So we got a new server rack at work and the AC fans it comes with are very noisy and we don't need anywhere near the airflow they provide.
I opened up the unit and there's four fans. If I put two of them in series (so each fan sees half the voltage) they run at a perfect speed for us. What I want to know is, is this a problem? Will they burn out and set our office on fire over the weekend?
> So we got a new server rack at work and the AC fans it comes with are very
> noisy and we don't need anywhere near the airflow they provide.
> I opened up the unit and there's four fans. If I put two of them in series
> (so each fan sees half the voltage) they run at a perfect speed for us.
> What I want to know is, is this a problem? Will they burn out and set our
> office on fire over the weekend?
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> When you say "AC" do you mean "Air Conditioning" or "Alternating Current"?
> If the former, you should be safe. If the latter, I wouldn't try it.
> On 18 October 2012 11:59, Spoz <mr.s...@gmail.com> wrote:
>> So we got a new server rack at work and the AC fans it comes with are very
>> noisy and we don't need anywhere near the airflow they provide.
>> I opened up the unit and there's four fans. If I put two of them in series
>> (so each fan sees half the voltage) they run at a perfect speed for us. What
>> I want to know is, is this a problem? Will they burn out and set our office
>> on fire over the weekend?
>> --
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What may be the case is that by free-running them in parallel you are
getting audible beating as the fans aren't running quite in sync. The
difference only needs to be very small to lead to beating.
By putting them in series they are forced to run in sync due to the peak in
current flow having to go through both motors at the same time.
Fwiw: I have seen fan shelves for cabinets that have 4 x 120 VAC fans, with
2 each in series, then with each series string in parallel. They rewire
them all in parallel for 120V (eg: USA, Japan).
--
Cef
On Oct 18, 2012 1:41 PM, "James Churchill" <pel...@gmail.com> wrote:
> They couldn't be AC induction motors, because halving the voltage
> wouldn't change the speed of them (as the speed is dependent on line
> frequency).
> I don't think there's any risk here.
> - James
> On 18 October 2012 12:34, tjhowse <tjho...@gmail.com> wrote:
> > When you say "AC" do you mean "Air Conditioning" or "Alternating
> Current"?
> > If the former, you should be safe. If the latter, I wouldn't try it.
> > On 18 October 2012 11:59, Spoz <mr.s...@gmail.com> wrote:
> >> So we got a new server rack at work and the AC fans it comes with are
> very
> >> noisy and we don't need anywhere near the airflow they provide.
> >> I opened up the unit and there's four fans. If I put two of them in
> series
> >> (so each fan sees half the voltage) they run at a perfect speed for us.
> What
> >> I want to know is, is this a problem? Will they burn out and set our
> office
> >> on fire over the weekend?
> >> --
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> Groups
> >> "hackerspace_brisbane" group.
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They work at a slower speed, although I think what's happening is they are slipping under load because of the reduced torque.
After reading that the lifespan of AC fans is reduced with reduced voltage we've decided to just get some DC fans from umart and use those instead, but I'm still curious as to why this might be the case?
On Thu, Oct 18, 2012 at 1:15 PM, Spoz <mr.s...@gmail.com> wrote:
> Alternating current fans.
> They work at a slower speed, although I think what's happening is they are
> slipping under load because of the reduced torque.
> After reading that the lifespan of AC fans is reduced with reduced voltage
> we've decided to just get some DC fans from umart and use those instead,
> but I'm still curious as to why this might be the case?
> To post to this group, send email to hackerspace_brisbane@googlegroups.com
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Wrong - lower voltage decreases current in a passive circuit. Only if
wattage is maintained at a constant level by an active circuit does current
go up (via reducing the resistance).
- James
On Oct 18, 2012 1:46 PM, "Jimmy Bowler" <denomina...@gmail.com> wrote:
> Lower voltage increases current which generated more heat and increase's
> resistance and current. AC theory 101
> On Thu, Oct 18, 2012 at 1:15 PM, Spoz <mr.s...@gmail.com> wrote:
>> Alternating current fans.
>> They work at a slower speed, although I think what's happening is they
>> are slipping under load because of the reduced torque.
>> After reading that the lifespan of AC fans is reduced with reduced
>> voltage we've decided to just get some DC fans from umart and use those
>> instead, but I'm still curious as to why this might be the case?
>> To post to this group, send email to
>> hackerspace_brisbane@googlegroups.com.
>> To unsubscribe from this group, send email to
>> hackerspace_brisbane+unsubscribe@googlegroups.com.
>> For more options, visit this group at
>> http://groups.google.com/group/hackerspace_brisbane?hl=en.
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On Thursday, October 18, 2012 1:46:01 PM UTC+10, Jimmy wrote: > Lower voltage increases current which generated more heat and increase's > resistance and current. AC theory 101
> On Thu, Oct 18, 2012 at 1:15 PM, Spoz <mr....@gmail.com <javascript:>>wrote:
>> Alternating current fans.
>> They work at a slower speed, although I think what's happening is they >> are slipping under load because of the reduced torque.
>> After reading that the lifespan of AC fans is reduced with reduced >> voltage we've decided to just get some DC fans from umart and use those >> instead, but I'm still curious as to why this might be the case?
>> To post to this group, send email to hackerspac...@googlegroups.com<javascript:> >> . >> To unsubscribe from this group, send email to >> hackerspace_brisbane+unsubscribe@googlegroups.com <javascript:>. >> For more options, visit this group at >> http://groups.google.com/group/hackerspace_brisbane?hl=en.
On Thu, Oct 18, 2012 at 12:59 PM, Spoz <mr.s...@gmail.com> wrote:
> I opened up the unit and there's four fans. If I put two of them in series
> (so each fan sees half the voltage) they run at a perfect speed for us. What
> I want to know is, is this a problem? Will they burn out and set our office
> on fire over the weekend?
AC fans most likely have universal motors in them, same type as
dremels and vacuum cleaners. They'll be fine in series. A small
mains-rated capacitor across each one may help with commutation noise,
like the Y1 caps from a switchmode supply.
On Thursday, October 18, 2012 2:26:45 PM UTC+10, Triffid Hunter wrote: > On Thu, Oct 18, 2012 at 12:59 PM, Spoz <mr....@gmail.com <javascript:>> > wrote: > > I opened up the unit and there's four fans. If I put two of them in > series > > (so each fan sees half the voltage) they run at a perfect speed for us. > What > > I want to know is, is this a problem? Will they burn out and set our > office > > on fire over the weekend?
> AC fans most likely have universal motors in them, same type as > dremels and vacuum cleaners. They'll be fine in series. A small > mains-rated capacitor across each one may help with commutation noise, > like the Y1 caps from a switchmode supply.
Yeah, they spin up fine at low voltage (it takes about 15 seconds to reach maximum speed) but they do spin slower, and they have almost no torque at all. They move enough air through the case, but blocking the intake almost stops them completely.
Like I said we've already ordered the DC fans for reliability but I'd like to know the theory behind it. Are they slipping because they don't have enough torque to "keep up" with the mains frequency? Does that result in greater current draw? Unfortunately I don't have any test gear with me at the office to confirm.
On Thursday, October 18, 2012 3:06:47 PM UTC+10, Triffid Hunter wrote: > On Thu, Oct 18, 2012 at 3:45 PM, Spoz <mr....@gmail.com <javascript:>> > wrote: > > Here is the datasheet:
On Thu, Oct 18, 2012 at 4:29 PM, Spoz <mr.s...@gmail.com> wrote:
> Yeah, they spin up fine at low voltage (it takes about 15 seconds to reach
> maximum speed) but they do spin slower, and they have almost no torque at
> all. They move enough air through the case, but blocking the intake almost
> stops them completely.
that's not good- they'll stop once a bit of dust builds up.
I'd be curious to see if the torque picks up if you put a small
capacitor across each fan
> Like I said we've already ordered the DC fans for reliability but I'd like
> to know the theory behind it. Are they slipping because they don't have
> enough torque to "keep up" with the mains frequency? Does that result in
> greater current draw? Unfortunately I don't have any test gear with me at
> the office to confirm.
I'm not familiar enough with AC synchronous motors to get into detail,
but if they have very little torque then surely the current has
dropped through the floor.. current is generally proportional to
torque with any type of motor
