Capacitors starting motors

Capacitor-start motor. A capacitor-start motor is a capacitor motor in which the capacitor phase is in the circuit only during the starting period. 

Single-Phase Operation. If single-phase power only is available, it is advisable to consider the use of single-phase to three-phase converters and three-phase motors. This avoids the use of large single-phase capacitor start motors, which are relatively expensive and contain a starting switch that could be a source of trouble due to failure or to the presence of flammable gas in the vicinity of the well,... 

Capacitor-Start Motors. Capacitor-start motors are similar to split-phase motors except that a capacitor is placed in series with the start winding to produce greater starting and accelerating torque (Fig. 5.104). After the start winding is removed from the circuit by a centrifugal or electronic switch, performance is identical to that of split-phase motors. 

FIGURE 5.104 Schematic diagram and speed vs. torque diagram—ac capacitor-start motor. 

The size of capacitors C, C l or C will depend upon the horsepower of the motor and the torque requirement of the load. For starting duty capacitors generally in the range of 30-100 pF and for a run of 2-20 //F will be adequate. 

Figure 25.7 Typical power and control scheme for A/l> starting of a motor with capacitor bank, illustrating capacitor shorting during A...

Permanent-split capacitor motor. A permanent-split capacitor motor is a capacitor motor having the same value of capacitance for both starting and running conditions. 

Starting Capacitance for a Capacitor Motor. The starting capacitance for a capacitor motor is the total effective capacitance in series with the starting winding under locked-rotor conditions. 

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. 

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 simple and reliable starting method, used by most modem fan motors and other low torque types is the "shaded pole" configuration. In Fig. 20.7, it can be seen that a small part of the metal or ferrite pole has a single turn of heavy copper wire around it. This acts like the short-circuited secondary coil of a transformer, as previously discussed on page 108. The inductance at that edge of the pole is decreased, similar to the effect of a small capacitor. The force vectors are quite complex. 

Two-Value-Capacitor Motors. Two-value-capacitor motors have both a switched-start capacitor and a run capacitor to improve full-load current, starting torque, and power factor (Fig. 5.106). Both are connected in parallel to the start winding, with the start capacitor disconnecting as the motor accelerates.These motors provide good overall torque characteristics and are quiet-running. 

Voltage too low at motor terminals (because of line voltage drop or voltage drop in wiring to motor) Starting load too high Shorted or weak capacitor Low temperature (below 0°F)... 

Indicated by humming sound when switch is closed. Check for loose wiring connections, whether starting switch inside motor is closed, or if capacitor is defective. 

It is important to understand why this twofold approach is needed. Prior to the 1980s, system failures could usually be identified as specific component failures (e.g., relay open circuit, capacitor short circuit, motor fails to start). However, since then the growth of complexity (including software) has led to system failures of a more subtle nature whose cause may not be attributable to a catastrophic component failure. Hence we talk of ... 

Three-phase motors have a high starting torque, high efficiency, and low current requirement The torque characteristics are described by NEMA electrical design, which is discussed in the next section. Three-phase motors do not use a capacitor, switch, or relay for starting. 

---