Current Controller for Low-Frequency Signal Injection and Rotor Flux Position Tracking at Low Speeds

Rotor flux spatial position can be tracked in an ac machine even at low or zero stator frequency if a low-frequency harmonic current signal is injected into its stator. The harmonic current injection is source of the rotor speed perturbations which induce voltage oscillations in the stator winding at the injected frequency. By analyzing the stator winding voltage response, it is possible to detect the rotor flux position regardless of the stator frequency. This paper presents a stator current controller that is suitable for imposing rotating or pulsating harmonic current injection and a method for tracking the rotor flux position in either induction machines (IMs) or permanent-magnet synchronous machines (PMSMs). The controller contains, in addition to the standard fundamental-frequency-based synchronous reference frame (SRF) current controller, two sets of harmonic current integral controllers placed in respective harmonic SRFs. Such an extended current controller simultaneously performs two important tasks: controlled harmonic current injection with zero steady-state error and separation of particular spectral components in the stator voltage (spectral/sequence decomposition) which contain the rotor flux position information. The theoretical analysis presented, based on perturbation theory and averaging techniques, gives general expressions which link the rotor flux position error in IM and PMSM to the harmonic current controller outputs. Two special cases with the rotational and pulsating harmonic current injections are considered in more detail. The validity of the theoretical analysis and the feasibility of the sensorless rotor flux position detection are experimentally verified.