The manual voltage / frequency adjustment is based on a voltage / frequency law with fixed voltage increase at low frequency. This adjustment is sometimes called “manual boost”. The voltage applied to the electric motor is not a function of its load and this adjustment allows to obtain a high torque at low speed but has the drawback of maintaining a high current in the engine at no load with risk of saturation and overload.
The automatic adjustment releases to the motor a voltage with automatic compensation of the voltage drop RS / I. Has a certain speed, the voltage supplied to the motor varies depending on the load. This setting is sometimes called “auto boost”. A regulation allows to change the coefficient, compensating to the most accurate the voltage drop RS I of the used motor. This adjustment allows to achieve a high torque, low speed, having a low vacuum current. On the contrary, it has the drawback of a slower response time. An overkill of compensation can again lead to a supercharged engine: increased load, increased voltage, increased current. https://www.mrosupply.com/hose-fittings/hose-reels/773726_5625ohp_reelcraft/
The particular setting: quadratic conjugate: The resistant conjugate filled by a centrifugal pump or a fan changes depending on the square of the speed.