Power factor correction experiment

[Besoeker]

Sticking my nose in where I was not invited ... Semantics ... no LINEAR relationship, but yes, a direct versus an inverse relationship. AFAIK, with an operating motor at a constant voltage, there is no case where an increase in current does not reflect an increase in torque load.

Don't have to be invited and I, for one, appreciate your input.

dkarst is right.
Move out of the normal operating range and you get current and torque going in opposite directions.
See for example post #177.
Above about 94% speed, current and torque do go in the same direction and, in the normal region of operation, which is 99% of sync speed upwards, the current is approximately proportional to torque which accords with your experience.
Below that 94% speed, the current goes up but the torque reduces. The inverse relationship.

 

[T.M.Haja Sahib]

I would agree that this is not a normal operating mode.

Thanks,dkarst for your interest to post again.I want to say if the load torque increases faster with speed than that of motor,operation of motor in the unstable region becomes possible.Whether this mode of operation is normal or not normal depends on whether the motor is not overloaded or overloaded respectively.

 

[T.M.Haja Sahib]

That calculation is invalid.

http://books.google.co.uk/books?id=...&q=steinmetz motor equivalent circuit&f=false

Look at Fig 6.6 on page 278 and you might be able to work out why.

The motor current at 0.55 p.u voltage and 0.30 slip may be worked out from the example 6.3 on page 284 of above reference.Do you agree ?

 

[Besoeker]

The motor current at 0.55 p.u voltage and 0.30 slip may be worked out from the example 6.3 on page 284 of above reference.Do you agree ?

So what do you work it out to be and how does it relate to the motor full load current?

 

[dkarst]

Thanks,dkarst for your interest to post again.I want to say if the load torque increases faster with speed than that of motor,operation of motor in the unstable region becomes possible.Whether this mode of operation is normal or not normal depends on whether the motor is not overloaded or overloaded respectively.

I guess everyone has their own definition but I would believe that 99% of the cage-type inductions motors in the world are operating everyday at a slip between 1% - 8% and therefore the proposed operation at ~35% slip here is "abnormal".... maybe there is a better choice of words but I'll leave that to the psychology majors.

I do agree you have hit on an important point, so is the motor overloaded? Is is drawing more than 1pu current?

 

[T.M.Haja Sahib]

I guess everyone has their own definition but I would believe that 99% of the cage-type inductions motors in the world are operating everyday at a slip between 1% - 8% and therefore the proposed operation at ~35% slip here is "abnormal".... maybe there is a better choice of words but I'll leave that to the psychology majors.

I do not know if the application of class D and class F induction motors constitute only
that 1%...........

I do agree you have hit on an important point, so is the motor overloaded? Is is drawing more than 1pu current?

For that matter let us examine the equivalent circuit diagram as suggested by Bes.By equating the expression for power 'lost ' in the load resistance [=current squared*rotor resistance referred to stator*(1/slip-1)] to one third of the mechanical load on the motor ,the motor current may be found out.Correct?

 

[Besoeker]

I guess everyone has their own definition but I would believe that 99% of the cage-type inductions motors in the world are operating everyday at a slip between 1% - 8% and therefore the proposed operation at ~35% slip here is "abnormal".... maybe there is a better choice of words but I'll leave that to the psychology majors.

I do agree you have hit on an important point, so is the motor overloaded? Is is drawing more than 1pu current?

If you look at the characteristics I posted at #177 you see that at rated voltage and 0.7pu or 0.3pu slip the current is nearly 7.0pu which, in my experience is in line with expectations. This means that at half voltage you would get about 3 pu current. Not good.

 

[dkarst]

I do not know if the application of class D and class F induction motors constitute only
that 1%...........

For hoists/cranes/punches etc, maybe you would choose a NEMA design D (not class D) motor as appropriate but even then operating at 35% slip is pretty far out. You have been discussing a centrifugal load and I think everyone would agree that a design D machine is not appropriate for that application.

I'm unfamiliar with a "design F" motor... it seems like you may be interchanging NEMA design letters (A,B, ...) which describe the torque/speed characteristics and the "class letters" which describe the temperature ratings of the insulation? Maybe you mean the kVA/HP code letters ?

 

[Besoeker]

I'm unfamiliar with a "design F" motor... it seems like you may be interchanging NEMA design letters (A,B, ...) which describe the torque/speed characteristics and the "class letters" which describe the temperature ratings of the insulation? Maybe you mean the kVA/HP code letters ?

It may be IEC nomenclature as used in UK. India, having been at one time been a British colony, has probably adopted/retained UK customs and practices.
Class F in this case refers to insulation class rated for 140degC in a 40degC ambient.

It is not uncommon for a motor to be specified thus:
Insulation Class : F
Temp Rise : B
Class B is a 120deC rise so gives some built in margin.

I just took this picture of a nameplate that happened to be on my desk.

It came off a cooling fan motor in our compressor shed that had failed owing to single phasing and inadequate protection. The installation was done by an outside contractor who should have known better.

 

[dkarst]

... "class letters" which describe the temperature ratings of the insulation?

I am familiar with class F insulation rating... but have not heard of the class D also mentioned, maybe that is a UK/India thing as I don't see it in NEMA?

At any rate, I guess I thought Mr. Haja Sahib was referring to the "design D" and its torque-speed/slip characteristics. I don't see how the insulation class relates to the torque speed characteristics (other than if you're going to operate in overload mode, it will take longer to fail).

 

[Besoeker]

I am familiar with class F insulation rating... but have not heard of the class D also mentioned, maybe that is a UK/India thing as I don't see it in NEMA?

Nor have I. C, F, and H are those that I recall encountering.

At any rate, I guess I thought Mr. Haja Sahib was referring to the "design D" and its torque-speed/slip characteristics.

.
You are probably right. I think it was post #151 (I'm too indolent to check it again so relying on memory) that mentioned 415V and 50Hz which is what we had here in UK before the meddling EU decided to change nominal voltage for "harmonisation". It served no purpose whatsoever. Voltages didn't change. And there was no need for them to anyway. Bureaucratic nonsense. OK. Rant over. Mods, please forgive my off topic moment.

I don't see how the insulation class relates to the torque speed characteristics (other than if you're going to operate in overload mode, it will take longer to fail).

Now that made me smile.

 

[T.M.Haja Sahib]

At any rate, I guess I thought Mr. Haja Sahib was referring to the "design D" and its torque-speed/slip characteristics.

Yes.I referred to NEMA design classes of motor and thought that we were all aware of that ambiguity and so omitted mentioning 'NEMA design'.

 

[dkarst]

I do not know if the application of class D and class F induction motors constitute only
that 1%...........

Yes.I referred to NEMA design classes of motor and thought that we were all aware of that ambiguity and so omitted mentioning 'NEMA design'.

Thanks for clarification, I thought you meant NEMA design letter (A, B, C, D) rather than insulation class but I am only familiar with these NEMA design lettters and not the "F" you mentioned.

For that matter let us examine the equivalent circuit diagram as suggested by Bes.By equating the expression for power 'lost ' in the load resistance [=current squared*rotor resistance referred to stator*(1/slip-1)] to one third of the mechanical load on the motor ,the motor current may be found out.Correct?

My apologies for taking us all on a side tour of motor insulation classes and NEMA design designators, I think Mr. Besoeker has estimated the running current to be ~ 3pu. Do you agree?

 

[T.M.Haja Sahib]

I think Mr. Besoeker has estimated the running current to be ~ 3pu. Do you agree?

My problem is whether the efficiency and power factor of an induction motor drops,when both the voltage and load are reduced.If the load only is reduced,both drop, when load is below 50% or so.But if both voltage and load are reduced,efficiency does not drop. Please see again the abstract of IEEE paper below
http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=4111787

It states that efficiency does not decrease when voltage is reduced in direct proportion to square root of load torque.The power factor curve is similar to the efficiency curve with few points below it up to load of 50% or so.
So the motor current by calculation in post #244 will not exceed full load current up to a load of 50% or so.

 

[iwire]

I think Mr. Besoeker has estimated the running current to be ~ 3pu. Do you agree?

My problem is whether the efficiency and power factor of an induction motor drops,when both the voltage and load are reduced.If the load only is reduced,both drop, when load is below 50% or so.But if both voltage and load are reduced,efficiency does not drop. Please see again the abstract of IEEE paper below
http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=4111787

It states that efficiency does not decrease when voltage is reduced in direct proportion to square root of load torque.The power factor curve is similar to the efficiency curve with few points below it up to load of 50% or so.
So the motor current by calculation in post #244 will not exceed full load current up to a load of 50% or so.

It was a yes or no question, why not just answer it?:?

 

[T.M.Haja Sahib]

It was a yes or no question, why not just answer it?:?

What!You can't see my emphatic 'NO' in it ?!

 

[iwire]

What!You can't see my emphatic 'NO' in it ?!

No, I see your typical misdirection and evasion, but carry on.

 

[mivey]

It was a yes or no question, why not just answer it?:?

iwire, have you stopped taping wirenuts? Yes or no?

 

[iwire]

iwire, have you stopped taping wirenuts? Yes or no?

Well anyone who is inordinately infatuated with polysyllabic obfuscation, preferring never to employ a less complicated syntactic arrangement of descriptive words when there exists a single expressive unit that amalgamates the multiplicity of morphemes comprising the simpler phrase. Among the manifold objectives of multisyllabic, holophrastic verbalism are those of: rendering the author's meaning indisputably precise yet simultaneously incomprehensible; demonstrating through superior orthography and lexical awareness that the writer is manifestly more erudite than the reader; disempowering intellectual challenge to the proponent's argument by using logomachinations to divert discussion to the establishment of the opponent's comprehension of the vocabulary as opposed to addressing the factual import of the treatise which, upon analysis, may well prove amphigorous. The obscurantist sesquipedalian is likely to compound the reader's difficulties by indulging in glossosynthesis, thus enabling the author to dismiss all opposing views as ultracrepidarious. In other words, a sesquipedalian is one who would call a spade a manuo-pedal excavationary implement would know the answer.

 

[mivey]

Well anyone who is inordinately infatuated with polysyllabic obfuscation, preferring never to employ a less complicated syntactic arrangement of descriptive words when there exists a single expressive unit that amalgamates the multiplicity of morphemes comprising the simpler phrase. Among the manifold objectives of multisyllabic, holophrastic verbalism are those of: rendering the author's meaning indisputably precise yet simultaneously incomprehensible; demonstrating through superior orthography and lexical awareness that the writer is manifestly more erudite than the reader; disempowering intellectual challenge to the proponent's argument by using logomachinations to divert discussion to the establishment of the opponent's comprehension of the vocabulary as opposed to addressing the factual import of the treatise which, upon analysis, may well prove amphigorous. The obscurantist sesquipedalian is likely to compound the reader's difficulties by indulging in glossosynthesis, thus enabling the author to dismiss all opposing views as ultracrepidarious. In other words, a sesquipedalian is one who would call a spade a manuo-pedal excavationary implement would know the answer.

I'll be right back. I'm taking a print-out of this and a dictionary over to the local community college for some help with sentence diagrams and creating some decomposition charts so I'll know how to respond.