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Power Triangle

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Dalam sistem AC, terdapat tiga jenis kuasa iaitu Average Power (P; Watt), reactive (Q, Var) dan Apparent Power (S, VA). Ketiga tiga ini berkait dan memainkan peranan masing masing dalam pengiraan kuasa AC.

Kuasa sebenar yang dikira untuk penjumlahan bil elektrik adalah bersandarkan kepada averaga power dan diukur menggunakan Joules atau kWh. Manakala Q memainkan peranan dalam mengatur sudut power factor. Sudut power factor yang rendah mengakibatkan arus yang berlebihan mengalir dalam kabel dan menyebabkan banyak losses. Untuk mengurangkan arus yang berlebihan ini selalunya, power factor mestilah besar. Pembetulan sudut power factor amat mudah dilakukan iaitu dengan menyambungkan kapasitor secara parallel dengan load.

Kenapa kapasitor? kerana kebanyakan beban rumah atau di industri kebanyakan adalah berbentuk induktif.

No-load test induction motor

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The test is performed at poly-phase voltages and rated frequency applied to the stator terminals. When motor runs for some times and bearings get lubricated fully, at that time readings of applied voltage, input current and input power are taken. To calculate the rotational loss, subtract the stator I2R losses from the input power.

theory of no load test of induction motor

Calculation of No Load Test of Induction Motor

Let the total input power supplied to induction motor be W0 watts.

Where,
V1 = line voltage
I0 = No load input current

Rotational loss = W0 – S1
Where,
S1 = stator winding loss = Nph I2 R1
Nph = Number phase
The various losses like windage loss, core loss, and rotational loss are fixed losses which can be calculated by

Stator winding loss = 3Io2R1
Where,
I0 = No load input current
R1 = Resistance of the motor

Core loss = 3GoV2

Passive Sign Convention for Power

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No doubt that most student will got confused on when to put positive sign or negative sign for average power.

In general, the positive sign is for element that absorb power from the circuit and negative sign for element that supply power to the circuit.

The image is one student ask me about how and when to use negative or positive. So my solution to her question is:

MOSFET

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A single MOSFET has two states: ON and OFF.

ON: current can flow both directions

OFF: current can flow one direction (source-drain, due to the body diode) and not the other direction (drain-source, because the FET is off)

A single MOSFET in series with a diode:

enter image description here

also has two states, ON and OFF.

ON: current flows one direction (drain-source, through the FET and the second diode), but not the other direction (source-drain, because the second diode is pointing the wrong way)

OFF: current won’t flow either way, because whichever way you look there’s a diode opposing the flow of current.

Two MOSFETs in series, pointing opposite directions:

enter image description here

have four possible states. ON-ON, ON-OFF, OFF-ON, and OFF-OFF. For this example, I’ll describe current flow with directions as seen in the picture above.

ON-ON: current can flow both directions through the circuit.

ON-OFF: current can flow from left to right (through the left FET and the right diode) but not from right to left (because of the right diode)

OFF-ON: current can flow from right to left (through the right FET and the left diode) but not from left to right (because of the left diode)

OFF-OFF: current can not flow in either direction, because with both FETs off all you have is two diodes blocking current flow in either direction.

So the third topology has the option of either blocking currents in both directions, or allowing current flow in either direction, depending on how it’s gated.