Capacitors induction motors

Recommended capacitor ratings for direct switching with induction motors, to improve power factor to 0.95 or better at... 

Refer to a typical distribution network shown in Figure 2.5..5 I. The capacitor is of maximum use when located as near to the load-point as possible, especially in induction motors, because ... 

If a large induction motor is switched on such a system it is possible that its rotor may lock up at the sub-synchronous speed and keep running at higher slips. This situation is also undesirable, as it would cause higher slip losses in addition to higher stator current and overvoltage across the series capacitors. 

Capacitor Motor. A capacitor motor is a single-phase induction motor with a main winding arranged for a direct connection to a source of power and an auxiliary winding connected in series with a capacitor. There are three types of capacitor motors, as follows ... 

Power-Factor Correction. The induction motors used for oil-well pumping have high starting torques with relatively low power factors. Also, the average load on these motors is fairly low. Therefore, it is advisable to consider the installation of capacitors to avoid paying the penalty imposed by most power companies for low-power factor. They will be installed at the individual motors and switched with them, if voltage drop in the distribution system is to be corrected as well as power factor. Otherwise they may be installed in large banks at the distribution center, if it is more economical to do so. 

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. 

Induction motors give rise to lagging power factor, which can be penalized in the tariffs applied by the utility company. Certain other items of industrial equipment can also contribute to low power factor. This is compensated by coimection of capacitors, either individually for each reactive load, or grouped at a common feed point in the network with banks of capacitors being connected and disconnected automatically in stages as required. Other possibilities for power factor correction include the use of synchronous motors for large drives. 

A method to improve power factor, which is t3 ically expensive, is to use a unity or leading power factor synchronous motor or generator in the power system. A less expensive method is to connect properly sized capacitors to the motor supply line. In most cases, the use of capacitors with induction motors provides lower first cost and reduced maintenance expense. Figure E-7 graphically shows how the total KVA vector approaches the size of the real power vector as reactive KVAR is reduced by corrective capacitors. Because of power factor correction, less power need be generated and distributed to deliver the same amount of useful energy to the motor. 

Just as the efficiency of an induction motor may be reduced as its load decreases, the same is true for the power factor, only at a faster rate of decline. A typical 10-horsepower, 1800 rpm, three-phase, design B motor with a full-load power factor of about 80 percent decreases to about 65 percent at half load. Therefore, it is important not to overmotor. Select the right size motor for the right job. Figure E-8 shows that the correction of power factor by the addition of capacitors not only improves the overall power factor but also minimizes the fall-off in power factor with reduced load. 

A surge capacitor offers a near-open circuit during normal operation (at or near the power frequencies) and a near-short-circuit to the arriving surges at surge frequencies, while the inductance of the motor windings (<> will... 

Transient — Subcycle disturbance in the AC waveform evidenced by a sharp, brief discontinuity of the waveform. This may be of either polarity and may be additive or subtractive from the nominal waveform. Transients occur when there is a sudden change in the voltage or the current in a power system. Transients are short-duration events, the characteristics of which are predominantly determined by the resistance, inductance, and capacitance of the power system network at the point of interest. The primary characteristics that define a transient are the peak amplitude, the rise time, the fall time, and the frequency of oscillation. Figure 1.12 shows a transient voltage waveform at the output of a power transformer as the result of switching-in of a motor containing power factor correction capacitors. 

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