Induction motors power factors

For direct coupled (bearingless) applications, the large airgap on the Induction motor lowers the motor PF. Induction motor power factor with large airgap can be as low as 0.85. 

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... 

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

HP Speed RPM Synch. Motor Efficiency Full Load 1.0 PF Induction Motor Efficiency Power Full Load Factor ... 

In addition to power factor considerations, synchronous motor efficiency is higher than similar induction motors. Efficiencies are shown in Table 7-1 for typical induction and unity power factor synchronous motors. Leading power factor synchronous motors have efficiencies approximately 0.5-1.0% lower. 

Power-factor can be rated at unity, leading, or even lagging. The synchronous motor can supply corrective kvar to counteract lagging power factor caused by induction motors or other inductive loads. 

Figure 14-10 compares the efficiencies of the synchronous and induction motors. For a synchronous motor designed with an 0.8 power factor, the motor delivers a leading magnetizing kva component equal to 60% of the motor kva rating. The power factor of an induction motor is always... 

Figure 14-10. Synchronous and induction motor efficiencies Full load efficiencies of high-speed 0.8 power factor synchronous motors in the ratings shown are 1-2% lower than unity power factor motors. (Used by permission E-M Synchronizer, 200-SYN-33. Dresser-Rand Company.)...

Review the types of motors proposed for a process plant with a qualified electrical engineer thereby evaluating whether the mix of synchronous and induction motors will help the net power factor for the plant, because a net lagging factor for plants means that all power to that plant will cost more than if the factor were unity or leading. From Brown and Cadickd ... 

The induction motor usually requires irom 0.3 to 0.6 reactive magnetizing kva per hp of operating load, but an 0.8 leading power factor synchronous motor will deliver from 0.4-0.6 corrective magnetizing kva per hp depending on the mechanical load carried. Thus, equal connected hp in induction and 0.8 leading power factor synchronous motors will result in an approximate unity power factor for the system. 

Fp = power factor also see PF = active power f = frequency,cycles/sec. hj = enthalpy of steam at inlet conditions, Btu/lb hj = enthalpy of steam at exhaust conditions, Btu/lb hp = horsepower, or HP I = current, amperes OR, = induction motor kv = kilovolts kwh = kilowatt hours... 

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. 

Electricity is normally charged for on the basis of power (kilowatts) and the supply authority must install plant whose rating (and therefore cost) is a function of the voltage of the system and the current which the consumer takes (i.e. kilo-volt-amps). The relationship between the two is kW = kVA x cos (j> where cos (j> is the power factor and is less than 1.0. In the case of loads which have a low power, factor the supply authority is involved in costs for the provision of plant which are not necessarily reflected in the kWh used. A penalty tariff may then be imposed which makes it economically worthwhile for the consumer to take steps to improve his power factor. Low power factors occur when the load is predominantly either inductive or capacitive in nature (as opposed to resistive). In most industrial circumstances where the load includes a preponderance of motors, the load is inductive (and the power factor is therefore lagging). Consequently, if the power factor is to be brought nearer to unity the most obvious method is to add a significant capacitive component to the load. 

Can use squirrel cage induction motors with minimal derating. Good power factor (with most designs). Speed increase possible... 

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. 

Electrical energy is furnished to the plant at 5000 volts, 50 cycles, three phase. Two 1300-kva. transformers step the voltage down to 115 volts. The 50-cycle power is converted to 500 cycles by alternators with a capacity of 36 kw. A single motor drives two alternators, and a self-induction coil is used to regulate the power factor and ensure operational stability. The 500-cycle current is stepped up to 10,000 volts for the tubular ozonizers and 18,000 to 20,000 volts for the plate type. 

A coil, or any similar device, is said to generate self-induction or, in brief, it creates a circuit with inductance. Electric motors, for example, are wired through numerous coils with high inductance. Thus, electric motors, if not corrected, can have very low power factors. 

The most usual situation that leads to low power factors is the use of inductive motors that, as the name implies, can introduce very large inductive reactance in the line. The electric load introduced by an inductive motor can be represented as a resistance and an inductance in series. This combination will have an intensity vector that will be delayed with respect to the voltage. 

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