Drives & Energy Efficiency (Cyclo, LCI, or VSI)

There seems to be some confusion. Power factor and efficiency are not the same thing.

• Efficiency is the ratio between the output power from the drive and the input power to it. The difference between these is the power needed by the drive to operate itself.
• Power factor is the relationship between the voltage waveform and the current waveform in a circuit. The cosine of the phase angle between them is the power factor.

So what was the question again, please?

Thank you PWSlack for your insight and comment. Cyclos typically operate in a pf range of 0.5 to 0.8 while VSI drives have a pf of 0.95. If pf is a sort of efficient utilization of voltage and current coefficient, are the VSIs more efficient? ...and by how much? Cheers,

As #1.

Maybe you missed this part of his post:

"Power factor and efficiency are not the same thing."

First you must define the size of system you are investigating this for. Nobody uses LSI drive on anything smaller than megawatt applications, so what's the point of considering it for something smaller? If you are thinking of MW applications, there are a lot of other considerations besides PF that should go into the issue.

Then NOBODY I know is using Cycloconverter drives any more at all, so again, moot point.

You also should be aware that the ".95pf" claim by VSI mfrs is somewhat disingenuous; the displacement pf is .95, but the distortion pf is always higher. Because of that, harmonic distortion mitigation techniques rob efficiency from the overall drive package, again something to throw into the mix.

Thank you all for your comments and help.Yes, I have 2x10MW propulsion motors driven by a Cycloconverter and the pf going in is bad, specially at low RPM. There is not much I can do there.Nevertheless, for future builds... I will also have 2x20MW propulsion motors and yes, I will opt for a VSI. Hence the pf will be much better.I also agree and understand that pf and efficiency is NOT the same. Nevertheless, do we agree that a pf of 0.5 is "bad"?I think it is bad because a pf of 0.5 is not as 'efficient' as a pf 0.9.... but please correct me if I am wrong.So, if my thinking that a power factor of 0.5 is 'bad'... how bad is it? Can I simply calculate how many more amps I am using and calculate the losses? (I frankly do not think it is so simple).

Thank you again for your support.

dear,

For your information, At low RPM, every motor in herently will opeate at low PF. ( Just check the manufacturer's PF - KW load curve.

But, for such a huge load, you definiteky like to reduce the cable losses / bus duct losses, there by reducing the bus current ( motor line current ) which, you may install Power Capacitors by Auto matic Power factor correction method, across the motor terminals / Motor terminal Bus. I have done the same thing for 2*250 KW + 3 * 55 KW motor loads.

I have maintained PF up to o.95 , in order to avoid un wanted Over voltages due to resonance - L-C / in stability, but we have tried with unity pf, worked perfectly , without any damages.

Hi, the question you asked is not one that has a really simple answer. All of the current designs have their own merits and problems. All can be really efficient if the drive is spec'd correctly. The cyclo is an efficient drive but, because the power factor is bad then the transformers drive and cables have to be spec'd for low losses (ie big and heavy) to keep the overall efficiency high. At the power levels you are talking about the motor design is going to be important. If you have a high torque rating (say 2X rated) then your losses when in that range may be High ! Induction motors are good and rugged and have good efficiencies at full load and below, but are bad at overloads. Synchronous motors can handle overloads better but this fact is generally used by the vendor to supply a smaller overall machine....which of course is less efficient than a larger more correctly sized motor! These days unless you really require a "rugged" and simple drive you won't use a cyclo, but big (20MW) grinding mill drives of the ring type are still a good bet for cyclo. All other big drives are now PWM at 3~4 kV or more. These drives are now pretty much the same on efficiency (the IGCT types are a bit worse on losses) , the fun is then choosing the motor type. Some Vendors can only supply synch motors to match their drives which is a limitation, others can supply both and let you choose. The bottom line is the cheaper the capital cost the less likely it is to be the most efficient drive.

Bob

Thank you Jeff for your insight... on both forum fronts.

I understand now it is a case by case study of matching all the pieces to study the efficiency for the complete system... rather than simply looking at pf.

Thank you all for your comments and wise insight.
I went back to the books, reviewed your collective intelligence, and I think I have a better understanding of where I was wrong.

I have more questions on efficiency now... but I will need to hit the books for that.
I see also that cyclos need more reactive load = larger equipment, while LCI with a pf closer to unity can fit in a system with smaller equipment.

It looks like the obvious choice is an LCI or CSI...
now the question is how to maintain optimal efficiency on the LCI/Sync motor combination... but the user manuals may reveal that.

Thank you again everyone for your insight.
Sincerely.
Eric

Thank you again to the group that replied to the questions I had and I am grateful for your insight.

I am now investigating how to determine the efficiency of a drive over time.

Does anyone know how to check the efficiency claims of the manufacturer? ...and more so, what parameters to check if the efficiency of the system changes?

Thank you,

argentox