When you order a new broadcast transmitter, one of the first questions will be about power. Is it single-phase or three-phase alternating current? If single-phase, it is 208 or 240 volts? If it is three-phase, is it Delta or Wye connected?
Don’t guess, measure it for yourself with a multimeter, as I described in a recent Radio World article. Use caution when measuring in an electrical service panel (Fig. 1).
You will find 240-volt single-phase service used for homes, most studios and many transmitter sites. Center-tapped, it has two 120-volt circuits for equipment racks and lights, along with plug-in devices and numerous wall-wart power supplies. Heaters and air conditioners often require 208–240 volts. That single-phase power is derived, by the power utility, from three-phase farther up the power line.
Phase
What is this thing about phase?
You might remember an article I wrote in 2013 about oscilloscopes (find it on the Radio World website by Googling “Your Scope Is a Tool for All Seasons”). That tool allows us to see voltage rising and falling at a periodic rate.
Fig. 2 shows an oscilloscope displaying a 60 Hz sine wave, which is the power we all use. With 60 cycles per second, it has a wavelength of 3,100 miles at the speed of light. That is 16.7 milliseconds of time between voltage peaks.
Divided by three, it is 5.66 milliseconds between a voltage peak on one phase of three-phase power and the next. Said another way, there is a 120-degree phase difference from winding to winding, for a total of 360 degrees when the three-phase cycle is complete.
Our nation’s electrical “grid” is three-phase because that is the most efficient way to send power over long distances. Locally, they often start with 13.8 KV or 4.46 KV, then step it down in transformers for 120- and 240-volt consumer consumption. Single-phase power comes from two legs of a three-phase circuit. The power company uses transformers called tubs or pole pigs on utility poles. The latest innovations, as you probably know, are ground-mounted transformers used with underground cables.
As mentioned, three-phase power has a 120-degree phase difference between each of the three lines/legs. This kind of power is made in generators that are much like giant electric motors, except the generator shaft is turned by a device like a water wheel or turbine in a hydro generating plant or even a nuclear facility. Motor-driven backup power generators work that way.
Three-phase power is used to feed large transmitters and other high-demand equipment like air conditioners efficiently. The wire size is much smaller, and electrical panels handle less current per conductor while delivering the same number of watts.
Delta vs. Wye
Fig. 3 shows Wye connected power. Fig. 4 shows Delta connected power. You can see why they are named that way.
A, B and C are the connection points to three-phase power. The only importance as to which wire goes to which terminal has to do with motors. Reversing two of the connections will make a three-phase motor turn in the opposite direction. Electric utilities are careful not to reverse phases accidentally as cities often use three-phase motors to pump water and sewage.
The lines next to each of the schematic symbols on power transformer windings indicate an iron core. As you may know, transformers temporarily store magnetic fields, created by passing AC through the windings. That field is then induced into other windings on that same transformer to output a higher or lower volage.
Transformers are heavy devices because they require a lot of iron to handle the magnetic fields. Copper wire windings add to the weight. Aluminum wire is used, in some instances, by power companies in their systems.
A power transformer capable of handling 1,000 watts weighed in at 42 pounds in my shop. It just goes up from there.
Wye
The simplest to work with is Wye.
The neutral in an electrical panel connects to the center of the Wye. You get 120 VAC single-phase power in all three directions. However, connecting this three-phase to a transmitter gives you 208 volts from A to B and to C.
Transmitters have taps to set incoming voltage on internal power transformers so the right high voltage can be achieved. Not a problem, but it’s easier to tell your transmitter manufacturer what you have so they can set transformer taps and do testing before shipping. This applies to solid-sate transmitters too.
You might be tasked with moving a transmitter to a new site. This is not just plug-and-play.
Check the manual and call the manufacturer for the best advice before turning the power on. There may be a gotcha awaiting your lack of diligence.
Some transmitters have three-phase blower motors. That means they turn according to the phase rotation. There’s a 50% chance a motor will turn in the wrong direction when the transmitter is turned on. As mentioned, reverse two of the three motor wires and it will turn in the other direction.
Just because the motor is turning, that doesn’t mean it is pushing air in the right direction. Best to check airflow before saying all is good.
I ran into this when a contractor replaced an air vent motor in a transmitter building. He walked away and the building overheated. There was no engineer supervision!
Delta
If your incoming power is Delta, it has what is called a wild leg or high leg.
Single-phase connection for 240 and 120 volts is between just two legs, A and C in this case. Neutral is a center tap on that feed. The third leg (B) is maybe 208 volts when measured to neutral. You won’t connect anything to that unless it is going to a transmitter or other three-phase device.
Fig. 5 shows a three-phase load center with a main circuit breaker at the top and two smaller three-phase breakers to feed equipment.
Note the three power lines entering from the top. One of those is marked red. It is the wild leg in this Delta configuration. Neutral is the conductor marked white to the right of the main breaker. It is connected to ground in the first electrical panel in a facility and remains separate from ground after that.
Fig. 6 is the same panel with the cover on. Wild leg circuit breaker locations are marked so they won’t accidentally be used for single-phase applications. There is a problem with that wild leg. Surge arrester protection to ground needs to be configured accordingly. Check the documentation carefully before connecting.
Open Delta
I consider open Delta to be a bad choice. It is used where there is only a single-phase power line to a site. The power company makes pseudo three-phase power using only two transformers instead of three. This is not very common. The configuration is notorious for causing damage to equipment when lightning hits. Ouch! It is best to avoid this by finding a single-phase transmitter.
If not, use a rotary phase converter from Kay Industries, Phoenix or a solid-state unit from Phase Technologies, just to name a few. They generate real Wye or closed Delta three-phase power from single-phase power.
In conclusion, you need to know what the incoming power is in order to configure equipment and surge protection properly.
