364induction Motors

Of the various types of motor, Fig. 3.12, induction motors (IMs) are practical up to about 30 kW and 15 000 rpm. Beyond these limits exhausting the rotor losses is generally a problem (at 1500 rpm megawatt level machines are commonly constructed). At 15 kW IMs run satisfactorily up to 100 000 rpm but special motor construction techniques are needed to give strength to the cage. Water cooling is used at high power. A typical specification, for example, might be 36 kW, 12 000 rpm, PF 0.9 at 400 Hz, 36 kW - 4 pole: efficiency 0.9 at 400 Hz, 36 kW, 380 V, 68 A line current; slip (pure aluminium cage) 50 rpm cold, 70 rpm hot - torque 29 Nm (0.7 tesla), hot rotor diameter 6 in, active length 4 in, stator 9 in OD, peak torque at low speed 100 Nm (1.5 tesla), rotor cooling 8 CFM compressed air at 17 Psi, stator cooling 4 litres water/minute.

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