triacs

I'm looking to switch a bank of electric motors on and off. The motors will range from less than an amp to about 6 amps a piece. In the application I am looking at, the motors are generally just plugged into a common household

20A circuit. With my circuit I don't want to impose any amp limits over what the circuit would normally do. I was switching this with a relay, but for some reason (probably me doing electronics as a hobby and not knowing what I'm doing) the relay failed after just a short time. I asked around and someone said I should use a triac. So the background info aside, is this the type of triac I would need?

I know next to nothing about triacs, just want to buy the right product first before I start working on actually hooking a circuit up for it. Amps is the main thing. I want it to be able to handle as many amps as the house circuit it works with, and the common 120V.

"Chris Gentry

** WTF are you up to ???

What sort of motors are involved?

Where did the amp ratings come from ?

Are you trying to switch them ALL together, at once ??

May not be possible to do that on a domestic circuit.

....... Phil

"Chris Gentry"

** You cannot possibly switch that much PUMP MOTOR load all at once.

Pump motors have a start up current surge of about 6 to 8 times their rated load.

So you MUST sequence them.

No wonder that relay burnt out.

....... Phil

I'm wanting to make a circuit that will make a 'slave' out of a common household outlet. This is to switch off large water pumps that would be hooked to an aquarium. This whole thing is going to be a 'feed' timer, so when I push a button the circuit will shut my pumps off for 5 minutes while my fish eat. Then turn the pumps back on.

Some of the pumps could be up to 6-7 amps each. In my present case, the pump I am using is only a single pump and is about 1 amp.

I just added them up based on the possible number of pumps used with the possible amp draw of each pump.

I'm wanting to switch the whole outlet off and on, and anything that is plugged into it. Based on two plugs per outlet, and 6-7 amps per pump, the total would probably only be about 12-14amps.

"Chris Gentry"

** As am I.

** Absolutely not - pump motor surges are way longer than triacs can withstand.

You did not READ what I actually wrote.

** What is hard to comprehend about the word "sequence " ??

You are just being a PITA.

Get a licensed electrician to do to job for you or else

FUCK OFF !!!

...... Phil

Like I said, I'm just a hobbyist, and the mechanical relay I was using in the last scenario was only rated at 10A, so I see what you are saying, and I'm surprised it worked as long as it did. On the datasheet

it has this information about surge current:

Non repetitive surge peak on-state current (full cycle, Tj initial = 25°C)

F = 50 Hz t = 10 ms 210A

F = 60 Hz t = 8.3 ms 200A

Does that mean it can withstand a surge of 200A while the motor starts up? Cause if even at a full 20A draw, 8 times that would only

be 160A. And I'm thinking that anyone using this will probably have less than a 10A pump motor on this in real life use. I'm building it

for myself right at the moment and I have a very small motor of about 1A that this will be used with. But if I decide to build one for friends

or sell this in the future? With this triac will I still have to sequence them, or find another/bigger triac? Or a whole new scheme to switch

them? The reason I'm asking is because I don't want to impose limits on this, only those limits that are already imposed by your house

wiring. If I have a 20 amp house circuit, am I wrong to say that you could plug (2) 6-7amp motors into it? Of course if that is the only

way to build this, then I will say you can only use this with X amp rated motor.

Thanks -Chris

To add to what you've already gotten...

Triacs aren't that good at switching inductive loads, like motors or relays. There are some helper circuits you can use to get them to work better with them, or you can look into alternistors, which act like triacs but are built like a pair of SCRs, and are designed for inductive loads. I use alternistors in my furnace controller.

Motor startup loads are on the order of *seconds*, not milliseconds. My air conditioner, for example, draws 125 amps for about five seconds before settling down to a constant 30 amps. A small pump could be a half second or so, it's however long it takes the pump to come up to speed.

A zero crossing triac-driving optoisolator is a good choice, but note the maximum current draw and make sure you put a series resistor in. Calculate the value based on peak amps and 170v. When you first switch an opto, it carries the whole load until the triac starts conducting. Note that this is on the order of a millisecond or so; you can use a silghtly lower *wattage* resistor if you want.

Contrary to popular belief, circuit breakers will not prevent all over-current situations. They trip when the accumulated overcurrent is sufficient to heat the house wiring, and thus normally allow the second or two of motor startup surge current. You have to base your current ratings on that, not just the breaker rating.

The type of mechanical switch that controls high current is called a contactor, not a relay. IIRC some have a high speed wiping contact, not a simple pressure contact, to deal with the arcing that happens.

If you have 6 amp motors, you shouldn't put more than two to an outlet anyway. 18 amps on a household outlet doesn't leave much margin for the startup surge, so even that breaker may trip, and if it's a 15 amp breaker (still common in houses), even two is pushing it. See rec.woodworking for recommendations about circuit capacity for power tools.

If you can give each motor its own triac/alternistor, you can spread the surge around more, and possibly start up the motors one at a time instead of simultaneously.

Save yourself a lot of grief and effort and just get a Solid State Relay. Find one for 30 amps and with whatever voltage control you need.

Like someone already pointed out - motors starting from a dead stop do require a lot of current for a second or two. SSR's are frequently able to handle that current.

Otherwise (if you like to tinker) Triacs or back to back SCR's or even single SCR's with bridge rectifier are ways you might do it. Lot of good ways to do it, how involved do you want to get? Are you solving a problem and want it over quickly, or want the satisfaction of learning?

You could always use a bank of SSR's to turn on pumps sequentially with very little circuitry so not all the power comes on at one time, or individual off the shelf solid state time delay relays, etc..

Lot of ways to skin that cat.

You could have the pumps activate after some other circuit turns on or off, with very little effort.

--

----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+
Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption =----

Chris.. I also had relay problems with a 1/3 horse power motor on a swamp cooler. Changed to a 25 amp solid state relay, and the thermostat controls the DC power supply to turn on and off. No more problems since (12 years)

In later posts, it says you were using a 10A rated relay; the problem is that you need a MOTOR RATED relay, not just '10A' contact rating, because of startup overcurrent and shutdown overvoltage.

A triac solution will drop (in the ON state) 2V or so, and that means

12W of waste heat. If the motor was usually-OFF, that'd be OK. But for a motor that's usually ON, go with a heavier relay. Consider putting a surge supressor (varistor) on the motor side of the switch, too.

"default"

** The current surge ratings of SSLs are not too great when used for motor start - particularly a PUMP motor that may be starting under a heavy load. See table on page 4.

For a 25 amp rated SSR, a 112 amp peak value surge cannot be tolerated for more than 200 mS.

A surge that lasts 3 or 4 seconds must not be more than 47 amps in peak value.

The OPs case is almost certainly outside those levels.

The SSR will fail.

...... Phil