Power factor correction experiment

Status
Not open for further replies.
iwire said:
Why would you want us to learn a falsehood?
Click to expand...
Yes, I did note the comment about learning.
Interesting I thought in that it was directed at someone with 40 years experience by someone who appears to have none........
 
Last edited:
T.M.Haja Sahib said:
See post #155.I want you to learn that energy saving by voltage reduction method depends not only on the reduction in internal losses of the motor but also on the reduction in the motor output (and hence reduction in water output in case of pump).If the case study in post #151 does not convince you of that, nothing else can.
Click to expand...

I calculate that less than 25% of the energy savings in post #151 is due to the reduction in motor RPM (directly from the "affinity law" that you point to so much - the relative difference in the cube of the two rpms). While not an insignificant amount, it is a minority of the power saved. Other effects produced 3x the power reduction of that due to the rpm reduction. Not a very good counter example.
 
SAC said:
I calculate that less than 25% of the energy savings in post #151 is due to the reduction in motor RPM (directly from the "affinity law" that you point to so much - the relative difference in the cube of the two rpms). While not an insignificant amount, it is a minority of the power saved. Other effects produced 3x the power reduction of that due to the rpm reduction. Not a very good counter example.
Click to expand...
Bear in mind also that it is 25% of a motor running at 30% load in the first place.
It's the wrong motor for the job.
 
iwire said:
Can you point to a single post in this thread that would dispute what I made bold above? Not one of us has claimed a reduction in current would result if a fully loaded motor has it's voltage reduced.

Again, going back in this thread you tried to claim a reduction in voltage to a motor always increases the current.

You were shown otherwise and are now trying to change the subject.
Click to expand...

Your objection is not in order:we are trying see whether any energy saving is there by connecting a suitable PFC capacitor across a pump motor;I started with a premise that there will be an increase in motor current with a decrease in voltage across it.That we all accept now that this is so,when the motor is near full load.I hope we may proceed further from that point.
 
T.M.Haja Sahib said:
Your objection is not in order:we are trying see whether any energy saving is there by connecting a suitable PFC capacitor across a pump motor;I started with a premise that there will be an increase in motor current with a decrease in voltage across it.That we all accept now that this is so,when the motor is near full load.I hope we may proceed further from that point.
Click to expand...

Why not just say you were wrong?
 
Besoeker said:
Yes, I did note the comment about learning.
Interesting I thought in that it was directed at someone with 40 years experience by someone who appears to have none........
Click to expand...
That you still do not grasp that the speed of the INDUCTION motor drops significantly with drop in voltage amazes me.The torque of the motor is directly proportional to the SQUARE of the applied voltage and from the torque-speed characteristic of the motor,you can easily see how greatly the speed is reduced for reduction in voltage.
 
iwire said:
Why not just say you were wrong?
Click to expand...

''you tried to claim a reduction in voltage to a motor always increases the current.''
How I was wrong?I did not put 'always' in my statement and was thinking we are all experts.
 
Last edited:
Besoeker said:
Interesting I thought in that it was directed at someone with 40 years experience by someone who appears to have none........
Click to expand...
I think half of your experience,I am having is enough to counter your statements.:D
 
T.M.Haja Sahib said:
That you still do not grasp that the speed of the INDUCTION motor drops significantly with drop in voltage amazes me.
Click to expand...
I ought to be equally amazed that you insist on that but, rather sadly, I'm not. It's just plain wrong.

Here are curves for an actual motor where we provided the control equipment. It shows performance at rated voltage and 80% of rated voltage.
Look very carefully at the torque curves for the two voltages.

TorqueandCurrentvs.jpg


How much speed difference can you discern at 0.3pu load torque? Probably not a lot.
So maybe if we zoom on a bit to get a better look:

TorqueandCurrentcloseup.jpg


Maybe not a lot there either so in we go again:

TorqueandCurrentevencloserup.jpg


Which part of this do you not grasp?
 
Besoeker said:
I ought to be equally amazed that you insist on that but, rather sadly, I'm not. It's just plain wrong.

Here are curves for an actual motor where we provided the control equipment. It shows performance at rated voltage and 80% of rated voltage.
Look very carefully at the torque curves for the two voltages.

TorqueandCurrentvs.jpg


How much speed difference can you discern at 0.3pu load torque? Probably not a lot.
So maybe if we zoom on a bit to get a better look:

TorqueandCurrentcloseup.jpg


Maybe not a lot there either so in we go again:

TorqueandCurrentevencloserup.jpg


Which part of this do you not grasp?
Click to expand...
See below.
http://www3.sea.siemens.com/step/templates/lesson.mason?motors:3:3:5
 
iwire said:
Your link is for special high torque motors, which is not what we were talking about. And I see nothing in it about voltage.
Click to expand...
I added it to show that an induction motor can operate at 14% slip, for example.
 
T.M.Haja Sahib said:
​See
http://nptel.iitm.ac.in/courses/IIT-MADRAS/Electrical_Machines_II/pdf/1_8.pdf
Click to expand...

Two points from your link:

The foregoing discussion shows that the induction machine, when operating from mains
is essentially a constant speed machine. Many industrial drives, typically for fan or pump
applications, have typically constant speed requirements and hence the induction machine
is ideally suited for these.
Click to expand...

The curves in Figure 27 would be very unusual for a real life machine. It would require a high resistance rotor or, in the case of a wound rotor machine, external rotor resistance. In either case it would be very inefficient. I suspect that the curves were drawn with exaggerated slip to provide clarity rather than accurately reflect what happens in real life.
The curves I provided are for an actual motor in operation on a centrifugal load. From those you can see that changing the voltage alters the speed by a vanishingly small amount.
Which what I have been telling you since post #118.
Even your own example in post #151 demonstrates that with a change in slip of <1%. And those alone should convince you that Figure 27 is unrealistic.

And here's another real life motor:

TorqueandCurrentanothermotor.jpg


Grasp it yet?
 
Besoeker said:
The curves I provided are for an actual motor in operation on a centrifugal load.
Click to expand...
My dear Besoeker,let me play with you more.

In your post #170,you naively marked the 0.3 p.u. load point on the straight,stable operating portion of the torque-speed characteristic of the motor.This would be correct,if the load is of the constant torque type.But,as you stated,the load is of variable torque type and its 0.3 p.u load point occurs on its curve (red line in your diagram) near the 0.75 p.u speed.Suppose your client asks you that he wants to operate his centrifugal load (per your statement)at 0.3 p.u load point.How are you going to do it? (Do not say by varying the frequency.It is not relevant to the present discussion.
 
T.M.Haja Sahib said:
My dear Besoeker,let me play with you more.

In your post #170,you naively marked the 0.3 p.u. load point on the straight,stable operating portion of the torque-speed characteristic of the motor.This would be correct,if the load is of the constant torque type.But,as you stated,the load is of variable torque type and its 0.3 p.u load point occurs on its curve (red line in your diagram) near the 0.75 p.u speed.Suppose your client asks you that he wants to operate his centrifugal load (per your statement)at 0.3 p.u load point.How are you going to do it? (Do not say by varying the frequency.It is not relevant to the present discussion.
Click to expand...
You have once again demonstrated your total lack of experience in this field.
I don't mean that unkindly. It's an observation based on your posts.

In post #151 you cited an example of a motor running at 30% 0.3 pu.
The figures you posted indicated about a 1% change in slip.
I picked a motor from a real application for which I have reasonably detailed data and an example of the performance characteristics of modern, well designed motors.
I put in your 0.3 pu torque to demonstrate that there is no appreciable speed difference with reduced voltage at that loading.
Since the speed difference is virtually negligible, the load characteristic is pretty much irrelevant.
But, evidently, you still don't understand that despite the real world data I have provided for you.

I understand that it isn't your field and I'm still willing to elucidate and educate in any way that can help your understanding.
But the beginning of understanding is the acknowledgment of ignorance.
You have to take that first step.
 
Status
Not open for further replies.
Top