Mains half cycle skipping to regulate heater element - Page 1
Forum section: electronics / general

author: ocset
author_rank: Super Contributor
timestamp: November 09, 2019, 01:12:50 pm
content: Hello,We have an electric heating element connected to the mains. We wish to regulate the temperature by skipping half mains cycles, as in the attached waveform (LTspice sim also attached).The power factor is great because mains current and voltage are always in phase. But is there any problem caused in the mains electricity system by this ON/OFF type waveform?

author: NiHaoMike
author_rank: Super Contributor
timestamp: November 09, 2019, 02:20:40 pm
content: Zero cross switching is the standard way to do it. Only downside compared to phase angle control (generates lots of EMI) is the slower response, but that's rarely important for heating applications.

author: Brutte
author_rank: Frequent Contributor
timestamp: November 09, 2019, 02:34:06 pm
content: What is the time constant of that thing that you heat?For fast control you have to use triac which requires some additional protection + drive. Things get simpler with dumb relay but then this is for big time constants.There are triacs which already have zero-cross detection but the cheapest version just uses raw triac + uC that does zero-cross detection.

author: ocset
author_rank: Super Contributor
timestamp: November 09, 2019, 05:50:07 pm
content: Thanks, the attached two waveforms show the skipping of 10ms half cycles…in one, every fifth half cycle is skipped, in the other, every sixth half cycle is skipped.As you can see, when every sixth half cycle is skipped, there is an overall DC current in the mains supply system…do you think this matters?

author: Kleinstein
author_rank: Super Contributor
timestamp: November 09, 2019, 06:15:27 pm
content: The overall DC contribution can be a problem, if the distribution transformer is relatively small. A DC component of the mains can cause problems with transformers (hitting saturation to one side). AFIAK there are limits how much such DC background is permitted. I would not worry about a 50 W soldering iron, but a 10 kW warm water heater.

author: IanB
author_rank: Super Contributor
timestamp: November 09, 2019, 06:33:36 pm
content: Quote from: treez on November 09, 2019, 05:50:07 pmWhy don't you skip whole cycles? Avoids the whole problem of power factor and DC bias.

author: Cerebus
author_rank: Super Contributor
timestamp: November 09, 2019, 06:40:12 pm
content: Quote from: Kleinstein on November 09, 2019, 06:15:27 pmQuite. The easy way to avoid a DC component is to zero-crossing switchwholecycles. For pretty much all applications where half-cycle skipping is acceptable in time constant terms, whole cycle skipping will also be acceptable.The only thing to watch out for in either scheme is the EMI generated by the 'sharp edges' where you turn the signal on from zero or off to zero, they can have surprisingly high frequency content if you're using a good, fast switch - easy to fix with a little bit of capacitive bypassing though. It pays to have your load well characterised (some heaters can have non-trivial inductance) and, if necessary, to take it into account in devising your EMI suppression and switch protection.

author: Gyro
author_rank: Super Contributor
timestamp: November 09, 2019, 06:51:00 pm
content: Quote from: IanB on November 09, 2019, 06:33:36 pmAgreed, that is the optimal (normal?) way of doing burst-fire control.

author: floobydust
author_rank: Super Contributor
timestamp: November 09, 2019, 07:36:45 pm
content: Normally, heater control is done using many cycles on, many cycles off PWM because a heating element has a large time constant (thermal mass). You would not skip a cycle.Net DC offset is not permitted in the high power heater standards to protect the distribution transformer. It depends on how many kW this is, hot water or what.

author: IanB
author_rank: Super Contributor
timestamp: November 10, 2019, 04:10:12 am
content: Quote from: floobydust on November 09, 2019, 07:36:45 pmIt might be the instant heat electric shower from the previous thread. If so the action should be reasonably fast or the water will run hot and cold.

author: floobydust
author_rank: Super Contributor
timestamp: November 10, 2019, 05:24:30 am
content: Quote from: Brutte on November 09, 2019, 02:34:06 pmWe don't know the application, wattage, or the thermal time constant.Your standard treez thread that wastes everyone's time guessing details. 10msec off time is nothing for a heater.

author: ocset
author_rank: Super Contributor
timestamp: November 10, 2019, 12:17:13 pm
content: Thanks, yes, its for an domestic electric shower.Thanks, do you think the attached mains input current waveform would pass EN61000-3-11?Its 32ARMS.If this were the mains input current waveform to a device, would it fail regulations?One whole mains cycle has been "sliced out" at the zero crossing, every 5 mains cycles.

author: ahbushnell
author_rank: Frequent Contributor
timestamp: November 11, 2019, 04:43:29 pm
content: Quote from: treez on November 10, 2019, 12:17:13 pmThat should work.

author: Phoenix
author_rank: Frequent Contributor
timestamp: November 13, 2019, 03:24:50 am
content: Quote from: treez on November 10, 2019, 12:17:13 pmThis looks like how I would go about making flicker if I intentionally wanted to.As for 61000-3-11, you have a simulation, put in the reference impedance and measure the result. Compare to the limits.

author: Syntax Error
author_rank: Frequent Contributor
timestamp: November 13, 2019, 09:16:36 pm
content: Are there any inductive results from pulsating a heating element with 8Kw of raw power? If so, would this set the current and voltage out of phase enough to break the triac in the blanking gap?

author: temperance
author_rank: Super Contributor
timestamp: November 15, 2019, 07:04:27 pm
content: Part from anything else: if this is for a commercial product and you have to pass some regulations, then pulse skipping can be ruled out as a solution because it will probably generate to much flicker. You might want to investigate this before you continue.As you might find out, passing CE regulations is not a matter of luck.

author: NiHaoMike
author_rank: Super Contributor
timestamp: November 16, 2019, 01:32:38 am
content: The voltage sag is going to be the same whether switched with a relay or a solid state switch. If the supply is up to spec, the sag should remain within limits. If not pulling a substandard supply out of tolerance is important, the only reasonable way is to actually reduce the load such as by dividing the heater into multiple parts and switching them separately.If you're worried about many units synchronizing and causing a huge peak load, you can randomize the switching based on a PRNG seeded by reading the LSB of the microcontroller's ADC many times on startup.

author: David Hess
author_rank: Super Contributor
timestamp: November 16, 2019, 03:29:03 am
content: At high power levels, half wave rectification can cause excessive saturation current in the transformer. If the power line cycling is random, then I would not worry about it.

author: Cerebus
author_rank: Super Contributor
timestamp: November 16, 2019, 11:53:48 am
content: Quote from: David Hess on November 16, 2019, 03:29:03 amNot to pick on you David, but I'm constantly surprised how quickly on this (possibly any) forum people start rehashing points already made or answers already given. The whole DC saturation/half cycle switching/full cycle switching thing was raised and dealt with in message #4et seq. It does seem to happen sooner/faster on here than on many of the other fora that I've participated in.If there were pages and pages of previous discussion I can understand how it might happen, but with all of 16 messages (less than a page) in total when David commented, it seems a bit surprising for it to happen - surprising to me at least. Do people just not read what's already been said before commenting?

author: splin
author_rank: Frequent Contributor
timestamp: November 20, 2019, 10:07:08 pm
content: Quote from: temperance on November 15, 2019, 07:04:27 pmFor 'it will probably generate to much flicker' I'd suggest a better phrase would be 'it will almost certainly generate too much flicker'. You can get a free simulator here:https://www.solcept.ch/en/tools/flickersim/If it's a one off then I wouldn't worry too much as very few people use incandescent bulbs any more and if they do the more flicker the better to encourage them to replace them!I guess some LED lights could also be susceptible to flicker though depending on the type of driver.

author: David Hess
author_rank: Super Contributor
timestamp: November 23, 2019, 05:06:37 pm
content: Quote from: Cerebus on November 16, 2019, 11:53:48 amI should have paid closer attention but if the forum software tracked and displayed which replies have been read, like email and news forums have for *decades*, then it would be easier for the user to know which messages are new and which they have already read before. Discussion forums on the internet have taken steps backwards in usability sinceEternal September.

author: fourtytwo42
author_rank: Super Contributor
timestamp: November 23, 2019, 06:52:43 pm
content: Quote from: David Hess on November 23, 2019, 05:06:37 pmI wouldn't get excited up about it, after all its a treez post and nobody reads those expecting anything serious

author: David Hess
author_rank: Super Contributor
timestamp: November 23, 2019, 07:05:33 pm
content: Quote from: fourtytwo42 on November 23, 2019, 06:52:43 pmI think it was Kleinstein's post he was referring to but I am not worried about it when the forum structure encourages it.

author: ocset
author_rank: Super Contributor
timestamp: November 24, 2019, 10:51:20 am
content: Interestingly, The following electric shower..https://www.mirashowers.co.uk/showers/electric-showers/mira-decor-dual-108kw/has an extremely fine resolution in its temperature setting, so it must be doing mains burst fire control…….there is pretty well no other economically feasible way to do fine adjustment temperature control in an electric shower.

author: Gyro
author_rank: Super Contributor
timestamp: November 24, 2019, 12:35:50 pm
content: More likely, a decent quality thermal wax capsule flow regulator, as used in most modern mixer showers.Manufacturers will go simpler and cheaper, rather than excessive complexity.

author: ocset
author_rank: Super Contributor
timestamp: November 24, 2019, 01:30:48 pm
content: Thanks Gyro,Though sorry because I should have made it clear that I am only speaking about “electric showers” here…….ie, that only have a cold inlet. (no mixing)The shower in question (2 posts up) has three flow settings, and a large number of temperature settings. Such a wide range of temperature setting could only be economically feasibly produced by burst fire mains operation, with variable burst lengths. Would you agree?

author: Gyro
author_rank: Super Contributor
timestamp: November 24, 2019, 02:58:04 pm
content: Yes I was aware that you were talking about an electric shower. I was saying that the same wax cartridge technology can be used (as is used in a mixer type) in order to maintain temperature across different flow rates as you switch between different sized shower heads - one of the features.I suspect that the three flow rate settings are in fact three coarse (step) flow adjustments accompanied by three selected element power configurations and the fine control is a finer continuously variable flow rate adjustment. As I said, a wax capsule would probably take care of the rest. It would have to be configured that way, otherwise on flow setting 3 it would freeze you and on setting 1 it would take your skin off!You can achieve a lot of variation with a three circuit element and sufficient flow control. I don't see anything to suggest a more advanced electronic system.

author: nali
author_rank: Frequent Contributor
timestamp: November 24, 2019, 03:15:11 pm
content: Quote from: treez on November 24, 2019, 01:30:48 pmNo, I'd concur with Gyro. Look at the drawing of the internals in the instructions; the temperature control goes into what looks very much like a mechanical mixer/regulator.

author: ocset
author_rank: Super Contributor
timestamp: November 24, 2019, 06:45:44 pm
content: Having re-looked at the shower of post #23, I now agree with your words. It’s the flow rate that is being adjusted to give a wide range of temperature outputs.However, there is a very big problem with this. It means that people may end up using a very low flow rate, which is very bad because it results in scaling up of the shower system….also, if your shower is on high power and high flow rate, then you quickly adjust the flow rate down…it can result in significant scalding…as well as temporary thermal shut down of the shower.So this is why mains burst fire control, using a fixed flow rate, (or perhaps just two very similar flow rates) would actually be the best way to control an electric shower.Please see point 13 (page 3 ) of the “Mira Décor” shower……Mira décor shower manual (£400 electric shower)http://resources.kohler.com/plumbing/mira/pdf/1365393-w2-a-mira-decor-installation-and-user-guide.pdf…this is diabolical “DO NOT switch the shower off and back on while standing in the water flow.”….when soaping up in an electric shower..it is essential to be able to switch the shower off….then back on again when you wish to rinse off the soap….Burst fire mains control is the way forward.

author: Gyro
author_rank: Super Contributor
timestamp: November 24, 2019, 08:52:36 pm
content: Sadly, scaling is inevitable in hard water areas. Even an indirect hot water cylinder fills up with limescale, and the heat exchanger coil in that case is only heated to 60-80'C by the boiler. So called Combi boilers need water softeners and still suffer short lives in similar situations, kettle need descaling frequently etc.The universal problem with all electric showers is that the water flow through the small heating chamber is so fast that the element dissipation an surface temperature must be ridiculously high in order to 'flash heat' the water.If a low temperature indirect water cylinder scales up, then you basically have no chance with and electric shower, no matter what the control method you use - you simply can't heat the water fast enough at low element temperatures. The 10.8kW output and surface temperature of an electric shower element makes an ordinary domestic kettle element look like a toy.With regard to scald risk (I note that the specification for the Mira shower that you keep referencing says:QuoteHigher quality shower models maintain the water flow through the heating chamber for a few seconds after the element is turned off, to draw off the excess heat and bring it down to a safe temperature (which also helps with a little with scaling). Burst fire simply isn't going to help you with this - you need to dump the residual heat of the element / heating chamber into flowing water when you turn it off.

author: ocset
author_rank: Super Contributor
timestamp: November 25, 2019, 07:57:43 pm
content: Thanks, yes I see your good points.What I would add is, that I have lived in some 35 different rental accomodations, and in virtually all of them, the electric shower had been pranged by the landlord to always run on maximum flow.The reason for this is because adjusting the flow rate of an electric shower can lead to problems…..because if one restricts the flow rate too much, too suddenly, then the heater element overheats…and even though there is a thermal shutdown, it never really operates fast enough, and so cumulative damage is done to the heater element and other parts.Another point about slow flow rates is that it increases limescale build-up….more so than faster flow rates.This is why shower temperature adjustment by flow rate adjustment is a poor, el-cheapo solution to the problem.

author: Gyro
author_rank: Super Contributor
timestamp: November 25, 2019, 08:27:49 pm
content: Yep, electric showers are a pain in the neck generally, even the ones with low flow shut-off.The one that maintained water flow, until the heat in the thermal mass of the element/heating chamber had dissipated, was in a holiday cottage that we rented in June. It seemed to fix the scald problem completely.Unfortunately it's a bit far to go back and check the make and model.