Sensorless control of switched reluctance drives

The switched reluctance motor (SRM) has concentrated stator windings and a rotor which does not have windings nor permanent magnets, see Figure 1. This leads to a low-cost and extremely robust construction, which gives the motor the capability to be operated at ultra-high speeds and in harsh environments. Presently, switched reluctance motor drives are used for automotive applications, house-hold goods, electric vehicles, HVAC applications, etc.

Although the machine has a robust structure and simple design, the control is quite complicated compared to the case of classical AC machines. An essential aspect in the control of the machine is that the rotor position is needed in order to guarantee proper commutation of the current between the different phases of the machine. In many cases, the rotor position is provided by means of a sensor mechanically mounted on the motor shaft. The presence of a sensor leads to a reduced reliability of the drive, especially in harsh environments, and to a higher cost of the drive. For small motors, the cost of the sensor may be as large as the cost of the motor itself. Worldwide a lot of research has been done in ways to provide the rotor position without the need of a mechanical position sensor.

Our research aims at sensorless control of switched reluctance machines in the low- to medium-speed range. The method is based on the triggering of electrical resonances in the drive system. The resonances originate from an energy exchange between the phase inductance of the motor and parasitic capacitances present in the motor winding, motor cable and semiconductor devices. Such a resonant circuit is schematically shown in Fig. 3 for the idle (unenergized) motor phase and converter bridge of Fig. 2. A resonance is triggered by the application of a very short voltage pulse (in the microsecond range) in the idle phase, as depicted in Fig. 4. In this way, the system is electrically excited in a way like a mechanical system is hit by a stroke. The excitation leads to an oscillation of the phase voltage at the resonance frequency of the system. Due to the position-dependent phase inductance, rotor position information can be extracted from the resonating waveform, see Fig. 5.

Compared to some classical sensorless methods for switched reluctance machines, the method developed at EELAB shows several advantages:

• The method works in a speed range from stand-still to rated speed (high-speed sensorless operation is still under research).
• For low-dynamic applications (estimation of commutation angles only), the method is highly robust against heavy saturation of the machine.
• The position information can be extracted from a large-amplitude voltage signal. Depending on the bus bar voltage used, the measured signal can have an amplitude of hundreds of Volts. Such a signal is less prone to disturbances compared to small-amplitude voltage or current signals.
• Due to the very short test pulses, no current flows in the idle phase winding. Therefore, no torque disturbance is generated. The additional loss due to the voltage resonances is negligable.

Contact

For more information, kristof.geldhof@ugent.be, alex.vandenbossche@ugent.be, Jan.Melkebeek@Ugent.be