Is PFC needed in motor controller (1ph bridge+caps)? CE and other markings - Page 1
Forum section: electronics / projects

author: krisRaba
author_rank: Regular Contributor
timestamp: June 17, 2021, 10:22:45 am
content: Hi,I would like to ask you about power factor in relation to regulations/markings like CE and other. Controller wattage is somewhere about 400W.Standard single phase bridge rectifier + capacitors result in low PF, somewhere near 0.6 as far as I know. I am not sure if it matters or not but low PF is here a result of current shape (current is drawn only on sinewave peaks to charge capacitors) and not phase shift as it can be in case of capacitive or inductive load where current is also a sinewave but with different phase.So rectifier + capacitors will produce harmonics because of weird current shape.Is it possible to meet requirements for CE or other markings with such topology and without PFC onboard?For examplePN-EN 60730-1:2012 "Automatic electrical controls for household and similar use"allows PF 0.6 for devices controlling inductive loads. So maybe PF itself is not a problem.I have found in some listing a document calledIEC 61000-3-2:2005, Electromagnetic compatibility (EMC) – Part 3-2: Limits – Limits for harmonic current emissions (equipment input current ≤ 16 A per phase). Unfortunately I don't have access to full text but Google finds some guides like this (page 3)https://www.dsce.fee.unicamp.br/~antenor/pdffiles/IEC.pdfWith EUT connected, the harmonic ratiosof test voltage shall not exceed:Limits Harmonic order0,9% - 3th0,4% - 5th0,3% - 7th0,2% - 9th0,2% - even, from 2 to 100,1% - from 11 to 40So harmonics can be a problem with such current shape, I think.But again - have you any experience with that? Should such controller have a PFC or it is not required?I have a similar controller with CE marking laying on my desk and it doen't have PFC inside. Measured PF is 0.6 as well. But declaration of conformity (CE) is related to electrical and thermal safetyNothing about power quality in their declaration

author: Wolfram
author_rank: Frequent Contributor
timestamp: June 17, 2021, 11:25:19 am
content: For a commercial product drawing 400 W, it would be very hard to get it certified without some sort of power factor correction. 61000-3-2 is most likely the relevant standard here as you found, depending on who the end-user is and what environment it will be used in. The limits in your post apply to voltage THD at the feedpoint, which is a function of harmonic currents and supply impedance (often expressed as the short-circuit ratio of the incoming power feed). This means that the limits in current THD depend on parameters of the electrical installation where the equipment will be used.IIRC, the standard has some conservative limits if the supply impedance is unknown, and if you comply with those limits then the equipment can be used anywhere without considering the short-circuit ratio of the power feed. Those limits are easy to find, they are even listed on the Wikipedia article for 61000-3-2https://en.wikipedia.org/wiki/IEC_61000-3-2. And yes, these limits make it practically impossible to draw more than a hundred watts or so with an uncorrected rectifier-capacitor load.If you plan on releasing a certified product, the cost of the standards will only be a small fraction of the investment. Still, you can save significant money here by purchasing nationalized standards from other EU countries that charge less for them. The content is often identical, and the price can be lower by an order of magnitude compared to buying them directly from IEC.

author: krisRaba
author_rank: Regular Contributor
timestamp: June 17, 2021, 11:57:47 am
content: Thank you for your answer. I have made a quick simulation and it looks like rect.bridge + caps generate quite high harmonics.It doesn't look too good

author: jbb
author_rank: Super Contributor
timestamp: June 17, 2021, 09:28:36 pm
content: I have an ugly suggestion: do you need to use a big cap and steady DC bus?If you’re using a DC or universal motor, maybe you could have a small DC link cap (eg 1uF) and accept a pulsating DC link (ie rectified sine wave). Motor current would then be pulsating - which causes torque ripple, vibration and noise - but maybe that’s a worthwhile trade off if it gets you away from a separate PFC converter stage.

author: jonpaul
author_rank: Super Contributor
timestamp: June 18, 2021, 05:05:44 am
content: To the OP: Suggest to Start by learning about PFC, a huge topic.PFC measurement, requirements and solutions are very well know, please consult the many books and papers and seminars on PFC, eg IEEE, PESC, PELS,An old colleague Dr REDL is a world expert and has dozens of fine papers and seminars.EU and other Customs now can block entry of non-compliant products, or even require proof of testing.If your firm is going to manufacture an industrial product, you should purchase all applicable standards for EMI, PFC, transient, safety.Search your market areas (where it shall be sold) for the requirements.Most USA/CA/EU standards are harmonized, so IEC, CISPR, UL, will be interchangeable.Using partial free excerpts off the Web is risky.The PFC harmonics requirements will apply to a 400 W device, and normally a PFC prestage is needed.Many off the shelf solutions are possible.Beware that any SMPS or PFC generate EMI conducted and radiated,Compliance with EMI, also transient susceptibility is applicable.Bon Chance,Jon

author: krisRaba
author_rank: Regular Contributor
timestamp: June 18, 2021, 07:00:58 am
content: Quote from: jbb on June 17, 2021, 09:28:36 pmI appreciate that you tried to be funny. Maybe some other time you will succeed...

author: krisRaba
author_rank: Regular Contributor
timestamp: June 18, 2021, 07:11:29 am
content: Quote from: jonpaul on June 18, 2021, 05:05:44 amThanks. I have already started to learn. Looking at the PN-EN IEC 61000-3-2:2019-04 I can see that there is no chance to meet class D requirements (Table 3) with bare bridge + capacitors and without PFC.I have found some interesting solutions for CCM PFC and it doesn't seem to be very complicated. The biggest problem is the time for redesign to fit all additional componentsAt first I was hoping for some solutions that can be added on input, as separate module. something like passive PFC, harmonics trap filters etc. But most of solutions found weren't very promising. Or mayby you know something that could work like that?

author: jonpaul
author_rank: Super Contributor
timestamp: June 18, 2021, 08:13:56 am
content: Hello again: KrisRaba, Very fine, great to learn!There are some integrated PFC plus DC PSU topologies, have not seen a motor inverter with integrated PFC.The designs use a boost PFC to achieve a 99% PF 360V bus and then whatever inverter or converter is needed for requirements.The preregulator of the boost PFC, often simplifies and eases the design of the inverter or converter as the DC bus is now perfectly regulated.Check IEEE transactions group IA - Industrial Applications and group PE = Power Electronics for articles and info.Prof Fred LEE at Virginia Polytechnique did a lot of work on power electronics...The PFC class needs to be checked, am unsure if your application is indeed class D.Bon Chance,JonPS: what country are you, dont recognize the flag (;-:)

author: Berni
author_rank: Super Contributor
timestamp: June 18, 2021, 09:03:23 am
content: Quote from: krisRaba on June 18, 2021, 07:00:58 amThis is something that a lot of products actually do. So its not a joke and it does improve the power factor.Most products related to lighting do this. Be it compact fluorescent bulbs, electronic ballasts for fluorescent bulbs, LED lamps, 12V supplies for halogen bulbs etc... So these lamps still exhibit a 100Hz hum in there light output, despite being powered from DC rectified mains.You can also find products with permanent magnet brushed motors that run from rectified mains and also skip the capacitor smoothing altogether. They likely reason that if they had used any other single phase AC motor the torque output would have the same 100Hz pulsation anyway. Its only 3 phase motors that actually have a smooth continuous torque output.There are more crude ways of improving power factor of a switching power supply. For example most PC ATX power supplies operate with similar power levels while most do not have any form of active PFC (except the high end expensive ones). They use a passive PFC in the form of a carefully tuned inductor to smooth out the rectified current. It's not the best performing solution but apparently it works well enough to pass standards.

author: mikerj
author_rank: Super Contributor
timestamp: June 18, 2021, 09:31:40 am
content: Quote from: krisRaba on June 18, 2021, 07:00:58 amWould you care to explain why you think this is a joke?

author: Zero999
author_rank: Super Contributor
timestamp: June 18, 2021, 12:27:43 pm
content: Quote from: mikerj on June 18, 2021, 09:31:40 amI don't see how that comment can be taken as a joke.If it's driving a 230VDC motor, from 230VDC, then you don't want a huge filter capacitor, because it would increase the voltage to the motor, towards the peak volage of 325VDC, which might damage it.How big is the motor? Does it need to have a smooth output torque? Using unfiltered DC is definitely a viable option.Is it possible to change the motor, to a three phase induction motor? There are plenty of off the shelf, 230V single phase input, three phase output, variable frequency drives available.Consider buying an off the self, power factor corrected power supply. It has the advantage of having an isolated output. If you design your own, a power factor correction stage is a boost converter, which will give a higher output voltage, than the peak maximum specified input voltage, so >360VDC, assuming you have 230V nominal +10% in. You'll then need to use a buck converter to reduce it to a regulated 230V, to power the motor controller. By the time you've done that, it will probably work out cheaper, just to buy a ready made power supply.

author: Circlotron
author_rank: Super Contributor
timestamp: June 18, 2021, 01:29:54 pm
content: Quote from: Berni on June 18, 2021, 09:03:23 amHopefully they actually use a real inductor...https://www.eevblog.com/forum/chat/dishonest-chinese-power-supply-_engineering_/

author: Siwastaja
author_rank: Super Contributor
timestamp: June 18, 2021, 01:33:53 pm
content: Quote from: Zero999 on June 18, 2021, 12:27:43 pmDid you read the OP? It seems obvious to me this is aboutdesigningwhat basically is a VFD (or if it is a brushed motor, the control is simpler, but regarding the question about PFC, it's the same); a device which has bridge rectifier and filter caps to form a DC link which is then converted using half bridge(s) to whatever the motor requires (ACIM, BLDC, brushed DC...)Seeing this description, the motor type basically has to be ACIM, PMSM/BLDC or brushed DC (series wound or permanent magnet). In any case, it doesn't matter regarding the PFC question.So suggesting off-the-shelf motor controller when the OP is interested indesigningone doesn't solve the problem at all.And no, the DC link caps do not "increase the voltage to the motor". The whole point of the motor controller is to regulate the power conversion.And yes, letting the mains ripple through may be a viable option. But seeing OP thinks it as a joke might suggest they are willing to create a good performance motor controller similar to commercial VFDs or other types of existing motor controllers. I guess...What really matters is getting more details from the OP; what they are actually building. For different types of devices for different markets, the rules are different.

author: jonpaul
author_rank: Super Contributor
timestamp: June 18, 2021, 02:12:17 pm
content: To the OP, krisRabaYou will get better response if you post a schematic or link for the motor controller.One issue with a passive PFC is the change in harmonics with torque (load) and perhaps speed of the motor.Jon

author: Zero999
author_rank: Super Contributor
timestamp: June 18, 2021, 07:24:07 pm
content: Quote from: Siwastaja on June 18, 2021, 01:33:53 pmYes, I did read the original post, several times and no, it isn't obvious to me at all. He just said motor controller and when someone says that to me, I automatically think of at brushed motor. If he'd said BLDC, induction, or VFD, then it would have been obvious.Good point about not worrying about the filtered voltage being too high, as the controller can just reduce the duty cycle to compensate. There's no need for a buck converter, as the motor controller already does that, using the motor's inductance.Either way, I wouldn't consider doing this from scratch, unless it's something very specialised.Searcing for power factor correction rectifier IC, generates some interesting results.https://www.google.com/search?client=firefox-b-d&q=power+factor+correction+rectifier+IC

author: Siwastaja
author_rank: Super Contributor
timestamp: June 19, 2021, 07:53:57 am
content: Yes but for the question, whether the motor is brushed or ACIM or BLDC doesn't matter the slightest. The commutation is different, but all types mentioned typically use a smoothed DC link in the controller, but all can use a ripple-y bus if you accept the resulting torque ripple. All these controllers do is regulate the energy transfer from the bus to the windings, and obviously you can only do it when you have voltage available on the bus. Brushed motor does the commutation (i.e., DC to AC conversion) for you on mechnical domain; for brushless motor types, you need to do that yourself, but the fundamentals about energy transfer into windings won't change.

author: krisRaba
author_rank: Regular Contributor
timestamp: June 21, 2021, 12:53:35 pm
content: Whoa, long discussion hereI used to the fact that after short time topics die by natural deathAnswering to some of your questions - I use a very specific multipole BLDC motor. It can handle up to 600V so I don't care about exact DC bus voltage but there are some other factors like components ratings, clearance and creepage distances etc. that limit meI haven't try to get rid of capacitors but I don't think the system would like it. Even with rather smooth DC link there are plenty of problems to solve, tune etc.So far I have decided to use a workaround - there are power modules available with integrated bridge and APFC inside. They aren't cheap but with low production volume it can be cheaper that way. Maybe in the meantime I will think how to integrate APFC circuit to the controler.

author: Zero999
author_rank: Super Contributor
timestamp: June 21, 2021, 02:40:35 pm
content: I was thinking about something like this.https://uk.rs-online.com/web/p/embedded-switch-mode-power-supplies-smps/1306952/Regulated 360V output.Power factor corrected.95% efficient.Not isolated though.If you want isolation, then there's this, but it's a little overkil, although the extra power might come in handy when it's accelerating.https://uk.rs-online.com/web/p/embedded-switch-mode-power-supplies-smps/8399698/Other suppliers are available.

author: krisRaba
author_rank: Regular Contributor
timestamp: June 21, 2021, 03:32:20 pm
content: Yeah, very similar to the one I have foundIsolation is not needed in my case and I hope it can be stressed with larger current on motor startup. We will see