or other source of excitation (can they?) or that an induction generator can be made to produce more vars than it consumes (can they?).

cbarns - The shorted turn does not provide any insight to me. In fact if you analyse a shorted turn which does not significantly perturb the overall flux, you would find that the flux which causes the circulating current in the shorted turn arises from the excitation component of the stator current. This flux is directly dependent upon the stator applied voltage (NOT the load-dependent rotor current).

 

[gjones33]

To electricpete.
Where I believe we differ in concepts is that a voltage or potential cannot produce a magnetic field. Current has to flow and current needs a circuit and only then will a magnetic field be produced. In the case of an induction generator the load provides a convenient circuit.

However, if a supply is not available it is well known that self-excitation of an induction generator can be obtained by connecting in parallel with a suitable capacitor and running up on residual.

Cheers
G

 

[gjones33]

Hi All,

On reading the post again, I think the concept of overlapping poles can be very misleading. When you think about it there is no such practical entity as overlapping poles. When a phase is energised it will produce however many pairs of poles as the design dictates. Each phase of a two pole motor will produce just two poles; each phase of a four-pole motor will produce just four poles and so on. If two phases are energised at the same time they will only produce the same number of poles as one phase. Two phases energised in a two-pole motor will only produce two poles but this time the poles will be phase shifted from the positions of the poles due to one phase. (The poles will be midway between the two if the voltages are in phase).

This phenomenon (energising two-phases) is used in stepper motors to produce half steps etc.

From the physics viewpoint magnetic fields comprise of lines of force, i.e a line joining points having equal field strengths. It follows lines of force cannot cross and therefore poles cannot overlap.

Cheers
G

 

[electricpete]

Good discussion!


gjones - I never said that voltage in the absence of current created a field. What I said was that the external voltage supplies the excitation current (vars into the generator) which creates the airgap flux/field. The magnitude of that flux is proportional to the magnitude of the terminal voltage. I felt it necessary to distinguish between the three fluxes: #1 - voltage-dependent excitation flux which creates the air-gap flux under normal conditions, #2 - load-depedent rotor flux, #3 - load dependent stator flux (equal and opposite to load-dependent rotor flux). The relationship between these three: total stator flux=#1+#2. Total flux=total stator flux +rotor flux =(#1+#2)-#3=#1. #1=(total stator flux)- (loaddepednet stator flux)=(#1+#2)-#2=(#1+#2)-#3. #1=(#1+#2)-#3 is the direct analogy to N1IM=N1I1-N2I2.

External capacitors certainly can fulfill the similar role as external voltage source since capacitors are sources of vars.

I remember hearing as you say that induction machines can generate without interconnection to a grid PROVIDED that caps are attached. But that is hard to understand. Slip no longer has any meaning since there is no syncronous frequency in the picture... only rotor frequency. So if there is no slip, how do we interpret the torque vs slip curve? What defines and controls the torque and power in such a machine? (just curious).

I never said that poles overlapped… only that pole-phase groups overlapped. (Please let me know if you want me to elaborate on the difference between a pole and a pole-phase group.) We need to consider the physical overlap of the stator windings in order to determine what stator flux from one phase will link an adjacent phase.

 

[electricpete]

gj - I will elaborate on the term pole-phase group. It is a group of physically adjacent coils connected in series which are associated with the same phase and the same pole.

Pole phase groups DO overlap in induction motors. That is a fact. I have already made the mistake of trying to explain that fact to someone who already understood it, so I won't try to explain it again. Let me know if you want to discuss more.

 

[gjones33]

To electricpete

Consider your #1 and #3. How can they be different? We agree it is current that produces the magnetic field so how can you have two values of current in the same coil at the same instant? You can see how it is very misleading to state and I quote "The magnitude of that flux is proportional to the magnitude of the terminal voltage." It is not correct the flux is proportional to the current which can only flow in one direction at a time, the error is then self evident. Similarly the load cannot produce a current or magnetic field.

If you adopt the concept that a current is necessary to produce a magnetic field then you will understand why it is not necessary for the induction generator to be connected to a source. It only needs a suitable electrical load and by load I mean a current sink which can be the grid or a capacitor.

There is no argument that the coils forming poles overlap. For example in a three-phase machine if there are two coils forming a pole then there is be a span of 4 slots between the inner coil, the next two slots taken up by the two coils of the previous phase and the next two taken up by the coils of the third phase, thus the coils overlap but the magnetic poles which was the subject of the original post do not.

Do we agree now?

Cheers
G

 

[electricpete]

gjones - I can think of no way to respond other than responding to each of your statements one by one.

Consider your #1 and #3. How can they be different?
If you accuse me of being wordy and hard to understand, then I plead guilty. It is unfortunate that I chose to introduce the numbered fluxes #1,#2#3 which are very non-descriptive. It adds precision but does not add clarity.

Let me try to clarify. Draw the equivalent circuit all referenced to the stator and you will see a node where the exciting branch is hooked. KCL at that node gives I1=Im+I2 where I1 is total stator current and I2 is rotor current, as referenced to the stator. This means that there are two components of stator current I1…. a voltage-dependent component Im and a load-depdent component I2=I1-Im. With simplifying assumptions (neglect leakage reactances) I2 will be in phase with applied voltage and Im will be 90 degrees out…. making it easier to distinguish these two components.

We can associated each of the fluxes with the currents discussed above: Flux #1 is associated with Im. Flux #2 is associated with I2, Flux #3 is associated with the load component of I1… specifically I1-Im (which happens to be equal to I2… flux #2).

We agree it is current that produces the magnetic field so how can you have two values of current in the same coil at the same instant?
We have two components of stator current (in the same stator coil at the same instant). One is magnetizing Im, one is load-related (I1-Im). Add them together and you get I1.

You can see how it is very misleading to state and I quote "The magnitude of that flux is proportional to the magnitude of the terminal voltage."
"that flux" was referring to magnetizing flux=airgap flux = flux#1. It is proportional to the terminal voltage under normal three-phase conditions. Doesn't seeem misleading to me. Do you disagree?

It is not correct the flux is proportional to the current which can only flow in one direction at a time, the error is then self evident.
No error is evident to me. Please be more specific.

Similarly the load cannot produce a current or magnetic field.
I have used the term load-related to refer to the current components and flux components which vary with load. (as distinct from the voltage-dependent flux exciting flux #1). The flux associated with the load current varies with changes in load.


If you adopt the concept that a current is necessary to produce a magnetic field…..
I wholeheartedly agree that a current is necessary to produce a magnetic field. I have never suggested otherwise.

then you will understand why it is not necessary for the induction generator to be connected to a source. It only needs a suitable electrical load and by load I mean a current sink which can be the grid or a capacitor.
The fact that an induction generator needs to be connected to either a grid or a capacitor suggests that vars need to be supplied to the induction generator to establish its field. The reactive vars flow into the generator in while real power watts flow out. Why should we suspect otherwise?

There is no argument that the coils forming poles overlap. For example in a three-phase machine if there are two coils forming a pole then there is be a span of 4 slots between the inner coil, the next two slots taken up by the two coils of the previous phase and the next two taken up by the coils of the third phase, thus the coils overlap but the magnetic poles which was the subject of the original post do not.
Magnetic poles may have been the subject of your "original post", but not mine. I first discussed overlapping of pole-phase groups. You second objected to my posting with a discussion of magnetic poles.

Do we agree now?
Getting closer I think.
Cheers!
electricpete

 

[rhatcher]

Wow guys, I am glad tomorrow is Saturday. I need the time off to catch up with the posts from the last two days. I am about 1/2 through now and will finish in the morning. This is certainly an interesting thread and if nothing else it demonstrates what I like about our field:

'When you go deeper into it, some questions get answered but then other questions pop up. Your mind plays tricks on you, then you play tricks back...' X-)

 

[jbartos]

Suggestion to Marke (Electrical) May 13, 2002 marked ///\\When you operate a motor off a three pahse supply and then remove one phase, the torque potential is dramatically reduced, but provided that the motor does not stall, the non driven phase will act as a generator generating a voltage that is correctly phased.
///Could you clarify the generated voltage in terms of wye and delta connections. The wye connection lost branch will have no current flowing in it. The delta connection branch will have current flowing across the missing voltage supply terminal. Therefore, the motor terminal voltage at the missing phase motor terminal will be different between wye connection and delta connection.\\ If the shaft load is very low, the gnerated voltage will be comparative in magnitude and phase to the missing phase.
///This statement needs a clarification in terms of the motor wye and delta winding connections.\\ This renders voltage monitoring devices next to useless unless the sensitivity is high and then there are nuisance trips due to supply variations. As the shaft load increases, the generated voltage reduces
///This needs a clarification in terms of the motor connection; especially, the wye connection.\\ and voltage monitoring devices can work.

 

[jbartos]

Suggestion to electricpete (Electrical) May 9, 2002 marked ///\\ Generated voltage on the open phase. That's a good question too. And will it depend on motor load? Hmm.
///There will be a different generated voltage on the missing phase terminal of wye connection and missing phase terminal of delta connection since there is no current flowing in the missing phase winding of wye connection and there is current flowing across the missing phase terminal of delta winding. This current will approximately be one half of the abnormally powered delta leg.\\\

 

[gjones33]

To jbartos,
Have you missed our post on induction generators.

When you loose a phase in a three phase induction motor you do not loose torque unless the motor is loaded to almost pull out torque. What happens is the motor slip increases, the BEMF falls and current is increased in the other two phases which may be sufficient to burn the motor out.

There will be a voltage close to the line voltage generated in the third phase whether it is wye or delta connected and this is a function of the rotor magnetic field and the speed of the rotor not the current in the phases as you suggest.
Cheers,
G

 

[electricpete]

gjones
you say "the slip increases" while jbartos says that "we lose torque". I say you're both right. If the torque speed curve decreases for all speeds (as is the case for loss of phase), the new operating point is at a higher slip. You're right that the change will not significant impact operation unless we reach breakdown torque.

I don't believe the discussion on induction generators sheds any light on generated voltages in the case of a lost phase. An induction generator recieves it's excitation (which establishes the airgap flux) from vars which are supplied by either an external power source or connected caps or both. This is the same as an induction motor. From this perspective, we might say that an induction generator really doesn't "generate voltage" (except for small amount due to residual rotor magnetism), it generates real power. The external var source supports the voltage.

If you wish to draw a direct analogy between an induction generator and a motor with open-circuited phase, you must ask the question what will happen to the induction generator terminal voltage when we remove the external var source (caps or grid). I believe the voltage would decrease to a small fraction of normal.

 

[electricpete]

The purpose of my last statement of last post was to illustrate that we can't draw any useful analogy between induction generator and induction motor with open phase which would explain the observation that voltage remains on the open phase. (I was not trying to argue that induction motor open phase does not generate voltage.)

 

[gjones33]

To electricpete,

Reading your last post,I now believe we have been on diferent tracks for a while. In one of my earlier posts I mentioned residual magnetisim of the rotor to explain how an induction generator can build up it's terminal voltage from stationary when driven by a prime mover without any electrical input power.

The rotor residual magnetism is the magnetisim left over from previous events and when the machine is rotated it builds up a voltage in the stator windings which is fed into a capacitor (or other suitable load) to phase shift the stator current and thereby the stator magnetic field so as to induce a voltage in the rotor bars, which, in turn produce the rotor magnetic field which, in turn interact with the stator magnetic field to produce a torque. Whew! I hope you followed that.

The capacitor does exactly the same job as a shorting turn on a shaded pole motor as charn24050 suggested in the post above. Just like rotor bars, the shorting turn produces a magnetic field which is out of phase with the main field.

Now when the motor is running on two phases, the rotor magnetic field is still induced from current in the other two phases and we therfore have an open circuit winding surrounding a rotating magnetic field, the stuff generators are made of.

The alternative is that if the voltage in the open winding is zero or low, then there is no rotor magnetic field and no bemf in the other phases either so the current will be almost v/r. Whoops!. The point I'm trying to make is that and induction machine operates as both motor and to a lesser extent a generator in the motor mode and as a gnerator and to a lesser extent a motor in the generator mode. It is all down to the direction and phasing of the currents in the stator windings.

Cheers,
G

 

[electricpete]

gjones - lots of info in the last post. I'm not sure if I disagree with any of it. I do object to repeated previous inferences that a motor with one open phase will generate a voltage by the same mechanism as an induction generator. The induction generator cannot generate any significant voltage without an external supply of vars... it is no different than a motor in that respect. Both have external supply of vars to create the magnetizing field.... but the torques and real power flows are reversed. The rotor residual magnetism plays only a minor role in the steady-state excitation of the machine.

I stand by all my previous statements in this post.

 

[electricpete]

I'll be signing off for a few days. Look forward to continuing te discussion after next Thursday.

 

[gjones33]

to electricpete,
Thank you for your post, I hope you enjoy your break.
Cheers,
G