The Power Factor Correction Capacitor - What it Does

Now I'll explain why we would connect a Power Factor Correction Capacitor to the motor's circuit. This gets sort of fun....not too complicated! We have previously learned that motor current is composed of 2 parts: "Load Current" and "Magnetizing Current". They add together inside the motor to make the total current delivered to the motor.

Magnetizing Current, establishes the magnetic field so the motor will spin, its current is constant and does not vary with load, it uses no energy and it's sine wave lags the voltage's sine wave by 90 degrees.

Load Current is zero if the motor is spinning with no load (theoretically, meaning no friction losses, wind losses, etc.) and increases with increased resistance to spinning. Load Current delivers energy to do the work (i.e. drives the pump) and the Load Current's sine wave is in-phase with the voltage's sine wave.

In the graphic below I show a very lightly loaded motor. It has a Power Factor of less than 0.3. Notice there is alot of magnetizing current compared to the load current.

We know that the load current is the one that delivers the power, so if we could eliminate the magnetizing current, the total current delivered to the motor would be less.

In the graphic below is the same motor with no magnetizing current being delivered, thus the total motor current equals the load current and the Power Factor on this line is 1.0, or unity.

So how do we get rid of it? How do we stop delivering the magnetizing current component along the cable feeding the motor? If you remember back in the "Adding Currents" section, I displayed the following graphic which shows two curves; one is magnetizing current and the other is that of a power factor correction capacitor which draws a current of equal magnitude.

The currents are 180 degrees apart from each other. Comparing them to the system voltage, the magnetizing current lags the voltage by 90 degrees and the Power Factor Correction Capacitor current leads (happens ahead of) the voltage by 90 degrees. Anyhow, they complement each other... when the magnetic field is building-up, it is demanding current. During that time the capacitor is discharging and supplying current. On the flip side, when the magnetic field is collapsing and kicking current back into the system, the capacitor is demanding current and building a charge.
A Match made in Heaven!
....a perfectly compatable couple.

What does this mean? If we connect a Power Factor Correction Capacitor near the motor, the capacitor and the motor can take turns feeding each other the current associated with the magnetic field. Now, the cable that connects the motor to its source of power needs only to deliver load current.

Next...lets learn the Lingo....Power Factor TERMINOLOGY.