• Mechanical loads are characterised by torque/speed curves.
• Single quadrant and multiquadrant load torque/speed curves are typical.
• Constant V/f supply IMs are suitable only for constant speed single quadrant loads.
• For single and multiquadrant variable speed loads, variable V/f supply IMs are required. They result is energy savings commensurable with speed control range.
• Three load torque/speed curves are typical: quadratic torque/speed (pumps), constant torque (elevators), and constant power (machine tool, spindles, traction, etc.).
• The standard IM design torque/speed envelope, to match the load, includes two regions: below and above base speed Ωb. For base speed full voltage, full torque, is delivered at rated service cycle and rated temperature rise.
• With self ventilation the machine overtemperature leads to torque reduction with speed reduction. For constant torque below base speed, separate ventilation is required.
• Above base speed−constant voltage and increasing frequency, the torque available decreases and so does the flux linkage in the machine.
• A 2/1 constant power speed range (from Ωb to 2 Ωb) is typical with standard IM designs at constant voltage.
• When an induction motor designed for sine wave power is faced with a notable harmonic content in the power grid due to presence of power electronic equipment nearby, it has to be derated. In general a harmonic voltage factor (HVF) of less than 3% is considered harmless (Figure 14.6).
• A standard sine wave IM, when fed from a PWM voltage source inverter due to the additional (time harmonics) core and winding losses, has to be derated. A derating of 10% is considered acceptable with today’s IGBT converters.
• Further on, the presence of a static power converter leads to a 5% voltage reduction at motor terminals with respect to the power grid voltage.
• Finally, an additional derating occurs due to unbalanced power grid voltage. The derating is significant for voltage imbalance above 2% (Figure 14.7).
• Induction motor specifications for constant V/f motors are lined up in pertinent standards. Nameplate markings refer to a miryad of specifications for the user’s convenience.
• Efficiency is the most important nameplate marking as the cost of losses per year is about 30 – 40% of initial motor costs.
• Standard and high efficiency motors are now available. NEMA and EU regulations refer to high efficiency thresholds (Table 14.2 and 14.3).
• Designs A, B, C, D, E reveals through their torque speed curves, the starting, pull-up, and breakdown torques which are important factors in most constant V/f supply IMs.
• Matching a constant V/f IM to a load refers to equality of load and motor torque at rated speed and lower load torque below rated speed.
• For variable speed drive, two pole pairs count motors at two different frequencies, one below base speed (flux zone) and one above base speed (flux weakening, constant voltage zone) may be used.
• For constant power large speed ranges Ωmax/Ωb > 3, very large breakdown torque designs are required (above 300%). Alternatively, the voltage per phase is increased above base speed by star/delta connection or a larger torque (larger size) IM is chosen.
• Design of an IM means sizing the motor for given specifications of power supply parameters and load torque/speed envelope.
• Main design factors are: costs of active materials, fabrication, and selling, capitalised costs, maintenance costs, material limitations (magnetic, electric, dielectric, thermal, mechanical), and special application specifications.
• The IM design features 5 issues.
• Electric design
• Dielectric design
• Magnetic design
• Thermal design
• Mechanical design
• IM sizing is both a science and an art based on prior experience.
• Dis2L output coefficient design concept has gained widespread acceptance due to Esson’s output constant and, with efficiency and power factor known, the stator bore diameter Dis, may be calculated for given power, speed, and stack length L per pitch τ ratio λ given.
• Further on, with given stator winding current density, airgap, stator teeth, and back core flux densities Bg, Bts, Bcs, the outer stator diameter is obtained. Based on this data, the stator/rotor slot sizing, wire gauge, machine parameters, performance, losses, and temperatures may be approached. Such a complex enterprise requires coherent methodologies, to be developed in subsequent chapters.
• The rotor tangential stress σtan = (0.2 – 4)N/cm2 is defined above as a more general design concept valid for both rotary and linear induction motors. This way there is no need to assign initial values to efficiency and power factor to perform the complete design (sizing) process.
• More on design principles in References [4−6].