Impact of Parasitic and Load Current on the Attenuation of Motor Terminal Overvoltage in SiC-Based Drives

In SiC-based adjustable speed drives, the high voltage slew rate (/) of the switching transitions results in excessive overvoltage at the motor terminals due to the reflected voltages across the drive power cables. Besides the cable length, the switching rise/fall times of the voltage pulses are a key parameter to quantify the motor overvoltage in PWM inverter-fed drives. These times are varying depending on the load current and parasitic elements of SiC MOSFETs, that is, a standard two-level converter typically results in a non-uniform overvoltage envelop at the motor terminals. This article analyses the switching mechanism of the two-level converter considering the impact of SiC parasitic elements and load current showing how they affect the motor overvoltage in cable-fed drives. The analysis is then extended to the mitigation of the motor overvoltage using quasithree-level (Q3L) modulation as a candidate filter-less approach with a T-type converter. The theoretical analysis is validated through experimental tests by using the Q3L T-type converter. The analysis and results show that the instantaneous load current value critically determines the peak motor overvoltage, while it allows either a full or partial overvoltage mitigation when the Q3L modulation is adopted

Mitigation of Motor Overvoltage in SiC-Based Drives using Soft-Switching Voltage Slew-Rate (dv/dt) Profiling

In silicon carbide (SiC) device based motor drives, the high voltage slew rate (dv/dt) associated with the fast switching transitions results in excessive motor overvoltage, due to the reflected wave phenomenon, which increases the motor winding insulation stress and causes premature failure while raises electromagnetic interference (EMI) problems. This article proposes a voltage slew rate profiling approach to mitigate the motor overvoltage in SiC-based cable-fed drives. The proposed approach optimizes the rise/fall time of the output voltage according to the cable length, without altering the switching speed of the SiC devices. The proposed profiling approach is implemented using a soft-switching inverter. The optimum rise/fall time that can significantly mitigate the overvoltage is derived using frequency and time domain analysis. The auxiliary resonant commutated pole inverter (ARCPI) is adopted as a soft-switching inverter to experimentally verify the proposed slew rate profiling approach for the overvoltage mitigation. The analysis and experimental results show that the motor overvoltage is fully mitigated when the output voltage rise/fall time is set as the cable anti-resonance period, i.e. four times of the wave transmission time along the cable. Further, the slew-rate profiling approach along with the ARCPI reduces the switching loss and improve the EMI performance at high frequency region.</div

Optimal PWM switching strategy for single-phase AC-DC converters

The thesis describes an optimal selective harmonic elimination strategy suitable for singlephase AC-DC converter-fed traction drives. The objective is to eliminate low-order supply current harmonics, including those injected into the supply due to load-side current ripple. Other advantages that the switching strategy has to offer over phase-control include improved supply power factor, reduced VA consumption for a given demand speed and load, reduced torque and speed ripple and smaller armature circuit smoothing inductance. The effect of field current boost on the dynamic response of the drive is also described. It is shown that field boost helps to reduce the speed rise-time by increasing the electromagnetic torque available during acceleration periods. Closed-loop control of a 4-quadrant DC drive is described and a comparison made between the performance of PID-control and pseudo-derivative feedback control. It is shown that pseudo-derivative feedback control has several advantages to offer, amongst which are ease of tuning of the controller gains and a superior performance following load torque disturbances. A laboratory size drive system was designed and built, and used to validate simulation predictions for both the switching strategy and pseudo-derivative feedback control. A microcontroller based hardware implementation of both the switching strategy and a digital pseudo-derivative feedback controller was adopted, with the switching strategy being implemented using an off-line approach of precalculating the switching angles and storing these in look-up tables. The armature voltage controller comprises a dual-converter employing IGBTs as switching devices. The use of IGBTs allows higher switching frequencies at significant power levels than would be possible if GTOs were used. It also simplifies the gate drive circuit design and minimises the need to use snubber circuits