Motors power-factor correction

With DC electric motors, power factor correction is sometimes used to improve the power factor of the drive [30]. This is done by incorporating capacitive components into the circuit. Capacitors produce leading reactive power whereas the phase-controlled rectifiers produce lagging reactive power. Thus, by adding appropriately sized capacitors, the power factor of the drive can be improved. 

Range 900-1,800 rpm. These speeds are used for pumps above 3,000 hp and for centrifugal compressors using speed increasers. The efficiency, power-factor correction, and other factors may favor motors below 3,000 hp. 

The usual synchronous motor power factors are unity (1.0) or 0.8 leading. Values of 0.7 or 0.6 leading will give more leading correction to an otherwise lagging system. 

C. Synchronous Motors. These are normally used only when speed constancy is important, power factor correction is desired, or application is very low speed. 

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. 

This chapter introduces the basic items of design and specification for the principal systems and components of an electrical industrial installation. Electrical supply systems are discussed with regard to interface with the supply authorities and the characteristics. Salient features of switchgear, transformers, protection systems, power factor correction, motor control equipment and standby supplies are identified and discussed together with reference to the relevant codes of practice and standards. The equipment and systems described are appropriate to industrial plant installations operating at typically 11 kV with supply capacities of around 20MVA. 

Capacitors as bulk units can be connected to the supply busbar via a fuse switch, molded-case circuit breaker of air circuit breaker. In this type of installation control is purely manual, and in cases of a reasonably constant load and where the amount of power factor correction is limited such a manually controlled system is perfectly adequate. The supply authority may, however, require to be informed that a capacitor bank is permanently connected to the supply. Capacitors are more generally connected either in banks controlled from a VAR sensitive relay or across individual loads (e.g. motors). 

When located at the motor the capacitor bank will be normally cabled from the motor terminal box, so that the size of the motor cable can then be selected on the basis of the reduced-power factor corrected current drawn by... 

For relatively small loads, the power factor correction equipment usually takes the form of static capacitors. In larger installations, it may be more economic to install an A.C. synchronous motor that, if its excitation is adjusted correctly, can be made to draw a leading current from the supply. In most industrial plants, the load is variable, and to gain the maximum benefit from the power factor correction plant this must be varied to suit the load conditions. 

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. 

FIGURE 3.19 Transient due to motor starting. The motor had an input capacitor for power factor correction, and the motor and capacitor were turned on simultaneously. 

Polystyrene capacitors have exceptionally low tan S values (< 10 q, making them well suited for frequency-selective circuits in telecommunications equipment. Polymer capacitors are widely used for power-factor correction in fluorescent lighting units, and in start/run circuitry for medium-type electric motors used in washing machines, tumble-dryers and copying machines for example. They are also used in filter circuits to suppress radio frequencies transmitted along main leads. Such interference noise may originate from mechanical switches, furnace controllers and switch mode power supplies it not only spoils radio and television reception but can also cause serious faults in data-processing and computer equipment. 

A tabulation of prices and ratings would lead one to the proper motor selection. The motor should be sized to operate at high efficiency over the anticipated duty cycle. This allows for a minimum power factor correction. In the low duty part of the cycle, a smaller motor Is more efficient. It Is strongly recommended that the power factor correction be used to make the motor operate at Its peak efficiency over the total cycle. Buyers have a tendency to overspecify the required HP which results In less than peak efficiency. 

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 frequently with 80 percent power-factor synchronous motors. The needed additional field supplies both additional pullout torque and power-factor correction 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 capability. 

Arrangements have been made for power factor correction at some time in the future if required. This is simply the provision of available space for the addition of capacitor banks to the operating circuits of the large motors and to the main substation bus. The "bus correcting" capacitor bank at the substation will in effect correct for the reactive power demand of the numerous small motors inuse throughout the site. 

Synchronous Motor Starting. This method is used for power-factor correction of heavy concentrations of induction motors. It is also used for constant-speed, slow-speed industrial drive applications and for maximum efficiency on continuous heavy loads in excess of 75 hp (55 kW). Three-phase ac power is connected to the stator and dc to the rotor (which has both a field and a squirrel-cage winding). 

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. 

Automatic correction is always recommended to eliminate manual dependence and to achieve better accuracy. It also elimintites the risk of a leading power factor by a human error that may cause an excessive voltage at the motor and the control gear terminals. 

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. 

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. 

Select standard size motor. A motor that is loaded to 85 percent by a 79.1-hp impeller will require a minimum size of (79.1 hp)/0.85 = 93.1 hp, which means a 100-hp (75-kW) motor. This motor and impeller assembly is correctly sized for conditions with the design gas flow. However, because of the gassed power factor, that is, P/P0 = 0.38, should the gas supply be lost for any reason, the impeller power would increase to 78/0.38 = 205 hp and seriously overload the motor. To avoid this problem, some method (typically electrical control) prevents motor operation without the gas supply. When the gas supply is off, the control either stops the agitator motor or, in the case of a two-speed motor, goes to a lower speed. 

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. 

Correct power factor by using energy-efficient motors or capacitors. 

Select standard size motor. A motor that require a minimum size of (79.1 hp)/0.85 = 93 motor and impeller assembly is correctly sized because of the gassed power factor, that is, P/Pq the impeller power would increase to 78/0.38 = this problem, some method (typically electrical supply. When the gas supply is off, the control two-speed motor, goes to a lower speed. 

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. 

Compute the fan speed and power input. Multiply the capacity-table r/min and bhp by the composite correction factor to determine the actual r/min and bhp. Thus, using data from Table 6.31, the actual r/min is (1096)(1.1147) = 1221.7 r/min. Actual bhp is (99.08)(1.1147) = 110.5 hp. This is the horsepower input required to drive the fan and is close to the 113.2 hp computed in the previous example. The actual motor horsepower would be the same in each case because a standard-size motor... 

The horsepower required to rotate a 55-in-diameter impeller (11-in blade width) at 68 r/min can be computed for the process fluid, using the technique described in Example 12.1. The turbulent power number is 1.37, from Fig. 12.1. The Reynolds number at 68 r/min becomes 161. The viscosity correction factor for this value is 1.35, from Fig. 12.2, which gives a power number NP of 1.37(1.35) = 1.85 for the design conditions. From the power number, impeller power can be computed P = 1.85(l.l)(68)3(55)s/(l. 524 x 1013) = 21.1 hp. With an 85 percent loading for the motor, a minimum motor horsepower would be 21.1/0.85 = 24.8 hp, so a 25-hp (18.5-kW) motor would be required. If the next larger standard motor is substantially larger than the minimum motor horsepower, the impeller diameter may be increased by an inch or two to fully utilize the available motor capacity. 

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