Why is the indication of the meter in the AC power supply circuit a valid value?

In the three-phase four-wire system, the load of each phase can be roughly divided into three situations: 1. Three symmetrical loads, the current of each phase is equal, and the phase difference is 120 degrees. The vector sum of the three-phase current is zero, i.e., the midline current is zero.

The cancellation of the center line will not affect the work of the three-phase circuit, which is the reason why the power supply adopts the three-phase three-wire system. 2. Three-phase asymmetrical resistive load, the size of the current of each phase is not necessarily equal, the phase difference is 120 degrees, and the vector sum of the three-phase current is not zero, but its size will not exceed the current value of the larger phase of the current in the three-phase. 3. Three-phase asymmetrical inductive and or capacitive loads, the size of each phase current is not necessarily equal, and the phase difference is not necessarily 120 degrees, at this time, the sum of the three-phase current is not zero, that is, the center line current is not zero, and the size of the center line current may exceed the current size of each phase. ...

The direction and magnitude of alternating current changes over time! It has 50 cycles of change per second!

Although its strength varies, there are highest and lowest values, but the hairspring pointer of a mechanical meter is inert!

It only reflects the average!

The AC voltmeter calculates the RMS value of alternating current by shaving the peaks and filling the troughs of various electrical components, and converting them into direct current. Therefore, the AC voltmeter does not display the instantaneous value of the alternating current, but the average value over a period of time, that is, the effective value. The RMS value of sinusoidal alternating current is one-half of the root number of the maximum value (i.e., the absolute value at the crest and trough).

AC rmsIt is a value used to measure the magnitude of alternating current. The effective value is a value used to measure the magnitude of alternating current. If alternating current passes through a resistor, heat and direct current occur in a cycle.

If the amount of heat that occurs through the same resistance at the same time is the same, then the amount of direct current is called the effective value of alternating current. In actual use, if the maximum value is used to calculate the work or power of alternating current.

Not suitable, because after all, there are only two moments in a cycle that reach this maximum.

Alternating current rms measurement method:

1.In the peak detection method, the peak value of the signal is measured with a peak detection circuit and divided by the crest factor ( to obtain the effective value of the signal.

2.In the rectification averaging method, the measured signal is rectified at full wave, and then an integral circuit is used.

Average the signals.

Multiply by the waveform factor ( to get the effective value of the signal.

3.In the true RMS method, the positive one uses a true RMS measurement circuit to directly measure the true RMS of the signal.

In the transformation method, the signal is sampled at high speed, and then analyzed by FFT to obtain the RMS values of the DC component, fundamental and each harmonic, and then find the square root of their sum of squares.

Finally, the effective value of the signal is obtained.

Refer to the content of Shiqing Lu: Encyclopedia - AC effective value.

Expression: <>

In the formula: em is the maximum value, is the electrical angle, e is the instantaneous value, and t is the time.

From the above formula, it can be seen that the magnitude of alternating current changes with time, the magnitude of the instantaneous value (a certain instant) varies between zero and positive and negative peaks, and the maximum value is only a momentary value, which cannot reflect the workmanship ability of alternating current.

If AC and DC are respectively passed through the same resistance, and both consume the same amount of electrical energy for the same amount of time (or produce the same Joule heat), then the value of this DC is called the value of the RMS of AC.

The effective values of electromotive force, voltage, and current for sinusoidal alternating current are expressed as e, u, and i, respectively. The average magnitude of the electromotive force, voltage, and current of the alternating current is its effective value. The rated value of the AC electrical equipment and the value indicated by the AC meter are also valid values.

The relationship between the effective and maximum values of the surface and sinusoidal currents is as follows

Other spike chords (voltage, current, etc.) can also be written in the form of the first expression with the Hu wheel at the beginning of the text:

Voltage and current also have instantaneous values, maximum values, and effective values. Generally, instantaneous values are represented by lowercase letters (e.g., u, i, etc.), and maximum values are represented by uppercase letters with subscript m letters (e.g., um, im). Valid values are indicated by uppercase letters (u, i).

The relationship between the maximum value and the valid value is:

Yes. Generally, it is sinusoidal alternating current, and the principle is to measure the maximum value and divide it by the root number 2. The scales of AC ammeters and voltmeters are made to display the effective values.

Yes, the relevant national standards stipulate that in the absence of special indication, AC voltmeters and ammeters are scaled and displayed with effective values.

Sine wave rms upeak 2 sqrt(2); Note: sqrt(2) is the open square of 2.

The RMS value of square wave is 2 * (1 duty cycle), if the point space ratio is 0%, it is u peak 2.

Triangular RMS upeak 2 sqrt(3); Note: sqrt(3) is the open square of 3.

In order to facilitate the measurement and calculation of alternating current, it is necessary to specify the amount of alternating voltage and the magnitude of alternating current, i.e., the effective value, in order to facilitate the measurement and calculation of alternating current. If the thermal effect of this alternating current is equal to that of a certain voltage of direct current, then the voltage of this direct current can be considered to be the effective value of this alternating current voltage.

is a valid value. The RMS value passes through the DC current and the AC current respectively on the same resistance, and after an AC cycle, if they consume the same amount of electrical energy on the resistance, then the magnitude of the DC current (voltage) is taken as the RMS value of the AC current (voltage), and the RMS value of the sinusoidal current (voltage) is equal to 1 2, about times of its maximum value (amplitude).

There is a big difference between a DC ammeter and an AC ammeter, and there is no way to exchange measurements, and for a high-current ammeter, the DC ammeter is actually just a 75 millivolt DC voltmeter (which measures the millivolt voltage on the shunt).

The internal resistance of the AC ammeter is very small (it measures the current of the short-circuit of the secondary of the transformer), which is short-circuited to the circuit, and the current can allow 5 amperes of AC to flow through.

Alternating current is a sine wave with periodic changes in amplitude and polarity over time, and its period is seconds. The voltmeter cannot be tested directly.

The amount of 50Hz AC power needs to be rectified and filtered to obtain a stable voltage or current. This stable voltage or current is equivalent to a steady DC. Therefore, the voltmeter ammeter measures the RMS value of alternating current, not the instantaneous value.

To measure the instantaneous value of alternating current, an oscilloscope is required.

The RMS value of alternating current is one of the physical quantities that characterize alternating current and is specified according to the thermal effect of the electric current. If they produce the same amount of heat at the same time, then the value of this DC is called the RMS value of the AC. Here are a few things to keep in mind:

A. The voltage of the power circuit is 380V, and the voltage of the lighting circuit is 220V, which refers to the effective value of the voltage.

b. The rated voltage value and rated current value marked on all kinds of electrical appliances using alternating current refer to the value.

c. The values measured by the AC voltmeter and the AC ammeter are the effective values of the AC voltage and current.

d. When calculating electric work, electric heat and electric power, the values of AC voltage and current substituted are effective values.

Explain in detail.

Let a periodic current i(t) pass through the resistor r, because the current is changing, the instantaneous power i 2r is different, and the heat generated in a very short time dt is i 2rdt, and the heat generated in a cycle t is .

ti^2rdt

If the magnitude of the DC current that produces equal heat through the resistor r is i after time t, then there is t

i 2rdt = i 2rt, which gives the rms value of the current i = [(1 t) ti 2dt] (1 2).

For sinusoidal quantities, let i(t)=imsin(wt+ )i= (1 2).

Because sin 2(wt+) = (1 2) [1-cos 2(wt+), i = (1 2).

im^2/2)^(1/2)

im/[2^(1/2)]=