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softstart after 3 phase bridge

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what solutions exist for limiting the current for charging the capacitors after 3 phase diode bridge in 380Vrms 3 phase system?
please give some pdf.

Average DC current of inverter load is about 40A.
 

Hi
You will have two way :
1- limitation before rectifier with a triac .
2- limitation after rectifier and with a mosfet or IGBT and a PWM . do you have any filter after your diode ? is your rectifier Gertz bridge ?
Best Wishes
Goldsmith
 

The easiest way is to have a second step resistor that is shorted out when the output voltage reaches some value. This is what high power transmitters use on their 25KV 8A high voltage supplies. You can put the resistors (one per phase) on the mains side. As the resistor(s) is/are only in circuit for only 1 or 2 seconds, they need not be capable of carrying their full power for any longer then the 1 or 2 seconds. So as a starting point, 400V at 40 A, so inrush limited to 80A, try 400/80 = 5 ohms, 50 W resistor with a 1 second timer or a voltage sensing circuit. You need high current contacts on your relay !!
Another way would be to use SCRs as your bridge and turn them on, earlier and earlier in the mains cycle until they are fully conducting over the whole cycle.
Frank
 

please give some info on using SCR on DC bus (550VDC ) for softstart and how we can keep on the scr when the load current is zero.

sometime load is working and sometime load is not working,when load is not working current is zero and SCR may become off.
 

please give some info on using SCR on DC bus (550VDC ) for softstart and how we can keep on the scr when the load current is zero.
Click to expand...
Hi again
Why SCR ? why not a simple IGBT ? behavior of SCR at DC is a bit suffering ( you will need another SCR and an snubber network to be able to turn it off at your desired time , and it will be cause of more price and lower frequency of operation ( lower speed of control ) . how much current do you need ?
All the best
Goldsmith
 

for softstart we need to turn on the SCR ,there is no need to OFF.

But at zero current of load scr will be off.

why we use IGBT instead of SCR for softstart?
 

why we use IGBT instead of SCR for softstart?
Click to expand...
Hi again
To increase efficiency and to simplify driver circuits . and to yield more precision .
Good luck
Goldsmith
 

please give a block diagram or pdf on it.
 

please give a block diagram or pdf on it.
Click to expand...
Hi
You can use a series IGBT , with your main power line ( + ) ( after your bridge ) and then a float driver such as IR2117. and then create a PWM , to drive that IGBT , D.C should have changes , from 10 percent , up to 100 percent , it's frequency is up to you , and your desired time delay .
Good luck
Goldsmith
 

After reaching 100% charge ,is there a need to PWM or make IGBT on continuously?


If IGBT is on continuously(after softstart) when load current is zero(load draws current periodically, 2second draws current and 2 seconds draws no current) ,will IGBT remain on?
 

After reaching 100% charge ,is there a need to PWM or make IGBT on continuously?
Click to expand...
Hi again
If VCE of your IGBT , be around +15 volts , it will be on till you give zero volts as VCE . it means if you want after charge , it be still on , you will need a constant voltage across it . ( don't you know how does an IGBT work ?)

All the best
Goldsmith
 

I know IGBT and I used them in H bridge, I need to start a test on IGBT as softstart.
 

I know IGBT and I used them in H bridge, I need to start a test on IGBT as softstart.
Click to expand...
Well
Do you know how to create a PWM which has variation in it's duty cycle ? it should vary from zero or 10 percent up to 100 percent and then be stable at that mode . ok ? do you know how to take average from that function ? you'd better to use an eliminator filter after that . i hope you got the point .
Best Luck
Goldsmith

- - - Updated - - -

By the way , my typing error in post #11 :
VGE of IGBT not CE . sorry because of that error .
 

I know how to generate a PWM using stm32f103 TIM3 and change the duty cycle in interrupt routine.

But I do not understand what is "take average from that function" please if possible explain more.
 

But I do not understand what is "take average from that function" please if possible explain more.
Click to expand...
Hi again ,
Average voltage of a square wave , will given by the equation : 1/T integral over Vsdt from zero up to ton . if you solve it , you'll have D.C*VS where , D.C is duty cycle and VS is , amplitude of square wave . it demonstrates , when D.C is variable , out put voltage of an average taker circuit will be variable too . and what is the average taker ? an LC low pass filter .
You understand ?
Best Wishes
Goldsmith
 

I know the math routine to integrate and get average of a squarewave,but my question is why I need to average ? for what purpose I should average please give your comments

with a link or pdf if possible for more details.
 

I know the math routine to integrate and get average of a squarewave,but my question is why I need to average ?
Click to expand...
Why you need to take average ? because high frequency components of PWm signal , can suffer the load ( especially motors ) . hence a usual term is , using an eliminator filter or as a matter of fact , integrating filter or as a matter of fact three stage LPF for three phase .
As you probably know each motor has a core and of course it made from iron . ( usually ) so , it can't tolerate high frequency components ( i hope you know about magnetic rules and how an induction motor does work . )

Good luck
Goldsmith
 

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