Determination of InductionMotor Parameters DC Test Determines R

Determination of Induction-Motor Parameters • DC Test – Determines R 1 – Connect any two stator leads to a variablevoltage DC power supply – Adjust the power supply to provide rated stator current – Determine the resistance from the voltmeter and ammeter readings ECE 441 1

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For a Y-Connected Stator ECE 441 3

For a Delta-Connected Stator ECE 441 4

Determination of Induction-Motor Parameters • Blocked-Rotor Test – Determine X 1 and X 2 – Determines R 2 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 ECE 441 5

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Simplified Equivalent Circuit Neglect the exciting current under blocked-rotor conditions – remove the parallel branch ECE 441 7

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. ECE 441 8

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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. ECE 441 11

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. ECE 441 12

Electrical connection for the No-Load Test is the same as for the Blocked-Rotor Test ECE 441 13

Determination of Induction-Motor Parameters • At no-load, the speed is very close to synchronous speed – the slip is =0, causing the current in R 2/s to be very small, and will be ignored i the calculations. • IM>>Ife, so I 0 = IM. ECE 441 14

The equivalent circuit for the no-load test is shown. Ignore ECE 441 15

Substitute X 1 from the blocked-rotor test to determine the value of XM. ECE 441 16

Example 5. 16 • The following data were obtained from noload, blocked-rotor, and DC tests of a three-phase, wye-connected, 40 -hp, 60 -Hz, 460 -V, design B induction motor whose rated current is 57. 8 A. The blocked -rotor test was made at 15 Hz. ECE 441 17

Blocked-Rotor No-Load DC Vline = 36. 2 V Vline = 460. 0 V VDC = 12. 0 V Iline = 58. 0 A Iline = 32. 7 A IDC = 59. 0 A P 3 phase = 2573. 4 W P 3 phase = 4664. 4 W a) Determine R 1, X 1, R 2, XM, and the combined core, friction, and windage loss. b) Express the no-load current as a percent of rated current. ECE 441 18

Convert the AC test data to corresponding phase values for a wye-connected motor. ECE 441 19

Determine R 1 Determine R 2 ECE 441 20

Determination of X 1 and X 2 From Table 5. 10, for a design B machine, X 1 = 0. 4 XBR, 60 = 0. 4(1. 0182) = 0. 4073Ω/phase X 2 = 0. 6 XBR, 60 = 0. 6(1. 0182) = 0. 6109/phase ECE 441 21

Determination of XM ECE 441 22

Determination of combined friction, windage, and core loss: b) Express the no-load current as a percent of rated current. ECE 441 23