limiting heavy charging current

[rathin2j]

hello everyone,how r u all? i m doing electrical engg. student and v hv a project work now in final year.... for that i had one query so i though of sharing and discussing with u all experts..

our faculty has given a problem statement that "

v hv to design such a circuit that limits the HEAVY charging current when a highly capacitive load is switched ON..

" according to our teacher this problem of heavy charging current is of high practical significance as many times it results in to a blast in capacitor.......

now frm my side for solution ,i came up with an idea [ though very rookie one ] of using a "zero crossing detector"!!..through this i m planing to implement such a circuit that turns ON the load only when the Input supply is at zero value [instantaneous],so there will b no voltage hence no potential difference and there will b no current!![eureka!! ha..ha..ha.. ]

now my question is ,is such a circuit possible using ZCD and can there b such electromechanical co-ordination? or are there any other ways to achieve such thing!!??

hope this much info is enough..thank u..

 

[rathin2j]

by the way i forgot to mention that the solution to the problem should be as much low cost and as simple as possible. i searched few electronics/power-electronics books and google also but i noticed that most of the circuits i found were using op-amp or a microprocessor/controller..which are i think difficult to interface with mechanical switch or relay... so i think perhaps v might hv to think of thyristor...!!

 

[Nigel Goodwin]

I would suggest you try looking up 'soft start' circuits, very high power audio amplifiers is a classic use for them.

 

[ericgibbs]

by the way i forgot to mention that the solution to the problem should be as much low cost and as simple as possible. i searched few electronics/power-electronics books and google also but i noticed that most of the circuits i found were using op-amp or a microprocessor/controller..which are i think difficult to interface with mechanical switch or relay... so i think perhaps v might hv to think of thyristor...!!

hi,
The ZCD would be the way I would choose, but trying to use a mechanical device at ZC is difficult to maintain ZC.

A thyristor has control only in one direction, current flow is blocked in reverse direction.
Look at the data for Triacs and SSR's [Solid State Relays]

 

[rathin2j]

nigel Goodwin: ok!sir,so u r suggesting "soft starter" and very high audio amplifier... excellent thought,i'l search it and will try to study it... thank u

ericgibbs:- yes sir, the day i received this project my eyes are on ZCD only. ...and in fact it's not necessary to use a mechanical device,i just told it as i wasn't knowing any other better option... because when i thought of thyristor... thoughts of thyristor firing ckt and gating circuits and the limited power rating all came in my mind,and found that it might make things difficult to understand so neglected it temporarily..and yes sir i m in touch of solid state Power devices such as IGBT, GTO, SITs and all.... i mentioned thyristor in general but thanks for pinpointing...

 

[KeepItSimpleStupid]

My initial comment is, that no where did the OP mention what we are talking about. e.g. Mains stepped up/down and being recitified and filtered by capacitors. In fact, AC wasn't mentioned at all, just DC. A better description of the problem is in order.

In any event, way back when thermal dime delay relays based on vacuum tube technology were used long ago. Varistors have been used. Resistors in the AC line with time delay short has also worked.

ZCT is bnot recommened to transformers and I don't quite know the reason why. Part of the problem with an inductive load is that voltage and current can be out of phase. A "dimmer" design for an incadescent lamp is much different than "a motor speed control".

For a lamp, you can basically turn it on at whatever place in the line you want and it will turn off at zero to maintain a particular duty cycle.

For a motor, you have to find the voltage turn on point and continue turning the unit on until the next voltage turn-off point because it will turn off at zero current and that could occur just after you turned it on.

Operating into a highly inductive load generally requires some sort of current limiting. Been there, done that.

I have designed an unconventional turn on system for an audio amp. It effectively had 40,000 uf of capacitance, 50 V rail and a 12 A winding (simplified). The design was not quite what I wanted and I really didn't have the space for another relay, but it works. The only problem is if a fuse blows in any of the rail supplies, I have to replace a resistor. The design worked flawlessley when i accedently reversed the position of the output devices. No damage, except the resistor and up to 5 fuses (4 DC supplies and a line fuse).

 

[Gary B]

The traditional solution is a ramp and pedestal soft start circuit. The ones I used to work on were built around a UJT that supplied the firing pulse to the phase controlled SCR pre-regulator. We are talking 5 or 6 components max. for the whole circuit.

 

[rathin2j]

i pardon my long silence,i was out of town and couldn't access internet...in regards to as mentioned by KeepItSimpleStupid:i would like to clarify that yes,it's a AC system,and in case of varistors/variacs yes,they are a very poor option.they are lossy and heating problem....

other wise i found many unclear points,so i rediscussed the question and found out few points which makes things little bit clear..points follows:-
1) it's a AC system(as stated before in this post)
2) intended use of this device is in power factor correction circuit used in power systems or industries,so The voltage rating of the system will be very high and as well as the power rating so,it makes use of semiconductor devise little bit difficult [as stated by the teacher] [],
3) the problem statement is given keeping in mind that

in power system- during capacitor are switched On/off frequently, and capacitor takes heavy current during each turning ON!! so eventually the insulation wares out and many times[almost always if not taken care] results in a blast!! so, intention is that to limit this problematic charging current and to protect the insulation

Gary b: sir,can u describe the circuit or do u hv it's circuit diag. something similar!! .. coz in our project v dont require to hv any latest or any state of art technology... so,if ur circuit is compatible,v can it's idea!![atleast, wont take whole ckt other wise it's unethical]

 

[Gary B]

No, I don’t have a copy of the circuit with me. It was used in a telephone switch power supply that we were selling in the early ‘70’s so you should have no problem finding a similar arrangement in standard circuits manuals. One thing, this is for slow starting 5,000 microfarad filter caps in AC to DC power supplies. I don’t know how good it would be on something that has to slow start twice every cycle.