chapter  16
37 Pages

INDUCTION MOTOR DESIGN ABOVE 100KW AND CONSTANT V AND f (Size Your Own IM)

Induction motors above 100kW are built for low voltage (480 V/50 Hz, 460 V/60 Hz, 690 V/50 Hz) or higher voltages, 2.4 kV to 6 kV and 12 kV in special cases. The advent of power electronic converters, especially those using IGBTs, caused the raise of power/unit limit for low voltage IMs, 400V/50Hz to 690V/60Hz, to more than 2MW. Although we are interested here in constant V and f-fed IMs, this trend has to be observed. High voltage, for given power means lower cross section easier to wind stator windings. It also means lower cross section feeding cables. However, it means thicker insulation in slots, etc. and thus a low slot-fill factor; and a slightly larger size machine. Also, a high voltage power switch tends to be costly. Insulated coils are used. Radial-axial cooling is typical, so radial ventilation channels are provided. In contrast, low voltage IMs above 100kW are easy to build, especially with round conductor coils (a few conductors in parallel with copper diameter below 3.0mm) and, as power goes up, with more than one current path, a1 > 1. This is feasible when the number of poles increases with power: for 2p1 = 6, 8, 10, and 12. If 2p1 = 2, 4 as power goes up, the current goes up and preformed coils made of stranded rectangular conductors, eventually with 1 to 2 turns/coil only, are required. Rigid coils are used and slot insulation is provided. Axial cooling, finned-frame, unistack configuration low-voltage IMs have been recently introduced up to 2.2MW for low voltages (690V/60Hz and less). Most IMs are built with cage rotors but, for heavy starting, or limited speed-control applications, wound rotors are used. To cover most of these practical cases, we will unfold a design methodology treating the case of the same machine with: high voltage stator and a low voltage stator, and deep bar cage rotor, double cage rotor, and wound rotor, respectively. The electromagnetic design algorithm is similar to that applied below 100kW. However the slot shape and stator coil shape, insulation arrangements, parameters expressions accounting for saturation and skin effect are slightly, or more, different with the three types of rotors. Knowledge in Chapters 9 and 11 on skin and saturation effects, respectively, and for stray losses is directly applied throughout the design algorithm. The deep bar and double-cage rotors will be designed based on fulfilment of breakdown torque and starting torque and current, to reduce drastically the number of iterations required. Even when optimization design is completed, the latter will be much less time consuming, as the “initial” design is meeting approximately the main constraints. Unusually high breakdown/rated torque ratios (tbe = Tbk/Ten > 2.5) are to be approached with open stator slots and larger li/τ ratios to obtain low stator leakage inductance values.