# Inverter Surge Capacity -Technique Behind it

An inverter's surge capacity indicates how it handles short-term overload before "tripping". Surge capacity is essential to start up some large loads, especially motors that need 2-3 times their running power to get going. The start period may be very short – a fraction of a second

I clearly know the inverter cannot withstand the 3 times of its load for a steady state current ,it can only a transient .

How do the technique attain ? How the circuit desgin ?

Can i use NTC Thermisitor for this purpose ?

Or can i put a time constant (capacitor for saving the over current for transient) accross the current sensing point at the micro controller section

• Too many questions over a subject that can be complex, or maybe it is simple and your lack of SMPS knowledge is confusing you. – Sparky256 Dec 22 '17 at 4:19

## 2 Answers

There are protection circuits associated with most practical inverters. The simplest protection circuit would just limit the maximum current to the maximum steady-state current. That sort of circuit would protect the inverter but would trip if the inverter was not grossly overrated for loads that have a surge at start, such as motors, compressors and the like.

However, the inverter circuit will not immediately fail due to short term loading somewhat in excess of what it can sustain indefinitely. It takes time for the parts to heat up to the point where damage occurs.

So a more sophisticated protection circuit that trips immediately on heavy overload or short and allows a moderate overload to persist for some time will allow the inverter to be used for such a load. You could use a state observer to model the thermal response if you have a microcontroller, estimating the real-time junction temperature (for example) based on measurable variables, and tripping at the appropriate time.

• i addede some questions – girikks Dec 17 '17 at 4:45
• +1 but... "allows a moderate overload to persist for some time will allow the inverter to be used for such a load." Hmm that is not always the case though, since part of the choking means the terminal voltage falls, a big motor will never accelerate enough and will stay at high start current indefinitely. That can result in the death of the motor and the invertor. – Trevor_G Dec 22 '17 at 15:47
• @Trevor In that case the protection circuit isn't doing a very good job. The point is that the protection circuit can allow brief surges above the maximum steady-state power output. It also has to respond to continuous overloads eventually. Like a slo-blo fuse. – Spehro Pefhany Dec 22 '17 at 16:08
• Yup, I just wanted to point out/add it is possible to get into a rather detrimental lock-up scenario. – Trevor_G Dec 22 '17 at 16:10
• @Trevor Sure, sometimes foldback current limiting can do that in a power supply that feeds things like switching converters that have negative resistance input characteristics, usually without killing anything, but it's still locked up and not working. – Spehro Pefhany Dec 22 '17 at 16:14

Here is for example the datasheet of one very popular inverter power module for low-voltage drives in the several-kW-range: skiip-12ac126v1

In fig.8 one can see that about 90% of Rthjs (both for IGBT and diode) is reached after about 0.4s. Therefore, after 0.4s (or much earlier, dependent on current) an overcurrent will quickly raise junction temperatures above nominal values (e.g. 120°C), and inverter lifetime will go down. If your overcurrent is high enough to raise junction temperatures above 150°C you destroy the power chips immediately.

The shorter the time of the overcurrent, the lower Rthjs, and the lower the rise of junction temperature.

The current controller of the inverter can limit the current. It is up to the designer of the inverter to find a compromise between acceptable overcurrent (size and duration) and inverter lifetime.