Hi,

I have posted this under the heading "electric motors" because I believe it is the closest topic I can find on this forum.
The problem I have is this:
I have a tri-phase AC generator,(6 KVA) with a busted AVR.
These AVR's cost a lot of $, and are very good at going "belly up", so I believe I should use a different approach to the field coil regulation.

I have mechanically added a 12 V car alternator to try and solve the problem, but the no-load - load difference is too big, and the voltage of said
car alternator is too low (in spite of the manual of the AC gen telling me it was 12 V)

So, I need to sense the voltage coming out of the generator constantly and crank-up or crank down the field coil voltage accordingly.
In the commercial version, this is done on one phase only,(with a dodgy triac circuit) hence the fact that one can't load these generators asymmetrically.
For now a similar solution would do.

I can either take the supply for the field coil from the generator directly (preferred) or from a separate field coil generator.

Understanding that this panel will/can't or shouldn't supply me with a "here-copy-this-solution" I would very much appreciate your
advise and insights in this problem.
The original AVR was covered in a black kit, that can't be removed, so there was no way I could study the design.
The Net provided some insights but it seems that proper diagrams of the AVR's are a "closely guarded secret"!
So, if you have a diagram of such a devise, even if it would only be suitable for a certain brand or type or mono phase AC generator
or any other wisdom regarding this circuit, I would very much appreciate your input.

many thanks!
Paddy

 

You can step down the output voltage of the generator with a transformer and rectify it. Then use an AVR (or whatever microcontroller you like) to measure the peak value of the rectified voltage and make the appropriate adjustment to the field voltage.

 

mik3-
The AVR he speaks of isn't a uC, its a Automatic or Adjustable Voltage Regulator, I believe.

Paddy lynch-
Did you remove the diode-bridge from the alternator? If not, you will only be getting single phase converted to DC.

The three phase from the alternator can be kept as such by, after removing the diode bridge, running the 3 wires out from there. As for voltage regulation after that point, that will stay in phase individually, I am inexperienced.

Am I correct in my assumption so far? You are looking for 3phase regulation?

You want to sense the voltage and in turn adjust the stator voltage to maintain levels on the output, correct?

 

In the commercial version, this is done on one phase only,(with a dodgy triac circuit) hence the fact that one can't load these generators asymmetrically.

No AVR in the world can regulate individual phase voltage of a generator, if user load the phases with different loading.

Remember there is only one aspect user can change to control the output voltage of all three phases, i.e. excitation, via the field winding.

And why do you have the impression that commercial AVR circuits are dodgy circuit?

 

Thank you all for your input:
The reason why I am convinced that the commercial AVR's are crap (pardon the French) is twofold.
First of all I have busted three of them, while treating them like fragile babies! and making sure all the phases were
loaded equally with ridiculously tiny tolerances!

Secondly the manufacturer of said generators, has now moved to permanent magnets in the generators (under 10 KVA)
just to get rid of the angry (future) customers!
This was done after moving the AVR out of their designated spot, because they couldn't stand any substantial T° increase
or vibration (on behalf of the manufacturer)

No, the AVR is no µ controller but indeed an Automatic Voltage Controller

yes I understand that it can never sense on phase an regulate the load for all three in a different way! (as there is only one field!)

The (tri phase) rectifier was included on the board that I have thrown a way (3 times) so, yes it has been removed.

I am aware of the fact that I am going to have to make an AVR, but at least if I have made it myself I can choose components/a design
that is a lot more tolerant that the commercial stuff that costs me over € 100 a pop!(and gets me nowhere)

So a circuit diagram of one(any one) would really help, to get an idea how the commercial vendors tackle this problem.
I have some ideas, but those combined with the knowledge of the standard circuits might get me there.

So far the reasoning I made is this:
-rectify the output (3-phase rectification)
-this voltage is a measure for the AC output
-this voltage is converted to a variable (V to f) AC signal
-the frequency of this signal will determine the output of a power supply (fed by the generated AC)
-this PSU will feed the field coils

further
I might incorporate a µ controller to regulate the whole process and to control some protective
circuitry.
In this way, I might not have to replace the thing every two weeks!


Paddy

 

It may help us if you would tell us the load(s) being incurred.

What are you connecting to each of the three phases?

 

The load on each of these phases is 1500 Watt, non capacitive, non inductive

Paddy

 

How is it possible that a commercial genset manufacturer can have its AVR failed three times each within such short two weeks period?

I would certainly complain to the manager and demand investigation and compensation for all my monetary losses. Send the failed AVR back for their examination too.

Tell them if nothing is forthcoming from them you will name their product and give details of the failures on a public forum.

 

You should be able to hook standard breakers to the lines to keep the damage from occurring. You treat it as a standard home wiring after that.

If your loads are not changing, I don't see why you would need anything more than a transformer. The genset could be run at full power and the items would use what they need. The breakers would keep excess power from being pulled from each phase, protecting the rest of the unit.

Are your loads AC?

 

doesn't solve my problem, and although it might make me feel a bit better, it still leaves me with no AC!

I do need more then a transformer, as there is control circuitry that switches loads, so the load is always well balanced, and that can't stand the higher AC when the load is switched of (or not on yet)

Also sometimes a similar load (in nature ) of 3 times a 1000 watt might be used.

So basically it has to do the complete "works of a commercial AVR"

Paddy

 

Ahh. you may want a shunt regulator. Or a diversion type controller. That will allow the sudden load changes to be 'slowed'. Any over voltages could be sent to a large resistive load to convert to heat.

 

Well....no
This would no doubt work, but it would have me inserting a large resistive load to
"buffer" the sudden changes.
What I want is that the load changes, which in turn cause the voltage changes are followed by an almost immediate
change in the field coil voltage.
Like the standard AVR works!

Basically it is a "programmable" power supply unit, that takes its "instruction" from the main AC output voltage levels.
I know how this can be achieved, (there are various ways) but I would like to see how this is done in the commercial units.

All commercial units work in a similar, if not the exact same way, so any example would do
Hence my quest for a circuit diagram....

Paddy

 

Patent search is where you can find what your looking for.

 

tried that...nothing detailed enough though...

TNX
Paddy

 

I hope it isn't too late....

If your genset speed controller works well, you don't really need an AVR. In such a case u should use a constant voltage DC supply at rated Exciter Voltage and current. (This is typically much higher than 12V you are using. eg:100V DC and the current is typically 5A to 8A. please check your alternator datasheet). This DC power can be arranged from within the genset by using a transformer on the main output and a rectifier bridge.

In case the speed controller doesn't work properly and the generator shifts its frequency too often (upon changing loads), u need an AVR to keep the output voltage within reasonable bound. This is achieved by increasing or decreasing the magnetic power of rotating field and hence by controlling the current in the rotor winding.

Although commercial AVRs utilize TRIACs or SCRs, however you can achieve the same result by using a power transistor or MOSFET using a PWM circuit.

The basic idea is to keep sensing the output AC voltage and cut off the field current as soon as the voltage crosses upperbound and similarly turn on the field current as soon as the voltage drops below the lower bound.
In no case the field current should increase the mximum rated Exciter current in the datasheet.

 

Hi Paddy, Check out the search & put in, (AC Sensing generator control) on page 4 there is a circuit for an AVR & how i got it apart & rebuilt it. Ime sure there may be some pointers there. Daryl

 

No AVR in the world can regulate individual phase voltage of a generator, if user load the phases with different loading.

Remember there is only one aspect user can change to control the output voltage of all three phases, i.e. excitation, via the field winding.

And why do you have the impression that commercial AVR circuits are dodgy circuit?

I am looking at repairing a 3-phase generator which has 4 slip-rings which I assume is to regulate each of the three phases

 

You should be able to hook standard breakers to the lines to keep the damage from occurring. You treat it as a standard home wiring after that.

If your loads are not changing, I don't see why you would need anything more than a transformer. The genset could be run at full power and the items would use what they need. The breakers would keep excess power from being pulled from each phase, protecting the rest of the unit.

Are your loads AC?

Post #9 is WRONG!! The whole reason a generator needs a voltage regulator is that the voltage will vary with the load and the speed and so the excitation that causes the voltage to be generated must be adjusted to hold a constant voltage. Circuit breakers are a good idea but they will not at all do anything to control the output voltage of the generator.

 

If the loads on the 3-phase generator are fairly constant then there is the option of using a system similar to what was used in older automotive alternator systems. It works like this: A series resistor is used to supply a small amount of current to the field so that some voltage is produced. There is also the normally closed contact of a voltage sensing relay in series with another resistor, to supply additional current to the firld. As the output reaches some preset maximum, the relay opens and the field current is reduced. When the voltage drops a bit the contact closes again and the output starts to increase.
In a more reliable version the relay is replaced by a suitable transistor to switch the field current on and off, and a comparator circuit is used to control the transistor.If the circuit is fast enough and the limits are set correctly then the generator voltage will stay within a few volts of the target level

 

This is a legacy post, resurrected!
If this is a 4 slip ring AC generator then the 3 phase output is from the slip rings, not the excitation source.
For a auto alternator it is for the field.
Max.