The load-torque estimator forms an inner feedback loop, as shown in Fig. 2. As an example, in pumps and fans, the load torque is approximately quadratic in the speed as f(ωm) = lω2m for ωm ≥ 0. A feedforward ˆτl = ˆlω2 meas can be used. If an estimate ˆB of the viscous damping constant is available, then ˆτl = ˆ Bωmeas can be introduced.
If ˆ f(・) = f(・), the feedback loop formed by the nonlinear load-torque contribution is canceled. This is an exact linearization of the system [39]. However, ˆ f(・) = f(・) is generally not a realistic assumption; exact knowledge of the speed–load-torque characteristic is fairly rare. Moreover, the speed-independent part τL is often unknown or only partially known, which makes a good estimate ˆτL unobtainable. An error is therefore unavoidable.