Induction motor Operation & Equivalent circuit

When the stator coils is energized a rotating magnetic field is created.
This rotating magnetic field cuts through the rotor and induces a voltage on the rotor bars.
The voltage on the rotor bars creates its own rotating magnetic field in the rotor.
The rotor magnetic field will try to line up wit the stator magnetic field and will cause the rotor to rotate.
The rotor magnetic field never catches up but follows slightly behind and the difference is known as slip,s.

s is a variable that is always changing. These are not actual resistance you can see and measure in the machine ; they are representations of the  machine. The equivalent circuit is similar to the transformer equivalent circuit but the difference is ( in drawing) is that there is an air gap and the stator and rotor impedance is not combined.

Transformer Type A solution

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Solution a)

Solution b)

Solution c)

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Short Circuit Test

In the short circuit test the secondary side of the transformer is shorted through an ammeter. The input voltage must be reduced to 10-15% of the open circuit / terminal voltage so that Isc ( short circuit current) does not exceed the rated current. The reading on the voltmeter is the Vsc. The no load current is approx. zero there fore Iin = I1, the input current is called Isc and can be measured using an ammeter. The reading in the wattmeter is the short circuit power Psc.

Using the values obtained in the in the experiment:

Psc/(Isc^2) = Rs or Req

Vsc/ Isc = Zs or Zeq

jXs = sqrt ( Zs^2 - Rs^2)

No Load test / Open Circuit test

In the open circuit test, the input voltage is the terminal voltage and is also the open circuit/ no load voltage and can be read using the voltmeter. The secondary side of the transformer is open circuited. The current I1 = 0 therefore Iin = Inl + I1 ....Iin = Inl. Using the ammeter one can obtain (open circuit current) Ioc = Inl. Using the wattmeter, you can find the open circuit power. A low power factor wattmeter is needed.

using the values obtained in the experiment,

Poc/(Voc x |Ioc|) = cos ɵ

Voc/|Ioc|cos ɵ = Ro

Voc/|Ioc|sin ɵ = jXm

Transformer Equivalent circuit

This circuit represent the equivalent circuit for the transformer. V1 is known as the terminal voltage. This is a per phase diagram or a one phase ¤ diagram. I will be using the symbol ¤ as phase from this point on. The transformer has 2 losses; No Load losses and Copper Losses. To obtain the parameters of the transformer 2 test must be performed; The No Load / Open Circuit test and the Short Circuit test.