Pass the PowerBars

Located near the MagLab, this substation draws electricity from the city’s power grid and distributes about 56 megawatts (MW) (or 56,000 kilowatts) to the lab. So at any given time, the lab has access to that amount of power. (By contrast, the typical load for U.S. home is a fraction of that – 2 to 4 kilowatts ). If we wanted to, we could turn on more than 800,000 60-watt light bulbs at once with that 56 MW. But since that would probably not do much to advance science, we run magnets with it, instead.

That power, in the form of 12,470 volts of alternating current (AC), travels from the substation to the lab’s power yard and main switchgear houses, where breakers distribute it to the lab’s electrical loads.

From there, eight 28-ton transformers take the current and step it down to a lower, more practical voltage. The transformers have two voltage taps, so the incoming 12,470 volts can be stepped down to either 520 or 640 volts (standard outlets in U.S. homes are 120 volts). Each of the transformers can distribute between 8 to 10 MW of power, depending on the voltage.

Electricity, of course, comes from electrons traveling through wire conductors. The friction of all those electrons bumping into each other generates an enormous amount of heat – think how quickly a little ol’ 40-watt bulb can burn your finger. To keep these transformers from overheating, each is filled and cooled with 5,676 liters (1,500 gallons) of soybean-based dielectric oil, which doesn’t conduct electricity.

By now the power is in the heart of the DC Field Facility. But it’s not ready to be fed to the magnets. If you haven’t guessed already from the facility’s name, that AC power, which works great in your home, needs to be converted to DC power (like what is used in a battery) for the magnets. AC power and resistive magnets are not a good combination; alternating current, as the name implies, flips direction some 60 times a second in the U.S. (50 times a second in Europe). The pole of a resistive magnet is determined by the direction of the current coursing through it. So if AC current was used with resistive magnets, their poles would flip every time the current flipped, making the magnets virtually unusable. How to rectify this situation?

Conveniently enough, devices called rectifiers convert the AC to DC, creating a smooth, stable flow of power rather than the incessant fluctuations of AC. The current is divided among four power supplies, each of which can provide 14 MW of 700 volts DC. Each power supply can service only one magnet at a time. Since our weakest DC Field magnet requires 17 MW of power – enough to power more than 425 homes – no more than two of our resistive magnets can operate at the same time. And when our biggest electricity hog, the hybrid magnet, is at full field, it draws on three of the power supplies, so no other DC Field magnet can run when the hybrid is in use. Magnet time must be scheduled so that demand for power never exceeds 56 MW.

To get from power supply to magnet, the DC current is conducted through solid aluminum bus bars located in the ceiling above the magnet cells. The current flows one way through one bus bar, and in the opposite direction through the other, creating a circuit between the power supply and each magnet, just like the circuit between the bulb and battery in your flashlight. The current direction through the magnet can be adjusted, depending on which way the researcher wants to orient the magnetic field.

The current surges through the coils of the magnet, made up of hundreds of Bitter plates, pancake-like sheets of metal pocked with holes (we’ll explain those in a moment). The more current pushed through the coils, the greater the magnetic field produced. The field is most intense in the hollow center of the cylindrical magnet (called the bore), which is where scientists put their experiments, using a long, narrow tube called a probe.

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