triac power design urgent help needed !!!

[mike theodore]

Hi

I need to modify design for a circuit to switch 3 heating elements in a
3-phase design.
The original design has 3 power relays to switch each heating element
on/off .
The heating elements are 2kw each .
My questions are .
Can i use triacs to switch the elements instead of relays ?.
What are the cooling requirements then ?.
Can i use pwm to adjust heating power ?
Since i will be switching large loads on/off (with pwm control) what
regulation problems
will i encounter and can these be solved ?
Has anybody experience and is willing to work as a consultant for this
design ?
Are there any application notes with pwm control for power loads ?


Michael

 

[martin griffith]

Hi

I need to modify design for a circuit to switch 3 heating elements in a
3-phase design.
The original design has 3 power relays to switch each heating element
on/off .
The heating elements are 2kw each .
My questions are .
Can i use triacs to switch the elements instead of relays ?.
What are the cooling requirements then ?.
Can i use pwm to adjust heating power ?
Since i will be switching large loads on/off (with pwm control) what
regulation problems
will i encounter and can these be solved ?
Has anybody experience and is willing to work as a consultant for this
design ?
Are there any application notes with pwm control for power loads ?


Michael

I suggest you read this
http://catb.org/~esr/faqs/smart-questions.html


and then this, which was on page 1 of a google search for Triac
http://www.fairchildsemi.com/sitese...i=sitemap+id&ia=1&text=triac&as=1&render=1&w=


martin

 

[linnix]

I suggest you read this
http://catb.org/~esr/faqs/smart-questions.html


and then this, which was on page 1 of a google search for Triac
http://www.fairchildsemi.com/sitese...i=sitemap+id&ia=1&text=triac&as=1&render=1&w=

The biggest one in this page is a 10A TO-220. I think he is going to
need a bit bigger, at least 15A.

 

[John Woodgate]

Hi

I need to modify design for a circuit to switch 3 heating elements in a
3-phase design.
The original design has 3 power relays to switch each heating element
on/off .
The heating elements are 2kw each .
My questions are .
Can i use triacs to switch the elements instead of relays ?.
Yes.

What are the cooling requirements then ?.

Read the triac data sheets. Triacs are cheap; use a generously rated
one, both for voltage and current. For Europe, a 600 V, 12 A part might
be good. Or even 800 V.

Can i use pwm to adjust heating power ?

You CAN, but your product won't meet EN 61000-3-2; it's specifically not
allowed to use PWM for heating above 200 W, unless the Class D limits
can be met, and I doubt that you can do that. See clause 6.1 of EN
61000-3-2.

Since i will be switching large loads on/off (with pwm control) what
regulation problems

You almost certainly can't use PWM, and you need to consider what
happens when you switch the loads manually.

will i encounter and can these be solved ?

2 kW/phase isn't all that high. Look at EN 61000-3-3 for voltage-change
limits your product must meet.

Has anybody experience and is willing to work as a consultant for this
design ?

I can help you with the standards issues. The e-mail address works.

Are there any application notes with pwm control for power loads ?

I haven't looked on the web recently, but TI used to have some.

 

[Rich Grise]

Hi

I need to modify design for a circuit to switch 3 heating elements in a
3-phase design.
The original design has 3 power relays to switch each heating element
on/off .
The heating elements are 2kw each .
My questions are .
Can i use triacs to switch the elements instead of relays ?.
Yes.

What are the cooling requirements then ?.

That depends on the on voltage of the triacs and the current through them.
Use a heatsink that has a low enough thermal resistance to keep the
junction temp. lower than the spec.

Can i use pwm to adjust heating power ?

Well, kind of - for this kind of app, we usually use "phase control",
which is very much like PWM, but is per half-cycle - it starts with
"dead time", then after a delay, you trigger the triac, and it conducts
for the rest of the half-cycle. Excellent way of regulating, especially
heaters.

Another technique is called "integral-cycle" control, where your control
circuitry fires the triac for a whole cycle, 360 degrees, but picks and
chooses _which_ cycle it triggers on. This cuts down on EMI and power
factor problems considerably.

Since i will be switching large
loads on/off (with pwm control) what regulation problems will i
encounter and can these be solved ?

If you design a good enough controller, there will be no problems
of any consequence, and any that you do encounter are easy to solve.

Been there, done that.

Has anybody experience and is
willing to work as a consultant for this design ?

Yes - just concatenate my first and last name and send it to the
Yahoo server.

Are there any
application notes with pwm control for power loads ?

I don't know yet - why don't we search google concurrently?

Cheers!
Rich

 

[Joerg]

Hello Mike,

I need to modify design for a circuit to switch 3 heating elements in a
3-phase design.
The original design has 3 power relays to switch each heating element
on/off .
The heating elements are 2kw each .
My questions are .
Can i use triacs to switch the elements instead of relays ?.

Yes. But they exhibit a different set of pathologies so be careful.

What are the cooling requirements then ?.

Needs to be calculated but for 2KW it's not going to be outrageous
because triacs are essentially switches.

Can i use pwm to adjust heating power ?

Your utility might not like it at all if you are planning to turn them
on only part of the phase. Above a certain load level they may require
muffling and power factor correction (PFC). Also, some control gear
might "go on the fritz" if you'd PWM without any filtering.

Since i will be switching large loads on/off (with pwm control) what
regulation problems
will i encounter and can these be solved ?
Has anybody experience and is willing to work as a consultant for this
design ?

It is good to realize early on when it's better to let a pro take over.
I have to congratulate you for that, many potential clients only realize
that after a mid-size disaster. Or not at all and then they give up.

I am pretty booked up right now but it'll help to let folks in this NG
know which company or at least where it is. With this kind of stuff
there'll likely be quite a bit of on-site work.

Are there any application notes with pwm control for power loads ?

The load level isn't so important because the techniques are all
similar. Only the device sizes and driver methods differ, really.
Unitrode has great app notes, some of which can now be found under TI
because they bought the company.

If this is just for a one-off installation and not a series I'd consider
contacting a local control gear place that sells and installs industrial
electronics. For example heater controllers for breweries, bakeries etc.

 

[martin griffith]

The biggest one in this page is a 10A TO-220. I think he is going to
need a bit bigger, at least 15A.

Maybe not, if you look at the OPs' post he has a european address, so
I concluded the was on 230voltish


martin

 

[Eeyore]

Hi

I need to modify design for a circuit to switch 3 heating elements in a
3-phase design.
The original design has 3 power relays to switch each heating element
on/off .
The heating elements are 2kw each .
My questions are .
Can i use triacs to switch the elements instead of relays ?.

Subject to certain considerations.

What are the cooling requirements then ?.
Modest.

Can i use pwm to adjust heating power ?

Not with triacs. At least not 'chopped' mains.

Since i will be switching large loads on/off (with pwm control) what
regulation problems

Where's the pwm ? Do you mean low speed - cycle by cycle as opposed to
'chopping' ?

will i encounter and can these be solved ?

See above.

Has anybody experience
Some.

and is willing to work as a consultant for this design ?

You're in Greece ?

Are there any application notes with pwm control for power loads ?

I'm sure IR have loads.

Graham

 

[petrus bitbyter]

Hi

I need to modify design for a circuit to switch 3 heating elements in a
3-phase design.
The original design has 3 power relays to switch each heating element
on/off .
The heating elements are 2kw each .
My questions are .
Can i use triacs to switch the elements instead of relays ?.
What are the cooling requirements then ?.
Can i use pwm to adjust heating power ?
Since i will be switching large loads on/off (with pwm control) what
regulation problems
will i encounter and can these be solved ?
Has anybody experience and is willing to work as a consultant for this
design ?
Are there any application notes with pwm control for power loads ?


Michael

Yes, it can be done, I ever build a one phase version. Supposing you've
220/240Vac resistive heaters, I'd go for solid state relays of 800V/16A
which zero cross switching. Derating you know. You can control them with a
pulse of 1s repetition time and a duty cycle 0-100%. So your control goes
from 0-100% in steps of 20W. (More or less will not be problem.) You'll
have to adapt the old relay drivers to drive the solid states and you will
need some heatsinks as the solid state relays will produce some heat.
Numbers to be calculated from the datasheets of the components involved.
Regulations very. Not only from country to country but even from town to
town but FAIK (half) cycle control for this relative small heaters will give
no problems.

petrus bitbyter

 

[Spehro Pefhany]

Hi

I need to modify design for a circuit to switch 3 heating elements in a
3-phase design.
The original design has 3 power relays to switch each heating element
on/off .
The heating elements are 2kw each .
My questions are .
Can i use triacs to switch the elements instead of relays ?.

Yes. You will probably have to consider the triacs as expendable
devices, since they will tend to fail when the heaters fail unless
they are well fused and very much overrated. If they fail, for
whatever reason short of massive over-current, they will tend to fail
"on". Consider this in your safety evaluation of the entire system and
whatever limit controls you deem necessary to assure no injury or
property damage is likely to occur over the life of the product.

What are the cooling requirements then ?.

Roughly 1W per ampere per triac. That's a fair bit of heat to get rid
of using natural convection. Fans (and liquid cooling) have other
problems.

Can i use pwm to adjust heating power ?

Yes, zero-voltage-switched PWM with a cycle time in the seconds is
preferable for control of most constructions of electric heating
elements (it's not practical for some that respond very quickly). One
downside is that it can cause annoying visible flicker in lights that
operate on the same electrical circuit. Another is possible thermal
fatigue induced failures of the triacs (die bond fractures, die
overheats, die fails on, heaters stay on at 100% power, other damage
may then occur).

Since i will be switching large loads on/off (with pwm control) what
regulation problems
will i encounter and can these be solved ?

You have not mentioned what you are trying to control. If it is a
closed-loop temperature control system then you have no particular
problem. Variations in line voltage result in proportional variations
in output voltage and the resulting output 'actuator' is quite linear.

Has anybody experience and is willing to work as a consultant for this
design ?

I might be able to help, but for a one-off it will be cheaper to buy a
packaged SCR power pack or several SSRs operated by the controller.

Are there any application notes with pwm control for power loads ?

Yes, more or less, see the power device manufacturers. You may not
find your exact application described, but it's just a combination of
the zero-voltage switching and the control.

Michael

Best regards,
Spehro Pefhany

 

[[email protected]]

give me a drawing / sketch and the proper solution can be found.

We have already experimented with the switching switching many types of
loads.The other important factor is the switching cycle - how often
will the load be switched on and off.

Triacs is by far better suited to switch loads than a relay.

Bing.

 

[mike theodore]

Guys thanks for the advices

Altough i'm quite experienced in some areas of electronics these high power
applications are not my field
and i don't want to spend too much time until i'm on the right track.
Since it's a production device i want to have clear all aspects before i
start any design .
My last question is if somebody has worked with board traces carrying these
currents and what types of
connectors to use .
Are special pcb's used ?. As i see the solutions are terminal barriers or
terminal blocks . Any opinions ?

 

[default]

Hi

I need to modify design for a circuit to switch 3 heating elements in a
3-phase design.
The original design has 3 power relays to switch each heating element
on/off .
The heating elements are 2kw each .
My questions are .
Can i use triacs to switch the elements instead of relays ?.
What are the cooling requirements then ?.
Can i use pwm to adjust heating power ?
Since i will be switching large loads on/off (with pwm control) what
regulation problems
will i encounter and can these be solved ?
Has anybody experience and is willing to work as a consultant for this
design ?
Are there any application notes with pwm control for power loads ?


Michael

I did something like that 15 years ago. The goal was to regulate the
temperature of circulating water in a loop at just shy of boiling at
atmospheric pressure. 6 KW element, about 6 minutes to circulate the
whole loop (back to the heater). They already had temperature sensors
(and chart recorders) on inlet and outlet that I could tap into.

I built a small PWM circuit that monitored both inlet and outlet.
When the returning water was 90 degrees C, I dropped out one relay and
went to one heater (one and 1/4 heaters effectively).

The total mass of water was about 20 liters - overshooting would be
disastrous because the system was closed, and there was too much
heating element for too small a volume. The purpose was to sanitize a
sterile water delivery system to rooms in a pharmaceutical
environment.

I used solid state relays to switch power, to a set of three mercury
relays to power to the elements - Today I'd just use SSR's to switch
the power directly. The system has been in service for all this time
with one failure - a solid state relay failed by not passing power to
the control relays.

Only safety was a large mechanical thermostat inside the heater (with
too slow a response to be an effective safety in my opinion). In
theory that thermostat was supposed to regulate the temperature but
wouldn't meet their specification - 98 C plus or minus 1/2.

 

[Eeyore]

Guys thanks for the advices

Altough i'm quite experienced in some areas of electronics these high power
applications are not my field
and i don't want to spend too much time until i'm on the right track.
Since it's a production device i want to have clear all aspects before i
start any design .
My last question is if somebody has worked with board traces carrying these
currents and what types of
connectors to use .
Are special pcb's used ?. As i see the solutions are terminal barriers or
terminal blocks . Any opinions ?

You may want to use '2oz' ( 70 um ) copper but there's nothing unusual about
pcbs carrying these currents. The relevant IEC / EN standard wil give you
guidance about trace separations.

Graham

 

[Joerg]

Hello Mike,

My last question is if somebody has worked with board traces carrying these
currents and what types of
connectors to use .
Are special pcb's used ?. As i see the solutions are terminal barriers or
terminal blocks . Any opinions ?

The highest I did was 100 amps although it was designed with plenty of
safety margin (medical electronics). It was a backplane. We used nickel
plating to mitigate any contact issues and be compatible with the lugs.
The lugs were then screwed onto the board with very heavy hardware, IIRC
it was 16mm bolts. Could have been 5/8" hardware but that's almost the
same. Strong spring washers made sure this would not come loose but we
also had sense wires that would have tripped off the power supply if it
ever did. In our case there was a lot of rough air shipment so stuff
could come unglued upon a rough landing etc.

We used six planes for the power traces and they were 2-3cm or so wide.
The land area for the cable lugs was about 3cm by 4cm. It was "peppered"
with vias to make sure all the planes were well connected. These vias
were done without thermal reliefs to minimize resistance. If you do that
you need to check the Gerbers right before board fab because it has
happened that people thought this was an error and wanted to edit
thermal reliefs back in. Mostly I make sure a technical note is sent
along explaining this to the fab house so nobody will get the idea to
"improve" things.

On a side note, the board to chassis contact areas must also be able to
handle the full worst case load current (short) in case something goes
wrong. You don't want to have a FR-4 board go up in flames. IIRC we used
nickel plating there as well and the chassis was also nickel plated at
those contact areas.

 

[mike theodore]

Jim hi
Thanks for the reply
One of the proposals I got is to use integral mode pwm so I fire the triac
the whole 50 hz cycle leaving some cycles off .
I think this can accommodate with the harmonics produced when adjusting the
phase angle since the triac will be activated
at zero crossings every time.
What is your opinion on this ?

 

[Joerg]

Triacs is by far better suited to switch loads than a relay.

Until the day when lightning strikes uncle Leroy's shack next door.

 

[John Woodgate]

One of the proposals I got is to use integral mode pwm so I fire the
triac the whole 50 hz cycle leaving some cycles off . I think this can
accommodate with the harmonics produced when adjusting the
phase angle since the triac will be activated
at zero crossings every time.
What is your opinion on this ?

There is a very long story about this an IEC/EN 61000-3-2, together with
the specification of the measuring instrument for harmonics, IEC/EN
61000-4-7. The latter was changed, to increase the measurement bandwidth
so as to include interharmonics (frequencies not exact multiple of the
supply frequency). Before this change (which has been suspended until
the problems are solved), the arrangement you propose was permitted, as
being 'symmetrical control'. But in fact control of this type may not be
symmetrical, and even if it is, it produces interharmonic currents that
the new measuring instrument takes as being even harmonics. The limits
for even harmonics are very low, so the equipment is very likely to
fail.

People have been working on this problem for about a year, and the full
resolution may be achieved in November. However, since there isn't yet
full agreement on measured values on what appears to be the same circuit
(it's not a round-robin test, though), there may be a delay.

The thing to do is to study the latest editions of EN 61000-3-2 and EN
61000-4-7, disregarding clause 7 of that, so that the wider bandwidth is
invoked, and make sure that the switching that you choose will allow the
product to meet the requirements of EN 61000-3-2.

 

[Michael A. Terrell]

Until the day when lightning strikes uncle Leroy's shack next door.

The little ole shack out back? ;-)


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[Joerg]

Hello Michael,
Yeah, with all the extension cords strung about ;-)