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Inverter size and in rush current

hopefulbull

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May 27, 2022
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When looking at sizing an inverter for a well how much of the sizing is to deal with the in rush current?

Do they make aftermarket soft start kits for standard A/C pump motors?
 
Yes, There are a few choices. I'm using a HyperEngineering Brand on a 4 ton Scroll compressor. Works very well and has been reliable for several years.
 
When looking at sizing an inverter for a well how much of the sizing is to deal with the in rush current?

Do they make aftermarket soft start kits for standard A/C pump motors?
It's best to study the specs on the inverter rather than attempt to use a rule of thumb. (EG 3x's the nameplate) I've seen inverters that wouldn't run resistive loads at 75% of thier nameplate and others that would start loads 5X's thier nameplate.

A quality inverter will publish those specs.
 
Is it a well pump motor that you want to start? AC well pumps with a start capacitor and a relay box are harder to start than the type that just have a built in capacitor (permanent split capacitor...PSC). The term "inrush current" is more fitting for a capacitor than a motor. An AC motor has a starting current that peaks right at the start to a value known as Locked Rotor Amps and then tapers off as the speed comes up. If the inverter shuts down when it sees LRAs, it never gets a chance to even try to start. So you must be able to deliver LRA for a half second or so and then sustain something less for the rest of the start.

There are two main type of inverters, low frequency and high frequency. Both produce 50 or 60 cycles but low frequency switch power to the input of a large iron transformer where high frequency work much like a switching power supply. Bottom line is low frequency tend to work better for motor starts.

I have a low cost low frequency inverter that is 230v rated at about 3000w but will deliver about 6000w for a motor start. It started two of my three 1/2hp pumps. The one that wouldn't start had one of those starter boxes and the relay just hummed. Recently I have installed 2, 3000w Growatt (high frequency type) in 230v split phase configuration. I have tested with one of the PSC type pumps and it worked fine.
 
If the inverter shuts down when it sees LRAs, it never gets a chance to even try to start. So you must be able to deliver LRA for a half second or so and then sustain something less for the rest of the start.
Actually you don't need to deliver LRA if a Soft Starter is installed.
 
Actually you don't need to deliver LRA if a Soft Starter is installed.
That is true, if the motor/load will indeed start. A soft starter limits the starting current and thus reduces the starting torque. So it will be motor design and load dependent on weather it actually works or not. Single phase well pumps can be WEIRD.

This actually happened to me. I bought a new pump and installed it. It wouldn't start. It was about 220 feet down in the well. As I pulled it up, I tested it again but at about 100 feet. It worked fine. Tried it again at 220, and No, wouldn't start. I called the company and after some time talking to them the guy asked, "What is your well diameter?" I said, 6 inch. He said, "Well that pump is for a 4" well, you do you have a sleeve on it?" (no....are you kidding?) I have worked for a major US electric motor MFG for all of my adult life and never imagined some start condition where a pump would need a restricted flow intake (by design). I pull the pump, put a 4 foot long piece of 4" PVC pipe (hung from the top of the pump) over the pump so it would think it was in a 4" well, and low and behold, it started and is fine to this day.

So when you start altering the start conditions, be ready for anything. I am sure soft starters will work just fine in many cases but I would not buy an inverter sized for a soft started motor until the motor was already starting okay on the starter and I knew what the start current curve looked like.
 
All great info. Thank you for the replies. Did stang the inverter it sounds like I need to start with adding a shift start device. Next measure my LRA (locked rotor amps) with the study start. Then I can Suze the inverter. Does that sound correct?

What kind of soft starts are people using? How long have you had them in service? Has anybody used a Hyper Engineering study start on a well?
 
All great info. Thank you for the replies. Did stang the inverter it sounds like I need to start with adding a shift start device. Next measure my LRA (locked rotor amps) with the study start. Then I can Suze the inverter. Does that sound correct?

What kind of soft starts are people using? How long have you had them in service? Has anybody used a Hyper Engineering study start on a well?
What is the HP rating of the pump? Does it have a starter box above ground and three wires going down the well?
 
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