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Topics - Ian

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1
Generators / Anyone speekee Chinglish ?
« on: September 09, 2007, 07:00:58 PM »
I have managed to get hold of a 3 phase STC 7.5 kW alternator. Unfortunately the manual is in "Chinglish". I guess the original Chinese characters have been put through an automatic translator. Below is a portion of the Chinglish text - concerning the wiring diagram and harmonic / elementary wave / excitation coils.

Whilst I think I can get the gist of the text, I am not sure I really understand what is being said. I wondered if any of you good fellows may be up to the challenge and see if you can put it into a more understandable form of English ?

My queries are....
1. What is an elemantary wave coil and what does it do ?
2. Why have THREE harmonic windings feeding ONE field coil ?
3. Why is only ONE of the harmonic coils connected to the "elemantary wave coils" ?
4. Why / how does the elementary wave coil only effect the UNLOADED voltage ?
5. Why have elemantary wave coils when there is a "field rheostat" fitted which should adjust the output voltage ?

Here is the Chinglish text...

The wiring principle chart is shown as in Fig 1.

In the diagram the Z4 Z5 and Z6 Z7 as for the elementary wave coil of the auxiliary winding of the difference turns. Both can reach to adjust unloaded voltage purpose vis change over right and reverses series connecting plate. The unloaded voltage rises when the Z4 Z6 connect to the Z7-Z8 . When the Z4 Z7 connect to the z6 z8 the unloaded voltage goes down. When the z4 connect to  the  z8 or the  z6 z7  connect  to  the  z5 z8 that the two elemenbeve wave coil will be used separately. The connecting plate has been connected afotre delivering. According to the needs  of the  users  it  can  be  changed  to  connect  elementary coil as above method.



STC-3KW-20KW   STC-30KW -50KW
Fig 1 wiring principle chart
1. Main winding   2. Harmonic coil of the auxiliary
3. Field rheostat   4. Elementary wave coil of the auxiliary winding
5. Silicon-controlled rectifier   6. Excitation winding
7. Winding of indicator   8. lndicator   9. Voltmeter


Regards,
Ian

2
Generators / AVR regulators
« on: September 08, 2007, 08:49:13 PM »
Both of my generators have AVR units built in.

One of the AVR units is wired in parallel ACROSS the field winding (as well as obtaining the reference voltage from the output coils).

The other AVR is wired in series with the harmonic windings - so the output of the harmonic winding passes through the AVR and then onto the bridge rectifier.

Both seem to do the job and work very well.

It occurs to me that the AVR wired in parallel (across) the field winding probably works by effectively introducing a variable resistance path and hence an alternative current path back to the other leg of the harmonic winding. So to reduce the output voltage, the current flowing in the harmonic winding will actually increase with an increased proportion of it sunk in the internal AVR circuitry.

However, the AVR in series with the harmonic winding will work in the inverse way to the other AVR - ie, it variably increases resistance to reduce current in the field coil and hence reduce output voltage.

I am no electronics guru and I may have got it wrong - but the second AVR method (in series with the field winding) appears to me to be a more elegant method. So why have 2 different types of AVR ? Comments anyone ?

Regards,
Ian

3
Listeroid Engines / Governor spring
« on: August 31, 2007, 10:32:49 AM »
Has anyone got a reliable method of determining the spring length and/or strength to optimise the Listeroid governor for a specific speed ? Failing that, how did you go about optimising yours ?

I am aware that there are a number of posts about correcting governor linkage geometry, correcting lazy / fouling governors, and others but I cannot see any where spring selection has been the topic.

Was the spring supplied with your Listeroid even in the right ball-park to give you the right level of governing without further modifcation ?

The spring supplied (and a further 5 springs I ordered as spares) for my JKSon 12/1 was much too heavy and offered very poor regulation. I have tried a good range of softer springs and have improved the situation but think it can be improved further still and am now seeking guidance from people who may have a better angle on it than me.

Regards,
Ian

4
Generators / Power Factor
« on: August 30, 2007, 04:35:46 PM »
I am out of my depth and am seeking some knowledgeable input please.

My grid tie setup takes the AC output of my single phase 230v 50 Hz ST alternator and full wave rectifies it in a bank of home made bridge rectifiers in parallel – all mounted on a massive heat sink. The resultant DC output is fed into the input stages of a 6 kW grid tie inverter, which houses something like 200,000 microfarad of electrolytic capacitors.

My query is about “power factor” and its correction. If I were to connect the alternator AC output directly to a capacitive load, I would expect to see the voltage and current waveform out of phase with each other. However, in my scenario the capacitors are effectively seeing only DC (albeit quite heavily rippled) so there is no phase to be out of alignment. However, current flow will be highest when capacitor voltage is lowest, and least when the capacitor voltage is highest – which is pretty close to what could be a DC equivalent of “power factor”.

So what is the generator actually seeing ? As I view it, the only thing the generator sees is the bank of diodes (a slight resistance) in the full wave rectifiers and what might be regarded as a variable load (dependent upon voltage).

So, is the generator seeing a capacitive load ? (I think so). If the generator IS seeing a capacitive load, is there any benefit in introducing inductors to make life a little easier for the generator and reduce losses ? If yes, where should the inductors go ? (On the AC side or on the rippled DC side)?

If the addition of inductors would be helpful, how would I go about calculating the inductance to add and what would the result be ? As I said, I am out of my depth, so if there is anyone out there who may be able to make any sense of these ramblings, and would feel able to point me in the right direction, then a bit more information will undoubtedly be necessary. I think I have covered the majority of the likely needed information below….

Alternator = ST 7.5 kVA wired as 230 volts 50 Hz at 1500 rpm. The windings have a static resistance of less than 1 ohm. The alternator is actually run at 1800 rpm with a measured AC rms figure of about 280 - 300 volts. The measured current is about 17 amps when the grid tie inverter is squirting about 4 kW into the grid continuously.

Rectification = 4 x 1000 volt 35 amp bridge rectifier diodes wired in parallel. In use, the heat sink gets only slightly warm. The unloaded voltage can go as high as about 430 volts DC. When under load, the measured DC voltage drops back to around 265 volts DC as measured by a digital multimeter; of course the voltage will be rippled but I do not have a scope to see what the ripple waveform looks like.

Grid tie inverter = 6 kW SMA Sunny Mini Central with about 20 x 10,000 microfard capacitors at the DC end. There appear to be some heavy current inductors on the motherboard input stages but I cannot tell what their inductance is; they consist of about 20 turns of very thick (4 -5 mm diameter) enamelled copper wire on an approx 50 mm diameter ferrite ring. Neither the inductors or the capacitors appear to get unduly warm in continuous use.

Any comments from anyone who understands what may be going on ?

Regards,
Ian

5
Generators / Fan
« on: August 30, 2007, 04:31:06 PM »
During a maintenance and bearing upgrade on one of my 7.5 KVA ST alternators I noticed that the fan was not running true; it was buckled. Whilst it was not a major issue I still decided to “correct” it. Having got the fan almost true I was ham-fisted enough to crack the fan casting. The crack is radial extending from the outside (where I was levering), propagating along the root of a fan blade, and terminating in the conical section behind the front bearing. The crack does NOT extend to the centre shaft.

Whilst I have used a 2 part epoxy adhesive to temporarily repair the crack (and it seems to work just fine), I intend to replace the fan.

The fan appears to be secured onto the rotor shaft by a combination of interference fit and keyway. There is a circlip in front of the fan (behind the front bearing) which I assume allows the fan to be pulled forward and removed. I also assume that the fan is impeded from moving further back towards the windings by a step in the shaft but this is not obvious. There appears to be a keyway of some kind in the shaft axially in line with the main shaft keyway but no obviously visible key.

So,….
1.   Has anyone actually tried to remove a fan and replace it ?
2.   Are there any tips on removing and replacing the fan ?
3.   Is the fan keyed ? How ?
4.   As the fan is a quite brittle alloy casting, is there a way of getting the fan off in one piece ?

Regards,
Ian

6
Generators / Heat
« on: August 30, 2007, 04:28:21 PM »
I grid tie the output from my 7.5 KVA ST alternator. I have tried two 7.5 KVA ST alternators on otherwise the same equipment. As I am based in UK, the alternator outputs are nominal 230 volts single phase 50Hz.

I full wave rectify the outputs and feed the resultant DC into the grid tie inverter. The inverter squirts about 4 kW into the grid. This is quite a demanding load for the alternators as power is only drawn at the peaks of the waveform. Whilst the rated current output is 32 amps, I suspect that the instantaneous output at the peaks will be maybe 100 amps or more due to the capacitors looking almost like a short circuit at this point.

Both alternators manage to happily supply this power all day but one of the alternators runs significantly hotter than the other under the same conditions. The cool running one has a case temperature of about 10 degrees C (maybe 20 degrees F) above ambient but the hotter one probably has a case temperature of about 30 degrees C (55F?) above ambient; it feels much hotter.

Both alternators have the same stator winding cross sectional areas and they are both equally clean with nothing to impede airflow.

Does anyone have any idea why one alternator should run significantly hotter than a similar one under these conditions ?

Regards,
Ian

7
Generators / Balance
« on: August 30, 2007, 04:26:20 PM »
One of my 7.5 KVA ST alternators has had an attempt to balance it at the factory. The balance is rudimentary at best; a hole has been drilled in the fan and a nut and bolt packed out with slivers of core laminations has been attached. However, my other ST alternator has had no obvious balancing done.

1.   If you have an ST alternator – has yours been balanced; how has it been balanced?
2.   At what level of imbalance would it become appropriate to correct it ?
3.   Is there an easy DIY way of determining balance if you do not have something like a lathe or set of spindles to hold the shaft ?

Regards,
Ian



8
Generators / Windings : enamelled copper cross sectional area
« on: August 30, 2007, 04:24:52 PM »
I am a little confused and am seeking clarification, please, from your good selves….

My ST alternators are rated at 7.5 KVA. The rated output current (at around 230 volts) is about 32 amps (stator). The field winding (rotor) is nameplate rated at 2.7 amps at about 62 volts.

The cross sectional area (diameter) of the winding wire is usually proportional to the current the winding has to carry. So, I would expect the stator windings (required current carrying capacity of 32 amps) would be substantially heavier than those for the stator (required current carrying capacity of 2.7 amps). But it is the other way round!

The 4 rotor windings have substantially larger cross sectional areas than the stator coils. I would have expected commercial sense to limit the rotor windings to the minimum required and also to limit centrifugal forces by keeping the rotating mass down. In my view, the cooling effectiveness of the rotating bobbins on the 4 rotor coils is good, and there is plenty of room left on the winding former so there is unlikely to be a heat build up in the middle of the field windings.

When I refer to the stator windings, I am talking about the main output windings and NOT the harmonic or “tell-tale” bulb windings, which, as one would expect, are of low cross sectional area compared to the main stator windings.

1.   Why are the rotor windings larger than the stator windings ?
2.   What am I missing ?

Regards,
Ian

9
Generators / Skewed stator
« on: August 30, 2007, 04:19:20 PM »
I have a couple of 7.5 KVA ST alternators. Having taken both of them apart I have been struck by just how similar they are in materials and construction despite them coming from different suppliers and with different brand names. Both of my ST alternators have skewed stators which surprised me; I was not expecting this.

Having followed this forum for about a year, and noting comments about poor waveform output, I was expecting a linear stator. I do not have a scope so I cannot see what the output waveform is like but I have had no trouble with any equipment so far and the skewed stator would lead me to believe that the waveform should be pretty good.

So …
1.   Are ALL ST alternators like this ?
2.   Has anyone put a scope on a skewed stator ST head and evaluated the output waveform ?

Regards,
Ian

10
Generators / Soft, dull, hollow knock sound
« on: August 30, 2007, 04:16:34 PM »
Hi, I have been lurking for about a year as a guest but have only just signed up as a forum member. I am based in UK and have a 12/1 JKSon Listeroid clone coupled to a 7.5 KVA ST generator situated in an engine shed about 20 metres from my house at the bottom of the garden and running on WVO (the liquid stuff) and old chip fat (the solid stuff). Whilst it is still all coming together, the plan is to provide CHP for my house. The power is grid tied and the hot water (thermosyphon engine coolant and exhaust gas heat exchanger) passes through to my house radiator circuit via a flat plate heat exchanger. I have about 200 hours running experience so far.

I have a couple of ST generator heads (only ONE is coupled to the engine at any one time). Both are remarkably similar in materials and construction (even down to the same braid and knot used to tie cables to the rotor spindle. I have a few questions about these ST heads and I hope that you knowledgeable folk will be able to answer them and enlighten and help me… I will ask each question in separate threads to try to keep the topic concentrated ….. so here goes with number 1:

Knock when turning the rotor slowly by hand
One of my 7.5 KVA ST alternators has been in quite hard use for about 200 hours from new. The front (fan) bearing has started to make a whining noise and I reckoned that it was time to put a decent set of bearings in to replace the Chinese originals. Turning the rotor slowly by hand after disconnection from the prime mover displayed a roughness in the bearings but also a kind of soft non-metallic knock as the rotor was either moved slowly continuously or too and fro. I thought this needed investigating too.

The bell housings came off relatively easily (hammer and a block of wood on the fan end bell housing and a large span puller on the rear end bell). Both bearings were left on the rotor shaft. The rear bearing came off OK with a simple puller with a bit of grunt but the front bearing was a pig and required a hydraulic puller and some very fine arms to get behind the bearing but not foul the fan. Both bearings had unacceptable roughness (but very little real play), and were packed with the dark, thick “yak fat” grease resembling axle grease through which the balls cut a track. These bearings have now been replaced with fully sealed pukka SKF bearings.

Each of the 4 windings on the rotor are carried on formers (I would call them bobbins) that resemble a yellow / cream plaster type material. These formers are located between the rotor shaft and the laminated pole cores. Each core appears to be secured to the shaft by 2 large cap screws. The formers, I think, should be held firmly in place by some angled metal brackets at each end of the former, axially in line with the shaft which I assume to be an interference fit between the former and a milled out pocket section of the shaft – although what actually retains them I do not know. These angled brackets appear to be firmly held in place.

The copper windings on the formers themselves appear to be well wound with enough lacquer to stop any movement of the enamelled copper wires; but not enough for my satisfaction so I will apply a lot more to make REALLY sure they cannot move.

There is no obvious movement of individual pole core laminations or the cores as a whole. They appear to be tight and secure.

Each of the 4 formers have about 5 mm (1/4 inch) of axial movement and about 1.5 – 2 mm of side to side movement about the pole core. There is NO vertical movement. I am assuming this movement is what is causing the soft knocking sound. The movement occurs under the weight of the formers and windings when the rotor is rotated. My guess is that when spinning, centrifugal forces stop the axial movement but I would expect the magnetic forces to cause the formers to be constantly chattering and twisting in the side to side lateral movement. Having said this, I see no evidence of powder from the gradual erosion of the plaster like former material caused by chattering that I would expect to see if it was actually happening.

So my questions are…
1.   Should I be concerned about the movement of these formers ?
2.   What should I do to arrest the movement ?
3.   What is supposed to firmly hold the formers and stop them from moving ?
4.   What, if any, damage may have been caused by the movement and how can I check for it ?
5.   Is there anything else that might be causing the dull, soft, hollow knocking sound ?

Many thanks for wading through a very long first post.
Regards,
Ian

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