Power Factor

[jzeeff]

In theory, if you have the power factor right, you have less heat in the generator, more efficiency, less vibration and more output capability.

If, for some reason, you are using a lister/generator  to run electric motors, 3 phase will be more efficient.

[oliver90owner]

3 phase will be more efficient

Err - only for 3 phase motors, though!!!

Regards, RAB

[Ian]

Again, many thanks for the replies.

I have now installed a 3 phase STC 7.5 kW alternator (from Listerclones.co.uk). Again, before putting this generator into use, I stripped it down and replaced the bearings with SKF sealed units and epoxied anything that might move about on the rotor, plus replaced all fixings with stainless steel versions. This unit was not "balanced" with any weights or drillings and is branded "Kapur". The rotor is a skewed type and all the stator slots are filled. It is wired at the stator as 3 phases and common neutral. All the crimps in the doghouse were good and the cable interconnects are very substatntial but flexible tinned multistrand type which must be good for 30 amps or so.

Using rectified power from the 3 phase generator, the SMA Sunny Mini Central will not connect to the grid under the standard "PV" MPP setting that it was very happy to do when operated from rectified single phase (it shows "offset" on the display panel which the manual indicates is a failure to synchroinise internal diagnostic settings) - but it works very well in "(Wind) Turbine" mode.

In use, the generator makes a pleasant (normal) humming sound and exhibiting no obvious vibration or imbalance; when outputting the full 6 kW, it hardly gets warm with the case surface temperature only getting about 10 degrees C (18 F)over anbient tempertaure.

Regards,
Ian

[jzeeff]

How warm did the single phase generator get?   I'm sure there is an efficiency gain here, but I have no idea how much.  I've heard 10% better for electric motors.

[Ian]

jzeeff, I have two single phase ST alternators and I have used both at some time. The first one I tried ran very cool (only a few degrees above ambient) and was only taken out of service due to bearing failure and a "clunking" sound when turned over slowly. The second alternator ran much hotter. I do not recall the actual numbers now but it was noticeably hotter - maybe 20 - 30 degrees C hotter than ambient.

jens, now I have gone to 3 phase, I assume the ripple in the full wave bridge rectified DC output to be within the stated <10% specification of the SMA inverter, so I simply take this DC and plug it straight into the inverter (no filter caps, no batteries, etc). I run the Listeroid at 540 RPM and the STC at around 1500 RPM (50 Hz here in UK) with the field rheostat turned right down so I see about 530 volts DC cold, unloaded (about 520 volts DC warm, unloaded) and around 495 - 505 volts DC, loaded.

I hope this helps.

Regards,
Ian

[Tom]

Jens,

I believe most wind turbines run 3 phase "wild" ac. So I'd imagine the output from a 3 phase generator looks rather "tame". With wild ac the voltage and frequency is all over the place depending how hard the wind blows.

[jzeeff]

I agree - an inverter built to take input from a wind generator should work fine being fed by a listeroid generator.

I also agree - if it runs fine without capacitors, don't use them.  They only mess up the PF.

[Doug]

There is always the danger of over thinking things.

That leads to over building things.

Then no matter how cool or well thought out we are left with the fact in many cases we never needed to go to such extremes to solve a problem that was amagined or suspected but not actualy verified and proven.

I'm bad for this I look for places things can go wrong and lead to incidents and accidents, but thats my job......

Best thing to do with a home brew generating set is build it, start it and test it.
If it proves to have some flaws work on them it there are things that are unsafe adress them.

But never forget you complicate things at your own risk

[jzeeff]

I should mention that the "Kill A Watt" meter will display PF.  I believe that this is only the normal kind caused by motors, not the harmonic kind caused by DC power supplies.

So you can get more effective power and efficiency  from your generator by checking this and adjusting with capacitors.

[Doug]

Yes that's a wise investment.

I think a little reading up and some theory books on AC power would be a good companion to the meter

[Jim Mc]

...I believe that this is only the normal kind caused by motors, not the harmonic kind caused by DC power supplies...

Mine reads power factor for both linear (such as a motor) and non-linear (such as electronic ballast) loads quite nicely.  No matter the cause, power factor is always the ratio of real to apparent power. 

[jzeeff]

You are right - it does measure both sinusoidal phase shift (motors)  and harmonic (power supply) power factors. 

[Doug]

You gate an SCR, it causes a phase shift, it delays current flow....
If it quacks like a duck, walks like duck its probably inductive.
And thats as far as I look lol.

[jzeeff]

I would argue that when you start talking about two waveforms that look quite different, phase shift is no longer the right term.

[Doug]

Unless the distortion is causing current to lag voltage all electronics do this to some degree.

Harmonics and distorted waves are a different matter when created by rotating electrical equipment or inverters. In that case you have bigger problems than PF