capacitor control in 2 pole alternaltor

[bahnfeldt]

i have a diesel driven 2 pole alternator (single pahse 230volt 50hz0running at 3000rpm it is bruhlesss and has a capacitor across the windings what is this cap for is it like a motor whinch has a cap for a start winding or aux winding

my inverter charger will on turn in the battery charger when connected to it i have anonther battery charger connected to the genset and it works fine although it has no elctronices to worry about

as it is abuld charger

the sine wave on the scope looks it bit peaky

not a nice as off main grid

is there any thing i can do to give a better wave form so that the eletronics in my inverter will see ot and turn battery charger on

the genset is rated at 2.7kva has noi engine governor as such

[Ungrounded Lightning Rod]

i have a diesel driven 2 pole alternator (single pahse 230volt 50hz0running at 3000rpm it is bruhlesss and has a capacitor across the windings what is this cap for

It provides the excitation and sets the frequency.

The capacitor plus the inductance of the winding form a "tank circuit" - a resonant circuit that "rings" at a particular frequency.

What happens next depends on the type of rotor you have in the alternator.  (I'll describe it as if it were a squirrel cage, then delta the description for a wound-coil-diode variant.)

The ringing coil/capacitor combination produces an alternating magnetic field.  If the rotor is turning at roughly the frequency of the ringing, the magnetic field from the ringing current will try to magnetize the rotor with a pair of poles.  If the motor is trying to push the rotor faster than the speed that would exactly match the ringing, the rotating magnetized rotor will push against the field (which will resist it) and pump energy into the rining (which will make it stronger).

Meanwhile, the slippage between the slightly-faster rotor and the field from the ringing coil "drags" the magnetization through the rotor.  This induces a (large) current in the squirrel-cage conductors.  This current maintains the field and resists its motion through the rotor.

Once started up (by a trace of residual magnetization in the rotor and its motion at or above the critical rate), the ringing and magnetization build up in an avalanche until they are limited by the "saturation" of the magnetic core material of the rotor and/or the stator.  (This provides the voltage regulation.)  After that point the rotation maintains the magnetization in the rotor and the ringing coil, pumping energy from the motor (by drag on the shaft) into the currents in the tank circuit and the squirrel cage to replace the energy lost to resistive heating.

With the rotor magnetized the correct amount and rotating at the correct speed, its magnetic field also generates a voltage in the output winding.  Current can be pulled from there, and when it is the current resists the motion of the rotor - forcing the motor to provide the energy that drives the current into the load.

If the load pulls two much current, one of two things will happen:  The rotor will slow down below the critical rotation rate to keep the tank pumped, and/or the load will start reducing the magnetic field of the rotor.  Pull too much current and energy isn't pumped into the tank fast enough to replace what is lost to resistance.  The ringing collapses, the rotor's magnetic field collapses, and the output voltage drops to almost zero.  (This limits the output current.)  Reduce the load sufficiently and the ringing will build up again.

Turning off the motor while the load is still attached MAY completely demagnitize the rotor as the generator slows down.  Then you have nothing to get the process restarted again the next time you start up.  You can restart things by "flashing" the rotor - driving a current through the output windings by connecting one wire to one side of a battery and striking the other wire against the other battery post.  This may work if you do it when the genny is stopped (creating some residual magnetization in the rotor), and it WILL work if you do it while the motor is running (getting current going in the rotor and starting the ringing-pumping before it collapses).  But use care if flashing when the motor is running, because the generator will go to full output in a fraction of a second, leaving you holding hot wires and/or with the hot wires stuck to the battery.

Squirrel cage rotors work.  But they have problems.  The big one is that they are lossier than they need to be (due to the slippage of the mag field through them).  Another rotor type has a coil of wire to create a magnetic pole pair at a particular place in the rotor, with a diode across the end of the coil.  The diode rectifies energy from harmonics and off-speed motion to get a pole started at a particular place.  Then the shape of the pole pieces combined with the current keep it pinned there, without slippage, as the ringing builds its strength.  Absent the slippage the excitation circuit only steals enough power to keep its coil ringing - you lose the losses that would otherwise drive the currents in the squirrel cage.

[Ungrounded Lightning Rod]

is it [the cap] like a motor whinch has a cap for a start winding or aux winding

No.  In a capacitor-start or capacitor-run the capacitor is connected in series with the coil and the series pair is hung across the input power, along with the run winding.

The capacitor causes the current in the aux winding to "lead" the current in the run winding.  (Ideally, it's a quarter-cycle early.)  With only the run winding operating you get an alternating magnetic field.  With the run and aux windings operating, you get two out-of-phase alternating fields at different angles, which amounts to a rotating field.

When the motor is stalled, a one-coil alternating field doesn't really give it a push in either direction.  It will tend to sit there and buzz.  Once it's spinning the rotor will be in the right position to be pulled toward the pole pieces during the high part of a cycle - but as it arrives the half-cycle is ending, and once it passes the next half-cycle builds up a field which again pulls it along.  So a rotating motor will keep rotating, pretty efficiently, unless you drag it to a stop.

A capacitor-start motor only operates the aux field during motor start.  A capacitor-run keeps it operating (typically at a lower strength than with a cap-start) even while the motor is running.  This gives the motor a stronger and smoother pull, since it gets a strong pull at all parts of the cycle, rather than only during two of the four quarter-cycles.

the genset is rated at 2.7kva has noi engine governor as such

Fine control is not needed because once the rotation rate reaches the correct value for the resonant circuit the alternator starts resisting further speedup.

But you may find some throttle control to make the motor drive harder when the load becomes large.

the sine wave on the scope looks it bit peaky not a nice as off main grid

Probably due to the saturation of the core material.

is there any thing i can do to give a better wave form so that the eletronics in my inverter will see ot and turn battery charger on

You can hang a capacitor across the output.  (Just don't make it large enough for the OUTPUT coil to resonate at a lower frequency than the excitation coil, or your output coil will start acting as an exciter and the generator will start running at a lower frequency.)

[bahnfeldt]

thanks lighting that makws sense what can i do to improve the sine wave to make acceptable for the inverters charger to see it and turn on

[bahnfeldt]

thats excellent

i will try the cap across output tonight

is there a lot of nosie in the frequency output because of the cap

i see there is a inverter style gensets being sold now they calm to have aclean output sinewave was woundering trying to build something simliar

[oztules]

Underground lightning Rod,

That is the best explanation I have ever seen on this subject.....

credit to you for this............oztules

[Flux]

I doubt that there is much you can do to improve the waveform, it is generally poor with that type of alternator, anything relying on iron saturation usually has a high in phase 3 rd harmonic ( peaky waveform) It should not be bad enough to confuse your battery charger.

It may be confused by the volts or frequency being out of limit. With these cheap alternators the volts are proportional to speed and if the speed is high the volts may be out of spec for the charger.

Many of these chargers are too clever to work from poor quality power sources. Also the high reactance of many small alternators will make battery chargers perform badly ( the peak of the waveform is lopped off and that is where most chargers extract their energy)

You may find it will work with a significant resistive load on the alternator such as a halogen spotlight but it will not help the fuel economy.

Flux

[bahnfeldt]

thanks flux

that has put to rest the debate some my elctricial colleauges

as a matter of fact the alternator wasn't that cheap and it German made (LOL)

whinch was purchased before the flood of gensets into new zealand of the cheaper chinnese gensets

the ones form china don't seem to perform any worse (for any given size)

kind proves you can't beat elctricial prinpcials where ever these machines are made

intresting eoough i have a stand alone battery charger as well it have has a cap across the ac input to it

when on one on line i have 235 volts ac(with charger switched off) when the charger on online my ac output goes to 256volt ac

i presume the cap in the battery charger causes this harmonic ??

[Flux]

Yes a capacitive load causes a rise in alternator volts. With enough capacitance it will self excite.

Even well designed alternators with AVR control of the field will loose control if too much capacitance is added. These things are generally designed for 0.8 of lagging, you never see ratings quoted for leading power factor.

Flux

[Flux]

Yes a capacitive load causes a rise in alternator volts. With enough capacitance it will self excite.

Even well designed alternators with AVR control of the field will loose control if too much capacitance is added. These things are generally designed for 0.8 of lagging, you never see ratings quoted for leading power factor.

Flux