INDUCTION MACHINE DESIGN FOR VARIABLE SPEED
Variable speed drives with induction motors are by now a mature technology with strong and dynamic markets for applications in all industries. Based on the load torque/speed envelope, three main types of applications may be distinguished:
• Servodrives: no constant power speed range • General drives: moderate constant power speed range (ωmax/ωb ≤ 2) • Constant power drives: large constant power speed range (ωmax/ωb ≥ 2) Servodrives for robots, machine tools, are characterized, in general, by constant torque
versus speed up to base speed ωb. The base speed ωb is the speed for which the motor can produce (for continuous service) for rated voltage and rated temperature rise, the rated (base) power Pb, and rated torque Teb. Servodrives are characterized by fast torque and speed response and thus for short time, during transients, the motor has to provide a much higher torque Tek than Teb. The higher the better for speed response quickness. Also servodrives are characterized by sustained very low speed and up to rated (base) torque operation, for speed or position control. In such conditions, low torque pulsations and limited temperature rise are imperative. Temperature rise has to be limited to avoid both winding insulation failure and mechanical deformation of the shaft which would introduce errors in position control. In general, servodrives have a constant speed (separate shaft) power, grid fed, ventilator attached to the IM at the non-driving end. The finned stator frame is thus axially cooled through the ventilator’s action. Alternatively, liquid cooling of the stator may be provided. Even from such a brief introduction, it becomes clear that the design performance indexes of IMs for servodrives need special treatment. However, fast torque and speed response and low torque pulsations are paramount. Efficiency and power factor are second order performance indexes as the inverter KVA rating is designed for the low duration peak torque (speed) transients requirements. General drives, which cover the bulk of variable speed applications, are represented by fans, pumps, compressors, etc. General drives are characterized by a limited speed control range, in general, from 0.1 ωb to 2ωb. Above base speed ωb constant power is provided. A limited constant power speed range ωmax/ωb = 2.0 is sufficient for most cases. Above base speed, the voltage stays constant. Based on the stator voltage circuit equation at steady state,
with Rs ≈ 0
Above base speed (ωb), the frequency ω1 increases for constant voltage. Consequently, the stator flux level decreases. Flux weakening occurs. We might say that general drives have a 2/1flux weakening speed range.