No load and Blocked rotor test to determine the values of the equivalent circuit parameters of a 3-phase induction motor.

The no-load test

The behaviour of the machine may be judged from the equivalent circuit of fig. 21(a). The current drawn by the machine causes a stator-impedance drop and the balance voltage is applied across the magnetizing branch. However, since the magnetizing branch impedance is large, the current drawn is small and hence the stator impedance drop is small compared to the applied  voltage (rated value). This drop and the power dissipated in the stator  resistance are therefore neglected and the total power drawn is assumed to be consumed entirely as core loss. This can also be seen from the approximate equivalent circuit, the use of which is justified by the foregoing arguments. This test therefore enables us to compute the resistance
and inductance of the magnetizing branch in the following manner.
Let applied voltage = Vs. Then current drawn is given by











Vs, Is and Ps are measured with appropriate meters. With Rm known from eqn. 10, Xm can be found from eqn. 9. The current drawn is at low power factor and hence a suitable wattmeter should be used.











Blocked-rotor Test

In this test the rotor is prevented from rotation by mechanical means and hence the name. Since there is no rotation, slip of operation is unity, s = 1. The equivalent circuit valid under these conditions is shown in fig. 21(b). Since the current drawn is decided by the resistance and leakage impedances alone, the magnitude can be very high when rated voltage is applied.

Grizzly G2527 Motor 1/3 HP Single-Phase 1725 RPM TEFC 110V/220V

Therefore in this test, only small voltages are applied — just enough to cause rated current to flow. While the current magnitude depends on the resistance and the reactance, the power drawn depends on the resistances.
The parameters may then be determined as follows. The source current and power drawn may be written as






In the test Vs, Is and Ps are measured with appropriate meters. Equation 12 enables us to compute(Rs + R ′ r). Once this is known, (Xs + X ′r) may be computed from the eqn. 11.

IEC 61972 Ed. 1.0 b:2002, Method for determining losses and efficiency of three-phase cage induction motors

 Note that this test only enables us to determine the series combination of the resistance and the reactance only and not the individual values. Generally, the individual values are assumed to be equal; the assumption Rs = R′r, andXs = X′r suffices for most purposes. In practice, there are differences. If more accurate estimates are required IEEE guidelines may be followed which depend on the size of the machine.

IEC 60034-12 Ed. 2.0 b:2002, Rotating electrical machines - Part 12: Starting performance of single-speed three-phase cage induction motors

Note that these two tests determine the equivalent circuit parameters in a ‘Stator-referred’ sense, i.e., the rotor resistance and leakage inductance are not the actual values but what they ’appear to be’ when looked at from the stator. This is sufficient for most purposes as interconnections to the external world are generally done at the stator terminals.

Circle diagram (Heyland diagram)
7.4.1 Locus diagram


Circle diagram of induction machines means locus diagram of their stator current.
Preconditions:
•U1 wird in reelle Achse gelegt,
•der Läufer ist kurzgeschlossen,
•R1 0.










which is a linear function of s and can therefore be utilized for construction purpose of the slip-line.
A tangent to the circle is to be drawn at I0, intersected by a line in parallel the –j-axis.
This line is called slip-line, which terminates at the intersection with the extension of the
current phasor I2. This line is divided linearly because of is proportional to the slip.
Besides the no-load point, a second point on the circle graph must be known, in order to
    define a parametrization.
InSinkErator ISS10029 3 Phase Commercial Commercial 1 HP Induction Motor Garbage Disposal SS-100
If the ohmic stator winding resistance needs to be taken into account, to apply for low power machines and power converter supply at low frequencies, an active partition is added to the circle of the locus diagram, which differs for location of center point and parameter assignment – not supposed to be discussed further.


Power in circle diagram

The opportunity to easily determine the current value for any given operational point is not the only advantage of the circle diagram of induction machines. Apart from that, it is possible to directly read off torque value M and air gap power PD, mechanical power Pmech and elektrical power Pel as distances to appear in the circle diagram.

THE PERFORMANCE AND DESIGN OF THE ALTERNATING CURRENT MACHINES: TRANSFORMERS, THREE-PHASE INDUCTION MOTORS AND SYNCHRONOUS MACHINES.

If R1 is equal to zero 01 R , the entire absorbed active power is equal to the air gap power
             PD, to be transferred across the air gap.






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