Deliberately slowing down heater warm-up



  • I have one of E3D's high temp heatbeds. Which is fine, in isolation my machine has no issue with it.

    The problem is I like to run my machines off a UPS, and the poor 500W UPS gets very cross at me during bed heat up, constant-tone. The bed will go from room temperature to 60C in about 10 seconds, so it's not like I have to deal with the tone for long... but deliberately overloading my UPS doesn't quite sit right with me.. and buying a more expensive, more appropriate UPS just for these 10 seconds seeems silly.

    So, I think I need to either need to find a way to current limit it. Or to do what I'm thinking is more likely, just have the duet tell it to heat up slower. And then just deal with the slightly longer heat up time.

    How would I go about that?



  • You could limit the duty cycle for the PID tuning.

    ref https://duet3d.dozuki.com/Wiki/Gcode#Section_M303_Run_heater_tuning and the P parameter.



  • Yeah, that did the trick. I think the relay I am using is 10A, so at 240v. Max power draw would be 2,400W. 500/2400 gives me roughly 20%.

    Tried P0.2 and it works a charm, no more tripping the overload alarm.

    It's obviously a lot slower now, but I still think it's heating up faster than my other printers. I can live with it.

    Thanks.



  • A number of things don't make sense here.
    Just because your relay is rated at 10A doesn't mean that's what you are drawing and a 2400W heat bed doesn't make sense. Even my 500mm * 500 mm heatbed is only I think 1200W.
    If you have the common 300*300 size, you are likely drawing in the 700W range.
    I would suggest running the heater straight off line voltage as a 500W UPS isn't going to last more than a few minutes anyway if you are running the heater from it. Use the UPS to protect the sensitive electronics instead.
    Chances are that during a power glitch the temporary loss of the heater will not affect things and on a longer outage the UPS won't be able to handle things for long anyway.
    Last but not least, a 20% duty cycle is extremely low. It might save the UPS a bit of effort and prevent over heating but it isn't going to change the fact that you are drawing too much current (in pulses) from the UPS. Instead of stressing the UPS due to overheating, you will stress it due to the nature of the current draw which it is not designed to handle. The constant cycling of too much operating current which also causes cyclic heating in the semiconductors will likely reduce lifetime of the UPS.



  • @jens55 Indeed something is seriously wrong. The thing that I picked up on was that the bed temperature can go from ambient to 60 Deg C in 10 seconds. Which would indicate a massively over powered heater.



  • @deckingman said in Deliberately slowing down heater warm-up:

    The thing that I picked up on was that the bed temperature can go from ambient to 60 Deg C in 10 seconds.

    or a very unfortunate thermistor placement?

    would need more data to comment on the rest but I probably wouldn't worry too much about it, its like the cold start inrush current causing the trip, which drops quite alot as things heat up. what makes most sense with respect to use the ups or not for the heated bed comes down to what sort of (duration of) outages you're trying to protect against, and what the consequences are of interrupting the heating.



  • Those E3D high temp 300 x 300 beds are 1100W. If it heats up to 60C in 10 secs, then sounds like 110V bed is used with 240V. In that case bed will draw more then 20A and power will be about 5KW



  • OMG, that would be perfect if you wanted to have a reliable way to burn down your house .....



  • @bearer I was thinking more of what the temperature might get up to after a few minutes if the bed Mosfet failed. Hope the OP has thermal cut-outs installed but I would still be very nervous about leaving that bed heater unattended.



  • I mean... sorry to concern you all. It says right on the heater 240V, 800W. 2400W was a guess based on what the maximum I thought it could even be since it wasn't blowing any fuses on tripping any breakers.

    I didn't time that 10 seconds, I just know it heats up to 60C fast.

    I'll run it off the mains without the UPS and time it, I used to have a power meter, but that's gone walkies.


  • administrators

    @andywm said in Deliberately slowing down heater warm-up:

    It says right on the heater 240V, 800W.

    Then its resistance should be 72 ohms. You could check that using a multimeter.



  • E3D give the specifications here https://e3d-online.com/high-temperature-heated-beds

    If it's 800 Watts, then it's 300mm x 200mm so about 1.3 Watts/cm^2 - arguable that's roughly over powered by a factor of about 3 times. They also list the maximum operating temperature as being 200 deg C but unless I missed it, I don't see any reference to a thermal cut-out to limit the temperature in the even of a control system failure.



  • @dc42 - yeah, 71.2 omhs measured across the bed power.



  • This just shows that my time estimation skills are awful. @deckingman

    It takes 31 seconds to go from 23C to 60C at full power. If 10 seconds is about 3 times overpowered, then funnily enough.. 31 seconds would be about right.

    Okay, what's the proper solution to this if I still want to use the UPS? Wire the relay to a second IEC C14 power connector and power them independently? Or do I need to fork out for an 800W UPS?



  • If it was me, I would power them independently. UPS for the electronics, straight line power for the bed heater.
    If you want to have the UPS cover the draw of the bed heater then do yourself a favour and go for something with enough head room such as a1200 W UPS.



  • @andywm said in Deliberately slowing down heater warm-up:

    I'd do what @jens55 suggested - oh wait, I already have! (although I chose a DC UPS).

    https://somei3deas.wordpress.com/2019/10/22/fitting-a-dc-uninterruptible-power-supply-ups-and-separate-5v-supply/

    and

    https://somei3deas.wordpress.com/2019/10/27/ups-battery-installation/

    That is to say, run the bed from the mains and the rest of the printer from the UPS. That way, in the event of a power outage, the UPS will keep the printer running for much longer because the bed has by far the biggest current draw. And it'll take some time for the bed to cool.

    I'd also fit a thermal cut-out if the bed doesn't have one (although if it was me, I wouldn't use a bed heater that is so over powered).


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