Today is the last day on a really fun job that has lasted the last 6 weeks. One of the research test cells where I work needed to be able to rotate their main motor backwards. They use that motor to test turbine blade geometry under various atmospheric conditions and rotational velocities. The turbines are used in jet engines. Since researchers view the rotation from the front of the turbine while aeronautics companies view the rotation from the pilot's viewpoint, in order to test private sector turbines, the motor must be able to turn counter-clockwise. To understand this, take a pinwheel and blow on it to get it spinning. Notice which direction it is turning while it faces you, then slowly rotate the stick until it's facing the other direction, and you'll see that from your new perspective, the pinwheel is rotating in the opposite direction.
In order to make any three phase motor reverse direction, any of two phases much be switched. That isn't too hard to do in a small motor, but this one is 15,000 horsepower. It can draw almost 1000 amps at full load. The main problem is that the high voltage cables that power the motor are 59 years old, and beyond their life expectancy by 10 years. With that in mind, we didn't want to have to move them very much, or very often. The trick was to come up with a way to switch phases, without having to move the cables every time.
The best method would have been to use a reversing disconnect switch, but the switch alone cost $80,000. The head engineer wanted a more cost-effective method, so I redesigned the existing switch to enable us to swap out pieces of buss (4" wide x ¼" thick copper plates that are used instead of wires) for clockwise and counterclockwise rotation.
The first thing to do is determinate the existing cables, so they can be tested. This picture shows the cabinet, with the cables removed from the buss.
Here is the other end of the cables, where they terminate on the motor buss. They are taped up with insulating tape to keep the 6,900 volts contained.
The cables tested out fine, so we continued. Since only two of the phases need to be interchanged, the third phase (also called Phase C) was put back on the same buss. We had lowered the cables to give us as much room as possible to form them where they needed to go, so we had to manufacture a short buss extension before landing the Phase C cables. A nice cup of java to start the day...
The next step was to start fabricating the other buss pieces. This is a buss bender. We used to to bend right angles in the copper plates. This was fuuuun!
We were meticulous with our layout and measurements. In all, there were 12 pieces of buss that had to be measured, cut, drilled or punched out, polished, and put together. My partner, Bill is cutting one of the straight pieces.
This is a picture of the Phase B buss for clockwise rotation. The cables will land on that angled plate, then the circuit continues up to the same spot the cables originally landed. You can also see the Phase C cables (with the blue tape). The three shiny vertical cables in the back are the incoming feeders.
Bill and I bent up the Phase A buss pieces next. This is the layout for clockwise rotation also, since the circuit goes from the cables right to where they were landed previously. You can see the 1½ inch offset that brought the buss closer to the side of the cabinet. This was to make it easier to land the first Phase A cable. The piece on the bottom with the holes punched in it is where the cables will be bolted in. I didn't like how this was looking because it would require me to bend the cables too far, violating the bending radius specification, so I moved that angled plate, and rotated it.
My ride is here! I'm flying to Raleigh, North Carolina to participate in the weekend long Eastern Balboa Championships! I won't be competing, but I'll be taking classes for 4 days, and dancing each night. I'll post more about this fascinating job when I get back.
