Thunderbolt Siren Restoration Blower Thunderbolt Siren Restoration Main Siren Chopper Chopper Housing Rotator Blower Siren Controls Sirens Main Page Civil Defense Museum Main		The Thunderbolt siren uses a blower unit consisting of a roots blower and a large electric motor to pump 250 cubic feet of air per minute at 6.5 psi. through the chopper. This gives the siren it's high sound output by "supercharging" the siren unit at the top of the pole. The blower of this first Thunderbolt I restored is a three phase A-series blower. The above left photo shows the blower frame with the motor, on the left, and roots blower, on the right. Air is drawn into the top of the blower through the box shaped cover on top of the blower. The blower outlet air pipe comes out of the bottom of the blower and goes out in between the bottom blower frame rails at the right end of the unit. The object with the round plates on top of the air pipe just to the right of the the blower is the blower pressure relief valve. This valve releases excess air pressure if the chopper is stopped and closing off the air flow through the horn or if the air flow is blocked for some other reason. The valve has weights that stack on top of it to hold the valve down until the pressure exceeds 6.5 psi. When the pressure exceeds this amount the valve lifts up and releases the excess pressure while the blower is running. This way the blower is protected from overloading. This motor and blower are not the original units that were with this siren. The original roots blower was completely rusted inside so I replaced it with a newer type smaller blower I got from another siren. This motor and blower came from a mid-1970s vintage Thunderbolt. The motor is a 7.5 hp 3-phase unit that checked good when I had it tested at a motor shop. I mounted the blower skid on a set of heavy casters to make the siren easy to move around. It actually worked very well. The siren was easier to move around on the casters than my big Matco toolbox. The above right photo shows the back side of the blower unit. You can see the air pipe and pressure relief valve to the left.I didn't repaint the blower frame. I just gave it a WD-40 and rag treatment. To see the other types of blowers that came with Thunderbolt sirens see the bottom of this page. Click Photo To See Larger. Here is the siren assembled, without the blower cover installed, sitting in my dump of a garage. The rotator can be operated in this arrangement because the weight of the blower and motor has more than enough weight to keep the thing from tipping as the horn rotates. This is how Thunderbolts were shipped from Federal. They were crated with the horns attached to the blowers in this arrangment. A-Series Thunderbolt siren blowers. This blower information is based on the blowers I have presonally dealt with and information in Thunderbolt siern manuals. There may be some other differences in blowers out there that I haven't come across yet. There is indeed a gasoline powered Thunderbolt siren but I have never seen any photos of the inner workings of one. The above photos show the early type (larger) and later type (smaller) of Thunderbolt A-Series blower units side by side. The difference is obvious from the photo. Federal went to the smaller size blower in the 1970s or maybe late 1960s. I don't know the exact year. The smaller blower runs at a much higher speed than the larger type. Notice the pulley sizes on each blower. When the blowers are running the smaller one really screams while the larger just putt-putts along. The larger type blower unit is very heavy. I would guess it's well over 700 pounds with the motor on the frame. At the time this photo was taken I had the blower motors at the motor shop for a checkup before sending them to Moss Lake, Texas. Both motors were in good condition. Both of these blowers use 7.5 horsepower 240 volt 3-phase motors. The above left photo shows two older A-Series three phase blowers with the motors installed. The above right photo shows an older style A-series single phase blower. The old style single phase blower uses a repulsion induction single phase motor which runs twice as fast as the three phase motor. Since the single phase motor runs faster the roots blower is half as large as the roots blower used on the three phase blower. I thought this would make a big weight difference in the blower but the single phase motor is quite a bit heavier than the 3 phase. The single phase blower still isn't nearly as heavy as the three phase unit though. The later model blowers use capacitor start single phase motors. B/C-Series Thunderbolt Blower. The B-series Thunderbolt changed to a little different blower configuration. The B-series used a vertically mounted roots blower with smaller double belts. This photo is of a blower removed from Dallas Fire Station Number 10 in March of 2009. The B/C Series blower is obviously very different from the A Series blowers in that the frame is far simpler. The blower cover box is quite a bit smaller than the A Series blower as well.