Energy
Let's see what electric power is and the different types that exist. To understand electrical power, we have to previously define the concept of energy, which is the ability to perform a job, or in the case we are going to treat, electrical energy is the ability of an electrical mechanism or apparatus to perform a job. When we connect any electrical device to the voltage delivered to us by the electricity supplier company, or to a battery, or if you prefer it to a battery, the electrical energy flows through the conductor, allowing us to turn on a lamp or a computer, or you can start a motor to move a machine.
Energy is not created or destroyed, it is only transformed, and in the case of electrical energy, that transformation is manifested in the octainment of light, movement, heat, cold, or any other useful work.
The energy used in performing that useful work is measured in "Joule", and is represented by the letter J.
Electrical power
Now, let's define the electrical power: Electric power is the speed of transformation of that energy. The energy can be perceived in many parts, it can be electric, as in this case, or hydraulic in the case of liquids, or wind in the case of wind, or caloric in the case of fuels. If we were talking about hydraulic energy, the power would be, for example, the number of liters per second to transfer a liquid. In our case, the electric power is the speed with which the electrical energy is transformed, and if the energy was measured in Joules, J, the power will be measured in Jolues per second: J / s, and the watt is used as a unit of power, represented by the letter W, and 1 watt (W) is equivalent to 1 J / s. It means that when 1 Joule is consumed in a second, we consume 1 watt of electrical power.
Active power
When we have a resistive charge connected in an electrical circuit, we call it active charge, and in this case, if we know the value of the electrical voltage and the current that circulates through the resistance, the power can be calculated in a simple way as the product of the voltage in volt (V), multiplied by the value of the intensity of the current that circulates through that resistance that is expressed in ampere (A).
If, as we saw, the power is measured in watt, the voltage in volt and the current in ampere, we have that 1watt = 1 volt x 1 ampere, or : 1 W = 1 V. 1 A
Calling P to the power V to the applied voltage and I to the current that circulates through a reactive load, we can write the formula: P = V x I
Taking into account that the voltage supplied by electricity companies is constant (or at least it would be desirable), then we can see that the higher the power of an electrical equipment, the greater the intensity of current that circulates through the circuit. Of course, as sometimes companies send us a voltage much lower than 220 V, or 380 V for those with three-phase, we see from the formula, that keeping the load constant, the current consumption increases proportionally to the voltage drop. This is what often causes some appliances, machines or electronic equipment to burn.
The companies that supply electrical energy, instead of billing the consumption in watt-hour, do it in kilowatt-hour (kW-h), If we have lamps on or any active charge, for a total of 1000 watt (for example 20 bulbs of 50 watt) for an hour, the clock that records the consumption, will increase by 1 kW-h, in that period.
If we want to know the consumption of any electrical device, we must look at the sheet metal that these equipment have, usually on the back; but beware, that when the voltage drops, the consumption is maintained; but it increases the current inversely proportional to the decrease in voltage. In many cases, if they are resisitive loads, the power is usually indicated in VA (voltampere), which is nothing more than the voltage multiplied by the current. Knowing the voltage, we know what the current that circulates will be. If we have 220 V, then the current will be the value of VA, divided by 220 V. But beware, this is only valid for resistive loads.
Reactive power
To calculate the power in certain equipment that works with alternating current, it is necessary to take into account the value of the power factor or cosine of "phi" (Cos Fi) that they have. This is the case of equipment that works with reactive load, which are consumers of electrical energy that use copper wire coils, for example motors. These devices are called reactive or inductive, as they have an inductance rather than a resistor.
In active loads, such as resistance, incandescent or halogen lighting lamps, electric heaters with nichrome wire resistors (NiCr), the power factor is equal to "1", which is the ideal value of an electrical circuit and is therefore not taken into account when we calculate the active power .
Equipment that has inductive loads, as in the case of electric motors, has a power factor of less than "1", in general between 0.8 and 0.98, so the working efficiency is lower and produce a higher energy expenditure. That's why electricity companies fine companies that have a low power factor or Cos Fi. Companies, to avoid such fines, use capacitive loads, which compensate for inductive loads. Capacitors are used that increase the power factor, trying to bring it to values close to "1". Also in the plates of the motors, they have indicated the power factor, in addition to the supply voltage, the frequency, the nominal current, the power, and the nominal speed.
To calculate the power of a device that works with single-phase altenra current, taking into account the power factor is.
P = V . I . Cos Fi
where P is the power in watt (W), V is the voltage in volt (V), I is the current in ampere (A), and Cos Fi is the power factor.
Let's look at an example: Suppose we have a single-phase engine of 1 hp, with these data: Power 0.75 kW, 1 hp, 1400 rpm, In = 5.3 A, Cos Fi = 0.91
P = V . I . Cos Fi that is to say that we will have: P = 220 V . 5.3 A . 0.91 = 1061.06 watt
And why does it give us 1061 watts, when the sheet of the motor says 0.75 kW, or 750 watts? is that in the catalog of that engine that I gave as an example, it appears that the performance of that engine in %, is 70%. It has a low performance, and if we multiply 1061.06 watt . 0.70 = 743 watt. Actually, in the catalogs, the approximate power is put, and in this case it was specified as 0.75 kW or 750 watt.
I take this opportunity to tell you that if we have the power in kW and we want to take it to CV, we multiply by 1.36, they can do the calculation: 0.75 kW x 1.36 = 1.02 hp. There is also a difference between HP and CV. HP, is an English unit that means Horse Power (Cavallos de fuerza) and CV is a French unit, which means Cavallos de Vapor. The difference is 1.39%, approximately 1.4%, that is, 1 HP = 1.014 hp.
In the example we gave, which we obtained 0.743 kW, the power will be 0.743x1.36 = 1.0105 hp, or 0.997 hp. Engine manufacturers give the power in HP, or in CV, in approximate form; but it is good to know that there is a difference between the units. For practical calculations, we can say that a 0.75 kW engine is a 1 HP or 1 HP engine.
We will define both units so that there are no doubts.
CV is equivalent to the force sustained for one second, necessary to move one meter away, a weight of 75 kg.
HP equals the force sustained for one second, needed to move one meter away, a weight of 76.04 kg
If we do 76.04 / 75, it will give us the value of approximately 1.014, of difference between both units.
Let us not despair of this difference because the two are used interchangeably. The important thing is to know that it is not the same, and that if we do not require much accuracy in the calculations, we can use any of those units. The most elegant thing is to speak in kW. To unify criteria, it was proposed to use a single, universal measurement that did not depend on the measurement techniques of different countries or continents, and the kilowatt, equivalent to 1000 watt, was chosen, being the watt a universal power measure. 1 kW is equivalent to 1.36 hp or 1,341 hp.
Apparent power
We already explained that there is active power , which is what we really hire from the electricity company, and it is the useful power , that is, the one that is really used in work. We also have the reactive power , which is consumed by motors or devices that have coils to produce an electromagnetic field, and constitute a load for the electrical system, which has to maintain both active and reactive loads. A capacitive charge, or capacitor battery, is also a reactive charge, contrary to inductive charges, and used to compensate for Cos Fi.
There is a third type of power, which is the apparent power , which is the sum of the active and reactive powers. This apparent power is the one that really has to be transmitted from the electricity generation plants to the factories, the houses, the businesses, to all the consumers and we have to size both the power plant and the distribution lines and cables for that total power . That is why low power factors are penalized, since there is a reactive power , which does not produce work; but that you have to transmit through the cables.
The active and reactive powers are not added mathematically, but vectorially, they are 90º outdated vectors, that is, the total or apparent power is the diagonal of both vectors. It is the hypotenuse of the tripangle, so knowing the Pythagorean theorem, we can make the calculations of these powers. I do not go into detail here because I do not think it is necessary for what we want to explain.
And since we talked previously about penalty by electricity companies, we contract with the electricity company a certain consumption, which is called contracted power ; but then we can consume that power, less than that and in some cases we exceed. That real power that we consume, is called demanded power . That is why the power demanded should never exceed the contracted power because we will be paying a penalty, that is, a value of the kW-h, higher than the one actually contracted.
