3000W Chinese Gensets Info.
professor95 wrote: EDIT ADDED 45/5/2013- When this thread started in March of 2005, I never expected to see it survive this long or amass the quantity of information that has been shared here. In...
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The response of a generator to load application as a function of the voltage regulation method.
That little 2000W peak generator that I complained about in the other thread did prove to be a benefit to my knowledge base after all. Something seems strange about how it responded to the application of a load. Something that was different from all the others I owned. The instruction booklet showed that it was a non regulated capacitive feedback excitation generator head.
But…….. When I took the end cap off, it had an AVR!!!!!!!!!
It provided an unmatched look at how an AVR style system responded to load transits. The generators small rotating mass amplified how it responded to small load changes. And it proved to be the answer as to why the generator could not handle the application of a 1200W resistive load. When I done thorough power test with a variac controlled load, I could smoothly test the outer limits of it’s operation and I found the answer to the problem.
The torque curve of a gasoline engine drops off when you move away from the optimal RPM range.
The function of the AVR will naturally increase excitation in response to a load increase to counteract output winding resistance.
The function of the AVR will also naturally increase excitation in response to a decrease in prime mover RPM. That is because…… With a constant excitation field, a generator output voltage will drop as the RPM drops. So, to maintain voltage, it increases excitation as the RPM drops.
The drive torque of a generator is the function of the RPM and the output wattage. Or roughly, TQ=W/RPM
On a resistive load, the wattage it pulls is constant as long as the RMS voltage stays the same, irrelevant of line frequency.
When RPM falls, The voltage stays the same. The wattage pulled by the load stays the same. That results in a dramatic increase in driving torque required to drive the generator head. While, at the same time, the engine’s torque output capability drops with the falling RPM.
It should be obvious that that creates a positive feedback situation. Dropping RPM, more load torque, less supply torque, which results in even less RPM. Final result, a stall.
On a non regulated head, the voltage drops when the RPM drops. That results in a reduced load, and a reduced drive torque. That is because the wattage of a resistive load drops or increases at the square of voltage. While the voltage output of a generator drops in direct relation to the RPM. So, an unregulated generator running at half rated RPM driving a resistive load, will require one half the torque it would require if it was running at rated RPM. A regulated generator running at half rated RPM will require twice the torque as it would if it was running at rated RPM.
So, how does that cause a problem with my generator?
When you apply a block load to it. It takes time for the governor to catch up. The speed that the engine drops to will be the determining factor as to the question of it’s ability to handle the load application. If the speed drops to a point where the required torque is greater than what the engine can deliver at that RPM, then the engine speed will drop to the point that it stalls. Even if it can easily handle the load when it is slowly applied.
On my generator in question. If I used a variac to slowly apply a 2KW resistive load. I could manage to load it to 1.4kw before it reached the ledge and fell into the feedback loop that results in a total stall. When it is sitting at that knife edge, all it takes is the littlest disturbance to stall it.
When I apply a load all at once, the generator can only handle about 800 watts without stalling.
I can apply 1200W of load if I apply the first 1kw in blocks of 500 and allow the generator to catch up. Then apply the last block of 200W.
When the generator goes over that knife edge and starts dropping into a stall, the only way I can avoid the stall is quickly dropping the load to 500 to 600 watts. Then the generator will recover and I can reapply the load.
If I remove the AVR and apply a regulated DC supply to the excitation field to manually control the output voltage. The disposition of the generator improves greatly. I can drop a 1.2KW load on it with ease and it won’t stall out. The voltage drops at first, but it comes back when the RPM comes up. I can even drop a 1.5kw load, on it and it will handle it. The output voltage drops some, which means I have to up the current to get 1.5kw, but it can handle it.
When I used the variac controlled 2kw load, I could push it past 1.5kw but the engine slowly drops in speed as you apply more load. The engine is obviously at full throttle.
It also seams to handle starting motors a lot easier when it is operating unregulated…. Well…. I was kind of expecting this result. The reactive current that a motor pulls will go up as the line frequency decreases, so long as the voltage stays the same. So, as the generator slows, the motor will start pulling more current, which piled on top of the other things going on with the generator, makes the positive feedback even worse. A non AVR generator will reduce it’s voltage as the speed drops. That prevents the motor from drawing more reactive current as the line frequency drops.
Remember. The metal in an electric motor’s armature is designed to operate at just below saturation. Reducing the line frequency in an inductive circuit caused a greater field to build up in the metal of that inductor. That is why you can not use an American 60hz transformer/motor in the UK 50hz. It will overheat from saturation.
In a non regulated generator, the armature is designed to operate at saturation. That is what regulates it’s voltage. The generator head will always saturate before the motor that it runs will. No mater what the operating speed.
In a regulated generator, the head is not even close to saturation. When the speed drops, and the voltage stays the same, the motors it runs will saturate before it does. That saturation will cause an increased load on the generator system and contribute to the stalling of the generator.
The regulated generator in question can output 190v before it saturates. That means that it can put out 120V all the way down to 37HZ which will cause saturation in almost any inductive motor load or transformer if the engine RPM drops off.
It is not as pronounced on larger generators because they have a larger rotating mass which gives the governor time to catch up.
A small generator with an AVR of this size with it’s low rotating mass is basically useful for running electronics and nothing else because it can not maintain speed with the application of a load over half it’s rated capacity (1500W). When it looses RPM it will avalanche into a complete stall. So it will stall if you apply a block load greater than half it’s rated capacity. The stall is a result of the AVR action compounding the problem of a drop in RPM upon the application of the load.
Now, if I could just find a way to convert it over to a non regulated head. The easiest way would be to build a regulated DC supply and set it to maintain a generator output voltage of 125V when it is unloaded.
That will give me something to think about……….
That little 2000W peak generator that I complained about in the other thread did prove to be a benefit to my knowledge base after all. Something seems strange about how it responded to the application of a load. Something that was different from all the others I owned. The instruction booklet showed that it was a non regulated capacitive feedback excitation generator head.
But…….. When I took the end cap off, it had an AVR!!!!!!!!!
It provided an unmatched look at how an AVR style system responded to load transits. The generators small rotating mass amplified how it responded to small load changes. And it proved to be the answer as to why the generator could not handle the application of a 1200W resistive load. When I done thorough power test with a variac controlled load, I could smoothly test the outer limits of it’s operation and I found the answer to the problem.
The torque curve of a gasoline engine drops off when you move away from the optimal RPM range.
The function of the AVR will naturally increase excitation in response to a load increase to counteract output winding resistance.
The function of the AVR will also naturally increase excitation in response to a decrease in prime mover RPM. That is because…… With a constant excitation field, a generator output voltage will drop as the RPM drops. So, to maintain voltage, it increases excitation as the RPM drops.
The drive torque of a generator is the function of the RPM and the output wattage. Or roughly, TQ=W/RPM
On a resistive load, the wattage it pulls is constant as long as the RMS voltage stays the same, irrelevant of line frequency.
When RPM falls, The voltage stays the same. The wattage pulled by the load stays the same. That results in a dramatic increase in driving torque required to drive the generator head. While, at the same time, the engine’s torque output capability drops with the falling RPM.
It should be obvious that that creates a positive feedback situation. Dropping RPM, more load torque, less supply torque, which results in even less RPM. Final result, a stall.
On a non regulated head, the voltage drops when the RPM drops. That results in a reduced load, and a reduced drive torque. That is because the wattage of a resistive load drops or increases at the square of voltage. While the voltage output of a generator drops in direct relation to the RPM. So, an unregulated generator running at half rated RPM driving a resistive load, will require one half the torque it would require if it was running at rated RPM. A regulated generator running at half rated RPM will require twice the torque as it would if it was running at rated RPM.
So, how does that cause a problem with my generator?
When you apply a block load to it. It takes time for the governor to catch up. The speed that the engine drops to will be the determining factor as to the question of it’s ability to handle the load application. If the speed drops to a point where the required torque is greater than what the engine can deliver at that RPM, then the engine speed will drop to the point that it stalls. Even if it can easily handle the load when it is slowly applied.
On my generator in question. If I used a variac to slowly apply a 2KW resistive load. I could manage to load it to 1.4kw before it reached the ledge and fell into the feedback loop that results in a total stall. When it is sitting at that knife edge, all it takes is the littlest disturbance to stall it.
When I apply a load all at once, the generator can only handle about 800 watts without stalling.
I can apply 1200W of load if I apply the first 1kw in blocks of 500 and allow the generator to catch up. Then apply the last block of 200W.
When the generator goes over that knife edge and starts dropping into a stall, the only way I can avoid the stall is quickly dropping the load to 500 to 600 watts. Then the generator will recover and I can reapply the load.
If I remove the AVR and apply a regulated DC supply to the excitation field to manually control the output voltage. The disposition of the generator improves greatly. I can drop a 1.2KW load on it with ease and it won’t stall out. The voltage drops at first, but it comes back when the RPM comes up. I can even drop a 1.5kw load, on it and it will handle it. The output voltage drops some, which means I have to up the current to get 1.5kw, but it can handle it.
When I used the variac controlled 2kw load, I could push it past 1.5kw but the engine slowly drops in speed as you apply more load. The engine is obviously at full throttle.
It also seams to handle starting motors a lot easier when it is operating unregulated…. Well…. I was kind of expecting this result. The reactive current that a motor pulls will go up as the line frequency decreases, so long as the voltage stays the same. So, as the generator slows, the motor will start pulling more current, which piled on top of the other things going on with the generator, makes the positive feedback even worse. A non AVR generator will reduce it’s voltage as the speed drops. That prevents the motor from drawing more reactive current as the line frequency drops.
Remember. The metal in an electric motor’s armature is designed to operate at just below saturation. Reducing the line frequency in an inductive circuit caused a greater field to build up in the metal of that inductor. That is why you can not use an American 60hz transformer/motor in the UK 50hz. It will overheat from saturation.
In a non regulated generator, the armature is designed to operate at saturation. That is what regulates it’s voltage. The generator head will always saturate before the motor that it runs will. No mater what the operating speed.
In a regulated generator, the head is not even close to saturation. When the speed drops, and the voltage stays the same, the motors it runs will saturate before it does. That saturation will cause an increased load on the generator system and contribute to the stalling of the generator.
The regulated generator in question can output 190v before it saturates. That means that it can put out 120V all the way down to 37HZ which will cause saturation in almost any inductive motor load or transformer if the engine RPM drops off.
It is not as pronounced on larger generators because they have a larger rotating mass which gives the governor time to catch up.
A small generator with an AVR of this size with it’s low rotating mass is basically useful for running electronics and nothing else because it can not maintain speed with the application of a load over half it’s rated capacity (1500W). When it looses RPM it will avalanche into a complete stall. So it will stall if you apply a block load greater than half it’s rated capacity. The stall is a result of the AVR action compounding the problem of a drop in RPM upon the application of the load.
Now, if I could just find a way to convert it over to a non regulated head. The easiest way would be to build a regulated DC supply and set it to maintain a generator output voltage of 125V when it is unloaded.
That will give me something to think about……….
