Induction motors loading effects

Figure 12.7 Effect of a large load variation on the response of overcurrent relays at the generators. The diagram shows the response when a large induction motor is started direct-on-line. Four different standing loads are shown.

It may be seen that if this method is used to control the speed of a standard, almost constant-speed type of induction motor, then the actual range of speed control obtained will, in fact, be small before stalling occurs. The situation could be improved by using a motor with a high rotor resistance as shown in Figure 14.2. The rotor resistance at full-load has been increased by a factor of 10 in order to demonstrate the effect on the torque-speed characteristic. 

Section 8.2.3 also considers the effect of an abrupt parametric fault in one phase of the inverter s load. In this case study, the load of the three-phase inverter is an RL-network in delta configuration often used in studies of three-phase PWM voltage source inverters. The study may be extended by replacing the RL-network by a sophisticated BG-model of an induction motor (see for instance [31, Chap. 8]). Studies of the three-phase diode bridge rectifier typically assume a resistive load in parallel to a filter capacitor. 

Speed control by way of adjusting power frequency is becoming more and more important for economical throughput or presstme capacity variation of modern process machinery. Several key parameters that must be considered when applying induction motors to adjustable frequency controllers include the load torque requirements, current requirements of the motor and the controller ciurent rating, the effect of the controller wave-shape on the motor temperature rise, and the required speed range for the application. 

Fig. 55. Effect of varying rotor diameter in brushless DC motors based on permanent magnets, (a) Situation for ferrite magnets under various constraints discussed in the main text. The arrow indicates increasing electric loading Q. (b) Envelope of feasible designs (hatched area) based on different permanent magnet materials. The arrows L and BH indicate increasing inductance (L) and increasing (BH)msi%, respectively. (After Howe et al. 1987.)...

Most installations have a low or bad power factor because of the inductive nature of the load. A capacitor has the opposite effect of an inductor, and so it seems reasonable to add a capacitor to a load which is known to have a lower or bad power factor, for example, a motor. 

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