1. Determination of Induction-Motor Parameters
DC Test
Determines R1
Connect any two stator leads to a variable-voltage DC power supply
Adjust the power supply to provide rated stator current
Determine the resistance from the voltmeter and ammeter readings
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3. For a Y-Connected Stator
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4. For a Delta-Connected Stator
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5. Determination of Induction-Motor Parameters
Blocked-Rotor Test
Determine X1 and X2
Determines R2 when combined with data from the DC Test
Block the rotor so that it will not turn
Connect to a variable-voltage AC supply and adjust until the blocked-rotor current is equal to the rated current
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7. Simplified Equivalent Circuit
Neglect the exciting current under blocked-rotor conditions – remove the parallel branch
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8. A 60-Hz motor would use a 15-Hz test voltage.
IEEE test code recommends that the blocked-rotor test be made using 25% rated frequency with the test voltage adjusted to obtain approximately rated current.
A 60-Hz motor would use a 15-Hz test voltage.
The calculated reactance is corrected to 60-Hz by multiplying by 60/15.
Calculated resistance is correct.
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11. How Is the Blocked-Rotor Impedance Divided?
If the NEMA-design letter of the motor is known, use Table 5.10 to divide the impedances. Otherwise, divide the impedances equally.
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12. Determination of Induction-Motor Parameters
No-Load Test
Determine the magnetizing reactance, XM and combined core, friction, and windage losses.
Connect as for blocked-rotor test (next slide).
The rotor is unblocked and allowed to run unloaded at rated voltage and rated frequency.
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13. Electrical connection for the No-Load Test is the same as for the Blocked-Rotor Test
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14. Determination of Induction-Motor Parameters
At no-load, the speed is very close to synchronous speed – the slip is =0, causing the current in R2/s to be very small, and will be ignored i the calculations.
IM>>Ife, so I0 = IM.
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15. The equivalent circuit for the no-load test is shown.
Ignore
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16. Substitute X1 from the blocked-rotor test to determine the value of XM.
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17. Example 5.16
The following data were obtained from no-load, blocked-rotor, and DC tests of a three-phase, wye-connected, 40-hp, Hz, 460-V, design B induction motor whose rated current is 57.8A. The blocked-rotor test was made at 15 Hz.
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18. Blocked-Rotor No-Load DC Vline = 36.2V Vline = 460.0V VDC = 12.0V
Iline = 58.0A Iline = 32.7A IDC = 59.0A
P3phase = W P3phase = W
a) Determine R1, X1, R2, X2, XM, and the combined core, friction, and windage loss.
b) Express the no-load current as a percent of rated current.
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19. Convert the AC test data to corresponding phase values for a wye-connected motor.
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20. Determine R1
Determine R2
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21. Determination of X1 and X2
From Table 5.10, for a design B machine,
X1 = 0.4XBR,60 = 0.4(1.0182) = Ω/phase
X2 = 0.6XBR,60 = 0.6(1.0182) = /phase
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22. Determination of XM
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23. Determination of combined friction, windage, and core loss:
b) Express the no-load current as a percent of rated current.
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