What are the challenges in designing high-speed electrical machines?

High-speed machines (>10,000 RPM) face challenges: mechanical (rotor stress, critical speeds, balance requirements, bearing selection - often magnetic or air bearings), thermal (concentrated losses, limited cooling surface), electrical (increased core losses due to high frequency, skin effect, proximity effect), and manufacturing (tight tolerances). Design trade-offs include: surface PM rotors with retention sleeves, limited rotor outer diameter to manage stress, careful slot/pole selection for low losses, and specialized insulation systems. Applications include turbomachinery, flywheel energy storage, and high-speed spindles.