Lister St 2 Cylinder Diesel Engine For A Generator

I've often wondered why the engine has to run at generator speed, even when only a trickle of 110 is needed. Could there be a sensor that would regulate the speed of the engine through some sort of transmission to turn the generator at the speed needed to supply the load?

Yes, I know it has to go 3600 to supply 110 volts. So. I'll bet some hot dog experimenter could do something solid state to convert low-RPM voltage to low amp 110 voltage, dependent upon draw.

Do this and solve some of the world's energy problems.
 
I've often wondered why the engine has to run at generator speed, even when only a trickle of 110 is needed. Could there be a sensor that would regulate the speed of the engine through some sort of transmission to turn the generator at the speed needed to supply the load?

Yes, I know it has to go 3600 to supply 110 volts. So. I'll bet some hot dog experimenter could do something solid state to convert low-RPM voltage to low amp 110 voltage, dependent upon draw.

Do this and solve some of the world's energy problems.
Actually it needs to run at 3600RPM to supply 60Hz. It would be possible to run the motor at its optimum speed for the load and use what would amount to a sort of VFD to convert what the generator then produced to 120VAC at 60Hz but that would make the generator much more complex and expensive. The savings would also not be very large: those engines are designed to run efficiently at constant RPM with varying load..
 
I've often wondered why the engine has to run at generator speed, even when only a trickle of 110 is needed. Could there be a sensor that would regulate the speed of the engine through some sort of transmission to turn the generator at the speed needed to supply the load?

Yes, I know it has to go 3600 to supply 110 volts. So. I'll bet some hot dog experimenter could do something solid state to convert low-RPM voltage to low amp 110 voltage, dependent upon draw.

Do this and solve some of the world's energy problems.


A conventional generator has to run at a fixed speed to provide 60HZ depending on the number of poles. A 2 pole requires 3600 RPM, a 4 pole requires 1800 RPM and a 6 pole requires 1200 RPM.

It is absolutely possible to do what you describe using an inverter. My Honda interter generator does exactly this. The motor runs at the speed needed to provide output for the load.
 
I may be missing something here, but how the heck do you turn a 3 phase motor into an alternator? You have to generate the magnetic field some way and a standard armature has no windings to create the field. Alternators have a wound rotor that a DC current is applied to, thus generating the magnetic field.

You could use a permanent magnet or wound field DC motor and an inverter. A brushless DC motor will output 3 phase also, they have a permanent magnet rotor.

.
.

You can get a three phase induction motor to 'self excite' and operate as a generator but you need to add a couple of strategically connected capacitors, or alternatively it has to be connected to an existing grid which will not be the case in this situation (if you had a three phase motor running normally off the utility network supply and you make it spin a bit faster it will reverse the power flow and act as a generator). There are some relatively large induction generator - I've seen 3MW ones.

The rated speed of 1450 is because in an induction motor the shaft speed falls below the 50Hz synchronous speed. When operating as a generator you would need to run at around 1550rpm to get 50Hz.

There is a good book "induction motors as generators" by nigel smith which will tell you everything you need to know. It is predominantly for using motor as generator in micro-hydro schemes but the same principles apply.
 
You can get a three phase induction motor to 'self excite' and operate as a generator but you need to add a couple of strategically connected capacitors, or alternatively it has to be connected to an existing grid which will not be the case in this situation (if you had a three phase motor running normally off the utility network supply and you make it spin a bit faster it will reverse the power flow and act as a generator). There are some relatively large induction generator - I've seen 3MW ones.

The rated speed of 1450 is because in an induction motor the shaft speed falls below the 50Hz synchronous speed. When operating as a generator you would need to run at around 1550rpm to get 50Hz.

There is a good book "induction motors as generators" by nigel smith which will tell you everything you need to know. It is predominantly for using motor as generator in micro-hydro schemes but the same principles apply.

I learn something new here every day. Thanks!
 
Even though an alternator produces AC power, internally it needs DC in the fields to produce the magnetic field.

If you disassemble your existing genset, you will find the output of the stationary windings (the "stator") is connected to a rectifier that sends DC into the rotating armature (the "rotor") via a pair of brushes and slip rings. By spinning the stable magnetic field within the stator, AC is induced at the frequency the rotor is spun at. There may be an adjustable voltage regulator associated with the rectifier. By varying the voltage in the rotor, the AC output voltage can be adjusted. The throttle (governor) is adjusted to get the correct frequency.

(Unless you have a brushless design, then there will be a large capacitor connected to 1/2 the output of the stator, an extra winding on the rotor and a rectifier between the two sets of windings on the rotor. Frequency is still engine speed dependent, but the output voltage is controlled by the size of the capacitor.)
Some of the cheaper and more heavily advertised gensets use permanent magnets for the field. The advertising tries make this sound like an advantage.
 
You could spin up a small automotive alt/gen directly from the engine to excite the field.
 
Sam, you raise a number of good points. You have received numerous good responses. I assume you are talking about being off the grid. Strictly speaking the conventional three phase induction motor is not going to work. As others have pointed out, in a grid connected application, a slightly over synchronous induction motor will push energy into the grid. This is called an asynchronous generator. The field excitation gets to the rotor by transformer action (which is just the relative speed difference between the rotating magnetic field of the stator, driven by the 50Hz grid and the speed of the rotor that is a few percent above synchronous speed - similar concept to how a motor works). Without the grid, there is no easy way to energize the rotor. This is a very simple and robust system, but it is limited to what percentage of the overall grid can be provided (as the percentage increases, the controls get more complicated).

The easy approach is to acquire an electrical machine that allows you to create a rotating magnetic field (preferably in a controllable way - but even that is optional, with enough electronics the energy generated can be fixed after the fact). There are a number of electrical machines that will accomplish this, but they are generally described as a "generator" (or some synonym).

The engine. Yes, you certainly can direct connect the generator. That is a very common approach. As other posters have stated, you simply need to operate the generator at the designed speed - so with a 4 pole generator to get 50Hz, the the generator need to operate at 1500 rpm. That is pretty slow for a Lister ST2. This is still not a big deal, you simply need to derate the horse power expectations (probably quite a bit). There is probably a power curve in the manual, or available on line. Remember that for a 10kW generator, you need nearly 15kW of engine, at the target speed (especially if you are starting a good sized motor). To get enough power, you will likely need to run the engine up around 2200rpm, again, check the power curve. Then you need to ratio down to the correct speed for the generator. Of course the common method is with belts and pullies (certainly numerous other options).
That is only one approach. Since you are off grid, run at your own frequency - how about 65Hz? That will make the engine happier. Then you lathe will simply operate faster than what the name plate says, sort of your own mechanical variable frequency drive (don't try too large a change or the controls will not like it). You could also run all the power through an inverter & frequency drive, but those devices do not like being too far off designed frequency (and you will run into quite a financial cost).

Once you get it sorted, let us know how you worked it out.

Regards, David
 
Just a side note...

There are many places one can buy a generator without the engine... only the actual generating part.
One place is the Northern Tool catalogue...

google Northern Tool

This may give you some ideas as to how to set your genset up.

I would work out the HP and RPM necessary, and do a set of pulleys with belts... to get the desired 50 Htz and watts output.
 
i have worked on the lister diesels many times- they are sweet little engines.
they were used for pumping and generator applications in my tenure
 
Back
Top