High Dynamic Sensorless Control of Induction Machines at Zero Speed Using Transient Excitation Technique

High dynamic controlled operation of induction motors implies the knowledge of the machine´s main flux position to be known at any time instant. Speed sensorless field oriented control schemes do not require a shaft sensor to determine the necessary flux position in the whole frequency range. This has the advantage of decreased costs and increased reliability of the drive. Although used in the higher frequency range, methods only based on fundamental wave models will deteriorate at low or zero fundamental stator frequency due to the lack of signal. Signal injection techniques are able to guarantee a stable operation even at zero frequency using inherent saturation or slotting saliency effects. This allows a calculation of the flux- or rotor position independent from the fundamental frequency. The injected signal can be a balanced polyphase high frequency excitation or, as it is used in this paper, a transient stimulation with voltage pulses. With the injection of additional signals into the machine, the resulting control signal obtained can be noisy what may cause problems for the control loop. To overcome this drawback, the scheme proposed in this paper combines both, a signal injection method and a fundamental wave method in the low frequency range and at zero frequency. The signal injection method output is used to stabilize the fundamental wave model what results in a smooth output much more suitable for the control scheme. The advantage of this structure is, that the dynamic performance is not reduced, as is the case with conventional filtering methods. Measurement results using a standard induction machine are given to show the performance of the proposed structure