US Patent 9452750 Methods, systems, and vehicles with electromechanical variable transmission

A method of operating a first motor/generator and a second motor/generator in a vehicle drive includes operating the vehicle drive in a first operating mode and a second operating mode, the first operating mode including operating the first motor/generator as a generator and operating the second motor/generator as a motor, the second operating mode including operating both the first motor/generator as a generator and the second motor/generator as a generator when a first power generation capability associated with the first motor/generator is below a first threshold level, the first motor/generator and the second motor/generator providing electrical power to the DC bus without doing either of (a) providing electrical power to an energy storage device or (b) consuming electrical power from an energy storage device.