Load power factor

The no-load test is a very informative method to determine the no-load current, core and pulsation losses, friction and windage losses, magnetizing current and the no-load power factor. The test also reveals mechanical imbalance, if any, performance of the bearings, vibration and noise level of the motor. 

Plant load power factor Requires sophisticated No synchronization problems Makes plant power factor > D D... 

Efficiency at full, and V2 load Power factor at full, and V2 load Winding resistance measurement Bearing inspection... 

SWBD load power SWBD load power factor... 

Utilities usually pass on to customers the costs of operating low-PF loads. Power factor maybe billed as one, or a combination, of the following ... 

Load power factor (PF) range crest factor (CF) PF 0.6 to 1 CF 3... 

The SPWM results in the loss of ZVS for each switch, twice in every line cycle. The extent of loss of ZVS is a function of the output current (Eq. (1)) and the load power factor (cos ( )). Eq. (2) is the expression of the available ZVS range (shown in Fig. 29) as a percentage of the line cycle. 

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. 

Power Supplies and Controls. Induction heating furnace loads rarely can be connected directiy to the user s electric power distribution system. If the load is to operate at the supply frequency, a transformer is used to provide the proper load voltage as weU as isolation from the supply system. Adjustment of the load voltage can be achieved by means of a tapped transformer or by use of a solid-state switch. The low power factor of an induction load can be corrected by installing a capacitor bank in the primary or secondary circuit. 

Field current is an important control element. It controls not only the power factor but also the pullout torque (the load at which the motor pulls out of synchronism). For example, field forcing can prevent pullout on anticipated high transient loads or voltage dips. Loads with known high transient torques are driven freqiiently with 80 percent power-factor synchronous motors. The needed additional field supplies both additional pullout torque and power-factor correc tion for the power system. When high pullout torque is required, the leading power-factor machine is often less expensive than a unity-power-factor motor with the same torque capabihty. 

A study of no-load performance suggests that no-load current, power factor and losses may vary in the following proportions, depending upon the type, size and design of the motor ... 

The declared efficiency and power factor of a motor are affected by its loading. Irrespective of the load, no-load losses as well as the reactive component of the motor remain constant. The useful stator current, i.e. the phase current minus the no-load current of a normal induction motor, has a power factor as high as 0.9-0.95. But because of the magnetizing current, the p.f. of the motor does not generally exceed 0.8-0.85 at full load. Thus, at loads lower than rated, the magnetizing current remaining the same, the power factor of the motor decreases sharply. The efficiency, however, remains practically constant for up to nearly 70% of load in view of the fact that maximum efficiency occurs at a load when copper losses (f R) are equal to the no-load losses. Table 1.9 shows an approximate variation in the power factor and efficiency with the load. From the various tests conducted on different types and sizes of motors, it has been established that the... 

Table 1.9 Approximate values of efficiency and power factor at three-quarter and half loads corresponding to values at full load...

Tests on load are conducted to determine the performance of the machine, such as its efficiency, power factor, speed and temperature rise etc. For all tests, a machine with load should be properly aligned and securely fastened. Load characteristics are obtained by taking readings at high loads, followed by reading at lower loads. This is usually carried out at 125%, 100%, 75%, 50% and 25% of the full load values. 

Several factors are important in the performance of a generator, and not the service conditions alone, as discussed for motors, in Section 1.6. In addition to service conditions, the operating power factor plays a significant role in the selection of a DG set, as noted above. The following p.f. conditions may occur in practice, depending upon the type of loads connected on the system. Refer to Figure 16.8. 

The power factor of a motor decreases sharply at loads lower than rated as discussed in Section 1.8. All the above factors, contribute to reducing the overall system power factor, which is sometimes seen to reach a low of 0.6 or even less on an LT distribution network. 

Shenoy, G., Reducing harmonic currents and improving power factor in installations having predominant thyristor loads. Siemens Circuit. XVII, No. 1 (1982). 

We will notice subsequently that series and shunt compensation are complementary. What a shunt capacitor cannot do, a series capacitor does and vice versa. On a secondary transmission system, say up to 66 kV, a shunt compensation may always be necessary to improve the power factor, as the load would mainly be indtictive. A series compensation may become essential, to improve the stability of the system, to cope with load fluctuations, switching of non-linear loads and voltage fluctuations occurring on the other power system or the grid to which this system may be connected. 

The power factor can be improved with the use of shunt capacitors at the load points or at the receiving end, as discussed above. It is not practical to have a near-fixed loading for all hours of the day. Moreover, there may also be seasonal loads which may upset the parameters considered while installing the capacitor banks. In such conditions the system may therefore have to be underutilized or run under a high risk of instability during... 

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