Slow starter for electric heater element - Page 1
Forum section: electronics / projects

author: Mike Jung
author_rank: Contributor
timestamp: April 04, 2023, 01:20:20 pm
content: I'm wondering if anybody here might suggest an idea ( schematics ) for a (3Kw) heater element slow starter. With some form of adjustable timing if possible. I have a small generator and it keeps hunting when powering the heater.

author: Swake
author_rank: Frequent Contributor
timestamp: April 04, 2023, 02:01:47 pm
content: Is that heater a purely resistive device?Can that generator handle that 3kW load? It might be rated on paper for that load but reality might be very different especially for cheap devices.If there are 2 elements you can start one and then the second with a simple delay.Inrush limiters exist but their action is very short in time, might not be enough for the generator to speed up again. More helpful with inductive loads.3kW dimmer is the next thing that comes in mind.Recovering the heat of that generator is likely 40%. Worth investing in an heat exchanger if used frequently.

author: Mike Jung
author_rank: Contributor
timestamp: April 04, 2023, 09:20:03 pm
content: Thank you for your kind suggestions but what I need is some adjustable "slow start" for the heater element. I can cobble some mechanical/electronic contraption but I would like a purely electronic solution.

author: Vovk_Z
author_rank: Super Contributor
timestamp: April 04, 2023, 10:05:11 pm
content: How hot is that heater? If it is not red-hot but 100-200 C hot then its resistange doesn't change much. So slow-start want help.

author: james_s
author_rank: Super Contributor
timestamp: April 04, 2023, 10:16:21 pm
content: As was said above, I'm not sure slow start is going to help you, is the generator actually rated for 3kW continuous? Is it in good working order? It's common for carburetors to get gummed up or exhaust spark arresters to clog and prevent the engine from developing full power. If you really do want a slow start circuit, a high power triac light dimmer ought to do the trick, modify it by replacing the pot with a capacitor and resistor.

author: BradC
author_rank: Super Contributor
timestamp: April 05, 2023, 12:21:11 am
content: Quote from: Mike Jung on April 04, 2023, 01:20:20 pmThat's a load change step to a mechanically governed system, it'll always oscillate until it settles. It should do that to a degree when the element cuts out also.If it were me, I'd go with james_s triac solution, but I'd control it with a small microcontroller to ramp up and ramp down. If you were really fancy you could control the ramp by looking at the frequency change as the generator loads and unloads to keep it as smooth as possible. That'd qualify as "purely electronic solution".

author: Mike Jung
author_rank: Contributor
timestamp: April 05, 2023, 12:39:54 am
content: Quote from: Vovk_Z on April 04, 2023, 10:05:11 pmThe problem is not the resistor. The problem is the generator..........................................

author: Mike Jung
author_rank: Contributor
timestamp: April 05, 2023, 12:44:49 am
content: Quote from: BradC on April 05, 2023, 12:21:11 amWell, that's the idea : to minimize the load change step. No step, no oscillation give or take a couple of variables.I'll try the triac solution.

author: Mike Jung
author_rank: Contributor
timestamp: April 05, 2023, 12:53:25 am
content: Quote from: james_s on April 04, 2023, 10:16:21 pmOf course is going to help. It'll allow the mechanical governor of the generator to catch up smoothly. The generator is rated 5Kw and the engine is in perfect condition. It's a new 4 stroke single cylinder Honda and I repaired lots of those when my son was racing go-carts. By the way, I tried to triac suggestion by hand and it works 100%. I'll try figure out how the pot is connected and mod it as per your suggestion.

author: TheMG
author_rank: Frequent Contributor
timestamp: April 05, 2023, 10:17:30 am
content: On a small generator with a mechanical governor a small amount of hunting in response to a step load is normal, but it should settle in a couple seconds.If it keeps hunting for a significant amount of time the governor needs adjustment. If the governor arm has several holes in it for the spring, try moving it to a hole further away from the pivot point, that will improve stability.

author: james_s
author_rank: Super Contributor
timestamp: April 05, 2023, 05:25:03 pm
content: Quote from: Mike Jung on April 05, 2023, 12:39:54 amYou misunderstood him. His point is that a heating element that gets very hot will have a lower resistance when cold and thus will have a larger inrush than steady state draw, that clearly isn't the case here.

author: james_s
author_rank: Super Contributor
timestamp: April 05, 2023, 05:29:23 pm
content: Quote from: Mike Jung on April 05, 2023, 12:53:25 amThe classic light dimmer circuit is pretty standardized, most will be something like this:https://makingcircuits.com/blog/simple-triac-dimmer-switch-circuit/I forgot it's AC though so I don't think the capacitor idea is going to work. You could replace the pot with an analog optocoupler (or make one with a CdS cell and LED) or make a microcontroller based dimmer, then you can program it to do whatever you want. You should be able to find some designs for those floating around, it's a fairly simple concept to monitor the zero cross point and then trigger the triac after a variable delay.

author: Mike Jung
author_rank: Contributor
timestamp: April 05, 2023, 07:36:18 pm
content: Quote from: james_s on April 05, 2023, 05:25:03 pmNo, I did not. While for many years I made a living through mechanical machining I used to be a fairly competent physicist.

author: Vovk_Z
author_rank: Super Contributor
timestamp: April 05, 2023, 09:21:45 pm
content: Ok. Possibly I haven't understand TS firstly. Then that's what jame_s is talking about - generator can't manage a large 3kV load change.There are a lot of ready-made soft-start modules on Aliexpress.E.g.soft startThey are mostly are described as "for motor soft start" but I guess they should work for resistive load too.

author: 5U4GB
author_rank: Super Contributor
timestamp: May 02, 2023, 09:47:01 am
content: These things are all over Aliexpress, under the designation SSR-ddWA-R1, dd = power rating, e.g.this one. However there's two issues with them, the first is that none of them indicate which of the two connections are the fire wire (= active) and zero wire (= neutral), every single diagram for them just labels them "a" and "b". Anyone know which is which?The other problem is that they have to be wired between the power switch and the load rather than being made up into a general-purpose external soft-start controller that you can plug an arbitrary load into, because if they come before the power switch then they're permanently energised and won't ramp up the voltage when the load is switched on. This means you need to find space inside the device/load for a honkin' great piece of electronics.You can also get thesedinky little three-wire thingsthat'll fit inside most devices you want to run, but I'd hate to think what's inside those, maybe a single thermistor? The half-brick sized ones at least have potted power MOSFETs (according to one teardown) and a fair bit of thermal inertia.