Fan heater motor start-up current? - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: April 06, 2023, 06:01:22 pm
content: Hi,Need to find smallest SMPS transformer that can power a 24V fan which is meant to blow hot air onto icey car windows. Its mains powered as the cars are taken into the garage.So VAC input is 100-265VAC. Output is 24V. Currently the fan is taking 8W at 24V, but they want to increase fan speed to 35000 rpm. Assume it’s a BLDC fan as they weren’t adjusting the voltage to adjust the speed. He adjusted speed by adjusting a pot on a control board.The guy reckons it will need 700mA for 35000rpm, but said that the SMPS should best be rated for 24V, 1A, since otherwise it will drop out when the fan starts up.Anyway, do you have any idea what kind of current the fan may take when it starts up, and how long it will draw such overcurrent? Sorry, I don’t know the fan part number.

author: ebastler
author_rank: Super Contributor
timestamp: April 07, 2023, 09:02:36 am
content: Quote from: Faringdon on April 06, 2023, 06:01:22 pmIcey car windows + hot air = mechanical stress = risk of cracked glass?Garage = above-freezing temperature = ice will melt on its own?I think the choice of SMPS is not the key question to be answered for this product concept...Also, you have talked about the fan -- but where does the heat come from? An electric heater will obviously take much more power than the fan.

author: Gyro
author_rank: Super Contributor
timestamp: April 07, 2023, 10:12:52 am
content: Stall the rotor, measure the current. It's that simple!

author: Faringdon
author_rank: Super Contributor
timestamp: April 07, 2023, 01:48:37 pm
content: Thanks, for how long will this overcurrent flow....presumably till it gets up to speed?Also, presumably i can just measure stator resistance?

author: Faringdon
author_rank: Super Contributor
timestamp: April 08, 2023, 10:51:24 am
content: Thanks, to be honest, i reckon that even if the power supply is limited to 700mA max, the fan will still start up, but the output volts of the PSU will be dragged down for a second or two?I mean, having a PSU rated for higher current than the nominal current of the motor sounds a bit unwise?

author: Zero999
author_rank: Super Contributor
timestamp: April 08, 2023, 11:19:37 am
content: Quote from: ebastler on April 07, 2023, 09:02:36 amAs long as the air isn't extremely hot, it shouldn't crack the glass. Air has a much lower heat capacity than water. Cars have hot air blowers to de-ice windows, but they only work whilst the engine is running, which is not possible to safety to in a garage, unless there's a hose from the exhaust pipe to outside, to get rid of the dangerous CO.In extreme, prolonged cold spells, the air in the garage can fall below freezing. It's unusual for it to happen in the UK, but does for at least a week in one of every five winters.Quote from: Faringdon on April 06, 2023, 06:01:22 pmDon't worry about the inrush current. It will be limited by the driver. Just select an SMPS rated to deliver the rate current, plus a little more, as it's always good not to run it at full capacity.

author: Gregg
author_rank: Super Contributor
timestamp: April 08, 2023, 07:49:19 pm
content: Faringdon / Treez:Once again you are attempting to get forum members to solve a problem with insufficient data or explanation. Please stop this behavior and organize your questions before posting; you may even find your own answers while attempting to get organized.Have you looked at fan curves for the device in question? Many fans require exponentially more power as their speed increases. Other things like back pressure are variable with application.Without such information the alternative is to experiment and measure the actual devices in their final configuration. It actually may take less time to get to work in the real world than to ask poor questions.

author: Faringdon
author_rank: Super Contributor
timestamp: April 09, 2023, 10:06:11 am
content: QuoteThanks, the thing is, its an offline smps and the smps controller has overload shutdown...so in other words, if it doesnt get into vout regulation within 300ms, then it will just shut down.QuoteThanks, the great members have provided superb advice so far.Wish i had a lab then i could just go in and do the test, but till a few days time, dont have any access to any stuff.Also, When youre in the lab, theres only a limited time slot, you have to "get ready" so to speak.I guess this is the way it is when you work for places, where their main profit is simply coming from "behind the scenes" direct Chinese importation....the "hands on" stuff is more just the "token" business, to "Oil the wheels" of the lucrative importation market.The name of the game in electronics these days, is to set up a token business, but then rip all your main profit from direct Chinese importation.....which you operate kind of "separate" to the token business.Ive worked for companies who even directly compete with their own "token" products with their vast Chinese imports.....and its all legal...and it makes loads of money.Operating a "token" electronics business makes it easier for you to operate the more lucrative direct Chinese electronics importation business. (for various reasons).BTW, i did have my own lab for 5 months...but it cost £800/month and i just couldnt keep affording it.

author: Gyro
author_rank: Super Contributor
timestamp: April 09, 2023, 10:23:29 am
content: Quote from: Faringdon on April 09, 2023, 10:06:11 amNot that old narative again. Give us a break.

author: jpanhalt
author_rank: Super Contributor
timestamp: April 09, 2023, 11:01:59 am
content: Quote from: Faringdon on April 09, 2023, 10:06:11 amI recall that, your questions about the floor, ESD, ventilation, soldering, violating the lease, and your contract to make a big SMPS...You have been asked to design and/or make a SMPS running from mains that produces 24V at 1A continuous. My advice is, "Do it." BTW, where are you going to build it?

author: Mark
author_rank: Frequent Contributor
timestamp: April 09, 2023, 05:16:01 pm
content: Based on the information provided, it is difficult to accurately estimate the current drawn by the fan during startup without knowing the specific fan part number or having access to its specifications. However, it is common for BLDC fans to draw higher current during startup, which can be several times the normal operating current.Assuming the fan draws 700mA at 24V during normal operation at 35000rpm, it is recommended to select an SMPS transformer with a slightly higher current rating to allow for the potential overcurrent during startup. A 24V, 1A transformer should be able to handle the normal operating current and potential overcurrent during startup.Additionally, when selecting an SMPS transformer, it is important to consider the efficiency and power factor requirements to ensure optimal performance and compliance with relevant standards. It is recommended to consult with a qualified electrical engineer or use specialized tools for transformer selection to ensure the best performance and safety for your specific application.

author: Faringdon
author_rank: Super Contributor
timestamp: April 10, 2023, 08:51:22 am
content: Thanks, supposinging we cvlamp the output current to 700mA....(nominal max)...will the BLDC then not start up?.......or would it just start up, but pull down the output voltage for a few seconds?

author: pardo-bsso
author_rank: Frequent Contributor
timestamp: April 10, 2023, 03:42:56 pm
content: Hey Faringdon,ignoring for a while the 35 000 rpm, every textbook on electrical machines from ~1900 onwards that you can find in archive.org will tell everything you need about motors and how to measure their parameters for any given operation condition (startup included).At most it's an hour of reading at a very slow pace.

author: Faringdon
author_rank: Super Contributor
timestamp: April 11, 2023, 09:38:28 am
content: Thanks, ayk, it willd epend on the BLDC driver used, and i dont even know the part number for that......but was just wondering generally, what kind of dip in vin is generally tolerated for startup

author: johansen
author_rank: Super Contributor
timestamp: April 12, 2023, 03:24:36 am
content: 24v at 1 amp is 10$ wall wart territory . What in the hell is going on?