Capacitors can be used when we need to improve the power factor. In this condition, three capacitors are usually used with three-phase load. At this time, we do not use Farad to calculate capacitor. We use Reactive Power instead, the unit is var. The purpose to add capacitors is to offset the effects of inductive load like motors or fluorescent lights. Then the load can be close to resistive load.
var = V² × 2πfC
In the above formula, V represents Voltage (V),f represents Frequency (Hz), C represents capacitance (F).
If we use kvar and μF as the units, then the formula becomes as below:
kvar ＝ V² × 2 πfC × 10^-6 ÷ 1000 ＝ V² × 2 πfC × 10^-9

We can use a capacitor to lower voltage because of the capacitive reactance that is generated at a certain frequency of the AC signals. The capacitive reactance can limit the maximum working current.
In the low potential power source, using a capacitor to lower voltage is not as safe as using a transformer. But the power source that we use a capacitor to lower its voltage is with small size and low cost. Because of the consideration of cost, this method is used widely in the power source, such as the electronic thermostat of a refrigerator or the on /off power source of a remote control. Compared with using resistance to lower voltage, using a capacitor to lower voltage can make very small heat loss in the capacitor for AC with 50/60Hz frequency. Therefore using a capacitor to lower voltage is better than using resistance to lower voltage.

For example, when the frequency is 60Hz, the capacitive reactance that is produced by a 1uF capacitor is about 2654Ω(XC=1/(2*3.14*f*C) ). When the 220V AC voltage is applied to two ends of the capacitor, the maximum current flowing through the capacitor is approximately 83mA. But there is no power consumption in the capacitor. If the capacitor is an ideal capacitor, the current flowing through the capacitor is imaginary current, it makes reactive power.

According to this feature, if we use a resistive element and a 1uF capacitor to make series, the voltage of the two ends of the resistive element and the power consumption generated by the resistive element depend entirely on the characteristic of the resistive element.

For example, if we use a 120V/10W bulb and a 1uF capacitor to make series and then connect with the 220V/60Hz AC voltage, the bulb will be bright. It will issue normal brightness without being burned. Because a 120V/10W bulb requires 83mA current (10W/120V). It coincides with the characteristic that a 1uF capacitor can generate capacitive reactance to limit the maximum working current. Thus using a capacitor to lower voltage is actually using capacitive reactance to limit current. The capacitor plays a role in limiting the current and dynamic allocation of the voltage of the capacitor and the load.

When using a capacitor to lower voltage, we should notice the following items:
1. We should choose an appropriate capacitor according to the current size of load and the operating frequency of alternating current, not the voltage and power of load.
2. When using a capacitor to limit current, it should be a nonpolar capacitor, not a electrolytic capacitor.
3. We can not use a capacitor to lower voltage in high power conditions because of insecurity.
4. Using a capacitor to lower voltage is not suitable for dynamic load conditions.
5. Using a capacitor to lower voltage is not suitable for capacitive and inductive loads.
6. When we need DC operation, we should use half wave rectifier as far as possible and meet the conditions of the constant load. Bridge rectifier is not recommended.

Before talking about this issue, we have to understand how capacitors save power. Motors are inductive load. When using motors, the current contains invalid current (wasted current), and the ineffective current phase angle of inductor is contrary to capacitors. Therefore we can add capacitors to make them counteract each other. This is why factories mount capacitors to enhance power factor and reduce invalid current so that they can save the power rate.

The higher the power factor is, the less the invalid current is. In general households, the power factors of electric appliances are very high. As electric appliances usually have capacitors inside to improve the power factor, so general households do not need any capacitors.(power savers)

Even though the power savers are flaunted that they have miraculous effect to save power, the truth is that they are just capacitors. This type of power savers has caused a whirlwind in the market, but consumers just bought a capacitor with a beautiful shell. It’s expensive without significant effect on saving power.