VSD (Electric Drives) & Energy Efficiency
VSD (Electric Drives) & Energy Efficiency
(OP)
There are a number of variable speed drive technologies out there. While some have a uniform power factor close to unity across the operational range (such as the VSIs), others go from anywhere around 0.4 to 0.8 (such as the Cyclos).
So, is any one type of electrical variable speed drive SIGNIFICANTLY more efficient than the other?
Thank you,
Eric
So, is any one type of electrical variable speed drive SIGNIFICANTLY more efficient than the other?
Thank you,
Eric





RE: VSD (Electric Drives) & Energy Efficiency
I am only familiar with VFD's and it may be true that some technologies waste more than others but, as far as I can see, given the same speed reduction in the load, the energy savings there should be pretty similar across the technologies.
RE: VSD (Electric Drives) & Energy Efficiency
Hence, does anyone know why and by how much? Is it really worth investigating?
www.MarineEngineeringManagement.com
RE: VSD (Electric Drives) & Energy Efficiency
RE: VSD (Electric Drives) & Energy Efficiency
www.MarineEngineeringManagement.com
RE: VSD (Electric Drives) & Energy Efficiency
Above that hp range, there are other designs with their own experts (not me!).
So, how you've got that 20MW divided up makes a lot of difference.
RE: VSD (Electric Drives) & Energy Efficiency
Curt Wilson
Delta Tau Data Systems
RE: VSD (Electric Drives) & Energy Efficiency
It seems we have to drag information out of you. Is this a single 20MW motor or 20 1MW motors or 200 10kW motors? It makes a differnce because LSI drives are only worth discussing if the motor is (are) in the MW range.
Cycloconverters are ancient history, don't waste time on them in my opinion.
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RE: VSD (Electric Drives) & Energy Efficiency
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If we learn from our mistakes I'm getting a great education!
RE: VSD (Electric Drives) & Energy Efficiency
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.
RE: VSD (Electric Drives) & Energy Efficiency
The cyclo-converter selects 'snippets' of the grid voltage and outputs them to the motor so that the motor voltage is proportional to speed and matches the EMF produced by the rotating externally excited rotor.
The snippets of the grid voltage that are delivered to the motor will be taken close to zero crossing of the grid sine-wave if the speed is low and progressively closer to the sine's peak as voltage demand (speed) grows when the motor is accelerating. This is what makes your PF bad at low speeds and better at higher speeds.
I wouldn't say that you are using more current than needed at low speeds. You do use the current needed to produce the torque needed by the propellers and that torque is never higher at low speeds than it is when running with full steam. And the power consumed by the cyclo converter/motor combination is not a lot higher than needed to move the ship. Actually, the cyclo converter is very efficient since there is just one set of thyristors between grid and motor and the switching losses are very low because the switching takes place at a low pace.
There is no reason to bother about a low power factor in the propulsion of a ship. At least not as long as it doesn't lead to increased current at low speeds. The reason is that you are using internal (to the ship) cables, bus-bars, transformers and switch-gear and needn't pay any extra for the low PF and you do not heat your equipment extra due to the low PF.
Instead, what you should worry about is the energy (equals oil consumed) consumption of the propulsion system. And that is quite low when you are running a synchronous motor from a cyclo converter.
It is true that you, perhaps, could reduce the current draw at low speeds and thereby reduce losses in the cables and transformers. But, since the current as such is already low at low speeds (affinity laws), the losses are also low (proportional to current squared).
You have a rather one-eyed view on this. As has been said several times in this thread, PF does not have anything (not much, anyhow) with efficiency to do. And as long as you are using a local and self-contained system and there's no penalty for the 'bad' PF, you really shouldn't worry.
Gunnar Englund
www.gke.org
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RE: VSD (Electric Drives) & Energy Efficiency
You said: "instead, what you should worry about is the energy (equals oil consumed) consumption of the propulsion system. And that is quite low when you are running a synchronous motor from a cyclo converter" ...how does the cyclo-synch propulsion at low speeds compare to a VSI-synch propulsion in terms of efficiency? How much better is it?
Cheers.
RE: VSD (Electric Drives) & Energy Efficiency
Thyristors have a rather low forward voltage drop and the switching losses are low. That makes the cyclo converter a very efficient plant.
The rectifier in the VFD input stage usually have a voltage drop at their rated current that is something like 1.2 - 1.6 volts. Two of them always work in series and that alone produces losses that are 3 times rated current.
Then, there is the ripple current in the capacitors in the DC link. That ripple current squared times the capacitor's ESR creates another component in the loss budget.
The IGBTs in a VFD switch at a much higher rate than the thyristors in the cyclo converter and, therefore, have higher losses. They also have conduction (forward) losses.
Sometimes, there are du/dt filters and (not very often) sine filters between inverter and motor. They also create losses.
Losses produce heat and that heat has to be removed. There are examples where air conditioning is being used to remove that heat and that consumes a lot of energy.
For motors in the 10+ MW range, I think that cyclo converters (or CSI) is a good choice and there is definitely no need to replace them with less efficient VSI systems.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: VSD (Electric Drives) & Energy Efficiency
I am surprised that you don't have flicker problems on your islanded generation system if you have two large cycloconverters. Most distribution utilities would be unhappy at the prospect of two cycloconverters of this size on their system because of the likely effects on other consumers.
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If we learn from our mistakes I'm getting a great education!
RE: VSD (Electric Drives) & Energy Efficiency
On a ship, it could easily be worth while to increase the size of some components to allow the low power factor since the gain in efficiency more than makes up for it. At 40MW of power, a gain of 1% in the efficiency is 400kW.
I agree with Gunner, the cyclo-converter should easily be the more efficient controller compared to a VSI. He did not mention the transformer or transformers in a cyclo-converter system, however a VSI that size quite likely has magnetics as well.
If you want to research a different drive topology then LCI should be the one you investigate. I don't believe a VSI is worth looking at for those power levels.
RE: VSD (Electric Drives) & Energy Efficiency
RE: VSD (Electric Drives) & Energy Efficiency
1) Efficiency is very high compared to IGBT/IGCT technology
2) 4Q operation as standard
3) more importantly on a big ship: reliability is excellent. They are bomb-proof when it comes to supply dips and short-circuits.
RE: VSD (Electric Drives) & Energy Efficiency
Another to add is that a LCI and synchronous motor make a good combination.
RE: VSD (Electric Drives) & Energy Efficiency
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If we learn from our mistakes I'm getting a great education!
RE: VSD (Electric Drives) & Energy Efficiency
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: VSD (Electric Drives) & Energy Efficiency
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
RE: VSD (Electric Drives) & Energy Efficiency
I dug out a recording from long time ago where grid voltage and current can be seen. This is at a rather high speed, so PF is good. The typical stepped current can be a problem if you have capacitance in the grid since it starts ringing and that can (did in this case) cause PD with ozone production if transmitted to MV side of transformer.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: VSD (Electric Drives) & Energy Efficiency
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