The Siren Board

Andys Live WX wrote:

(I wonder if around 3:00 is what it sounded like to be near one of the CLMs with the "winterization" option that had them "idle" at 50 rpm instead of ever shutting down...)

I've definitely never heard of such thing.

Some CLMs in Northern Canada were made with an idling option so the motor wouldn't freeze up. The CLM "Mailbox siren", which had three models, had this feature.

Some CLMs in Northern Canada were made with an idling option so the motor wouldn't freeze up. The CLM "Mailbox siren", which had three models, had this feature.

Wow... I guess what I'm saying is that I didn't know you could lower the speed on an induction motor without using a variable frequency drive. Because it's not the volts that determines how fast it spins. That is a good idea if it actually worked.

Low voltage will make an induction motor spin slower, but it will also produce lots of heat and can burn up a motor.

I'm still curious as to how they managed to make those motors "idle". They could have used a wound rotor but those motors aren't cheap.

Wow... Just... Lost of words.

Low voltage will make an induction motor spin slower, but it will also produce lots of heat and can burn up a motor.

Yeah, I know that because in the fall we are running 5 motors on the farm that runs our drying setup. 3 of them are 3-phase, and the single phase service coming into the farm is too small. The voltage drop is unbelievable when all those motors are running at once. That is why we burn up motors all the time. It dims all the lights in the house too.

How mamy Db do these sirens push?

Well, this is just a wild-arse guess here, but maybe they used a small single-phase motor on low voltage to keep the chopper (and thus the main motor) rotating, with a setup like the flywheel-and-Bendix Gear system used on car starter motors so as to avoid damaging it when the main motor is kicked in?

Alternatively, they *might* have basically used a "pulsed" power feed to the main motor, sort of akin to a growl test, but with much shorter duration, and the "50 rpm" figure was actually an average, with it being allowed to spin up to, say, 75 rpm under power, then spin down to 25 rpm before it applies power again. This might be simpler to do (I'm pretty sure it could be handled entirely by a centrifugal switch like the one for the emergency brake on elevators), but I'm not sure how hard it would be on the motor to have the power constantly flipping on and off.

EDIT: Just spoke with an E.E. friend, who said that the extra motor would be electrically the simplest, but mechanically the most complex and expensive. "The idea of pulsing three-phase is unappealing, but would be mechanically the simplest, I think. Undervolting seems like it would just be asking for trouble. In theory you could convert to DC and reconvert to three-phase at lower frequency... hrmm.. DC chopper? But the power circuitry would be a real biotch. Relays would wear fast, I suspect. Actually think it over, straight 3-phase for normal use and a bit of power pulsing for low-spin might be the way to go. Yes, the circuitry would get a bit crazy, but but you only have to design the beast once."

It's definitely an interesting engineering challenge, in any event. Not a particularly difficult one, but an interesting one in selecting how you'll do it, particularly when you limit yourselves to 1950s technology.

I wonder why they couldn't have just taken the two-done chopper from their omnidirectional siren and stuffed it in the rotating model. Still a neat design, though.

The true 10/12 port CLM is 125 dBs, so this one is probably around the same. As for the Idling, it was operated from the local control of the siren. They idled the motors with an idling transformer that would supply a low voltage to the motor, to prevent freezing up in the winter. There also was a transformer switch to control the speed of the idle, could make it faster of slower. I know the sirens had overload protection and would auto trip if there is too much heat build up, then you would just reset the button, so the motors wouldn't burn up. I know the 10/12 and 20/24 had it, not sure if the rotating ones did because I haven't seen an idle switch on any of my controls, just a on/off/auto which controls the siren itself, maybe they were built in I don't know.

(post removed; I completely blanked on what had been said earlier in the discussion. Carry on!)