View Full Version : How to calculate the inrush current for a inverter

April 6th, 2006, 11:30 AM
Hi All,

I have to find out how much 17x1,5kW and 3x2,2kW inverters use in inrush.

The 1,5kW inverter have needs a power supply capacity of 4,5 kVA and the 2,2kW inverter needs 5,5kVA.

It is 3phase and I have 400V between the phases

Total is 93kVA and gives me 134Amps.
But how do I calculate the inrush current.
With inverters there are a very little inruch current but I do not know the formula of calculating it.

Can someone please help me on that.

Thank you


Gerry M
April 6th, 2006, 12:45 PM
I would check with the drive manufacturer.

April 6th, 2006, 02:09 PM
I would check with the drive manufacturer.

Yes I would do that, but the are closed until to tomorrow and I would like to calculate the total currunt for the switchboard tonight.

April 6th, 2006, 02:13 PM
Well, it shouldn't be greater than a motor used with a contactor so using that number would put you in the ballpark albeit high.

April 6th, 2006, 02:19 PM
Using a drive you should not have inrush current, you will either use it in volt/hertz or scaler, each should have linear output current. And, with the drives being capacitive in nature they also help correct power factor.

April 6th, 2006, 02:23 PM
So when for example the 1,5kW inverter is rated 4,5kVA it would be 6,5A = inrush?But I was thinking that the inverter would have some inrush, because the always requeres som fuses that are bigger than the Ie.
That would be because of some Ist.

April 6th, 2006, 02:50 PM
Because inverter have caps on their internal DC buss, I gotta believe there will be some inrush current. Question is, how much? Did you check the mfg's website for any online documents?

April 6th, 2006, 02:54 PM
Any info I could get was the power supply regurement in kVA. But no info on inrush.

Think I have to wait til tommorow.

Thanks anyway


April 6th, 2006, 04:15 PM
When i spec in drives I still use the 125% overload to spec in breaker sizes just as a rule of thumb. I have never seen any inrush current on a drive, as a q&a , I always use a fluke 43b power quality analyzer it measure inrush. As for the precharge caps, when power is applied to caps they act as a short ciruit until they reach full charge,so i doubt that they will act as inrush.

April 6th, 2006, 04:45 PM
I guess it depends on how you define inrush. If the bus caps initially act as a short circuit, and draw current accordingly, where does this current come from?

Most of the AC drives I have come across have a precharge circuit built in to the power circuit. This limits the maximum available current level supplied to the bus until the bus voltage is near its maximum level. I have heard numbers like 95% of full charge tossed around. After that point full current is allowed. This is why it is a bad idea to frequently cycle power to an AC drive. you will cook the precharge circuit.

A precharge circuit may be why nswu1 isn't seeing any trouble with inrush current to the drive. Another possibility is that the trip curve on his circuit breaker is such that the short term inrush doesn't cause a problem.


April 6th, 2006, 05:02 PM
...As for the precharge caps, when power is applied to caps they act as a short ciruit until they reach full charge,so i doubt that they will act as inrush.

I define that as inrush. Be it inductive or capacitive.

April 7th, 2006, 07:38 AM
An inverter will have a very small idle current and a specified full load current. The input supply fuses etc are based on that full load value.

When you power up the drive, even tho it is idling, the current will go up to charge the DC bus caps over the normal idling level. It will not go so high as to exceed the full load input current.

As a result, if you size your supply per the instruction manual for full load current, you will not see any higher levels than that for power-up inrush.

The bottom line to this is that there is no higher inrush current to deal with.

April 7th, 2006, 08:09 AM
One word of caution. Many drives that may not have much inrush during normal cycling will have tremendous inrush on power up. Some as high as 2 to 3 times the drive rating. While this won't matter for you running calculations, it will matter depending on what type of circuit protection you use. Fused it will not be a problem. But some circuit breakers will react too quick and trip on power up. I have seen this happen on an application with about 20 drives. Power the system up and every breaker would trip. After continuously turning the breakers on till the capacitors were charged, the breakers would stay on.

April 7th, 2006, 08:37 AM
Dariusch you are correct in your question. These drive will have an inrush current that would blow the fuses IF you size them to the nominal motor specs.

In some plants I work, the electrician have the knowledge to size the fuses for the motor nameplate.

For instance, if the motor nominal current is 4 Amps, they will install a 5 Amps fuse (time delay of course) but when they do this on a system that has Inverters we get a call for the fuses sometimes melt.

We have had arguments with many. Then they will accept installing fuses rated for the nominal current of the Inverter. Same problem, the inrush kills them, specially when the guy switches the main disconnect ON and Off a few time in a short period.

In the Inverter manual there is always a recommended fuse size and this takes into effect the inrush of this particular brand.

I've tested the inrush a few times and its really short, not like a motor starting. We had gotten 22 Amps for less than 0.5 second on a 1.5Kw drive.

To size the main breaker for this is out of my ball park. Sorry. But its a genbuine good question.

We usually install the fuses to protect the wires not the drive. So for a 14 guage wire we install 15A.

Not the best but accepted by inspectors :(

April 7th, 2006, 08:52 AM
What is the drive manufacturer? Have you looked on the internet to see if they have manuals?

April 7th, 2006, 08:54 AM
Just looking at a few drive manuals. They give you the recommended fuse size. One doesn't recommend breakers, the other gives you the breaker rating and type

April 7th, 2006, 10:11 AM
I happen to have an AC Tech drive manual here. They say to size the input fuses at 1.5 times the drive rated input current (covers 150% output for 60 seconds) but no smaller than 10A. The 'no less than 10 amps' clause leads me to believe there is some precharge current to consider.

I think with AB drives the recommended input input fuse size matches up with the supply current for the 1 minute overload rating.

Dariush, all you can really do is use the drive manual to determine the protection level. There really is no consistent way to calculate this number. It's pretty much drive depoendent.


April 7th, 2006, 02:37 PM
Sorry guys but an inrush current into a drive should never exceed the short-term output overload rating of that drive. 22 amps for .5sec on a 1.5kw drive!!!! Something is seriously broke!

How could you ever use semiconductor fusing on such a drive? In fact, I doubt that ordinary Class J fusing at 10amps would work.

A 460V 1.5kw (2hp) drive would likely have a continuous output rating of around 3 amps. 150% overload would raise that to 4.5amp.

Any drive with functioning precharge circuitry on the DC bus will not exceed that 4.5amp figure at 1.5kw. If you've got more than that, you've either got a defective quality drive (the precharge circuit is not working) or you've got a poor-quality drive with no precharge circuit at all. Either way, if it was my responsibility, I'd change the drive out for one that was designed right or was working right.

April 7th, 2006, 02:52 PM
Just one other observation about input currents on small drives from a nasty experience I just had.

Most of you know that I intensely dislike floating delta power supplies. I recently started up a pair of 1hp 460V drives on such a system (I should have know better but the supply tested balanced so I thought it was grounded wye). We disconnected the RF noise filter that is internal to the drive to meet CE listing requirements since this can destroy the drive if the power supply happens to operate corner-grounded even for a short time. As soon as power was turned on and before the drive ever entered run mode, one of the three fuses (10amp) would release. Several fuses later, we decided to lift the drive off the back plate and remove all ground references to the drive.

Now the fuses all held. Checking with an ammeter, we found over 15amps of current ready to flow from the frame of the drive to ground. After further investigation, we finally found that the building had various places and equipment that had leakage from the 460V phases to ground which totalled up to 15+ amps. The virtual ground that the drive must have to do ground fault detection on its output leads apparently was the only available return path to the power network for these leakage currents accounting for the fuse problems.

We added a drive isolation transformer 460/460 delta/wye and grounded the center of the wye. Now the drives had a nice clean forced-balance power supply and no further drive fuses released. Instead, occasionally, now, the 30amp fuses on the input to the isolation transformer release instead. Efforts to get the building owner to eliminate leakages have been unsuccessful.

See why I just detest those floating delta systems. And, for this thread, my point is that the currents that flow may not always be drive related. They may be passing to ground thru the drive ground detection system and fooling everybody! It definitely fooled me for about three months of trial and error---and angry customer!

April 7th, 2006, 11:51 PM
Don't worry about inrush current in the inverter.
Inverter with pre charge capacitor only need few current flow to charged.
So you only need to calculate the total current of the motor and apply the fuse 150 % higher ( NH fuse ).