a specific type of helical gear reducer

helical gear reducer are effective in energy savers in pump and fan applications; "they enhance process operations, particularly where flow control is involved. VSDs provide soft-start capabilities, which decrease electrical stresses and line voltage sags associated with full voltage motor start-ups, especially when driving high-inertia loads."

combined gearbox are a specific type of VSDs; they are controlling the rotational speed of an alternating current (AC) electric motor by controlling the frequency of the electrical power supplied to the motor.