Starting is a challenge for any traction drive. During the travel motion, the motor rotates and thus the current flows into all three motor phases. The current is distributed evenly over time among the three transistor pairs in the inverter. At a standstill, only individual transistors are active, which can overload them. As a result, the inverter must significantly reduce the current and thus the torque for self-protection. The full current is usually only available starting at a speed of around ten Hz.
The limiting element is the internal temperature in a power semiconductor, i.e., the respective chip temperature. At a very low speed of the electric motor, the 3-phase electric current also “wanders” very slowly. In the worst case, only two of the six built-in transistors have to carry the current on their own during a standstill. So there is no “swapping” among the transistors. At this operating point, the permissible phase current must therefore be limited to about 1/3. This applies regardless of the manufacturer of the inverter. This also holds true for the VP600 family. For a very short time, the full current can also be run at a standstill. Classically, full current is available only starting at seven to ten Hz of electric speed.
The ARADEX LowSpeedBoost function is based on a proprietary model for calculating the chip temperature in real time. We adapt this model to each type of power transistor used and validate it through elaborate in-house measurement series.
As a result, a significantly higher current and thus torque is available, especially in the range of 1-5 Hz, which is of interest for starting. And also for starting from zero to one Hz.