Chopper Rotator


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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 chopper unit at the top of the pole. The blower of this first Thunderbolt I restored is a three phase A-series blower.

This 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 thus 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 originals 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. (See bottom of Thunderbolt Restoration Main Page) 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 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 wipe with a rag treatment. To see the other types of blowers that came with Thunderbolt sirens see the bottom of this page.

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 is more than enough to keep the thing from tipping as the horn rotates. This is how Thunderbolts were shipped from Federal. They were crated with the chopper/rotators attached to the blowers in this arrangment.