ST Genset Flicker

[(unknown)]

I have a 15Kw ST generator driven by a single cylinder 24Hp chinese diesel.  The unit is working fine and producing the electricity for our home and farm running on veg oil.  One issue I have though is with the slight flicker with the lighting.  Is there any easy way to smooth out this issue?  The unit has a regulator for voltage that has a trim pot that is adjustable from the outside and can be locked down with a nut.  The voltage is running 120VAC per leg (running 240VAC into the house) and the hertz is around 61 no load and gets down to around 60.5 under load.  Will a capacitor bank wired into this smooth out the flicker?  I do notice when the hot water heater comes on and the system is loaded down there is a lot less flicker than when things are basically coasting along.  Any help would be greatly appreciated.

Thanks,

Trav.

[BigBreaker]

Capacitors would smooth DC but your generator, like most generators, is outputting AC.  You can't just drop a big cap across two wires and solve a problem like this.

I think the issue might be in your AV waveform.  Do you have any idea if your genhead is making a high quality sinusoidal wave?  Is the flicker 60hz also? is it constant frequency or random?  When did this flicker start?  Perhaps it's not the gen but the lights that are misbehaving.  Do they work properly on mains?

I'm assuming that your flickering lights are fluorescents, yes?  It might be worth trying a few different types of lighting units.  It's the electronics that drive the bulb, the ballast, that would be finicky.  Trying a few alternatives out might yield useful information.  An oscilloscope is really the only way to be sure that the waveform is clean.  60 hz is slow enough that you could get by with something less than a scope but it would be rather kludgy.

[christopher]

If its a single its probably the power pulse on combustion.A heavy flywheel on the gen head may help, or an AVR if not already fitted.

[Flux]

Not enough details to be able to help you. Single cylinder engines invariably cause flicker, it's a problem of engine torque pulsation. Normally this is acceptable with machines with a big flywheel, if the flywheel is not big enough then it may be irritating and you are stuck with it unless you can add a flywheel to the alternator.

However this type of flicker becomes worse as the engine is loaded and you remove more energy and the flywheel smoothing becomes less. You I suspect have a different problem.

I suspect your alternator has an avr and the avr is suffering from ripple lock up and the field is being fed with low frequency blocks of current, this will also cause flicker and on higher load the avr stabilises and the frequency of the field pulsation is higher or ceases and you end up with a fairly constant field.

Unless you know quite a lot and have access to an oscilloscope you will struggle to even confirm this let alone cure it.

If you have an analogue dc voltmeter you could look at the field volts and if there is a low frequency flicker on the meter it is probably happening. A digital meter will just confuse you completely.

If this is happening it may be an unstabilised avr resulting from poor design or not being set up. If it doesn't have a stabilising pot you won't easily sort it. If it is ripple lock up in the avr error amplifier it again is a design issue and without detailed circuits and some basic experience you will not cure it.

Unfortunately I know virtually nothing about these ST alternastors, the early ones were a copy of early British designs and were compounding types that self regulated without an avr. They will not suffer from this problem. I suspect that are now copying a very early and crude avr that works well enough in maintaining constant volts but is not doing it in a way that leads to a smooth output. You tend to get what you pay for in the end.

Flux

[(unknown)]

Thanks for the very helpful posts.  Also, my apologies for the duplicate post (my mistake).  I was wondering about the single cylinder aspect of things myself.  May be something I just have to live with.  It is not a deal breaker in the least but was hopeful some sort of filtering may smooth it out a bit.  I am wondering if there is any kind of line conditioner available or that can be built that may help? Anyhow, thanks again everyone!!

Trav.

[SparWeb]

That exists, but I doubt you would want one.  First off they can be very expensive (15k$ for 15kW), they are terribly massive and they always consume power from the generator, even when you are not.  I have a 5kW model and it would always draw 12 amps when I plug it in.  I can't actually use it and regret having bought it (used+cheap thankfully).

If you're still curious, look up "Sola constant voltage transformer".  They get used in hospitals and labs where equipment power can't be allowed to flicker, and that cost outweighs the cost of the equipment.

[Flux]

There is also another possibility that I didn't cover and that is a cyclic governor hunt on the engine.

You can prove whether it is the engine or the alternator by mechanically loading the engine while maintaining the alternator on light load. If you can load the engine with a plank of wood levered against the flywheel you can load the engine without affecting the alternator. If the flicker reduces with increased load then it is an engine problem and most likely you have to live with it.

The ferro resonant constant voltage transformer may filter this out but I am not sure that it will. it's certainly not a practical solution.

Flux

[artificer]

The AVR's on the ST heads are infamous for being inexpensive and sensitive to frequency variations.  One solution is to get an aftermarket AVR.  One review of an AVR is here: http://www.hightechhicks.com/AVR%20Evaluation.pdf

www.power-tronics.com has a bunch of AVR's.  You can select an appropriate one.

Using the aftermarket AVR's has tamed the flicker on some Liser(oid) engine powered ST heads.  Considering they run at 650rpm vs your 1800rpm, they should work even better for you.  They also clean up the output wave form compared to the self exitation coils of the ST head.

Less flicker under load hints that its the AVR, and not the typical single cylinder engine problem with flicker.

Good Luck

Michael

[Ungrounded Lightning Rod]

They could also make things WORSE.

Constant-voltage transformers are resonant devices - leaky transformers with an extra secondary winding paralleled by a capacitor to resonate at the line frequency and keep the output flux loop just saturating on the peaks.

While they'll do a great job smoothing a stable AC source at or very near their resonant frequency, they'll actually worsen the regulation if the source has a frequency instability or takes a large phase hit.  (The latter can actually make them essntially DOUBLE their output voltage for a short time.)  This is part of the reason they have been abandoned in favor of other regulation schemes.

If you have a torque-pulsation issue resulting in an RPM variation great enough to foul up the output voltage, it means the frequency (and thus the phase) is varying from cycle to cycle by a non-trivial amount.  This is essentially a continuous series of phase hits.  A regulating transformer MIGHT improve it or MIGHT make it considerably worse, depending on the details of the phase instability.

[dnix71]

Spar, would your unused transformer make 208 into 240? We have a 4KW engraving laser at work that doesn't work right because it needs 240 and we only have 208 in the building. We bought a buck transformer for a laminator because we had to. Management doen't pull permits unless they have to and we keep buying stuff for 240 because no one bothers to check first. A/C's twice, laminator, laser and forlift battery chargers :>(

[dnix71]

Looking up Chinese gensets matching your description, there are many that are salient pole with brushes.

One service manual says flickering lights can be caused by an uneven air gap (loose bearings or poor manufacture).

The wave form may be spiky also if the core is oversaturated. The spikes can make your lights flicker.

[TroubledGus]

Sounds like a Cheap AVR.

If this unit has a stand alone harmonic winding ( leads marked Z1 and Z2 ) try it and see if there is a still a flicker from light to heavy loads.

How is the AVR wired in this ST ?

[SparWeb]

208V comes from having 2 phases of a 3-phase service tapped, doesn't it?

The CVT is the wrong thing to correct that - you have to shift the phases from 120 degrees apart to 180 degrees apart.  I know 2 ways to fix that: run a rotary converter that runs a 3-phase motor to drive a 240V generator, or use a 3-phase conversion transformer that has a 240 V secondary (or rather "secondaries", because it's a complicated thing wound on a "figure-8").  Either one would be CHEAPER than a CVT, too, if they were to be bought brand new!

[joestue]

Connect a 32 volt 600VA transformer as a voltage booster for the 4KW laser.

Cheapest way I can think of to get a 208:32 volt transformer is to rewind a 240:120 volt transformer, or get two identical MOTs and place them end to end, (the "I" pieces are not used) and wind your own secondary.

Running the primaries in series at 208vac will significantly reduce the operating temp of the core, (most of the heat generated is core loss when run at full voltage).

Properly built, it will last as long as any other transformer, if it still runs too hot, toss an extra 10 or 20 turns on the primary.

BTW, voltage boosters, typically with 5 and 10% taps are available.

[veewee77]

If you are pulling a few lights with a 15KW genset, very likely the flickering is caused by very light load. A 15K genset will run best (ans smoothest) at about 2/3 of it's rated maximum. With just a few lights, and as others have said a one-lung-er, it will be unstable at low to moderate loads.

My advice is to just deal with it. Flickering lights is better than no lights!

Doug

[wolfman]

If you are using cfl lights or electronic ballasts try using a bridge rectifier and 220uf 250volt cap to provide them with filtered dc the lights I have used on cheap mod sine inverters have had flicker issues and this has removed the problem.