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I have been trying to understand the concept of reactive power in power systems. From what I have read so far, I understand that any inductive load like a motor needs some reactive power to function (set up the magnetic fields to work etc.). Now I also understand that, while transmitting the current required for this motor to work, the transmission line also has to carry the current required for the reactive power. Now this increases the loss, as it is given by \$I^{2}R \$. Now, capacitors are used to help generate this reactive power, (as they dissipate power when the inductor consumes it) and are hence placed near the load to reduce the reactive power that needs to be transmitted. I have the following questions:

  1. Is my thought process correct? Am I right in my understanding of reactive power?

  2. If we already know the value of the inductance of the motor, why don't motors come attached with capacitors that make them less of an inductive load? If we can approximate the inductance of the coils can't we find a capacitance that cancels the effect and makes the net impedance value close to zero? Why do we need to have separate stations built for this when we can try to eliminate this at a device level.

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3 Answers 3

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  1. Is my thought process correct? Am I right in my understanding of reactive power?

Yes. It may help to consider the reactive "power" as sloshing back and forth between the inductive and capacitive loads and, when fully balanced, never leaving the building. That leaves only the real power travelling on the supply.

  1. If we already know the value of the inductance of the motor, why don't motors come attached with capacitors that make them less of an inductive load?

Cost and control.

If we can approximate the inductance of the coils can't we find a capacitance that cancels the effect and makes the net impedance value close to zero?

You can for a given load. Changing the load will change the current and the required correction will therefore change.

Why do we need to have separate stations built for this when we can try to eliminate this at a device level.

It can make sense to do this with motors some distance from the supply - on a large site, for example. If the load is fixed - a pump, perhaps - then the amount of correction required is constant and a fixed capacitor can be attached.

In most cases there is variable loading and "load discrimination" is applied to calculate the worst-case PF correction required with certain assumptions for the maximum likely load to be on at any one time. (e.g., Only one of a pair of pumps - run and standby - would run at any time.) Usually a capacitor panel is attached to the distribution board with a controller to switch in and out capacitors to maintain a certain power factor. This results in less capacitors being required - both in quantity and total capacitance value - and hence is more economical to install.

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Is my thought process correct? Am I right in my understanding of reactive power?

You are essentially correct except for:

(as they dissipate power when the inductor consumes it)

Should be: "as they store energy when the inductor releases it and release energy when the inductor stores it."

There are several reasons that motors don't come with capacitors attached.

  1. The motor does not have a convenient place to put them.

  2. They would add cost to the motor that the purchaser may not need to incur.

  3. Having the capacitors connected all of the time could cause the motor not to stop as quickly when shut off. The stored energy can help keep the motor running.

  4. It may be more cost-effective to have just a few larger capacitors for many motors.

  5. That is the way it has been done for the last century, from the time no one realized that adding capacitors could save energy and free up system power capacity.

  6. Some motors are electronically controlled and do not need capacitors.

  7. The need for capacitors must be handled a bit differently if there is harmonic distortion in the power system. Harmonic voltages in the distribution system can cause resonance oscillation currents between the capacitors and the motor and transformer inductance. One remedy is to add resistors and inductors to groups of power-factor correction capacitors so they can also serve as harmonic filters.

There are probably some additional reasons.

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  • \$\begingroup\$ Can you explain the last point? \$\endgroup\$ Jun 17, 2020 at 13:33
  • \$\begingroup\$ See revised answer. \$\endgroup\$
    – user80875
    Jun 17, 2020 at 14:02
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Reactive Power is simply power into a non-resistive load. That could be either inductive or capacitive. If you are driving a motor, that is an inductive load. The power grid will have to deliver current (energy) to create the magnetic field required by the running motor. This magnetic energy is "stored" in the motor, until it is turned off, then the field collapses and returns the current to the grid.

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