Capacitors in three-phase motor circuits are mainly applied for power factor correction, starting, and voltage filtering. Applying capacitors raises the power factor from 0.75 to 0.95, decreases reactive power by 66 kvar, saves on electricity costs, and increases system stability.
Power Factor Correction
Among the industrial power-consuming devices, most of them are three-phase motors, and most of those motors are inductive loads. Since inductive load has the characteristic of current lagging voltage, this then results in a decrease in power factor. Power factor is one of the most important parameters indicating the efficiency of a power system, and the more unfavorable it is, the greater the transmission of reactive power becomes, which eventually means more losses in transmission and reduced efficiency of the grid; overheating or even equipment damage may also result.
In general, the power factor is about 0.7 to 0.85 for an average three-phase motor, while theoretically, the power factor must be 1. Otherwise, capacitors are generally paralleled with the motor circuit in order to provide the necessary capacitive reactive power contribution to balance out the inductive reactive power generated by the motor, which would improve the power factor.
A three-phase 100-kW motor with a power factor of 0.75 would consume approximately 66 kvar reactive power. This can be brought to 0.95 or even higher by installing the proper value of a capacitor that would compensate for the reactive power losses. The result would be that the effective power-i.e., the active power of the system will come closer to the total power-picot apparent power, significantly reducing the transmission of reactive power. This not only reduces the load off the grid but also brings down the cost of electricity, as most power companies charge extra from users whose power factor falls below a certain standard.
Phase Shift in Single-Phase to Three-Phase Conversion
Even in the case when a motor itself is designed for three-phase power, sometimes it may be required that the motor be connected to a single-phase power supply. Here, capacitors play a vital role in the phase conversion process. By using the phase-shifting function of a capacitor, a single-phase power supply may simulate an additional phase and form a “pseudo three-phase power supply” for the running of three-phase motors normally.
Phase converters, such as rotary-type devices, use capacitors in an attempt to simulate three-phase power by providing a shifted voltage to the stator windings of the three-phase motor. Capacitors are generally connected between the main and auxiliary winding to provide a phase shift of about 90 degrees so that the motor has enough torque to start. As the motor operates, capacitors continue to provide phase compensation so the motor maintains normal rotation.
With a well-designed capacitor, the efficiency of such a single-phase motor for ordinary applications of about 7.5 kW can achieve as high as 85% or even higher, almost catching up with the efficiency under three-phase power. Although such a conversion is far from replacing the stability and continuity of output provided by the three-phase power supply, it may be an effective and realistic option where a three-phase power supply is unavailable.
In these applications, the selection and arrangement of the capacitors become critical. The capacitance and voltage rating of the capacitor must be selected to match the load requirements of a motor; otherwise, this will result in faulty motor startup or unstable operation of the motor. Furthermore, this phase shift can create some harmonic interference; hence, the overall anti-interference capability of the system has to be considered.
Starting Capacitors
Starting capacitors may be applied in three-phase motors: main starting capacitors in would-rotor motors or special types of motors. The motors need larger starting torque; hence, starting capacitors can provide high-capacity current for a moment to assist the motor in overcoming its inertia at the start.
A would-rotor motor of rated power 15 kW may require over 200% of rated current during starting. Without starting capacitors, it may not draw enough current to produce a sufficiently strong magnetic field to begin its turning; in fact, it may not start at all. Starting capacitors, on the other hand, discharge momentarily at the instant of motor start-up and instantly provide capacitance well into several hundred microfarads, developing a robust magnetic field that helps the motor achieve its rated speed quickly.
Starting capacitors operate only for the starting period and automatically get disconnected once the motor has gained its normal speed. If a starting capacitor remains in operation for a more extended period without being timely disconnected, then either the motor would operate unsteadily, or it may be damaged. In practice, contactors or relays are used to connect or cut the starting capacitor.
It is also worth noting that start capacitors have shorter life spans compared to the usual working capacitors due to their exposure to high-current surges. They thereby require checkups and replacements regularly in order to ensure that the system will run smoothly for a long time.
Voltage Filtering and Noise Suppression
Two high-voltage fluctuations and high-frequency noise during motor operation are two important factors affecting the efficiency of its operation and the life of the motor. Capacitors can also serve as filters—smoothing out voltage fluctuations and filtering out from the power grid high-frequency, thus protecting normal motor operation.
Motors connected to unstable or flickering electrical power systems often suffer from overvoltages, which sometimes cause overheating in the windings, deterioration of the insulation, and even burnout in the motor. Capacitors absorb these transient overvoltages and surge currents, converting them to reactive power, thus lessening their effect on the motor.
Motors produce high-frequency harmonic noise during turn-on or in normal operation. These, if not treated, may spread to the power grid and interfere with other equipment. Capacitors can be combined with inductors to form filters that eliminate such high-frequency components and ensure a purer voltage waveform for the system.
The most common situation of noise interference is when the motors working in a certain factory interfere badly with the power grid at the moment of startup, thereby affecting the operating precision of other instruments. After adding an appropriate filtering device of capacitors in the motor circuit, effectively suppressing the noise, the THD of the grid waveform was reduced by about 50%, which has significantly improved the power grid environment.
Reducing Voltage Drop Due to Load Changes
In some applications, three-phase motors draw highly variable loads, such as large-size fans or pump systems. When the load suddenly increases, the current drawn by the motor increases instantaneously. Consequently, the grid voltage sags, which affects the normal working of other equipment. Capacitors assist in instant discharging to make up for the voltage loss of the grid, thus reducing the extent of the voltage sag by maintaining system voltage stability.
A very large 30 kW fan motor can vary current from 80% to 120% with changes in load. In the case of a system uncompensated by any capacitor, voltage drop during an increase of load can rise to more than 5%, thereby changing the quality of power with other equipment. On the other hand, when a sufficient amount of capacitors are connected in parallel, the voltage drop can be easily controlled within 2% and sufficiently stabilize system voltage.
This voltage-stabilizing effect is very important for systems with several motors running in parallel. Imagine several motors operating at the same time on a production line; if one motor suddenly experiences a different load condition, it can momentarily fluctuate the voltage in the grid and, therefore, affect the stability of the other motors. Because the response characteristic of a capacitor is rapid, it provides reactive compensation in the instant of load variation, hence diminishing the voltage fluctuation amplitudes and maintaining voltage stability in the whole system.
