Need help for inrush current managment in relay operation

[shelsd96]

Hi,
I am designing a relay circuit for AC load(inductive). I need inrush current protection design concept, according to the spec there can be 2x65A 200uS inrush current. I am completely new this inrush management. can I use inductor capacitor combination for inrush current management. I dont want to use NTC for this because of it reset time.

 

[enjunear]

No direct experience with this problem, but power thermistors were the one thing that was rattling around in the back of my mind (the other alternative I recall was some big, complex active circuits with triacs). Here's a link to a place that makes the thermistors and discusses their use for in-rush mitigation.
Surge Current Causes and Prevention - Ametherm

 

[shelsd96]

I appreciate your effort Enjunear. But I want more detailed. Assume the load is connected to 230V,50Hz and the max current is 5A. its a inductive load.

 

[varunkant2k]

You may try to use one R and C series combination in parallel to output load. ( RC value u can make equal of inrush current period). This is a very effective method for inrush.
You also can try using negative feed back switch. Where switch can draw maximum currnet at the time of inrush current other wise it will be off.

 

[Syncopator]

I have seen this circuit used to limit inrush current from a d.c. supply - which didn't need D1 of course.

At switch on, the current is limited by the series R. After a time determined by the value of R and C the relay operates and its contacts short out the series R.

 

[shelsd96]

Hi guys thanks for your response and for spending your valuable time for me.
Syncopator the schematic you post, is good i think i may have to rework on it. Let ask you one thing, instead of mechanical relay if I use semiconductor switch what ll happen.?
one thing am going to implemnet is a zero crossing detection i think that will limit the inrush current to some extend but still i need a inductor to avoid the current in the start up .. DO you guys have any idea how to design the inductor inrush current limiter

 

[Syncopator]

Since you are controlling a.c., a semiconductor switch would imply a triac. I don't doubt that it would work, but I prefer the relative simplicity of the relay.

 

[shelsd96]

I have Lenard how the inductor behaves when the current changes through it. But I would like to know how to figure out the component values for it, I mean How to select a Inductor value to limit inrush current.

Suppose the inrush current rating is 100Amps/200uS. 230V AC inductive load

V= L di/dt can give induced voltage in the inductor, But how can I select inductor limit the 100A to a lower value( 5Amps)

 

[varunkant2k]

V= L di/dt can give induced voltage in the inductor, But how can I select inductor limit the 100A to a lower value( 5Amps)

Hi Shelsd96, to reduce Inductor limit either you need lower voltage difference across inductor or increase Inductor value or reduce the ramp-up time.
But all these criteria depends upon the switching frequency and your transient load.
So for design you have to figure out the criteria for ripple voltage and ripple current your design can accept.

 

[Matt-Ametherm]

use a NTC thermistor.

 

[goldsmith]

Dear shelsd96
Hi
one of the good ways that is available , is using a high value series resistor and a triac as bypass ! when your circuit is stable ( after transient time ) your trigging system can trig the triac thus the resistor will be out of your circuit at steady state .
Best Wishes
Goldsmith

 

[tpetar]

If not some secret what size is that relay ? I dont see where is goal of it with relay coil sorry.

You should concentrate concept and question on inrush current of some device/load.

You need softstart.

Usually softstart is used for heavy load to eliminate high inrush currents of loads, such is transformators and other inductors. Just like Goldsmith mention.

Softstart you can make it for small money.





- - - Updated - - -

If you need higher amperage use contactors, there is lots of contactors contact variants.

For sure 2 x 65A is not the job for some fresh relay from market ready to die.

;-)

- - - Updated - - -

In post #5 you have similar thing from Synchopator.

- - - Updated - - -

See triac softstart example, of course you will use adequate power triac and other parts for your design :

**broken link removed**


You should know that triac dissipate power and relay/contactor not (if we dont count relay/contactor coil). And you will need appropriate heatsink for up to 40-60W of triac dissipation.

 

[tpetar]

You can use such this triacs module for higher amperage like 190A-300A-400A and up :

For lower amperage 40-60A use BTA serie in RD91 or TOP3, they are isolated triacs.