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Old 12-09-2009, 08:06 PM   #1
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Default Pulse Width Modulation

Is anyone familiar with PWM settings. I have a controller from work that has PWM outputs. I am not sure where to set the Zero Width, Max Width and the Period. The minimum time I can set is 0.1 second. Do some SSRs have limitations on these settings? If I understand this correctly I could set the Zero to 0.3 ,the Max to 0.7, and the Period to 1.0. Would this mean that the first 0.7 sec it would be on and the last 0.3 sec it would be off at a command of 100%? If this is true then 50% command would mean on for 0.35 sec and off for 0.65 sec? I have been in building automation for a few years now and I have not been exposed to PWM yet.

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Old 12-09-2009, 08:48 PM   #2
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Is anyone familiar with PWM settings. I have a controller from work that has PWM outputs. I am not sure where to set the Zero Width, Max Width and the Period. The minimum time I can set is 0.1 second. Do some SSRs have limitations on these settings? If I understand this correctly I could set the Zero to 0.3 ,the Max to 0.7, and the Period to 1.0. Would this mean that the first 0.7 sec it would be on and the last 0.3 sec it would be off at a command of 100%? If this is true then 50% command would mean on for 0.35 sec and off for 0.65 sec? I have been in building automation for a few years now and I have not been exposed to PWM yet.

Hopefully coderage will chime in. I am curious about some of this too. Something he posted in another thread was that if the pwm is pulsing too fast it could create more heat from the SSR and maybe even cause it to fail over time.

This is a very interesting topic for me. I have used PWM's in other applications. I have built one to control my boil kettle element, but I have not used it yet. Still building.
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Old 12-09-2009, 08:52 PM   #3
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Does the PWM signal have to be VDC? Are there SSRs with AC inputs?

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Old 12-09-2009, 09:02 PM   #4
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I think the whole point of the relay is that you can't just switch ac with a low volt device such as a PID or other controller. So I would doubt there is an AC to AC SSR. I could be wrong I have never looked for one.

Other wise you would just switch the element(or whatever) with the line voltage directly.

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Old 12-09-2009, 11:49 PM   #5
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There are AC coil AC Load SSR's so becareful when ordering, also turn on time and turn off time can vary for each relay but most are below 100 ms which is the minimum width you can set your controller to. Depends on if it is a leading or trailing PWM it could be off for .3sec then on for .7 every 1 sec period but that should not matter, I am not familiar with this type of controller, I am use to just setting the duty cycle, but effectivly you are doing the same thing you would just have a duty cycle of 70%

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Old 12-10-2009, 02:44 AM   #6
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I read the instruction manual on Auber's website. Since my SSRs are coming from them I think I will go with their recommendations.

"6.7 Cycle time “t”
Parameter T can be set between 0.5 to 125 seconds (t=0 represent 0.5 second). It represents the speed at which the output is turned on and off. When the output device is a mechanical relay, the cycle time should be set >4 second. To increase the service lifetime, it is recommended to set it between 20.0~60.0 seconds. However, longer time might reduce the accuracy of the control in a fast responding system. When the output is logic level that controls a SSR, the cycle time can be set at 2 seconds or less for better performance"

So, from reading this it sounds as though my Period needs to be two seconds.
The Zero Width could be 0.2 seconds and the Pulse(or Max) Width could be 1.8 Seconds. So for every 2 seconds that pass the SSR will be enabled for 1.8 sec at 100% command. At 0% the SSR will be enabled for .2 sec out of every 2 seconds. If by leading you mean "on" first, then yes it it a leading PWM. There will be a PID driving this PWM. The controller is a Honeywell Plant Controller.

Here is a pic of how they define PWM
http://customer.honeywell.com/techli...gif/m14542.gif



Sorry the pic stinks.

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Old 12-10-2009, 02:50 AM   #7
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Quote:
Originally Posted by snowrs View Post
There are AC coil AC Load SSR's so becareful when ordering, also turn on time and turn off time can vary for each relay but most are below 100 ms which is the minimum width you can set your controller to. Depends on if it is a leading or trailing PWM it could be off for .3sec then on for .7 every 1 sec period but that should not matter, I am not familiar with this type of controller, I am use to just setting the duty cycle, but effectivly you are doing the same thing you would just have a duty cycle of 70%
If your Duty Cycle is set to 70% then what is your cycle time?
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Old 12-10-2009, 03:03 AM   #8
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My first thoughts, none of which really answer your question but if you have the make and/or model number of the controller maybe we can help more.

Every time I've had to specify PWM settings, it's either been in terms of period and high time, or frequency and duty cycle. I'm not familiar with the nomenclature your controller is using.

For something with as slow a time constant as refrigeration, I'd be surprised if cycling as fast as 1 second is even desirable-- I've written controllers for heaters (heating big honkin' hunks of quartz in optical bandpass filters) to within two tenths of a degree C, and I used periods in the ten second range.

There are relays with AC control inputs, but remember that a relay is still just a glorified switch-- if current is flowing at the input, the switch is in one position, if current isn't flowing, the switch is in the other position.

Edit: aaaaand you posted an answer while I was typing. Let me read all that.

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Old 12-10-2009, 03:32 AM   #9
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Now I'm just confused. You quoted the Auber controller manual but are using a Honeywell controller? Some of the Auber controllers have an internal relay, so you wouldn't need an external one.

That cycle time parameter you quoted is for the controller, not for a relay. It determines how often the controller switches on or off.

I'm no help at all...

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Old 12-10-2009, 03:37 AM   #10
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I was just quoting the Auber PID because I am using a Auber SSR.

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