Generator regulation question

[dave313]

Hello all, I have a question about a brushless single phase AC generator.

I acquired an old light tower with a 240 vac 8 kw Dyna generator. The voltage "regulator" consisted of a tap off the windings thru a diode and then into the field stator winding. The stator resistance is about 3 ohms and I think it runs around 12 vdc or so.

I installed a SX-460 AVR with the appropriate series resistor to meet the minimum 15 ohm required by the regulator. The AVR puts out 4 amps continuous and 6 amps for 10 seconds.

The problem I have run into is that after a 4-5 kw load the field current seems to be maxing out around 4 amps and the regulator starts to current limit dropping the output voltage as the load increases.

My options are to get a larger AVR (some have a 4 ohm minimum and output 7-16 amps) or having the field stator rewound to get 10-15 ohms that most of these regulators seem to like.

Since I do not know what the max field current is on the stator, a larger AVR might burn out the field. Also if I rewind the stator I do not know what "surprises" may be in store for me.

Having little experience with this kind if generator I would appreciate any recommendations or advice you can offer.

Thank you

 

[waross]

Do you see any capacitors anywhere? That is in addition to the lamp ballast caps.
Many of the light tower generators use a capacitor scheme to excite the generator. To raise the voltage, add more caps. The mfd to Volts out relationship is fairly linear in the range of interest.
There are several unique voltage regulation schemes that are not compatible with an AVR.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[iceworm]

Disclaimer: I've been involved with a lot of generators/AVRs - just none of them were single phase. However, I am an avid build anything I want at home. So the following is the direction I would head if it were me.

Q1: You use the term "field stator winding" Does this gen not use a rotating field with stationary output windings? Certainly possible. I've never seen one that wasn't DC.

Q2: If this is a rotating field, does it use brushes?

Q3: Do you know if the generator was working when you got it? Would it put out full load at full voltage?

Maybe the brushes are bad. Or the field winding is shorted. You really need to know the alternator is good.

In any case, looking at the issue as you stated:
I'm guessing the point of using an external voltage regulator is to stabilize the no-load/full-load output voltage.

First you need some information on exactly what the gen field requires:
I would measure the field voltage and current with the AVR/Series Resistor as it is now.
Consider reinstalling the original resistor and tap and measure the field voltage and current at full load?
Possibly the AVR is limiting on output voltage and is not putting out the required 4A (or the field needs more than 4A at full load)

A larger AVR would not concern me. It should not put out any more current than the field needs to keep the output voltage up. - unless the AVR fails and goes to full output. The ones I work with have adjustments to limit the output current. I don't know if the ones you are using have these adjustments. Perhaps they are limited by the value of the series resistor.

Disclaimer: I don't know anything about winding/rewinding generators.
As for rewinding the field: Sure. The field heed a specific magnetic field to make full voltage at full load. That is just ampere-turns. You want less amps - it needs more turns. Which means smaller wire - has to fit in the same size slot. But now you are designing generator magnetic circuits - Be out of my league.

Considering the economics of rewinding versus changing voltage regulators - I'd be looking at:
1. Figuring out exactly the requirements of the field at full load
2. Getting the regulator that will do that.
3. Design protection to limit current in the event the AVR fails.

Nice science fair project. And you are the engineer of record.

ice



Harmless flakes working together can unleash an avalanche of destruction

 

[dave313]

Thank you for the quick replies.

waross: There are no caps in the circuit, the lighting ballasts have been removed. The original way the gen was excited was a tap off one of the windings and a diode to give the field low voltage dc. As a light tower with 4kw worth of lights (static load), the diode regulation worked well but in a dynamic setting the diode regulation is worthless. The generator output is good with the AVR I have but regulation above 4-5 kw drops off.

iceworm: This is a rotating field brushless generator. I never had it produce full voltage at full load. With the diode regulation, OC voltage is about 260 vac and under a 5kw load the output would drop to about 210 vac. With the AVR it produces 240vac until about 4-5kw then the output voltage starts to drop off because the AVR is maxed out.

I use this generator as a backup to an off grid solar powered home. This gen charges the batteries thru 2 inverters and any other ac loads when running. It has served me well for 3 or so years now.

I know that I need more field current to reach full output but my concern is the field excitation limits. Since I do not know how much current the field can carry, I worry if I get a larger AVR I might fry the windings and then I will have to rewind the thing anyway.

The next time I am at the generator I will get the current and voltage readings at given loads and let you know but it will be a couple of weeks at least.

One of the things that bugs me about my current setup is the series resistor needed for the AVR. If 4 amps is running thru the 3 ohm field then 4 amps is running thru the 12 ohm resistor. I think the AVR output voltage maxes out around 60 volts which equates to almost 200 watts dissipated thru the 12 ohm resistor. Seems like a lot of wasted power.

Thanks again for your replies.

 

[waross]

The ballast capacitors were probably boosting the voltage. My light tower was capacitor excited but when used as a standby source with the lights off and the ballast caps out of the circuit, the voltage was low.
I digress.
This solution will probably work.
I'll let you do the math and some cut and try.
Get a small do-nut CT. A 50:5 Amp CT is a good starting place.
You will probably need more than 5 Amps. No problem. wind 10 turns of suitable wire in the proper direction and connect it in parallel with the existing 5 Amp winding. You can then wind more turns of the secondary wire to change the ratio.
Here's where the math comes in.
Determine how much current is needed through the brushless exciter field to give full voltage at full load.
Configure your CT to develop the required current when full load current is passing through the CT window.
You may use more than one pass through of the primary conductor.
Use the diode system to develop voltage at low load levels. Connect the CT excitation system through another diode so that the CT system takes over and increases the excitation as the load increases.
An alternate method is to drop the CT current across a suitable resistor. Use a bridge to develop DC and connect the resulting DC in series with the diode system. As the load increases the CT system will increase the excitation in proportion to the load current.
I have seen this system used on large commercial sets. The largest was 400 kW/500 KVA.
It was a long time ago and I don't have the diagram any more. But it can be made to work and is lot cheaper than an SX-460.
You should get around 5% regulation with the added advantage that you can over excite to compensate for line loss.
A note on changing CT ratios.
We set up the CTs for a SCADA system. The constraints that we were given:
For all motors, a 5 Amp input shall equal an indication of 150% full load current.
For all small motors, 100:5 Amp CTs shall be used.
To get the proper ratio, turns were added on both the primary and on the secondary. In some cases secondary turns were reverse wound to cancel turns.
We were faced with problems such as:
A motor has a full load current of 8 Amps. 150% is 12 Amps. By adding primary turns and adding or subtracting secondary turns, onvert a 100:5 Amp CT to a 12:5 Amp CT.
The point is, do-nut CTs are incredibly versatile.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

Bill,

is lot cheaper than an SX-460

But SX-460's are only like $25..


Aren't light towers like (4) 1000W metal halides? Why would the maker use an 8kW generator if they're so cheap they use a diode for the regulation?

My point is are you sure dave94143 that it's an 8kW generator, because you'd be having the same problem with a 4kW generator you're trying to get 5+kW out of.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Hi Keith. What a price shock. I just checked Ebay. As little as $19.95. I haven't bought an SX-460 for over 10 years. They used to run $200 to $400.
The light towers typically have four 1000 Watt fixtures. 6 kW sets are common.
8 kW sets are not uncommon. They are chosen when it is anticipated that there may be a need for some auxiliary power as well as lighting.
As you know, caps can be used to excite an induction generator. Caps will also boost the voltage of a synchronous generator. It is common for the voltage to drop on these sets when the fixtures are off.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dave313]

itsmoked: The nameplate states that it is 8kw even though it had 4 1kw lights.

waross: I like the idea of using a CT to regulate the output. There must be a downside to this somewhere. I have attached some info about my generator, it's not the exact model but it's close.

Dave

 

[waross]

The downside is cost. Despite the ridiculous prices charged for replacement AVRs, the cost of the AVR to the gen set manufacturer must be less than the cost of the hardware for a CT based excitation system.
The factory scheme used a constant voltage transformer to provide basic excitation and the CT based system to add load compensation.
Add to that a couple of adjustable wire wound or ribbon resistors to derive a voltage from the current.

Another application of CT excitation is field forcing. Voltage collapse under fault conditions is a recognized characteristic of generators. Voltage collapse leads to insufficient current to trip breakers under fault conditions. A common circuit for larger diesel generators is a CT based boost circuit. The fault current through a CT easily develops enough secondary voltage across a resistor to boost the excitation and increase the voltage and current under fault conditions.
These systems often used a conventional AVR for fine voltage control with the boost adding excitation voltage under fault conditions.
If you can find a boost circuit diagram, it will work for your application.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[iceworm]

Hello all, I have a question about a brushless single phase AC generator.

Tell me you added the part about it being brushless.
I really try hard to read the posts. I'll be minorly irritated with me if I missed that.

Okay - I'm over that.

Maybe, as Keith and Bill said, it is just a 5KW generator end - regardless of the nameplate.

As for the power loss in the resistor:
Yes, that is what I saw. I looked at the sx460 specs, 90V, 6A for 10 sec, 4A continuous. Into a 15ohm load, 4A => 60V => 192W for the 12 ohm resistor. In engineering terms, that's one honking resistor.

So where to go from here?
If you need the 8KW and burning up the windings in testing is an option - go for it. Jack up the field current. Bill's solution is certainly elegant - have not seen that before - nice.

The option to add caps to boost the voltage is a good idea, but consider the output current goes up, adding heat to the generator end, but you are not getting any useful power for the added current. That's making the assumption your load is essentially unity pf.

My reasoning is:
As a light tower, the load is 4KW, but highly leading reactive. So the field current was purposely limited because the fixture caps will boost the voltage. Along with the boost in voltage comes the reactive current adding to the real power current (yes, trig is involved) Maybe that is why there is an oversize generator end for the load.
Just curious: Is the generator showing any signs of running out of horsepower when loaded up to 5KW? Have you tried to push the load up to 8KW to see what the voltage goes to?

However, I'm thinking, if it is a 5KW unit, rewinding the field is not going to change that.

As I said, nice science fair project. I'm interested in how it comes out.



Harmless flakes working together can unleash an avalanche of destruction

 

[waross]

Hi ice. A couple of points for your interest.
A few years back I was in the market for a light tower. I must have looked at over a hundred specs on kijiji and various auction sites.
For a tower with 4 x 1000 Watt fixtures a 6 KW set was pretty well standard.
However there were some with 8 KW sets.
No problem accepting an 8 KW generator end.
These sets are generally overpowered so KVA = KW. The limit is not the rated KW but the KVA, even if the KVA is not listed on the nameplate remember
: Assume 100% PF and KVA = KW.
Normally the KVA is 25% higher than the KW.
Power factor. Although most ballasts tend to have a lagging power factor, these ballasts may have added capacity to intentionally boost the excitation.
Don't forget; These are purpose designed and built machines. No problem for the maker to bulk order ballasts with greater than normal capacity.
Rewinding is an interesting concept.
The only part that would need rewinding would be the field of the brushless exciter.
That is fairly small and should be easy to rewind.
An interesting factoid about the number of turns on an magnet. Amp turns depends most on the size of the wire used for the winding.
More turns will not increase the strength of the electro magnet.
Example:
One turn of a given size wire equals 1 Ohm. With 100 volts applied, the current will be 100 Amps. 1 Turn x 100 Amp. 100 Amp turns.
Two turns of the same size wire equals 2 Ohms. With 100 volts applied, the current will be 50 Amps. 2 turns x 50 Amps. 100 Amp turns.
Fifty turns of the same size wire equals 50 Ohms. With 100 volts applied, the current will be 2 Amps. 50 turns x 2 Amps. 100 Amp turns.
More turns of the same size wire will increase the resistance and lower the current but there is usually not enough space in the slots for that.
More turns of a smaller wire will increase the resistance but the voltage must be increased to maintain the Amp turns.
Remember, twice the number of turns will be four times the inductance with a corresponding greater time constant. To much increased induction may affect stability.
Myself I would either change AVRs or use a CT circuit. It depends.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dave313]

iceworm: Sorry, I did not add "brushless" to my first post. I did add an attachment a few posts back that explains a little about what I have. I really don't need all 8kw but 6kw is not out of the question. The gen is paired with a 2 cylinder diesel engine and it does drop a few RPM when loaded, I think it goes from 61hz OC to about 59hz at 4-5kw. Unfortunately I have not loaded it to 8kw as yet, that will be a good test.

waross: OK, you convinced me not to rewind the field but I could remove it and see if I can get a measurement on the windings, that will give an indication of the current carrying capabilities. If I think the field can tolerate more amps I will just go with a bigger AVR. The CT sounds interesting but experimenting with that sounds like more trouble than it's worth. AVR's have other advantages like low hz - over hz cutout.

I was leaning toward a bigger AVR anyway but I always like to hear other options. One of the problems are the inverters, when charging the batteries they switch around 20khz and that causes other problems with some of my electronics. Also when max charging is taking place, the line voltage at the inverters drops to the lower limit (110vac) and reduces the charge output to stay at that level, there is a similar drop at the gen.

I forgot to mention that I use the 240vac from the gen and run it to a 240 to 120v transformer because of the several hundred feet between the gen and the inverters. Before I had this gen I was using a little 5kw gen and I would change the taps at the transformer to make up for the line loss.

Dave

 

[iceworm]

Dave94 -
Did you look close at the schematic you posted?
The connection to F1, F2 really looks strange.

Harmless flakes working together can unleash an avalanche of destruction

 

[dave313]

iceworm: I noticed that too. On the gen it's wired the same way, it seems that they are only using one diode of the bridge. They use the same bridge throughout the gen. With the AVR in place, the bridge is just a backup.

Dave

 

[waross]

Typographical error or drafting error.
The AC corner of the bridge is grounded and the F2 connection should cross without connection to the - bottom corner of the bridge.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[iceworm]

dave94 -
Yes, that is what I saw. Odd they would use half-wave rectification.

Bill -
Yes that is the other guess.

Other issues:
F2 is showing a ground, which puts the F2 return current back to the Bat negative and also back to the neutral. In retrospect, that's okay. The battery current returns to the battery; and the current developed by the two windings tapped onto the neutral, returns to the neutral.

I'm not seeing what the two windings tapped off the neutral are doing. Any clues?

I'm also seeing a zener across the Gen field. Any chance that is conducting too soon - limiting the current to the gen field? Where I'm headed on this one is maybe check the rotating diodes. I don't know about small generation, but it does come up on the stuff I work. It is something we check every year (or maybe every three years - can't remember)

ice



Harmless flakes working together can unleash an avalanche of destruction

 

[dave313]

There was a 2 to 1 spade lug adapter on the AC terminal in question. It seems that they used that instead a terminal block for the wiring.

The windings off the neutral: The first winding goes to charge the starting battery and the second is for the field.

The zener is used as a surge suppressor from what I remember. It could be going bad but they usually fail shorted. I am going to see if I can get part numbers off the components and replace the whole lot. They are all connected with tarnished spade lugs so I will solder in the new parts to avoid this BS in the future. Good clean connections are always the way to go.

Dave

 

[iceworm]

2 to 1 spade lug adapter on the AC terminal in question:
So the F1,F2 rectifier is connected half-wave?

Windings:
Okay - see it now. Missed the diode next to the battery - aaarggg. So the left winding supplies a half-wave rectified 12V (nominal) to the battery. Both windings, in series, supply field current to the F1,F2 rectifier - maybe 24V.

Yes, verify the F1,F2 voltage and current. If the voltage is 24V, the current to F1,F2 could get up in the 8A range. I don't know what the half-wave rectification would do to that.

Zener:
Of course you are correct. Failure mode would be short, likely quickly followed by open. Still, I would recommend checking the Rotating Exciter diodes. I don't know about small generation, but we have had a few of these open on our stuff.

I can see why you want a real voltage regulator.

Keep us posted. I'm interested.

ice

Harmless flakes working together can unleash an avalanche of destruction

 

[waross]

Yes, if you can find a suitable regulator at a reasonable price that will be the way to go.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dave313]

ice: F1 and F2 are running half wave with the diode. The windings act real funny too. If I remember correctly, the battery winding puts out about 16-18 vac OC no load and when I run a 4kw load (with the AVR) that goes to around 22 vac. The field winding ends up providing around 12 vdc to the field stator.

When I first got the thing, the output was somewhere around 180 vac. One of the rotating diodes was bad and there was an unused diode pair on that bridge so I used that to get to full voltage.

In a few weeks when I have the time to visit the generator, I will get a bunch of measurements at different loads, AVR vs diode and let you guys know. With these readings I should be able to calculate the field current requirements at different load levels.

Thanks again for all your help.

Dave