Capacitor calculation for DIY stand alone genset

[Chris_DK]

I am building a homemade genset consisting of a Lister SR2 and a belt driven asynchronous motor as generator. I am in no way proficient in the electrical field, so this is also a learning project. I have tried to calculate the right size for capacitors as this will not be connected to the grid.
I've found two different equations on the web.
Equation one returns 108 uF
Equation two returns 62,5 uF

Could anyone say which is correct or come up with a valid equation of size of capacitor, before I start throwing money at useless parts :-).
I am planning to build a redundancy setup with 2 engines and 2 gennys on a common shaft, to mitigate breakdowns and service, so a valid equation would be very nice.
Thanks
Chris

[oldgoat]

Don't forget you will be wiring these in delta so you only need 1/3 of the capacitance. Then the total of 64Ufd sounds about right by my back of the envelope calculations.

[oldgoat]

These values will only be valid for a set load. If it varies or draws reactive power the capacitor bank will have to be varied to compensate.

[Chris_DK]

Thanks,
As I understand I'll need a rheostat for variable loads, is that correct?

[oldgoat]

This type of generator is only useful for fixed loads(preferably resistive) as the regulation changes with load. If you connect reactive loads like motors with a lagging power factor you will have to increase the size of capacitors to compensate. A rheostat won't help
Have a look at https://en.wikipedia.org/wiki/induction_generator

[scott p]

If I am reading that motor tag right the voltage is 400 or 690 volts and what phase

[Chris_DK]

@oldgoat
thanks for the link, my initial idea was to make a genset out of "scrap" parts that are easily and cheaply available and the make a building manual, so anyone with basic skills could build one cheaply without falling in the  same pitfalls that I am already falling into. I can see the idea is not ideal and mayby slightly waste of effort - anyhow I have booked a couple of hours with a professional electro-guy and maybe he can guide me to the right way to do it or convince me to buy an off the shelf generator head.
I am still a bit puzzled as what parts make a off the shell generator do the job, to my understanding those are also based on asynchronous gennys - is that the avr or ..?? Have other people taken this route and succeeded?

[Chris_DK]

If I am reading that motor tag right the voltage is 400 or 690 volts and what phase

Correct me if I am wrong - my electrical knowledge is just above the zero mark and I obviously need to learn more ;-).
I understand the  motor tag this way:
~3 means three phase
delta 400 means three phase 400V
wye 690 means 230V between phases

[oldgoat]

In your proposed setup the armature is made up of a ring of copper bars that are a short circuit. The stator ( the outside windings ) induce a current in these bars (by transformer action and set up a magnetic field in the bars which tries to keep up with the 50 c/s current in the stator. In your setup there only adjustment for load is to change the value of the external capacitor.                                                          In a proper generator the rotor has DC fed to the windings and provides a rotating field. Adjust the current flowing through this and you can control the voltage of the generator usually the voltage is fixed by the regulator and it adjusts the current to keep the voltage constant.    Delta means that the full line voltage ( nominally 415v ) is measured across each winding the The windings are shown as a triangle. The other connection method is Star these connections are shown with the 3 windings going to a common centre connection the voltage between the winding and the centre point is 240v. Google star and delta windings and you will see what I mean.  It is a bit more complicated than what I have described by that is a good starting point

[CS6_owner]

You have a motor with 400V coils.

This means that in delta, grid voltage needs to be 400 volts. This is the "normal" grid voltage in a modern factory.

If you have a 690V grid, you would need to configure the motor in star (is not used).

This is a normal configuration for "new" motors. You can thus use a star/delta configuration to limit the startup currents on a 400V grid.

Asynchronos motor are ideal to use as generator while grid connected. Sadly, this is illegal (for safety reason) in most parts of the world.

It could be used not grid connected (delta configuration), if you have a constant load (in star configuration with 230V loads), like an electrical heater. This electrical heater could be a star configuration of eg 3 1 kW heaters.

Using this motor as generator, you NEED a balanced load across the 3 coils. The way an unbalance would be solved is here not available (the neuter line in the center of the star is used to solve the unbalanced loads). If you would use the motor as generator with a not balanced load (eg 1Kw, 2kW and 3 Kw heater) not grid connected, you would get eg 180V on 1 heater (not a big problem) 260V on another and maybe 290 V on the third (big problem).

If you would have 3 400V heaters, then you could put both generator and load in star configuration. You could then connect both centers of the star with a neuter line, and thus use an unbalanced load. This would give other problems, mainly overheating of the coil that has the largest load and cogging. Not advised, and also not so easy to get 400V monofase heaters.

Using an asynchrone motor as a grid tied generator, you don't need the capacitors. The grid supplies the needed reactive power. And the load is almost always balanced.
Using it standalone, you do need the capacitors. And when you change the load, you need to change the value of the capactor or the voltage and frequency would drift.

Some reading:
http://www.gohz.com/induction-generator-synchronization-with-grid#:~:text=The%20grid%20feeding%20powering%20the,will%20deliver%20power%20in%20sync.
https://www.qsl.net/ns8o/Induction_Generator.html
https://www.smokstak.com/forum/threads/induction-generator-from-synchronous-generator.20046/#post-89860
https://www.smokstak.com/forum/threads/motor-starter-for-induction-generation.161917/
https://www.survivalistboards.com/threads/homemade-induction-generator.24036/
https://www.mytractorforum.com/threads/induction-generator.91257/
http://users.telenet.be/b0y/

PS: Do this on your own risk. I'm not responsable if you get killed...

 

[scott p]

Hats off to CS6-owner for a well said post. Pretty much laid it out. Looks like a single phase motor would, for simplicity’s sake, be a better way to go.

What about a three phase generator, can you run a load off one coil with say a star winding?

[CS6_owner]

Hats off to CS6-owner for a well said post. Pretty much laid it out. Looks like a single phase motor would, for simplicity’s sake, be a better way to go.

What about a three phase generator, can you run a load off one coil with say a star winding?

All I said above is for a 3 phase motor/generator.



If you want to feed a heavy mono phase load, you can use (with a synchrone 3 phase generator) a "dog leg delta". This is used to make sure the generator is balanced. Never use just 1 coil to feed a mono phase load. See: http://server.idemdito.org/techtalk/elec/stroomgroep/monofasig.htm (in dutch)

Apparently you can use a mono phase asynchronous motor as generator. See: http://server.idemdito.org/techtalk/elec/stroomgroep/asynchroon.htm (in dutch)