How do Inverters behave at their limits....?

lazza

HI Forum

Apologies for having been away for so long. The truth is we've had a load of installations and havent had time to stop. When we actually get paid for all (spanish are notoriously bad for paying up) maybe i'll buy that van I need

Anyway in one of the installations something interesting has come up and i'd like to ask you guys.

We installed a small off-grid system for a weekend rural cottage.. it included an 800W Victron pure sine wave 24V inverter as consumption was pretty low (lights, tele, not much more). However, the owner hadnt mentioned the 1HP water pressure pump he was installing... which meant the 800W inverter was at its limit. In start-up it pulled 63A (1.5kW) and then settled down to 35A (around 800W). This seemed strange as the pump was only 1HP and it appeared to be pulling more than its maximum power irrespective of start-up.

The following week we switched the 800W inverter for the larger 1200W, now that we knew the system would include a water pressure pump. Unsurprisingly, once installed, the inverter had absolutely no problem in powering the pump. But what did surprise me was that when I measure the current draw, it showed that in start-up it was now only pulling 30A (700W) and then settled down to around 20A (500W).

My conclusion is that the smaller 800W inverter was at its limit and tried pulling more and more current from the batteries to try to provide the power to the pump... anyone know if this is correct, and if not, why did I note such a difference in the power draw from the batteries between the 2 inverters?

Cheers
Larry

Cariboocoot

Re: How do Inverters behave at their limits....?

It's one of those fun things!

First of all, a "1 HP" pump isn't going to draw 746 Watts (1 electrical HP). I've got a 1/3 HP pump that will pull that running. No joke. Depending on the actual install (lift, pipe resistance, pressure, weight of water on the outlet) these things can be all over the board for current draw. My 1 HP septic pump pulls only slightly more Watts than the water pump because it doesn't have to work as hard.

You are quite right that the inverter was working too hard. The closer to maximum load, the efficiency can go down. More current drawn for a given Watt output = more power going to heat. What's more, two "same size" inverters from two different companies can pull different amounts for the same load depending on the individual design.

Margins. Big, wide margins. Lots of tolerance. Plenty of room for "oops". That's what makes 'em work.

westbranch

Re: How do Inverters behave at their limits....?

Interesting, that inverter is 800VA/ 700W at 25*C http://www.victronenergy.com/upload/documents/Datasheet%20-%20Phoenix%20Inverter%20180VA%20-%201200VA%20-%20rev%2005%20-%20EN.pdf
with a peak of 1600W. Doesn't state a time for the peak to last. How long did it take the output to drop to 700W?

BB.

Re: How do Inverters behave at their limits....?

Also remember the Watts vs VA issue:

Power = Watts = Volts*Amps * PF = VA * Cosine of the current angle

So, even if the motor is not drawing 1,500 watts starting, it may be drawing 1,500 VA -- Basically the Inverter still is working just as hard (VA for an inverter output stage is similar to Watts for output stage in terms of AC current)... But the Battery Bank is really delivering Watts only.

It is not unusual for a motor to run at PF = 0.67, and probably a lot worse when starting. And having a surge current of 3-5x rated running current is probably the normal situation too.

If you have an inverter with higher VA output, the pump motor will spend "less time" in the starting current load phase--And be "easier" on all of the hardware.

-Bill

mike95490

Re: How do Inverters behave at their limits....?

My 1/2 hp well pump, at 70' lift, pulls 1,000 w from the inverter (according to the inverter's data panel) You have power factor and real life losses to factor in.

lazza

Re: How do Inverters behave at their limits....?

Thanks for your replies, brilliant as always! That all pretty much adds up. I'll keep a close eye on any installations with pumps in the future and measure their real consumption prior to installing.

Cheers
Larry

Robin Gudgel

Re: How do Inverters behave at their limits....?

How are you measuring Meters are probably not fast enough to give you the real current draw. You also should be looking at the voltage drop while starting the pump. IT is normal to draw 3 or 4 times the run current to get the pump started.

waynefromnscanada

Re: How do Inverters behave at their limits....?

That supposed one HP pump only drawing 500 watts is really interesting. Now the question - - is the 500 watt reading way out of wack, or has the manufacturer greatly exaggerated the HP rating. Please please don't tell me that water pump manufacturers are starting down the same road of exaggeration that home air compressor manufacturers have been on for well over the last decade - - doubling, tripling and more, the real HP ratings to promote sales.

Cariboocoot

Re: How do Inverters behave at their limits....?

Traditionally motor manufacturers have use mechanical HP @ stall rather than any sort sensible rating.

Me I'd use the running HP @ rated speed. That would inevitably be less than the numbers they get now, but it would be more realistic. I'm almost 100% certain that my 5 HP shop vac never consumes 3730 Watts - based on the fact it hasn't tripped a 15 Amp breaker yet.

unicornio

Re: How do Inverters behave at their limits....?

lazza wrote: »

HI Forum

Apologies for having been away for so long. The truth is we've had a load of installations and havent had time to stop. When we actually get paid for all (spanish are notoriously bad for paying up) maybe i'll buy that van I need

hi, lazza, i thing that maybe is a good thing that your customers do not pay you in advance, with the (apparently) very low profile of project execution: the inverter is clearly undersized, and 1200va too ... that facility also requires 2500/3000 VA inverter (and may be a better manufacturer) to going to run smoothly and within acceptable safety margins ...

Believe me, I'm also in Spain and not think it's very wise to generalize an entire country ...so I have no problem with my clients, but I recognize that with the level of assurance that I offer, they have no arguments to not to pay, but certainly we need to hire well, and respect the law ......