The three-phase electric power calculation formula includes three kinds of power, active power P, reactive power Q, and apparent power S. For symmetric loads, the three power calculation formulas are relatively simple, the relative measurement is relatively simple, and only one power signal is measured.
For applications where high accuracy is required, I must measure the three-phase power using the two- or three-table method.
The cosine of the phase difference (Φ) between voltage and current is called the power factor and is represented by the symbol cosΦ. In terms of value, the power factor is the ratio of active power to apparent power, ie cosΦ=P/S
The three power and power factors cosΦ are a right angle power triangle relationship: two right angle sides are active power, reactive power, and the oblique side is apparent power.
Active Power Square + Reactive Power Square = Apparent Power Squared. In the three-phase load, these three kinds of power always exist at the same time: Apparent power S=1.732UI
Active power P=1.732UIcosΦ Reactive power Q=1.732UIsinΦ Power factor cosΦ=P/S sinΦ=Q/S
If the power supply voltage is AC three-phase power, the voltage of each phase is 220V.
It is known that the rated voltage of the motor is 380V and the rated current is 15A. I ask, :
1. When the three-phase asynchronous motor starts at the star, does the electric power calculation formula be: root number 3*U*I* power factor, U is 380V or 220V?
2. When the three-phase asynchronous motor is running in the angular shape, is the electric power calculation formula: root number 3*U*I* power factor, U is 380V or 220V?
1"The power supply voltage is AC three-phase power, the voltage of each phase is 220V, the rated voltage of the motor is 380V, the rated current is 15A (it should be 15KWâ–³), the motor can be changed to Y to adapt to the three-phase 220V operation. The calculation formula U=220V, the voltage is low, the current is large, and the power remains unchanged.
2" When the three-phase voltage is 380V, the three-phase asynchronous motor is calculated in the original connection method regardless of Y connection or â–³ connection, and its calculation formula U=380V.
3" When the three-phase voltage is 380V, when the three-phase asynchronous motor is changed to the Y connection method, the winding is originally subjected to 380V, and the winding can withstand the 380V voltage after changing the Y connection. Double (ie 380 & TImes; 1.732 ≈ 660V), the winding can withstand 660V voltage running at 380V, its root number 3 needs to increase to the third power. Namely: I=P/(1.732&TImes;1.732&TImes;1.732&TImes;U×cosφ) The calculation result should be the original 1/3 current or power
The power factor is the ratio of the active and reactive currents. Because the motor needs active and reactive current excitation, when the motor is fully loaded, the active current is the largest, the reactive current is the smallest, and the power factor is about 0.85. When no-load is used, the active current is small, the reactive current is large, and the power factor is between 0.4 and 0.7.
Generally calculated to take a power factor of 0.78 or 0.8
P=1.732×U×I×COSφ=1.732×0.38×17×0.78=8.7(KW)
Assuming that the power factor of the alternating current is 1 and the phase voltage is 220 volts, multiplying 220 by the phase current is the power of this phase. The power of the other two phases is obtained in the same way, and then the three-phase power is added and obtained. Note that you cannot multiply 380 volts by current when calculating.
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