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Killing SSR (relays) - Need to understand

jwest7788

Mar 11, 2016
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Okay,

Here's the setup:

I have a project I am working on that uses heating coils in a kiln type setup.

The whole thing is 240V 25-30amps

I'm using a PID to control the elements, as I am hoping to hold precise temperatures inside the kiln.


Something about my setup is killing SSR (relays) My relays are rated to 40A, 240V output, and the PID/input site is something like 3-32V.

The switching and everything works fine, kiln heats up for awhile, but then within an hour, the SSR fails and gets stuck open.



Though the SSR does have a heat sink with thermal paste, but heat could potentially be the cause.

However, I feel like the issue is the sudden stopping and starting of such a large load on the SSR, as the PID flicks the SSR on and off several times a minute when it's trying to maintain the correct temperature.

As a novice with electrical projects I want to confirm:
Is the switching of the load a potential issue? --> Using the water analogy, I picture a "water hammer" type event, where the sudden switch off causes a voltage spike, or something similar?

Any tips or solutions? I have gotten this far just reading various web pages, but now that I've killed 3 SSRs, and it's beginning to get expensive, I need help!

Thank you! Seems like a nice forum you have here!

JW
 
Last edited:

Arouse1973

Adam
Dec 18, 2013
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Hi JW
Do you have a schematic of your circuit we can see. Welcome to EP BTW.
Adam
 

jwest7788

Mar 11, 2016
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2016-03-11_1416.png

Here's a mockup. Sorry, Not an actual schematic as learning the symbols is something still on my to do list.

I'm sure this will generate some confusion, but I'll be here to answer questions, and I can grab pictures later if needed too.
 
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Minder

Apr 24, 2015
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Your heating coils are drawing maximum current when cold, so if you are switching on/off this is causing maximum stress on the SSR.
It sounds like your controller is using burst firing to control a PID loop or ?
You may need to get a higher rated SSR or mock something up, circuit wise with a higher rated Triac.
M.
 

AnalogKid

Jun 10, 2015
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Also, the heaters might be producing an inductive kick when turned off. A 240 V or 277 V transient suppressor across the output won't hurt. Many SSRs turn off at zero crossings and that should reduce this effect, but without more information this is a reasonable guess.

ak
 

Minder

Apr 24, 2015
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I see you have added a layout, as suspected it appears to be a PID burst firing set up.
Is this a commercially installed system or custom built?
M.
 

jwest7788

Mar 11, 2016
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Inductive kick has been something on my mind, but had no idea what it was called. A transiet suppressor, like this thing?
http://www.maplin.co.uk/p/transient-suppressor-metal-oxide-varistor-cp76h -- Any tips on findind one of these in Canada? Or somewhere that ships here I guess?

Is this a commercially installed system or custom built?

This was once a residential Kiln for firing pottery, but it's since been modified. Portions of it were broken when I took ownership of it, and it's mostly amateur work trying to get it working with temperature control.

Your heating coils are drawing maximum current when cold, so if you are switching on/off this is causing maximum stress on the SSR.

Again, it works for awhile, but then later fails, not sure if super important info, but the coils are usually pretty hot before it fails. (This is a reason I think heat may still be an issue?)



Right now my (probably misinformed) gut is telling me to focus on the transient supressor thing. Is the link above what you're referring to?
 

Minder

Apr 24, 2015
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Personally I don't think it is a inductive issue, I suspect it is more of a current issue and needs some empirical testing to find out the actual current that is occurring.
But a heat sink on the SSR's should also help.
M.
 

dorke

Jun 20, 2015
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1) Have you measured the heater current?
2)have you measured the SSR case temperature?
3) What type of SSR are you using (brand, model, datasheet)?
4) You hoist a Canadian flag, isn't your line voltage at 120V?
how come you have a 240V heater?
 
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Minder

Apr 24, 2015
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4) You hoist a Canadian flag,isn't your line voltage at 120V?
how come you have a 240V heater?
Canada/US is dual voltage 120/240 single phase, large demand systems and appliances are 240v fed.
M.
 

duke37

Jan 9, 2011
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There was a thread some time back on trouble with a kiln. The matter was not fully sorted but it appears that the bare heating elements were mounted in the ceramic insulation and considrable leakage occurred both when cold due to damp and when hot due to ceramic conductivity.
Some investigation is needed to detemine current consumption.
 

BobK

Jan 5, 2010
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I hope the wiring of the SSR is not as shown. Note that the AC voltage is going to one of the control inputs.

Bob
 

jwest7788

Mar 11, 2016
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hey Everyone, Thank you for the prompt replys!

I'll take some measurements in the coming day or so.

Can anyone point me to what exactly I am measuring?
Voltage across heating coils?
Voltage across SSR?

I don't think my beginner multimeter can handle the amperage involved..

Not sure how to proceed. Anyone have a tip or two for me?
 

Minder

Apr 24, 2015
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You really need a clamp on ammeter they come as attachments for DMM's.
M.
 

dorke

Jun 20, 2015
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hey Everyone, Thank you for the prompt replys!

I'll take some measurements in the coming day or so.

Can anyone point me to what exactly I am measuring?
Voltage across heating coils?
Voltage across SSR?

I don't think my beginner multimeter can handle the amperage involved..

Not sure how to proceed. Anyone have a tip or two for me?

You need to measure the current Through the heating element.
You are correct about your DMM- they usually up to 10A.

Like minder said,
you need a high current(100A and above)clamp-on like here
There are many to choose from.
The professional,good and safe ones, are not cheap.

There is a way to estimate the current without it :
monitor your KW*H meter(electric company installed ) with only the heater connected,and calculate the current from it.

Like so:
Say you monitor the meter(start reading,end reading) for 6 minutes.

KWH=(end reading-start reading)*10.
Amperes=KWH/(line voltage)
 

GPG

Sep 18, 2015
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Inrush at switch on may be a problem. Make your own SSR using a zero crossing driver.
 

Minder

Apr 24, 2015
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Measure the heating element at the SSR input, some meters also have a peak store feature.
M.
 

dorke

Jun 20, 2015
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Measure at the AC leads coming out of the SSR.
You actually need to see all the current that the SSR is "carrying".
The one you posted the link to has a Max reading which is a great thing in this case use it as well.

Like so:
1. Connect the clamp and turn one the meter in 200/400A
MAX mode.
2. Activate the heater with the SSR.
the above will give you the max reading(inrush).

3. Now put the meter in normal mode while the heater is on,
this will give you the normal average current.
 

jwest7788

Mar 11, 2016
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I've ordered the clamp on flow meter now. Expecting it on Tuesday, so will measure then and get back to everyone!

Appreciate the assistance with this. :)

JW
 
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