question

Danakil avatar image
Danakil asked

Compressor Inrush current pulse width

Hi everyone,

Do you have any idea of the duration of compressors inrush current ? I know (from this post: https://community.victronenergy.com/questions/4454/peak-power-of-victron-inverters.html) that a Multi/ Quattro can hold 200% the nominal current for 600ms (30 cycles - 50Hz).

But how long does an inrush peak current last?

I ask this question because in my sizing, I am 70% over the quattros nominal capacity during the inrush current.


Thanks in advance.

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2 Answers
ben avatar image
ben answered ·

There's no single answer. It will depend on the type of motor and the circuitry placed in front of it. You might be able to talk to your manufacturer, but more likely, you'll have to test it yourself.

You're powering a huge compressor with this inverter. It would make more economic sense for you to buy the right size of inverter for the steady-state demand, and then add a soft-start capacitor to reduce the inrush demand as you test and find it necessary.

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Danakil avatar image
Danakil answered ·

Hi Ben,

Thanks for your answer. I kind of expected this answer... But is there any order of time ? Is it more like microseconds, milliseconds or seconds ?


Unfortunately, I can't test it. A customer asked me to size a system and I don't know the compressors models.

It is not just one compressor, there's AC and some resistive load, with an average power factor of 0.9 if everything start together which is OK good.

But if most of the compressors start together, the quattros will be over load during these inrush current.

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ben avatar image ben ♦ commented ·

The inrush current curve is complex and will have different (larger) values in each of those timeframes you mention, typically.

How many ACs do you have? Where did you get the inrush number you are using to arrive at 170% of capacity?

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Danakil avatar image Danakil ben ♦ commented ·

In this set-up, I've got these inductive loads :

  • 10x small AC of 336W
  • 2x medium AC of 2500W
  • 4x pump of 2000W
  • 15x small fans of 55W
  • 4x fans of 120W
  • 1x laundry machine of 3000W but the engine needs 800W to spin

I took a factor 4 for the AC inruch current and 3 for the pump, I've got these numbers from the tables on the internet.

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ben avatar image ben ♦ Danakil commented ·

And what inverters were you planning on going with?

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Danakil avatar image Danakil ben ♦ commented ·

So far, 6x MultiPlus II 5000/48

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ben avatar image ben ♦ Danakil commented ·

Assuming for a moment that these are all on the same phase, my guess is that you will be totally fine unless you have something strange that might encourage a dozen units to start at the exact same time. Even a couple of units spinning up simultaneously is not going to come close to exceeding the surge capacity.

I have four smallish 1500W air conditioners in my setup. Their startup demand is roughly 4kW for about 1.5s. I'm sure in the very first cycle the current inrush is much higher, but our power supplies have "outrush" capacity, too.

I think you're going to be fine, or you're very close. Others with more experience with larger setups might chime in as well.

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Danakil avatar image Danakil ben ♦ commented ·

I also have a 11.5kW resistive load taken with a 60% bulking factor (otherwise 19kW).

The math gave me 21kVA (@60% bulking factor) in normal operation.


1.5 seconds is pretty long ! So according to the doc, I should size at 150% capacity ?

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ben avatar image ben ♦ Danakil commented ·

Oh, so you have a lot more loads, my bad.

I don't know what a "bulking factor" is.

What "doc" are you referring to?

I'm not sure 1.5s is that long given that the demand is only 2.5x. Most of these inrush demands are approximations, a steeply downward-sloping curve averaged over an interval.

Strictly speaking, inrush is quite complex: locked rotor rating, back EMF, stability of the driving power supply, voltage drop, other inductance in the circuit... all of these can affect the interaction, and so we just have rule-of-thumb multipliers to quickly arrive at a good sizing without putting everything on the scope.

For your size system, you need help from someone with more practical experience than I have to validate those rules-of-thumb, I think. (I don't think you're going to find the answers you really want anywhere in the Victron specs.)

Good luck!

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Danakil avatar image Danakil ben ♦ commented ·

Yes that the word I was looking for: a rules-of-thumbs (thanks to improve my english)

The bulking factor is a coefficient based on the probability that all the electrical equipments could run at the same time.


The documentation I was talking about is this one:

https://community.victronenergy.com/questions/4454/peak-power-of-victron-inverters.html


Thanks for your time

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ben avatar image ben ♦ Danakil commented ·

Your English is excellent. Sorry I can't help you further.

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