Strange behavior of the extractor blower at the stock ramp of Carmel hospital « on: May 11, 2025, 04:39:31 AM » Author: dor123
When the extractor blower at the stock ramp of Carmel hospital being turned off and back on by the emergency generator of Carmel hospital, behaves like this: https://www.youtube.com/watch?v=7lTKYye_pis What is the reasons?
« Last Edit: May 11, 2025, 09:06:50 AM by dor123 »		Logged

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	Re: Strange noise of the extractor blower at the stock ramp of Carmel hospital « Reply #1 on: May 11, 2025, 05:18:06 AM » Author: AngryHorse
That sounded to me like they threw it in reverse?, either that or it’s got some form of DC break on it?, before the emergency kicks in?
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	Re: Strange noise of the extractor blower at the stock ramp of Carmel hospital « Reply #2 on: May 11, 2025, 05:27:03 AM » Author: dor123
What is happening is that the motor slows down, than suddenly breaks and than speed up. There is a delay for several seconds before the breaking and speeding up. It don't spinning backwards. Why this is happening.
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	Re: Strange noise of the extractor blower at the stock ramp of Carmel hospital « Reply #3 on: May 11, 2025, 05:31:04 AM » Author: AngryHorse
They must have to almost stop it before the emergency kicks in? Wonder if it’s something to do with disturbing the sine wave from changing to another supply? 🤔
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	Re: Strange noise of the extractor blower at the stock ramp of Carmel hospital « Reply #4 on: May 11, 2025, 08:35:34 AM » Author: dor123
@Ash, @RRK, @Medved?
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	Re: Strange behavior of the extractor blower at the stock ramp of Carmel hospital « Reply #5 on: May 15, 2025, 03:15:17 AM » Author: Medved
It looks like it is driven from some VFD, programmed to soft start the motor. What then may happen is the power cut causes the thing to reset, so after power gets back on it starts from low frequency, assuming the fan is starting from still. But because the fan is still spinning by inertia, the VFD sequencer is not aware of it and is not able to detect it, but starts from the low frequency anyway, it then results into slowing it down to the speed corresponding to the low frequency and then speeding back up as the "startup" sequence continues. I would be afraid such sudden slowdown may cause overvoltage within the VFD (when slowing down, the motor acts as a generator) and also huge current surges in the output power stage, unless it really has an active braking capability (braking resistor, intentional lossy motor drive mode to dissipate the power in the motor itself,...). The way how it slows down sounds to me like not much controlled, so really rises suspicion the thing is not designed for this. Sounds fishy to me. What i would expect is the thing to be programmed so to extract controller supply from the spinning motor and so prevent the reset and follow the motor speed (until it really stops), so the restart rampup would then continue in a smooth way from the speed the thing was at at the moment the power gets recovered or the thing is commanded to restart. But there are many ways to treat these brief power interruption, some of them may lead to this behavior and the whole thing being designed for that, so in that case there won't be any issue at all...
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	Re: Strange behavior of the extractor blower at the stock ramp of Carmel hospital « Reply #6 on: May 15, 2025, 07:56:48 AM » Author: dor123
Indeed there is no 50hz humming from the motor, but what is the advantage of using VFD on it, if it always spinning at the same speed and never being slowed or fastened? Also: The HVAC cooling towers fans and pumps don't behaves like this when restarted, despite having VFD as well.
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	Re: Strange behavior of the extractor blower at the stock ramp of Carmel hospital « Reply #7 on: May 15, 2025, 01:40:07 PM » Author: Medved
There are very diverse types of VFDs, plus many could be set up different way. Why to use VFD? In an HVAC you have to carefuly balance the airflow through various blowers, so you keep pressures between the building interior and outside, or between different sections correct. Otherwise you will have problems with doors being hard to openin or close, door closers may not work reliably creating a fire hazard (the fire door won't isolate the sections how it is supposed to, when not closed). Tradditionally the blowers had beld drive, with one pulley adjustable so you may adjust the gearing ratio so the blower speed and output. These pulleys were quite a pain from reliability perspective (they tend to wear out, create quite a load on the bearing which then tend to fail sooner), so once there was an option to use direct drive blower without any gearing or pulleys, the industry went for it. And the VFD is there the thing allowing to adjust the blower speed as needed for the building air balance.
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	Re: Strange behavior of the extractor blower at the stock ramp of Carmel hospital « Reply #8 on: May 15, 2025, 02:02:13 PM » Author: dor123
I asked about the extractor blower, not the HVAC cooling towers, why it have VFD if it never changes speed. The fans and the pump in the cooling tower, indeed changing speed, sometimes they run slow and sometimes runs fast. But the extractor blower runs always at the same speed, so why it needs a VFD if its operation speed is never changed.
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	Re: Strange behavior of the extractor blower at the stock ramp of Carmel hospital « Reply #9 on: May 15, 2025, 02:48:43 PM » Author: Medved
What extractor is it? I though it is an air extractor from the building (cooking fumes extractor,...), so also a part of the HVAC system. HVAC is not only the air cooling or heating, but all the air management together, with pushing in fresh air via filters, extracting dirty air from the building, temperature and humidity control, safe emergency exit air handling,... Those needs to be balanced, the extractors balanced with the blowers pushing fresh air into the building, otherwise a pressure differential forms, causing all the problems with doors or so. And even when speaking about some other air manipulation, very likely that needs to be balanced as well (like forced flue extraction with forced air induction on furnaces,...). The speeds are adjusted when the system is being balanced after the system installation or overhaul, then the settings stay fixed. So the VFDs are adjusted once and stay that for the whole life of the equipment (so till the overhaul or some big changes). It is not being altered during operation.
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	Re: Strange behavior of the extractor blower at the stock ramp of Carmel hospital « Reply #10 on: May 23, 2025, 11:52:24 AM » Author: dor123
@Medved: This extractor sucking air from one of the clinics of Carmel hospital, day hospitalization I think.
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	Re: Strange behavior of the extractor blower at the stock ramp of Carmel hospital « Reply #11 on: May 24, 2025, 12:25:36 AM » Author: Medved
Any such extractor blower is "paired" (aka balanced) with some other blower (or some part of its capacity, if tgat is serving more things) pumping fresh air back in (often called Make up air function). That is how the "Ventilation" part of the "Heating, Ventilation, Air Conditioning" system works. And this pairing, aka balance, needs to be precisely adjusted, otherwise it will either create too high overpressure (when the Make Up Air has higher flow than the Extraction) or underpressure (when the extractor is too strong). It is practically impossible to just design the system upfront, assemble it and get the balance right, the unknown and tolerances are way too high, so it always needs to be adjusted once the building is build and even equipped and with all the furnture, often also finetuned when it starts operating and people and materials are moving in and out (opening and closing the entrances/exits, as well as a lot of people in the corridors are quite significant factor upsetting the balance). And this balance could be set by various means. Installed higher power blowers and then use chokes in the ductwork are the simplest way, but most energy hungry (blowers consume more than required) and noisy (the choked flow creates extra noise). Very often belt driven blowers, with one of the pulleys adjustable (probably the most common) allows to adjust the blower speed so used power to just what is needed without any added restriction, so they consume only what is really needed to move the set amount of air. But they must be stopped and partially opened before readjusting, plus the adjusting place is at the blower itself, so usually at an awkward place to access so makes the balancing a bit more tedious. Recently the VFDs became cheap enough to be used for this adjustments. It is efficient, but also convenient to adjust, from some better accessible spot but mainly without the need to shut down, disassemble and reassemble the covers for each readjustment try. Plus (probably not the case you are showing) it allows to be used with systems where part (e.g. a kitchen fryer hood,...) s are turned on/off on demand, without the need to have separated blowers and sometimes even ductwork for each work station.
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	Re: Strange behavior of the extractor blower at the stock ramp of Carmel hospital « Reply #12 on: May 24, 2025, 07:39:35 AM » Author: dor123
I think that this blower working alone without pairing with another blowers. The nearby blowers, that don't operating, are smoke blowers. I've never seen the working blower changing speed. I think that its VFD used only to soften the starting. I hope @Ash would join to this thread.
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I"m don't speak English well, and rely on online translating to write in this site.
Please forgive me if my choice of my words looks like offensive, while that isn't my intention.

I only working with the international date format (dd.mm.yyyy).

I lives in Israel, which is a 220-240V, 50hz country.

	Re: Strange behavior of the extractor blower at the stock ramp of Carmel hospital « Reply #13 on: May 24, 2025, 11:15:07 AM » Author: Medved
It can not be alone, it would upset the pressure balance. It does not have to be paired with other blower 1:1, but it could be just paired with the corresponding capacity. Could be a shutter flap, could be altering another blower speed settings, could be by working alternatively with other extraction fan/fans (when this blower is turned on, those other are shut down and vice versa). It is also possible this blower settings was kept on the initial default (usually the normal mains frequency), because the system happened to be balanced without touching this adjustments. But a first upgrade or component replacement and the need to adjust the output of this blower may pop up. All this are just the reasons why you could find VFDs seemingly useless, with fixed speed settings and operated just by turning off and on the incoming power. And it also could be just a soft start, but I doubt tyat was the reason. In an HVAC the need to have the possibility to adjust the output is the way more likely explanation. The soft starting is then just a nice bonus, not the main motivation to install complexities like a VFD. The refrigeration compressors for that HVAC are way stronger shock load (even when using VFD themselves, just the sheer power of a single such unit) than such blowers and the electrical system needs to handle that, even when on generators, such blower won't cause any problem, maybe beside turning on all installed in the building at once but that uses to be handled by delayed starts. And back to the topic: The blower is operated on a VFD for some reason or other, and that VFD does not have that well sorted out the hot restart (when it is powered up when the motor is still spinning) behavior, so much I won't even be surprised it causes some internal overstress and it won't last long.
« Last Edit: May 26, 2025, 01:08:48 AM by Medved »		Logged

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