This is at one of those sites with three phase open delta power. Needless to say, the transformer is toast, perhaps the entire transmitter too. This will be another fun transmitter scrapping project. I was thinking about this; over the last five years, I have scrapped at least ten to fifteen old tube transmitters. The old tube types are going away fast, as are those that can still work on them.
I alluded to this in an earlier post: Open Delta three phase service. Some transmitter sites are fairly remote and three phase power is not available. Occasionally, with lower powered radio stations, this is acceptable because those transmitters can be configured to run on single phase power. However, almost any transmitter above five kilowatts or so will require three phase power. This is the case at the WQBJ transmitter site in Palatine Bridge, NY. The site is located in the middle of farm land and only has single phase service. The nearest three phase service is several miles away and the utility company wants several hundred thousand dollars to upgrade the line.
The station is a class B FM with a six bay full wave spaced antenna. Even so, the TPO is 17 KW, which makes some type of three phase service a requirement.
The main transmitter is a Broadcast Electronics FM30B, which is now 25 years old.
The backup transmitter is a CSI FM20T, which is almost forty years old.
Rather than do an open delta service, which is not desirable for several reasons, both transmitters have their own rotary phase makers. From a reliability and redundancy standpoint, this is the right way to equip this site. The rotary phase makers are essentially a motor generator combination which takes the split phase power and generates a third phase.
The phasemaster is is a 40 KVA unit and is connected to the backup CSI transmitter
The Roto Phase unit for the main transmitter is actually two 40 KVA units connected in parallel through dry core isolation transformers. Incidentally, the Roto Phase units need to have their bearings changed every ten years or so. This requires the units be disconnected, placed up on their end. To get the old bearing out, the housing has to be cooled with liquid CO2. Both units are due for new bearings soon, which should be a pleasant job indeed.
Several months ago, I drove up to an FM transmitter site, looked up at the utility pole and saw this:
Three phase open delta is a bad hombre. Most, if not all, transmitter manufactures will void the warranty of any transmitter connected to a service like this. What is perplexing is it appears that all three phases are available on the primary side, why would this be necessary? Perhaps it was not always so at this location. Regardless, this was the source of power for 20 KW FM transmitters since 1958 until we moved it to a new building last month.
According to a GE publication on transformers, open delta 3 phase power is undesirable because:
Although this connection delivers three-phase currents which are approximately symmetrical to a three-phase symetrical load, the currents flowing in the high voltage circuit are not equal nor are they 120 degrees apart. The maximum safe output of the bank operating in this manner is 58% of a 3 pot Wye/Delta bank. The system is grossly unbalanced, both electrostatically and electromagnetically.
Schematically, it looks like this:
Regular 3 phase delta looks like this:
Most utility companies will not hook up 3 phase delta on the customer side anymore because the the “high” or “wild” leg, which as shown in the diagram runs a good deal higher than 120 volts to neutral. Hook up a high leg to a single phase 120 volt piece of equipment and wait for the power supply to blow up. Also true with 277 volt lighting circuits, as my assistant once found out with the Coke Machine in the break room. New 3 phase service will almost invariably be 208 wye unless there is some very compelling reason, which is fine.
There are many ways to get around three phase open delta, perhaps the best is a rotary phase converter. This piece of equipment will take a 240 volt split phase and add a third leg. These legs will not be 120 degrees apart, as they would be in a true three phase, however, they will be close enough that 3 phase motors and transformers will be happy.
This leads to an unbalanced voltage/current condition which needs to be accounted for in the design of the unit. The second way to do this is to power a three phase generator with a split phase motor. This will completely isolate the 3 phase equipment from the utility service and provide for true three phase power.
The downside to any motor/generator or rotary converter is moving parts and conversion inefficiencies. At any transmitter site that uses this type of equipment, either a backup power converter or a lower power split phase backup transmitter should be installed. With all mechanical things, eventually this will need to be repaired and it would suck to be off air while that is happening.
Regardless of any of that, this particular service is about to be disconnected permanently. Good riddance.