Author Topic: Question for MedVed, on MBFT, blended lamps  (Read 8038 times)
AngryHorse
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Question for MedVed, on MBFT, blended lamps « on: April 19, 2011, 11:25:50 AM » Author: AngryHorse
This summer I will be going the the rallies again with my Petter AVA stationary engine, running its 3 phase generator.
I have over the years run all different discharge lamps, as well as GLS with this set up, this year I have built a new display using some of my 500watt Philips ML lamps.

The lamps are from July 1974, and all new ones, but am I right in thinking these lamps have a high inrush current on starting?
My engine has a small 3 Phase generator on of 2.4KW, (its not a generator as such, but a converted induction motor!), but I have it generating on 3 phases.

I plan to run 3 lamps on each phase of the motor, so it will be balanced with 500watt per phase, they will also be turned on, one at a time.
It has a 6amp MCB, and 3 phase RCD protecting the generator, do you think I will encounter any problems with this set up?, I have run one of these lamps before, but on its own.
Thanks, Rich
« Last Edit: April 19, 2011, 11:31:59 AM by LinearSLI/H » Logged

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Re: Question for MedVed, on MBFT, blended lamps « Reply #1 on: April 19, 2011, 11:54:11 AM » Author: Medved
The inrush current does not differ from regular incandescents (as it is in fact the incandescent ballast causing the inrush current).
So if the generator is able to generate enough current (steady state) to power it up, don't worry, the inrush current would be then limited by the generator impedance, but it would still be enough for proper start.
In fact the current limiting would reduce electrode stress, as they have to handle the inrush current as well...
With this i would first connect all lamps and only then start the engine.
By the way, how is controlled the generator output voltage?
I ask, as these lamps (mainly their life) are very sensitive to the supply voltage variations, so it should be quite accurate, but such passive induction generator have very poor voltage control and on top of this the generated voltage is (at the nominal frequency) higher then nominal voltage of the induction motor.
Induction generator excited by simple fixed capacitor compensation (what you are in fact using), the voltage is limited only by rotor saturation flux, what mean the voltage is proportional to the frequency (so speed minus rotor slip), while the motor is likely designed to not saturate at mains...
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Re: Question for MedVed, on MBFT, blended lamps « Reply #2 on: April 19, 2011, 01:03:34 PM » Author: AngryHorse
Thanks for the quick reply, The voltage can be regulated just on the engine speed, and I can get this very steady, with no fluctuations in the voltage of each phase.
Here is a link to the stationary engine forum, with my thread on how the motor is set up
http://www.stationary-engine.net/forum/showthread.php?t=7206
Thanks again, Rich
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Re: Question for MedVed, on MBFT, blended lamps « Reply #3 on: April 19, 2011, 03:00:08 PM » Author: Medved
Maybe one extra note for the generator connection: I would rather use Delta connection and connect in this manner all loads (240V would be between phases, there would be no Neutral). The reason is, then the 3'rd harmonics (quite strong with all discharge lights) would then "circulate" between phases (assume fully symmetrical load) and would not load the generator winding. Result would be lower voltage distortion (caused by the distorted load current).
Then all capacitors should have (sqrt(3)) times the capacitance, rated for 250VAC...
Other advantage would be fully isolated supply (so single fault does not cause current) and lower phase voltage to the ground (only ~140V)

Anyway (in the star configuration) is better to keep the Neutral not grounded (insulated source), as classical protection schemes (e.g. overcurrent circuit breaker) may not work correctly due to quite high source impedance (the generator is not able to deliver enough current to trip it).
What is good feature on portable generators: Connect engine shut-down relay between the Neutral (in the Delta you should make it artificially using three equal impedances connected as Star between phases) and grounding rod. In case a leakage develop from any phase to ground, voltage appear on this (artificial) Neutral and so shut down the engine (interrupt/short ignition on gas, stop fuel injection on diesel,...).This system could be made very sensitive for real faults (<5mA of fault current; at the same time it will limit it to <10mA, what mean quite low health risk in case of shock), but still quite immune against false tripping (require the voltage imbalance to be really high).
« Last Edit: April 19, 2011, 03:13:21 PM by Medved » Logged

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Re: Question for MedVed, on MBFT, blended lamps « Reply #4 on: April 20, 2011, 09:59:34 AM » Author: AngryHorse
Interesting, thankyou for that, this spec came from a mechanical/electrical engineer, and his info is for star connection.
I have never tried it in delta, the grounded neutral was a mistake on my part, and has now been sorted, my ground rod is in the ground by 2foot, but some debate, on the forum, that unless the groung is very wet, its hardly likely to work well.
This induction motor, on my engine, I have found to be more `steady`, when running, that a proper made single phase generator, but as its running slightly slower than its machine speed, it only runs at 45HZ.
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Re: Question for MedVed, on MBFT, blended lamps « Reply #5 on: April 20, 2011, 01:31:17 PM » Author: Medved
The star is necessary, if you have to be compatible with existing 3-phase wiring (e.g. backup generator for a home - you can not alter the wiring when switching from mains to the generator and back).
But you have fully independent system, so you have full freedom to select the one more suitable for the particular need.

The poor grounding is the reason, why not to rely to traditional home installation protection methods, as these are designed only for well grounded neutral on the source side and so require quite high fault current to develop in order to act properly (GFCI require 30mA, while the poor grounding may limit it below 20mA, but this is still dangerous).
The voltage monitor is then the means to detect the insulation fault and so avoid potentially dangerous situation (should another fault develop, it may really shock someone). Remember, that you are operating not well protected installation outdoors (where it may start raining), where touching live wire is of not as big problem, so failure current limitation (that's why not grounding) and a sensitive detector/shut down (the voltage imbalance sensing) are quite essential for the safety...

If the engine shut-down is not possible (e.g. on pure mechanical diesel), with induction generator is enough to disconnect the generator from the capacitor bank - the voltage would then disappear from the whole system, include the generator itself (so even internal generator fault would be covered).
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Re: Question for MedVed, on MBFT, blended lamps « Reply #6 on: April 22, 2011, 04:26:26 AM » Author: AngryHorse
One of the problems I have had on this system is, like you say, disconecting the capacitor bank from the motor, if the load droppes off, (i.e, a mcb trips), the engine speed rises, and if not shut down fast enough, will burn the capacitors out, (as their voltage goes above 440volt), but have not found a way to do this yet.
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Re: Question for MedVed, on MBFT, blended lamps « Reply #7 on: April 22, 2011, 10:17:23 AM » Author: Medved
One of the problems I have had on this system is, like you say, disconecting the capacitor bank from the motor, if the load droppes off, (i.e, a mcb trips), the engine speed rises, and if not shut down fast enough, will burn the capacitors out, (as their voltage goes above 440volt), but have not found a way to do this yet.

That is, why the protection should not disconnect the load, but mainly disconnect capacitors from the generator.

One quite safe shut-down method for the stand-alone induction generator system is simple short circuit crow-bar. In case you detect a faulty condition, you short out all the outputs.

Don't forget, then your system behave by far differently then normal mains, so you should be very careful: Lot of things work in different way, so protection working on mains may become dangerous with this generator system. And so you would need to make some appropriate protection devices yourself. (short circuit crow-bar, voltage imbalance shut down, latched capacitor disconnect,...)

By the way, your engine does not have any speed governor?
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Re: Question for MedVed, on MBFT, blended lamps « Reply #8 on: April 22, 2011, 10:49:59 AM » Author: SeanB~1
If you want to do voltage limiting a pair of spark gaps ( like those dual devices you find in almost any telephone protector) across the capacitor will limit voltage, though the limiters will burn out within seconds.

 For shock protection a monitoring unit for an isolated supply would be a better bet, as they are designed for use on a isolated supply, and have a very low current that is used to detect accidental contact or degraded insulation ( or you touching the bare wire) which gives both a warning and the ability to disconnect the supply. These are mostly used in medical areas on isolated supplies.
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Re: Question for MedVed, on MBFT, blended lamps « Reply #9 on: April 22, 2011, 12:05:37 PM » Author: Medved
If you want to do voltage limiting a pair of spark gaps ( like those dual devices you find in almost any telephone protector) across the capacitor will limit voltage, though the limiters will burn out within seconds.

The issue is, then the overvoltage can take for way more then seconds: If the governor is weak (or not present at all), the engine rev up on disappearing load (e.g. burned filament of the incandescent or response of the load disconnect). And it takes quite long for the "operator" to notice this and shut down the engine (or reduce it's speed) and during this time the "spark gap" would already melt... Don't forget, then these capacitors alone have the "self-healing" feature and they will likely survive way more severe overvoltage then those spark-gap protectors.
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Re: Question for MedVed, on MBFT, blended lamps « Reply #10 on: April 23, 2011, 05:57:06 AM » Author: AngryHorse
The engine dose have a speed govenor, but I have wound it right off, as its a bit slow at reacting to load change, (with it being Diesel, it has a very heavy flywheel).
I control its speed with a fine adjusting screw on the pump bar, this way, I can control the voltage to the nearest, (5 volts), for priscise running of discharge lamp circuits.

This `reving up` problem, happend on a smaller set up I had, a 60watt GLS filament broke, blowing a 3 amp fuse, and causing the engine to rev up.
Sadly, at the time, I was talking, and by the time I could shut the speed down, the motor had run over 440volts, and taken the capps with it, not a big problem, I just put a spare set on, just a pain when it happend.
« Last Edit: April 23, 2011, 05:59:12 AM by LinearSLI/H » Logged

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Re: Question for MedVed, on MBFT, blended lamps « Reply #11 on: April 23, 2011, 01:06:10 PM » Author: Medved
The heavy flywheel should only help to maintain constant speed - it does not like it's changes by itself, so then the requirement for governor response time are eased (the fast load changes are then covered by the flywheel; this is exactly the reason, why the flywheel is so heavy on constant speed industrial engines).
If the governor have troubles to maintain constant speed (e.g. the speed is cyclically increasing and decreasing, not controlling the throttle in full range,...), i would expect the governor mechanism not moving freely (dirt, improper lubrication, excessive wear of some components,...).
The speed adjustment should be normally done only on the governor (flying balls position, spring tension,... - depend on it's design; it should have there some adjustment elements); the screws on the regulation rod on the injection pump are there only to limit the maximum fuel dose (so it correspond to the amount of air in the cylinder) and should be set so, the engine just does not make thick black smoke at full load (to be on the border; less dose limit mean it's not able to operate at full power, more mean it would only excessively smoke at overload and waste the fuel).
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Re: Question for MedVed, on MBFT, blended lamps « Reply #12 on: April 23, 2011, 06:20:39 PM » Author: AngryHorse
Ah, I wasn`t clear on the injector screw, its not for the fuel mixture, it one that I fitted with a bracket, it just touches the brass block on the end of the injector rod, to stop the engine speed from slowing down too much.
Thank you again for the info. ;)
« Last Edit: April 23, 2011, 06:22:21 PM by LinearSLI/H » Logged

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Re: Question for MedVed, on MBFT, blended lamps « Reply #13 on: April 24, 2011, 12:54:05 PM » Author: Medved
?? I didn't get, where you adjusted it: You said it was on the fuel pump and not on the governor, but it was not the maximum dose limitter.
As i understood, you have Diesel engine, correct?
Then on the pump itself i've seen only two adjustments: Injestion advance angle and the maximum power fuel dose.
And then on the governor is then the speed adjustment.
But on many pumps the governor is integrated into common block with the injection pump...
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Re: Question for MedVed, on MBFT, blended lamps « Reply #14 on: April 24, 2011, 06:23:19 PM » Author: AngryHorse
Sorry mate, I`m making this confusing :P, the engine is 1952 Diesel donky engine, and has a simple injector pump made by BRYCE, to increase the fuel injected to the engine, there is a sliding bar in the middle of the pumps body, one one end is the govenor spring, (that I have wound right off), and on the other end is a small brass block, with my made screw, touching the end, so I can manually set the speed, by how far it opens the pump.
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