Power measurement. Mechanical power formula and definition

that is, the product of force vectors by the speed of movement - and there is power. What is it measured in? According to the international SI system, the unit of measurement of this quantity is 1 watt.

Watt and other units of power

Watt means power, where one joule of work is done in one second. The last unit was named after the Englishman J. Watt, who invented and built the first steam engine. But at the same time, he used a different value - horsepower, which is used to this day. One horsepower is approximately equal to 735.5 watts.

Thus, in addition to watts, power is measured in metric horsepower. And with a very small value, Erg is also used, equal to ten to the minus seventh power of Watts. It is also possible to measure in one unit of mass / force / meters per second, which is equal to 9.81 watts.

Engine power

The named value is one of the most important in any motor, which can be of very different power. For example, an electric razor has hundredths of a kilowatt, while a spacecraft rocket has millions.

Different load needs different power to maintain a certain speed. For example, a car will become heavier if more cargo is placed in it. Then the friction force on the road will increase. Therefore, to maintain the same speed as in the unloaded state, more power is required. Accordingly, the engine will eat more fuel. All drivers are aware of this fact.

But at high speed, the inertia of the machine is also important, which is directly proportional to its mass. Experienced drivers who are aware of this fact find the best combination of fuel and speed when driving, so that less gasoline is consumed.

Current power

How is power measured? In the same SI unit. It can be measured directly or indirectly.

The first method is implemented using a wattmeter, which consumes significant energy and heavily loads the current source. With it, it is measured from ten watts or more. The indirect method is used when it is necessary to measure small values. The devices for this are an ammeter and a voltmeter connected to the consumer. The formula in this case will look like this:

With a known load resistance, we measure the current flowing through it and find the power as follows:

P \u003d I 2 ∙ R n.

According to the formula P \u003d I 2 / R n, the current power can also be calculated.

What it is measured in a three-phase current network is also not a secret. For this, a familiar device is used - a wattmeter. Moreover, it is possible to solve the problem of how electric power is measured using one, two or even three devices. For example, a four-wire installation would require three devices. And for a three-wire with an unbalanced load - two.

Executed for a certain period of time, to this period of time.

Effective power, engine power given to the working machine directly or through a power transmission. Distinguish between useful, full and nominal E. m. of the engine. Useful is called the E. m. of the engine, minus the power consumption for actuating the auxiliary units or mechanisms necessary for its operation, but having a separate drive (not directly from the engine). Full E. m. - engine power without deducting the indicated costs. Rated E. m., or simply rated power, - E. m., guaranteed by the manufacturer for certain operating conditions. Depending on the type and purpose of the engine, electromagnetic engines are established, which are regulated by standards or technical conditions (for example, the maximum power of a ship's reversible engine at a certain crankshaft speed in the case of a ship astern - the so-called reverse power, the maximum power of an aircraft engine with a minimum specific fuel consumption - the so-called cruising power, etc.). E. m. depends on the forcing (intensification) of the working process, the size and mechanical efficiency of the engine.

Units

Another common unit of measure for power is horsepower.

Power in mechanics

If a force acts on a moving body, then this force does work. The power in this case is equal to the scalar product of the force vector and the velocity vector with which the body moves:

M- moment, - angular velocity, - pi, n- rotational speed (rpm).

Electric power

Electric power- a physical quantity that characterizes the rate of transmission or conversion of electrical energy.

S - Apparent power, VA

P - Active power, W

Q - Reactive power, VAr

Power Meters

Notes

see also

Links

• The influence of the form of electric current on its action. Magazine "Radio", number 6, 1999

Wikimedia Foundation. 2010 .

See what "Power (physics)" is in other dictionaries:

A science that studies the simplest and at the same time the most general patterns of natural phenomena, the principles and structure of matter and the laws of its motion. The concepts of F. and its laws underlie all natural science. F. belongs to the exact sciences and studies quantities ... Physical Encyclopedia

Examples of various physical phenomena Physics (from other Greek φύσις ... Wikipedia

I. The subject and structure of physics Physics is a science that studies the simplest and, at the same time, the most general patterns of natural phenomena, the properties and structure of matter, and the laws of its motion. Therefore, the concepts of F. and its laws underlie everything ... ... Great Soviet Encyclopedia

High Energy Density Physics (HED Physics) is a branch of physics at the intersection of condensed matter physics and plasma physics, which studies systems with a high energy density. Under high ... Wikipedia

Electrical power is a physical quantity that characterizes the rate of transmission or conversion of electrical energy. Contents 1 Instantaneous electrical power ... Wikipedia

Electrical power is a physical quantity that characterizes the rate of transmission or conversion of electrical energy. Contents 1 Instantaneous electrical power 2 DC power ... Wikipedia

This term has other meanings, see Intensity. Intensity Unit MT−3 SI units W/m² ... Wikipedia

Wattmeter (watt + gr. μετρεω I measure) measuring device, designed to determine the power of an electric current or an electromagnetic signal. Contents 1 Classification 2 Low frequency and DC wattmeters ... Wikipedia

Power is a physical quantity that shows how much energy moves inside the electrical circuit of a particular equipment. What is it, in what units is it expressed, what is the power measured in, what devices are there for this? More on this and more below.

Power is a scalar form of a physical quantity, which is equal to the rate of change with the transformation, transmission or consumption of system energy. According to a narrower concept, this is an indicator that is equal to the ratio of the time spent on work to the period itself that is spent on work. It is designated in mechanics by the symbol N. In electrical science, the letter P is used. Often you can also see the symbol W, from the word watt.

Power

Useful, full and nominal in the engine engine differ. Useful is the power of the engine, with the exception of the costs that are spent on the operation of all other systems. Gross is the specified force without deductions, and nominal is specified and guaranteed by the factory.

Additional Information! It is worth noting that there is also the power of sound and explosive sound. In the first case, this is a scalar value associated with sound waves and sound energy, which is also measured in watts, and the second is associated with the energy release of TNT decomposition.

Basic concept in the tutorial

What is measured

An obsolete unit of measurement is horsepower. Answering clearly the question of what mechanical power is measured in, it is worth noting that according to modern international indicators, the unit of power is the watt. It is worth noting that the watt is a derived unit that is related to others. It is equal to a joule per second, or a kilogram times a meter squared divided by a second. A watt is also a volt multiplied by an ampere.

It is important to note that watts are divided into mega, kilo and volt amperes.

Formulas for measurement

Power is a value that is directly related to other indicators. So, it is directly related to time, force, speed, force vector and speed, force and speed modulus, moment of force and rotation frequency. Often, when calculating the electrical power variety, the formulas also use the Pi number, the resistance index, the instantaneous current with voltage in a particular section of the electrical network, active, apparent and reactive power. The direct participant in the calculation is the amplitude with the angular velocity and the initial current strength with voltage.

Electrical

Electrical power is a quantity that shows the speed or transformation with which electrical energy moves. To study the instantaneous electrical power characteristic in a certain section of the circuit, it is necessary to know the value of the instantaneous current and voltage and multiply these values.

To understand how much is the active, full, reactive or instantaneous reactive power indicator, you need to know the exact numbers of the current amplitude, voltage amplitude, current angle with voltage, as well as angular velocity and time, since all existing physical formulas reduced to these parameters. The formulas also use the sine, cosine of the angle, and the value 1/2.

The concept of electrical power

hydraulic

The hydraulic power indicator in a hydraulic machine or hydraulic cylinder is the product of the machine pressure drop and the liquid flow rate. As a rule, this is the main statement, taken from the only existing formula for calculation.

Note! More algebraic and engineering rules can be found in the applied science of fluid and gas motion, namely hydraulics.

DC and AC

As for the power of direct current with alternating current, they are most often referred to as an electrical variety. There is no specific concept for the two varieties, but they can be calculated based on the available algebraic settings. So, direct current power is the product of current strength and constant voltage, or twice the value of current strength and electrical resistance, which, in turn, is calculated by dividing double voltage by ordinary resistance.

With regard to alternating current, this is the product of the current strength with the voltage and the cosine of the phase shift. In this case, only the active and reactive varieties can be easily counted. You can find out the total power value through the vector dependence of these indicators and the area.

To measure these indicators, you can use both the above devices and the phase meter. This device serves to calculate the reactive species according to the state standard.

The concept of variable power current

In general, power is a value whose main purpose is to show the strength of a particular device and, in many cases, the speed of activity, interacting with it. It is mechanical, electrical, hydraulic and for direct with alternating current. It is measured according to the international system in watts and kilowatts. Instruments for its calculation are a voltmeter, a wattmeter. The basic formulas for self-calculation are listed above.

Power is a physical indicator. It defines the work done over time and helps measure energy change. Thanks to the unit of measurement of current power, the high-speed energy flow of energy in any spatial interval is easily determined.

Calculation and types

Due to the direct dependence of power on the voltage in the network and the current load, it follows that this value can appear both from the interaction of a large current with a low voltage, and as a result of the occurrence of a significant voltage with a small current. This principle is applicable for transformation in transformers and for the transmission of electricity over long distances.

There is a formula for calculating this indicator. It has the form P = A / t = I * U, where:

• P is an indicator of current power, measured in watts;
• A - current work on the chain section, calculated in joules;
• t acts as the time interval during which the current work was performed, is determined in seconds;
• U is the voltage of the circuit section, calculated in Volts;
• I - current strength, calculated in amperes.

Electric power can have active and reactive indicators. In the first case, the power force is converted into another energy. It is measured in watts, as it contributes to the conversion of volts and amperes.

The reactive power indicator contributes to the occurrence of a self-inductive phenomenon. Such a transformation partially returns energy flows back to the network, due to which there is a shift of current values and voltages with a negative impact on the power grid.

Definition of active and reactive indicator

The active power force is calculated by determining the total value of a single-phase circuit in a sinusoidal current for the required time period. The calculation formula is presented as an expression P = U * I * cos φ, where:

• U and I act as the rms current value and voltage;
• cos φ is the angle of phase shift between these two quantities.

Thanks to the power activity, electricity is converted into other types of energy: thermal and electromagnetic energy. Any electrical network with a current of a sinusoidal or non-sinusoidal direction determines the activity of the circuit section by summing the powers of each individual circuit gap. The electrical power of a three-phase circuit section is determined by the sum of each phase power.

A similar indicator of the active power force is the value of the transmission power, which is calculated by the difference between its fall and reflection.

The reactive index is measured in volt-amperes. It is a value used to determine the electrical loads generated by electromagnetic fields inside an AC circuit. The unit of measurement of electric current power is calculated by multiplying the rms value of the voltage in the network U by the alternating current I and the phase sine angle between these values. The calculation formula is as follows: Q = U * I * sin.

If the current load is less than the voltage, then the phase displacement is positive, if vice versa, it is negative.

Measurement value

The basic electrical unit is power. In order to determine what the power of an electric current is measured in, it is necessary to study the main characteristics of this quantity. According to the laws of physics, it is measured in watts. In production conditions and in everyday life, the value is translated into kilowatts. Large power scale calculations require conversion to megawatts. This approach is practiced at power plants to generate electrical energy. Work is measured in joules. The value is determined by the following relations:

Consumer power strength is indicated on the appliance itself or in the passport for it. By defining this parameter, you can get the values ​​​​of indicators such as voltage and electric current. The indicators used indicate what the electrical power is measured in, they can act as wattmeters and varmeters. The reactive power of the power indicator is determined by a phase meter, a voltmeter and an ammeter. The state standard of what the current power is measured in is the frequency range from 40 to 2500 Hz.

Calculation examples

To calculate the current of the kettle with an electric power of 2 kW, the formula I \u003d P / U \u003d (2 * 1000) / 220 \u003d 9 A is used. To power the device into the mains, the connector length of 6 A is not used. The above example is applicable only when the phase and current voltage. According to this formula, the indicator of all household appliances is calculated.

If the circuit is inductive or has a large capacitance, then it is necessary to calculate the power unit of the current using other approaches. For example, the power in an AC motor is determined using the formula P \u003d I * U * cos.

When connecting the device to a three-phase network, where the voltage will be 380 V, the powers of each phase separately are summed to determine the indicator.

As an example, consider a boiler of three phases with a power capacity of 3 kW, each of which consumes 1 kW. The phase current is calculated by the formula I \u003d P / U * cos φ \u003d (1 * 1000) / 220 \u003d 4.5 A.

On any device, an indicator of electrical power is indicated. The transmission of a large power volume, used in production, is carried out along lines with high voltage. Energy is converted by substations into electricity and supplied for use in the power grid.

Thanks to simple calculations, the power value is determined. Knowing its value, you can make the correct selection of voltage for the full operation of household and industrial appliances. This approach will help to avoid burnout of electrical appliances and protect the power grid from power surges.

We all come across electrical appliances every day, it seems that our life stops without them. And each of them in the technical instructions indicates the power. Today we will figure out what it is, learn the types and methods of calculation.

Electrical appliances connected to the mains operate in an alternating current circuit, so we will consider the power in these conditions. However, first, let's give a general definition of the concept.

Power is a physical quantity that reflects the rate of conversion or transmission of electrical energy.

In a narrower sense, they say that electrical power is the ratio of the work performed over a certain period of time to this period of time.

To paraphrase this definition less scientifically, it turns out that power is a certain amount of energy that is consumed by the consumer over a certain period of time. The simplest example is an ordinary incandescent lamp. The rate at which a light bulb converts the electricity it consumes into heat and light is its power. Accordingly, the higher this indicator is initially for a light bulb, the more it will consume energy, and the more light it will give.

Since in this case there is not only the process of converting electricity into some other ( light, thermal, etc.), but also the process of oscillation of the electric and magnetic fields, a phase shift appears between the current and voltage, and this should be taken into account in further calculations.

When calculating the power in an alternating current circuit, it is customary to distinguish active, reactive and total components.

The concept of active power

Active “useful” power is that part of the power that directly characterizes the process of converting electrical energy into some other energy. Denoted by the Latin letter P and measured in ( Tue).

Calculated according to the formula: P = U⋅I⋅cosφ,

where U and I are the rms value of the voltage and current of the circuit, respectively, cos φ is the cosine of the phase angle between voltage and current.

IMPORTANT! The formula described earlier is suitable for calculating circuits with, however, powerful units usually use a 380V network. In this case, the expression should be multiplied by the root of three or 1.73

The concept of reactive power

Reactive "harmful" power is the power that is generated during the operation of electrical appliances with an inductive or capacitive load, and reflects the ongoing electromagnetic oscillations. Simply put, this is the energy that passes from the power source to the consumer, and then returns back to the network.

Of course, it is impossible to use this component in business, moreover, it harms the power supply network in many ways, therefore they usually try to compensate for it.

This value is denoted by the Latin letter Q.

REMEMBER! Reactive power is not measured in conventional watts ( Tue), and in reactive volt-amperes ( Var).

Calculated according to the formula:

Q = U⋅I⋅sinφ,

where U and I are the rms value of the voltage and current of the circuit, respectively, sinφ is the sine of the phase angle between voltage and current.

IMPORTANT! When calculating, this value can be both positive and negative, depending on the phase movement.

Capacitive and inductive loads

The main difference between reactive ( capacitive and inductive) loads - the presence, in fact, of capacitance and inductance, which tend to store energy and later give it to the network.

An inductive load converts the energy of an electric current first into a magnetic field ( during half a half cycle), and then converts the energy of the magnetic field into electric current and transmits it to the network. Examples are induction motors, rectifiers, transformers, electromagnets.

IMPORTANT! When operating an inductive load, the current curve always lags the voltage curve by half a half cycle.

A capacitive load converts the energy of an electric current into an electric field and then converts the energy of the resulting field back into an electric current. Both processes again proceed for half a half-cycle each. Examples are capacitors, batteries, synchronous motors.

IMPORTANT! During capacitive load operation, the current curve leads the voltage curve by half a half cycle.

Power factor cosφ

Power factor cosφ ( read cosine phi) is a scalar physical quantity reflecting the efficiency of electrical energy consumption. Simply put, the coefficient cosφ shows the presence of a reactive part and the value of the received active part relative to the total power.

The coefficient cosφ is found through the ratio of active electrical power to apparent electrical power.

NOTE! In a more accurate calculation, the nonlinear distortions of the sinusoid should be taken into account, however, they are neglected in conventional calculations.

The value of this coefficient can vary from 0 to 1 ( if the calculation is carried out as a percentage, then from 0% to 100%). From the calculation formula, it is not difficult to understand that the greater its value, the greater the active component, which means that the performance of the device is better.

The concept of total power. Power Triangle

Apparent power is a geometrically calculated value equal to the root of the sum of the squares of active and reactive power, respectively. Designated with the Latin letter S.

S = U⋅I

IMPORTANT! Apparent power is measured in volt-amperes ( VA).

The power triangle is a convenient representation of all the previously described calculations and relationships between active, reactive and apparent power.

The legs reflect the reactive and active components, the hypotenuse - the total power. According to the laws of geometry, the cosine of the angle φ is equal to the ratio of the active and total components, that is, it is the power factor.

How to find active, reactive and apparent power. Calculation example

All calculations are based on the previously mentioned formulas and the power triangle. Let's look at the problem most often encountered in practice.

Typically, electrical appliances are marked with active power and the value of the cosφ coefficient. With these data, it is easy to calculate the reactive and total components.

To do this, we divide the active power by the coefficient cosφ and get the product of current and voltage. This will be full power.

How cosφ is measured in practice

The value of the cosφ coefficient is usually indicated on the tags of electrical appliances, however, if it is necessary to measure it in practice, they use a specialized device - a phase meter. Also, a digital wattmeter can easily cope with this task.

If the obtained coefficient cosφ is low enough, then it can be compensated practically. This is done mainly by including additional devices in the circuit.

1. If it is necessary to correct the reactive component, then a reactive element should be included in the circuit, acting opposite to the already functioning device. To compensate for the operation of an induction motor, for example an inductive load, a capacitor is connected in parallel. An electromagnet is connected to compensate the synchronous motor.
2. If it is necessary to correct non-linearity problems, a passive cosφ corrector is introduced into the circuit, for example, it can be a high inductance choke connected in series with the load.

Power is one of the most important indicators of electrical appliances, so knowing what it is and how it is calculated is useful not only for schoolchildren and people specializing in technology, but also for each of us.