Blocked rotor test

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Blocked rotor test is conducted on an induction motor. It is also known as short circuit test or locked rotor test or stalled torque test.[1] From this test short circuit current at normal voltage, power factor on short circuit, total leakage reactance, starting torque of the motor can be found.[2][3] The test is conducted at low voltage because if the applied voltage was normal voltage then the current flowing through the stator windings were high enough to over heat the winding and damage them.[4] The blocked rotor torque test is not performed on a wound rotor motors because the starting torque can be varied as desired. However, blocked rotor current test is conducted on squirrel cage rotor motors.[5]

Method[edit]

In the blocked rotor test, the rotor is locked.[6] A low voltage is applied on the stator terminals so that full load current flows in the stator winding. The current, voltage and power input are measured at this point. When the rotor is stationary the slip,  s = 1 .[7] The test is conducted at  1/4 ^{th} the rated frequency as recommended by IEEE. This is because the rotor's effective resistance at low frequency may differ at high frequency.[8][9] The test can be repeated for different values of voltage to ensure the values obtained are consistent. As the current flowing through the stator may exceed the rated current, the test should be conducted quickly.[10] By using the parameters found by this test, the motor circle diagram can be constructed.[11]

Calculations involved[edit]

Short circuit current at normal voltage[edit]

 I_{S} is the short circuit current at voltage  V_{S}
 I_{SN} is the short circuit current at normal voltage  V
 I_{SN} = I_{S} \times \frac {V} {V_{S}}

Short circuit power factor[edit]

 W_{S} is the total input power on short circuit
 V_{SL} is the line voltage on short circuit
 I_{SL} is the line current on short circuit
 cos \phi_{S} is the short circuit power factor
 cos \phi_{S} = \frac {W_{S}} {{\sqrt{3}} {V_{SL}} {I_{SL}}} [12]

Leakage reactance[edit]

 Z_{01} is the short circuit impedance as referred to stator
 X_{01} is the leakage reactance per phase as referred to stator
 Z_{01} = \frac \text {short circuit voltage per phase} \text {short circuit current} = \frac {V_{S}} {I_{S}}

 W_{cu} is the total copper loss
 W_{c} is the core loss

 W_{cu} = W_{S} - W_{c}
 W_{cu} = {3} \times{{I_{S}}^{2} {R_{01}}}

 R_{01} = \frac {W_{cu}} {3{I_{S}}^{2}}

 X_{01} = \sqrt {{Z_{01}}^{2} - {R_{01}^{2}}}

See also[edit]

References[edit]

  1. ^ "Motor testing methods". Electronic Systems of Wisconsin, Inc. 
  2. ^ "Blocked rotor test". Sakshat virtual labs. 
  3. ^ de Swardt, Henk. "The Locked rotor test explained". Marthinusen & Coutts (Pty.) Ltd. 
  4. ^ "Motor testing methods". Electronic Systems of Wisconsin, Inc. 
  5. ^ Industrial Power Engineering and Applications Handbook. Newnes. 
  6. ^ "Blocked rotor test". Sakshat virtual labs. 
  7. ^ B.L. Theraja, A.K. Theraja (2010). Electrical Technology volume 2 (Twenty third revised multicolour ed.). S. Chand. p. 1317. ISBN 81-219-2437-5. 
  8. ^ Knight, Dr Andy. "Electrical Machines". Department of Electrical and Computer engineering, University of Alberta. 
  9. ^ Pitis, CD. "femco squirrel.doc". Femco mining motors. 
  10. ^ "Motor testing methods". Electronic Systems of Wisconsin, Inc. 
  11. ^ Deshpande, M.V. Electrical Machines. PHI Learning Pvt. Ltd. 
  12. ^ Deshpande, M.V. Electrical Machines. PHI Learning Pvt. Ltd.