ssr diversion effects on compressors

[toothy]

Hello

I have a diversion question or maybe I'm just being crazy.

I have 2 Outback VFX3648's and am diverting AC to water heater elements with a SSR from AUX2 waste not high, volts -0.3 width 3. I've read what I can find and watched Chris Olson's video's and think I have it pretty well dialed in. I've gotten it to where it doesn't surge the inverter and only flickers the fluorescent lights a small amount. I have one concern however, I was standing next to my dehumidifier and the compressor kicked on and it made strange noises, like a V-8 running on about 4 cylinders and didn't smooth out.

Does anybody think this style diversion could effect the power to the point where it could do bad things? I have no particular love for the dehumidifier but the fridge and freezer are a different matter. I take it Aux2 waste not high get's me the most output but would some other setting be better from a easy on compressors standpoint and still get me descent harvest?

It is also quite possible I just heard a cheap compressor doing cheap compressor stuff and now I'm having cerebral intercourse with the issue. I just want to quit burning wood for my domestic hot water or at least increase my time between burns.

Thanks
Wade

[ChrisOlson]

You could try using Waste Not Hi on AUX1 with the SSR and set the delay/hold times fairly short.  The continuous cycling on/off won't hurt a SSR and it might stop that problem with your refrigeration compressors.

I think the problem that some folks are having with some inverters is due to the fact that the inverter can't regulate voltage as fast as the SSR switches on/off.  And the on/off cycling of the AC SSR is not as smooth as the PWM would like it to be because they are zero cross - meaning once they start conducting current it can't stop until the sine wave crosses zero.

It could possibly help to use a smaller load too, so the PWM will tend to want to keep it on more instead of trying to regulate the voltage to  the load.
--
Chris

[toothy]

Thanks Chris

Yesterday after I posted, as I have in the past, I tried different settings, with the usual surging and such. I setteled on one 240V 4500 Watt element on Aux 2 at -1.2 v 3 width. That seemed to provide  a 1500w load on the inverter. I have 3/4500 watt 240V elemants in one of my 500 gallon heat storage tanks.

I believe I'm going to abandon this diversion stuff for a bit until I get rid of my oddball X brand 600V charge controller and my  new Classic shows up about mid week, sorry I know you have a certain affinity for some of their hardware, that thing just bends me out of shape.

I have in the past, as you suggested, tried Aux 2 diversion with 1 element wired at 120v as a light load 1.1Kw test and nothing seemed different. I was hoping to maximize the diverted heat, that is the whole point especially up here.

As a side note I kept my ear to the fridge and freezer and there seemed to be no abnormalities, didn't have the dehumidifier on.

Thanks again Chris I'll fiddle about some more as soon as the new controller is up an running

Wade

[ChrisOlson]

For whatever reason I used to use -.7 offset with 24V and 3V width to stop our PSX-240 transformer from humming or making weird noises.  On the 48V system I've gone to -3 offset to get the most power from the arrays and back to using 1V width.

The less the width is, the faster or more infinite the regulation seems to be.  Our new system doesn't have any problem with the 1V width.  But I am using 2,000 watt elements with 240V power and a single pole SSR switching just one leg of the split phase.  I don't know if that makes any difference or not.  I don't think so.  But on a good day there is very little regulation going on during the final stages of absorb - a volt meter on the water heater shows 235-242 VAC, so I think the AUX is just pretty much solid "ON".

When AUX2 first becomes active as the bank voltage rises during Bulk stage I can hear a slight change in sound from it in the PSX-240 (now being used for split-phase leg balancing on the Power Distribution Panel).  If I turn the transformer off then the sound from the inverter changes due to the "pulsing" but again no light flickering.  So I have left it at 1.0V width because it seems to be much faster and more accurate regulation than 3.0V width.

So in conclusion, from when I first started using this feature, I have had to use a different setting on three different inverters we have had in the last six months.  It is obvious that "one does not fit all".
--
Chris

[cpfl]

Chris,

Following on from your comment, I think the "one fit does not fit all" adage is very true. In fact, in my experience, I would say that "one fit does not fit one".

I experimented with PWM on AUX2 with an SSR to my 120v 1500w water heater and I found that my settings of -0.9v offset and 3v width never quite got the balance right. The 3v width was necessary in order to minimize the CFL pulse, so that was set in stone. The -0.9v offset was a compromise. The offset when set too low (closer to 0.0v), would only redirect a small amount of power to the water heater and waste available power. When set too high (closer to -3.0v), it would redirect too much power to the water heater and cause the charging stages to take longer.

In order to extract the optimal amount of power from the panels and minimize impact on the charging stages, I decided to bring some extra hardware into operation.

My initial solution was to have the Raspberry Pi dynamically change the offset every 60 seconds depending upon the current charge state. The states ABSORB and FLOAT result in the offset being decremented by 0.1v (down to -3.0v) increasing the power redirected until the state enters BULK or FLOATM, and the states BULK and FLOATM result in the offset being incremented by 0.1v (up to zero volts) reducing the power redirected until the state enters ABSORB or FLOAT.

It sounds complicated but did seem to substantially optimize the redirection of available power to the water heater.

I believe that an even better solution would be to not monitor the state but monitor the battery voltage and keep it within a specific range of the target voltage by tweaking the offset and using a smaller interval between polls,  but that is an experimental tweak for another day.

Chris.

[ChrisOlson]

I've found, though, that even at -3.0 offset it might be "tickling" the SSR at 3 volts below charge stage but it is not taking much power to the aux load.  It has to get up to about 1 volt below the charge stage setting to really start using any serious power.

The thing is, if you have only marginally "extra" power you need only a marginally sized aux load to use it on the perfect day.  The more extra you have the bigger your aux load can be.  It still takes some "tuning" to match the load to the extra available power.

If you throw too big of an aux load at it, it will do as you describe - lengthen the charge time.  But there's a way around that.  On the average day when it's diverting some power tweak the charge stage setting up until you get actual required absorb voltage with the PWM operating.  Measure the voltage with a DVOM at the bank.  Then in the TWEAKS menu enable Divert Count so it counts down the timer whenever the AUX port is active, even though the voltage might be below the (abnormally high) charge stage setting.
--
Chris

[boB]

I experimented with PWM on AUX2 with an SSR to my 120v 1500w water heater and I found that my settings of -0.9v offset and 3v width never quite got the balance right. The 3v width was necessary in order to minimize the CFL pulse, so that was set in stone.

You might want to check the output voltage of the Magnum inverter somewhat closely.  Either with a Fluke
meter (or equivalent) with a minimum voltage hold function OR an oscilloscope set somewhat slow so
you view the envelope of the 60 Hz output voltage to see if it is a voltage regulation issue.  The ME
inverters are not known for extremely fast regulation like the Outback and some other inverters are.

boB