Homebuilt genset...

[DrivingMissLazy]

On good quality gensets with good electronic regulators, output voltage is controlled almost instantaneously by field current. A good regulator will control the voltage so that you see no movement on an analog voltmeter going from no load to full load.

Dip and overshoot are the two common expressions that define the response of the alternator to application and removal of full load. It may be as much as 20% depending on the design of the alternator and the regulator. Many times I utilized a significantly over sized alternator to reduce this problem.

Frequency is a function of speed and there is always some time lag between applying a load and the engine responds to the governor. Just like your bus engine, you have to set the no load speed a little high so that you have the correct speed at full load.

Correct. Frequency regulation is typically very poor on small sized gensets. On larger units we typically used a gear tooth counter on the flywheel to detect and correct frequency.

Most things are not particularly sensitive to frequency. The frequency from you utility company varies during the day as the load changes. Their target is a 60 hz average over 24 hours. A 2 hz change in frequency will give you a 3.3% change in motor speed but most electronic equipment has an internal power supply that is not sensitive to input frequency.

This was correct in the older times but anymore the frequency is very stable since the electrical grid is connected thru out the country and frequency variations are in the thousands of a hertz and corrected plus or minus to an atomic clock frequency every night at midnight.

Richard

[TomC]

I have a Powertech 10kw powered by a Kubota 4 cylinder and a Marathon brushless alternator (big).  The governor is very sensitive in that it is set at 62 hertz at no load and at full 10kw output is at 59.5.  On any Kubota, there are special springs you can install in the engine governor for maximum rpm control at 1800rpm-original springs are for 3600rpm. 

Setting and having a close control over hertz in alternating current is very important for running motors-like A/C's-otherwise with a slow running generator can cause undue current draw, overheating of the motor, and possible failure of the motor.  Other sensitive appliances are anything electronic.  About the only electrical appliance that is not sensitive to hertz will be a heating element or a light. 

Personally believe that the generator is the heart of the house part of the bus.  There is nothing more frustrating than a generator that is finicky, consumes a lot of your time and money just to keep it running.  On the other hand, having a quality built generator like the Powertech, or Wrico, it is pure pleasure when all you have to do is to change the oil and filter periodically and they just run and run without a second thought.  Do yourself a favor and spend the money on a good quality name brand generator-you'll be thanking yourself every time it is running hours on end with no problems.  Good Luck, TomC

[DrivingMissLazy]

I have a Powertech 10kw powered by a Kubota 4 cylinder and a Marathon brushless alternator (big).  The governor is very sensitive in that it is set at 62 hertz at no load and at full 10kw output is at 59.5.  On any Kubota, there are special springs you can install in the engine governor for maximum rpm control at 1800rpm-original springs are for 3600rpm. 

Setting and having a close control over hertz in alternating current is very important for running motors-like A/C's-otherwise with a slow running generator can cause undue current draw, overheating of the motor, and possible failure of the motor.  Other sensitive appliances are anything electronic.  About the only electrical appliance that is not sensitive to hertz will be a heating element or a light. 

Personally believe that the generator is the heart of the house part of the bus.  There is nothing more frustrating than a generator that is finicky, consumes a lot of your time and money just to keep it running.  On the other hand, having a quality built generator like the Powertech, or Wrico, it is pure pleasure when all you have to do is to change the oil and filter periodically and they just run and run without a second thought.  Do yourself a favor and spend the money on a good quality name brand generator-you'll be thanking yourself every time it is running hours on end with no problems.  Good Luck, TomC

In my opinion Tom, you are 100% right on.

Richard

[Stan]

Richard: One more question on the brushless alternator. Are there two separate rotors or are all the windings in the same slots.

Your comment about 20% voltage swing on load change. I hope that didn't occur for more than 1 or 2 cycles. On a slip ring type rotor, they will correct that fast. I used to rebuild regulators for a company that rented 200 - 600 kw plants and they wouldn't accept the regulator if they could see a 6" panel meter move when they dumped on full load in a load bank.

Many generating stations connect into the grid on DC lines so they are isolated from a standard frequency on the grid. I have a friend who is a shift engineer at a power plant with steam turbines. I will ask him how they control the speed to that kind of tolerance with changing load.  He has told me how many degrees tolerance they have when switching a generator onto the system. (but I don't remember).

[DrivingMissLazy]

Richard: One more question on the brushless alternator. Are there two separate rotors or are all the windings in the same slots.

Yes, they are two completely seperate units on the same shaft. The exciter is significantly smaller and is really another brushless alternator. In the 500 watt range I believe, but it has been many years since I have been involved with them.

Your comment about 20% voltage swing on load change. I hope that didn't occur for more than 1 or 2 cycles. On a slip ring type rotor, they will correct that fast. I used to rebuild regulators for a company that rented 200 - 600 kw plants and they wouldn't accept the regulator if they could see a 6" panel meter move when they dumped on full load in a load bank.

20% is typically worst case and most of the time it is better than that, depending on what the actual load is and how well it will tolerate the dip. Recovery from dip or overshoot was typically measured in milliseconds.

In many cases I might start with an alternator that was double the size required to reduce the dip. In other words I might use a 500KW alternator when the maximum connected load would only be 250KW. Therefore a 100% load change would be 250KW on a 500KW unit. The computers back in the 70's were very sensitive to sudden voltage or frequency changes. I sometimes added a large flywheel to prevent frequency variation when a sudden load was placed on the engine genset.

On AC motor gensets I utilized synchronous motors so that the frequency would not vary if there was a large load change. Synchronous motors run at exactly 1800 rpm regardless of load or voltage change as long as it is within the operating parameters of the motor.

Many generating stations connect into the grid on DC lines so they are isolated from a standard frequency on the grid. I have a friend who is a shift engineer at a power plant with steam turbines. I will ask him how they control the speed to that kind of tolerance with changing load.  He has told me how many degrees tolerance they have when switching a generator onto the system. (but I don't remember).

For more detailed information about the North American Power Grid you might want to review this site:
http://en.wikipedia.org/wiki/North_American_Electric_Reliability_Council

NERC was formed back in the 60's and I was an active member at one time many years ago.
This pretty well describes the system and how all the AC power grids throughout the country are now tied together into one massive grid. Individual power plants no longer have much effect on the overall grid.

Richard

[TomC]

On brushless generators, or alternators, to better picture what they look like, when you take the end cover off the alternator, what you'll see is a small alternator (about the size of a truck alternator, but without the case) mounted on the end of the rotating armature windings.  So the brushless exciting small alternator pumps current into the rotating armature windings that create the magnetic field that is picked up by the stationary field windings mounted in the case that creates the electricity.
On a slip ring type, the battery voltage or 12v is pumped directly through slip rings into the armature, where it creates the magnetic field that is picked up by the surrounding stationary fields.  Good Luck, TomC