Generators for Drives 1

[jcarrr]

Hi,

I am going to build some engine driven alternators units specifically to power a couple drives, operating at different output frequencies, totaling about 15 KW per generator. Long engine and generator life is important. I want to chase load with engine speed in order to minimize wear, fuel consumption and noise. It seems to me that it would be possible to avoid some of the problems inherent with using generators in this mode by having them wound for this specific application. The motors will ramp from zero torque and speed

Intuitively, well maybe a little more than intuitively, it seems that high frequency, lots of phases and high harmonic content might all contribute to a good relationship between the generator and drive. Not the usual desirable criteria for a genset.

I would appreciate all the thoughtful input you care to share and am looking forward to the discussion.

John C.

 

[DougMSOE]

If I understand you correctly, you want to use the generator as a variable frequency drive?

 

[jcarrr]

No quite, We configure gensets to operae as variable speed drives. This particular need is for the operation of existing drive installations or or to meet the need to supply more than one dirve from a single generator. This generator needs to supply constant voltage input power to the rectifier section of one or more drives. A 600 + Volt DC machine is one answer to the problem, but I am as uncertain about the ability of a DC machine to handle the unique requirements of the rectifier section any better than AC equipment. It seems best to start with equipment that can be reconfigured. Any discssion is welcome.

 

[ScottyUK]

jcarr,

One consideration to bear in mind is that if you ramp the generator speed, you change the frequency of the output. Most drives have a rectifier / filter / reservoir stage designed for operation at 50Hz or 60Hz. If you drop the frequency, you will find that the voltage ripple on the DC bus will increase, potentially causing problems with the output stage and / or the control section if the voltage troughs are low enough. If you drop the frequency substantially, you will have to consider increasing the size of the reservoir capacitors. You should also look at the low-frequency ripple current rating of the capacitors. I had a quick look and couldn't see it quoted for, say, 25Hz because such a frequency is unusually low and doesn't occur in a 'normal' mains application.



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[jbartos]

Suggestion: The original posting closely approaches to Navy shipboard power generation and propulsion systems. It may be a good idea to tap into vast resources in that area rather than starting completely at the beginning. US Navy order a development of 15-phase medium voltage squirrel cage induction motor for the ship propulsion in upper 90s. It runs very smoothly. It needs a 15-phase ac motor drive.

 

[UKpete]

Regardless of the machine type, the output voltage will be proportional to speed (ok, some limited compensation might be possible). To maintain a constant dc link voltage in the drive, presumably you will need to put a buck or boost converter after the rectifier.

Don't engines prefer to run at constant speed? e.g. on hybrid vehicles they run the engine at fixed revs regardless of power demand.

 

[jbartos]

Suggestion: Alternatively, a generator prime mover can be controlled to vary generator output and the generator motor output. This will eliminate the motor drive and a lot of problems.

 

[jcarrr]

Thanks for the interesting comments. Here are some further thoughts.

We do configure some alternators to operate as variable speed power supplies. They generally are setup to operate between 30Hz and 75Hz with constant Volts/Hz outputs. They function very well. Starting from reduced speed they can start a pump load of their rated capacity with an engine of the same capacity

The power supplies this query is concerned about must grab power from engines that are primarily performing another task that forces their speed to vary. The maximum turndown from top speed is about 40%.

The 15 Phase naval application is an interesting demonstration that every thing in the world need not be single or three phase. I once saw some old, out of use, mining equipment that I eventually deduced was a three wire two phase system. Two phases in 90 degree quadrature, not a 180 degree relationship.

I believe that by controlling excitation carefully I can keep the DC link voltage to the drive within acceptable limits. The loads and engine speed do not change rapidly. This will take careful implementation and a good protection plan.

I suspect that the engines in hybrid applications must operate, and have the opportunity, to operate at a fixed speed because they must produce maximum recharge power at maximum efficiency any time they are running just to make the whole exercise worthwhile. They probably operate at full manifold pressure whenever they are loaded.

400 Hz military equipment seems like it is worth a look for a first try for my experiments. It is usually dirt cheap, high quality and no big loss if there is a wreck. Any thoughts here?

I think that the introduction of a substantial third harmonic would be beneficial. I know this has been bred out of generators for the last hundred years but some one might have suggestions fo restoring it.

 

[aolalde]

For constant DC rectified voltage from a variable engine-generator with speed changing from 1800 to 1080 rpm, a high frequency (420- 252 HZ) 28 poles, 3 phase synchronous generator will provide the simple solution. (Aircrafts supply and Army).
A voltage regulator will keep the ac input voltage into a 3 phase full wave rectifier. The ripple change with those frequencies will not be so drastic.
To get maximum power per volume of generator, build full pitch windings that will get induced all the harmonics due to the field distortion.