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.
Pulse Width Modulation
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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.
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.
Other wise you would just switch the element(or whatever) with the line voltage directly.
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%
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/techlit/images/gif/m14542.gif
Sorry the pic stinks.
"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/techlit/images/gif/m14542.gif
Sorry the pic stinks.
If your Duty Cycle is set to 70% then what is your cycle time?
goose_on_fire
Well-Known Member
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.
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.
goose_on_fire
Well-Known Member
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...
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...
Hermit
fuddle
That would be more than a tad hard on the compressor. In systems using capillary tubes you generally want to be off a minimum of 3 minutes. Most can't start under the head pressure in less time.
the pwm out is not really intended to be used with a relay controlling a ac motor. pwm is really for controlling resistive heating elements, dc motors or current limiting in motor windings. An ac motor really wants ether a thermostat (full on or off as regards to a set point) or a vfd, not really applicable in refrigeration.
ther should be a way of changing the output, really all you need is a thermostat
ther should be a way of changing the output, really all you need is a thermostat
smizak
Well-Known Member
Yep. SSR for the most part do not like PWM signal applied with an inductive load. Even with snubbing circuitry, I have seen those things pop when switched mid-cycle and connected to a motor.
Most SSRs are zero-crossing, meaning the only turn on and off at the zero voltage part of the AC swing. Any PWM signaling would have to be synced with the AC phase and you could only basically rectify the power.
I think my original question was misleading. This is going to control a heating element in a RIMS heat exchanger. I am already controlling my refrigerator for fermentation. I do it based on temp. My original question was if SSRs came with DC and AC input signals. By this I mean does the signal turning the relay on need to be a DC signal or an AC signal. I have found that you can purchase either just as snowrs has stated. I know that my contacts will only be switching AC. My question is more about PWM setup and configuration. Thanks for all the help though.
ok, tracking now.
Zero Width, Max Width and the Period;
you zero with is the minimum pulse width. in a heating element at the low end (near 0%) there is a certain amount of current that will flow before the element gets hot.
this is used to reduce the the lag between controller hand element. this can be seen with a dimmer and a light bulb. slowly turn up a dimmer and you can get some knob turn before you see any light.
Max with is the same concept but on the near 100% side of things.
Period is how long each cycle will take to find switching freq = hz = 1/period
another thought, some circuits need a minimum pulse widths to register. that would also work.
Zero Width, Max Width and the Period;
you zero with is the minimum pulse width. in a heating element at the low end (near 0%) there is a certain amount of current that will flow before the element gets hot.
this is used to reduce the the lag between controller hand element. this can be seen with a dimmer and a light bulb. slowly turn up a dimmer and you can get some knob turn before you see any light.
Max with is the same concept but on the near 100% side of things.
Period is how long each cycle will take to find switching freq = hz = 1/period
another thought, some circuits need a minimum pulse widths to register. that would also work.
with out knowing the model of you ssr and specifics of your heating element it is imposable to answer that.
1. we need to know the units, milli- micro- seconds?
2. on you ssr there should be a max switching frequency. 1/period should be less than that with a suitable margin.
3. to set you zero width look on your relay and find the minimum pulse width or on time. that is you starting point, it shouldn't be smaller that this. it is possible that the relay min pulse is way smaller that the controller can up out and be ignored.
as far a taking up the deadband near 0% you should be able to manual command an output% and slowly bump it up until you can measure a temp rise on the element. use that to figure out how wide the zero pulse should be.
you can probably leave zero at the min pulse width of the relay and you will be fine.
if you have problems maintain a constant temp that is a sign that you need to just zero.
1. we need to know the units, milli- micro- seconds?
2. on you ssr there should be a max switching frequency. 1/period should be less than that with a suitable margin.
3. to set you zero width look on your relay and find the minimum pulse width or on time. that is you starting point, it shouldn't be smaller that this. it is possible that the relay min pulse is way smaller that the controller can up out and be ignored.
as far a taking up the deadband near 0% you should be able to manual command an output% and slowly bump it up until you can measure a temp rise on the element. use that to figure out how wide the zero pulse should be.
you can probably leave zero at the min pulse width of the relay and you will be fine.
if you have problems maintain a constant temp that is a sign that you need to just zero.
