62 the addition of electrical hardware out of the drive itself, except for a reconfiguration selector, which may be electrical or mechanical, depending on the specifications of the carmaker. Electromechanical isolation With traction inverters, battery packs and charging systems operating at ever higher voltages, 1500 V contactors are being used to protect a vehicle and its users in the event of a short-circuit or accident. These are electromechanical relays that stay connected until there is a problem, when they open to cut the circuit. These are used in the battery disconnect unit (BDU) for all varieties of electric platform, from passenger vehicles to offroad and agricultural equipment and electric shipping, where there are high-power battery packs, as well as the associated charging systems. “Initially, we came from developing rail systems, and we have 3000 V and 2000 A, but this is too big for automotive, so now the learning curve has taken us to 1500 V and 350 A,” says Pavel Tomashev, product manager, contactors at Schaltbau. One of the key challenges with a contactor is arcing when the contacts open. This requires a fast response to prevent the arcing from welding the contacts shut. “That means we know how to handle the arc reliably and we know a lot about efficiency with the lowest possible contact resistance. The main body is made from copper, but the contact points have silver tin-oxide tips that are welded to the contactor body, and it has a much higher stability and oxide stability over time.” That’s where the controller board, called the economiser, provides the pull to keep the contactor connected in a process that takes less than 100 ms. “What is also important is that we have a current sensor and temperature sensor alongside the voltage sensors, so that contact pressure remains stable. To avoid the contact pressure drop from current change, we do the temperature correction and adjust the current,” Tomashev adds. “Arcing is always there to break the current flow. Welding is another type of problem. You can see welding at the startup with a high in-rush current from the capacitor on the inverter, and the contactor has to survive that process. Pre-charge circuits are widely used, but if the pre-charge doesn’t work, the contactor still needs to open. “The silver tin oxide has a higher melting point, so it is more resistant to the welding, and we use a coil drive that reduces the contact bounce. As long as you make the contact quickly without bouncing, [this] is the way to make the contact. On the mechanical side, contact springs prevent bounce, which keeps the contacts closed. “The contact is not fixed on the armature. There is a certain level of freedom to get overtravel and this is crucial to avoid contact bounce. There is a powerful spring with a flexible connection between the armature and the contact to prevent the contact from bouncing,” he adds. High-power contactors used for renewable energy systems such as wind turbines are often filled with an inert gas, but for e-mobility applications, including electric ships, there is an advantage to having contactors that are open to the atmosphere, Tomashev says. “They are open-air insulation, so there is no special gas, no hydrogen, no vacuum,” he explains. “What is important in a failure case with a shortcircuit current of 5-10 kA is huge energy that generates an electromagnetic force that pushes the contact back, opening the contact. We call it levitation during the short-circuit. “We handle that in a similar way with a high melting-point material, and there is no risk of explosion with open air, as the high current creates a lot of extra pressure, heating the air to a temperature of 1000 C for milliseconds. “That is a problem for sealed contactors that cannot let the pressure out. That’s what helps us contribute to electrical isolation, even in the case of a shortcircuit, to open the main contact circuit to ensure proper isolation, based on our experience with contactors in rail systems installed under the railcar body. With all the humidity, dirt and snow, over decades we have seen no issues with that.” These contactors provide safety and efficiency, in that there is less risk of welding and no risk of explosion, and they use an efficient coil drive and main circuit. “Our calculations on charging station applications and general operation profile show contactors with a resistance of 100 µΩ versus the industry average of 200 µΩ can save more than the price of the contactor in less than a year, and the efficiency boosts the performance of the chargers or the battery packs,” says Tomashev. At the heart of isolation design for e-mobility is the safety of the people and the equipment. As more systems become electrified with ever-higher power levels, isolation technologies will play an ever-greater role. In a variety of ways, these are responding to the need for more efficiency, safer systems and new architectures. Deep insight | Isolation technologies January/February 2025 | E-Mobility Engineering The CT2 contactor for isolation (Image courtesy of Schaltbau)
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