Repair of Honda EM650 petrol generator 2

[bantam175uk]

I am trying to repair a Honda 650W petrol generator. after replacing the 8uF capacitor which terminates the 'condensor' winding with a similar but 10uF capacitor I got electrical output. however it is about 700VAC instead of the rated 240VAC. I am guessing that the capacitor value is not that critical and the problem is probably in the governor. Can anyone help? Is the output speed related? I have a circuit diagram which I could post if I new how?
regards
John.

 

[waross]

On many small petrol gen-sets, the voltage is dependent on both the value of the capacitor and the speed.
If this is a 3000 RPM rated set, you probably have an internal problem. 700 volts is too much to be accounted for by either a 25% larger capacitor or overspeed, (unless the governor is completely trashed and the motor is capable of about 7000 RPM wide open).
Try slowing the set down until it's putting out about 260 volts. Then find a way to check the frequency. An old electric clock with a synchronous motor drive is a good cheap method. You compare the time to a known good time-piece over a minute or so. A simple ratio will give you the frequency.
At this voltage it should be about 40 to 45 Hz.
If the frequency is close to this value at 260 volts, you have a governor problem and an oversized capaciter. If the frerquency is considerably different then you probably have internal problems.
If the generator is ok internally you can set it up like this;
Adjust the speed until the clock is reading about 63 to 65 seconds to a timed minute. This gives you a little overspeed to allow for governor droop when the set loads up. Change capacitor value to get the voltage you need at rated speed. Rated voltage plus 10% or 15% more or less at no load.
This assumes that the set is in good condition.
yours

 

[itsmoked]

That 8uF is a bazaar value which makes me think it might be important though waross' 25% point seems valid.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[nbucska]

What are you measuring the Vtg with ?

Plesae read FAQ240-1032
WEB: <

http://geocities.com/nbucska/>

 

[bantam175uk]

Thanks Gents. I have done a couple of things today. Firstly by cutting the speed right back I can get 260VAC and frequency around 45Hz both measured using a Digital Multimeter. It's a bit difficult to back off the speed accurately because the governor lever/spring is holding the throttle open. So from what you said the culprit looks like the governor.
The second thing I did was to visit someone who has a similar, in fact almost identical machine. When this starts up the govenor lever moves from the Throttle fully open position to a much slower speed.
However going back to my m/c the govenor lever doesn't move, hence the high speed and high voltage. I will strip the govenor and see what's going on.
Just another couple of questions about the principles of this m/c. Can you explain what this auxiliary 'condensor' winding and the series 8uF capacitor is actually doing? Is it in some way affecting the exitation of the (brushless) rotor? How is voltage control achieved with load changes?
If it would help I have a circuit but I don't know how to post diagrams?
John.

 

[waross]

When you get the governor fixed and replace the capacitor with an 8Uf or a couple of 4's in parallel you should be good to go.
respectfully.

 

[itsmoked]

Or a 3.3 and a 4.7 in parallel.

Posting pictures: faq238-1161

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waross]

Hi itsmoked
You have probably noticed that I am not familliar with standard capacitor sizes.
Can you suggest to the original poster what standard sizes he should look for if he wants to make small voltage adjustments. I'm sure you have figured out that the voltage is proportional to the capacity.
thanks
respectfully

 

[itsmoked]

Sure waross. I was just pointing out that a 3.3uF in parallel with a 4.7uF (both standard values) would add to give you exactly 8uF +/-tolerances.

And since 8 anything uF is pretty rare, if the OP wants to hit 8uF he could do it that way.

I still agree that 10uF shouldn't make that much difference but some generator regulator circuits are pretty bazaar/novel.


Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waross]

Hi itsmoked
An aquantance of mine had several belt drive capacitor controlled generators at a remote sport fishing camp. He had no electrical background but he found out by trial and error that the voltage was proportional to the capacitor value.
The voltage is also proportional to the speed.
Based on an original value of 8 uF, a .4uF in parallel would give a 5% boost in voltage. What are the closest standard values?
Thanks Keith
respectfully

 

[bantam175uk]

Thanks for help so far. Here is the Circuit Diag

http://i1.tinypic.com/rmnss0.jpg

Bsides being interested in fixing the generator I am keen to understand the theory. I have some electrical understanding so you don't need to start from scratch. The features that are puzzling me are:-
We have an auxiliary stator winding terninating in the 8uF capacitor (the capacitor winding) what is the purpose of this winding? I imagine it must be something to do with rotor exitation?
Also can anyone explain how the voltage and frequency are controlled as the load increases?
Regards.
John.

 

[itsmoked]

Bantam; The frequency is a function of the mechanical construction of the generator(number of poles) and the speed that it turns at. Generally for a small generator your engine would need to drive it at 3600RPM to achieve 60Hz. Hence why small generators are horrible screaming noisy things that wear out quickly.

Yes it does have to do with rotor excitation. It is the excitation! So I "think" that the cap is an important part of the regulation.

The frequency is maintained as the load increases by the governor advancing. Since all that means is the engine must continue to run at the same speed regardless of load the governor is called a constant speed governor and needs have no electrical aspect at all. Often they do but it isn't strictly necessary.

Someone else will need to explain the voltage regulation of this generator..

 

[itsmoked]

Waross;
Standard caps are these values with different multipliers(powers of 10).

Example 100uf,10uF,1uF,0.1uF,0.01uF,0.001uF (often as 1,000pF), etc, etc.

Here is the general situation. Like anything you can always buy "something" else with enough money. Caps often have low tolerances. +/- 50% is common with some types.

If a company wanted 8.0uF caps it would probably pay a cap company to sort all the 8.0uFs out of a lot of 8.2uF caps.

Here are typical values.
1.0
1.5
2.2
3.3
3.9
4.7
5.6
6.8
8.2

Of these:
1.0
4.7
Are by far the most common used.

Followed by
1.5
2.2
6.8

Much less often would be
3.3
3.9
5.6
8.2

Note.. There is no 8.0

To answer your question specifically what's close to 0.4uF?
Answer: 0.47uF but note whatever replacement cap's tolerance as a common one is -20%+80%

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waross]

Thanks Keith
The original poster should now have all the information he needs to adjust his voltage. It looks like a .47 uF will give a voltage increase of between 4.7% and 10.6% Good choice.
respectfully

 

[ccjersey]

I have always assumed that this relatively simple system of regulation is based on a magnetic coupling between the main and auxillary windings in the stator. If current goes up in the main windings, more voltage is induced in the auxillary.

But, I cannot see how the magnetic coupling between the main and auxillary windings will result in an induced voltage since both the windings are fixed in relation to each other. If the auxillary winding were moving through the magnetic field of the main winding, it would experience an induced voltage.

It may be that the auxillary winding is simply a source of excitation current relatively independent of the main windings and less succeptable to voltage dip under load.

 

[waross]

Hello ccjersey;
I think your remembering a different exitation system.
The diagram shows the capacitor winding connected only to the capacitor. When the frequency switch is operated, more capacity is added. There is no direct connection to the main winding.
It appears that an emf is induced in the capacitor winding that charges the capacitor. When the capacitor discharges it induces a current in the rotating field.
It would appear that the time constant of the field is adequate to recharge the capacitor.
The current in the capacitor winding is determined by the value of the capacitor.
This is an educated guess but the assumptions seem to fit the observed charicteristics of this type of generator.
Comments welcome.
respectfully

 

[UKpete]

The stator core flux is set by a combination of both the main and auxiliary winding currents in the stator, and the rotor mounted winding current. The auxiliary winding (with the capacitor load) therefore aids the excitation without supplying all of it.

So I think the purpose of the auxiliary winding feeding only a capacitor is that it provides a simple external way of adjusting the excitation. This is required when changing frequency, e.g. from 50Hz to 60Hz, the speed of the engine needs to be increased by 20% (to give the new frequency) which would also result in the voltage going up by 20%. So by reducing the capacitance, the excitation is reduced and the voltage is maintained at its previous level.

Incidentally, when large 3-phase synchronous generators are connected to unloaded lines (inherently capacitive) this can cause over-excitation i.e. voltage surges, hence reactors are connected to the line too. I for one can't provide an easy explanation of why capacitive current aids the stator core excitation, but I can accept that it happens.

The equivalent effect can also be seen in a synchronous motor. If the rotor is over-excited the motor draws capacitive current i.e. it has a positive power factor.