AC induction Motors
Since AC induction motors have been established as the preferred choice for industrial motors and the need for reduced power in all aspects of design is ever present, the efficiency of these motors is critical. An optimized slip control mechanism is the key factor for achieving the required torque and efficiency performance in an AC induction motor. To achieve slip optimization, a tightened control loop with highly integrated control logic becomes essential.
Most of the current solutions are based on processors that run fairly complex software programs to target efficient operation. Processor bandwidth and computation time in a software-based system significantly limit the responsiveness and, therefore, the power efficiency of these solutions. A more powerful processor implementation can give improved processing time and better power efficiency, but incurs additional cost.
Implementing control algorithms directly into FPGA logic gates can be a cost-effective alternative, with faster response times to I/O. Intellectual property (IP) can be acquired for design, and reprogrammable FPGAs allow for system upgrades as techniques improve. However, external processors and flash memory lookup tables (LUT) are still performance bottlenecks.
Highly integrated mixed-signal FPGAs with embedded nonvolatile memory and a soft or hard processor core provide the ideal solution for tightening the control loop and accelerating slip control processing, thereby improving AC motor efficiency. Since all processing power, LUTs and direct control algorithms can be integrated into a single reprogrammable chip, the solution is more accurate, efficient and cheaper than traditional solutions.
AC Induction Motor Basics
In AC induction motors, three-phase electrical power supplied to the cage or static part of the device (stator) is converted into mechanical power in the rotating part of the device (rotor) through electromagnetic induction. There is no direct supply to the rotor. Current in the rotor is induced by the rotating magnetic field in the stator. Current in the rotor creates a magnetic field, which interacts with the rotating magnetic field in the stator causing the rotor to turn. The slip is defined as the ratio between the speed of the rotor and the speed of the rotating magnetic field in the stator.
Maximum efficiency in the induction motor is achieved when the rotor is turning almost as fast as the magnetic field in the stator - a slip close to zero. But in order to deliver increased torque for higher loads, a higher slip may be preferred as depicted in the diagram below. Optimized slip control uses algorithms to find the ideal balance at any point in time for either max efficiency or max torque. See Figure 1 for efficiency/slip and torque/slip curves.
Figure 1: Torque and Efficiency vs. Slip.
