Guys, A mate of mine is installing a Civil Defence siren at one of the local fishing villages outside of Ashburton. This is a 3kW 400V motor with a single ended impeller unit on the end of the motor shaft. My question is, does it matter which way the motor rotates the impeller?. I replaced the motor on the local fire-station siren here last year some time and never really thought about the direction of rotation, it obviously works in the direction I chose, but is the rotation direction really that important?. We are after all just talking about a fan that makes noise by virtue of the holes in the case that houses the impeller. What's your take on this?. Your input would be most appreciated.
All I can say is "test it!" I've tested a few such things, and, if direction matters, there will be quite a difference in the sound. If it sounds like a cheap fan, you've got it backwards! In my experience, the ones with a direction also had arrows molded into the case- not that you want to be that close when you test. (What if you've got it right?)
#145537 - 06/09/0601:22 AMRe: Fire siren rotation direction
The Rotor (or Impeller) should spin so the Air is drawn in through the Intake Cone (into the center-most part of the Rotor), and expelled out through the tuned holes in the Stator (the assembly which houses the Impeller). Running the Motor to spin the Rotor in this fashion allows the Rotor to chop the air into pulses.
A little more descriptive, for fun! -
The first job of the rotor is as a centrifugal fan. It pulls air into the siren axially through the intake, and blows it out radially through the holes in the stator.
The second job of the rotor is to chop the incoming air stream into impulsive bursts. The rotor is segmented by vanes that periodically cover and uncover the holes in the stator. Each time the rotor and stator holes align, a burst of air is forced through. The frequency of these bursts is the pitch of the siren.
Some "Cold War Era" Civil Defense Sirens used in the US were the easy-to-identify "Federal Thunderbolts". These had a rotating Parabolic Radiator (AKA: Horn) and were FRIKKEN LOUD!!! The Blowers for these Sirens were remotely located on the Ground, contained within an enclosure - affectionately named "The Blower Box".
When I was a youngster (back in the 1960-1970's), there was a Civil Defense Siren installed about 100 feet from our House.
This was a Federal Model BT-10.
It was an extremely LOUD device, with an eerie sound (on the beginning of "War Pigs" by Black Sabbath, the Model BT-10 can be heard, mixed in with other sirens - the BT-10 is the one with the higher pitched dual tones).
Anyhow, spin the rotor to draw in air through the blower's impeller blades, and expel the air out through the tuned ports.
It will work the other way, but not as well; plus the air will not be chopped into beat frequency pulses.
Scott " 35 " Thompson Just Say NO To Green Eggs And Ham!
#145538 - 06/16/0604:58 PMRe: Fire siren rotation direction
Thanks Scott, interesting but very true, I will add that info to my memory banks.
I always thought that when the Ø rotation was correct, the sound was louder than when the siren was going the wrong way.
Our Fire Brigade siren works very reliably and I have not done any electrical work on it yet, exept installing a timeclock to control the ON / OFF times at night, or during day hours for the day time Yellow Watch.
edited for typo's Raymond
[This message has been edited by RODALCO (edited 06-16-2006).]
The product of rotation, excitation and flux produces electricty.
#145540 - 06/17/0601:51 AMRe: Fire siren rotation direction
Ray, Is Titirangi a Composite station?. I just noticed you used the term Yellow Watch. I haven't had time to even go and look at the siren lately, but if it is the type I'm thinking it is, it'll just be a matter of getting a 3 phase supply and giving the thing a whirl (after notifying the locals). Or maybe not, that could steal all the fun out of the exercise(Just kidding) . Interesting you should mention Siren timers Ray, we recieved a complaint from a person who recently moved into the street here, that the siren should be turned off after a certain hour at night. We were thinking about this as a Brigade some years back. My only fear is the pager system failing, mind you, the new Flex Pagers are more reliable than the old Firecom pagers. It would just seem rather strange turning out to a fire-call without the siren blaring. Maybe I'm getting old and set in my ways already!.
#145541 - 07/08/0602:44 AMRe: Fire siren rotation direction
I went and had a look at the siren this morning and any markings of direction or the name-plate were long gone. So off out into the country we went (myself, the Principal Rural Fire Officer and Dick our Civil Defence liasion guy) I called the PoCo before hand and told them what I was going to do and had the full blessing from the Chief Electrical Inspector. I wired the siren up as I thought it should go and tested the wiring with a megger, it all checked out OK. Took a temporary off of somebodies transformer and livened the siren controls. I hit the start button and it sounded OK. So I turned it off and then reversed two of the phases. The rotation was opposite and from what I heard from under the earmuffs it made little difference. We did get the attention of a local farmer or two though. Dick held a Decibel meter at 5 meters away. It peaked at 165dB(A) on the first test and 160dB(A) on the second. So that was our trip out into the country-side. The siren was marked with the former rotation direction inside the terminal case and will be mounted up on a pole next weekend.
#145542 - 07/09/0606:14 AMRe: Fire siren rotation direction
Hi guys, I have been working lots of OT for the POCO because of cable faults and substation problems hence my absense at ECN.
Thursday night 06 July 06 I had some time upon my sleeve for internetting, our 11 kV 230/400 Volts POCO powerpole outside got hit by a truck with an enormous explosion and then power of for 10 hours. Luckily I had some charged 12 Volts substation batteries in my garage to give the family some light after I had ensured that everything outside was safe and the repaircrews were in attendance, to replace the pole and fix the damage outside.
Ok our fire station is Yellow Watch which is manned from 07.00 h till 17.30h Mo - Fr. We as volunteers man it all other hours and weekends, stat holidays. We man 2 trucks and a van. Sometimes during the day comcen put the bells down for 692. Just in case if some guys are around when a greater alarm occurs.
As far as I'm concerned the siren should be on all times 24/7 but other powers dictate that it is on only at certain times controlled by the time clock.
In case the pagers fail, there is an override switch which will make our siren go 24/7 as well.
The product of rotation, excitation and flux produces electricty.
#145543 - 07/11/0604:08 AMRe: Fire siren rotation direction
Gidday there Ray, I've never been part of a composite Brigade and haven't really known anyone that has. Hmm, 0700-1730, that would be a sweet shift to be on, considering the alternative Career shifts. These days Ray, it is all about noise pollution, I fail to see how a fire siren and a young fella with his car stereo are different. At least we turn the siren off eventually. I hope you had your camera out after that truck hit your pole!.