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Blocked rotor test

In the blocked rotor test, the rotor is locked securely enough that it cannot break free.^[5] A low voltage is applied on the stator terminals so that there is full load current in the stator winding, and the current, voltage and power input are measured at that point. When the rotor is stationary, the slip, ${\displaystyle s=1}$.^[6] The test is conducted at ${\displaystyle 1/4}$ the rated frequency as recommended by IEEE, because the rotor's effective resistance at low frequency may differ at high frequency.^[7][8] The test can be repeated for different values of voltage to ensure the values obtained are consistent. As the current through the stator may exceed the rated current, the test should be conducted quickly.^[4] By using the parameters found by this test, the motor circle diagram can be constructed.^[9]

Calculations involved[edit]

Short-circuit current at normal voltage[edit]

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

Short-circuit power factor[edit]

${\displaystyle W_{S}}$ is the total input power on short circuit
${\displaystyle V_{SL}}$ is the line voltage on short circuit
${\displaystyle I_{SL}}$ is the line current on short circuit
${\displaystyle cos\phi _{S}}$ is the short-circuit power factor
${\displaystyle cos\phi _{S}={\frac {W_{S}}{{\sqrt {3}}{V_{SL}}{I_{SL}}}}}$^[9]

Leakage reactance[edit]

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

${\displaystyle W_{cu}}$ is the total copper loss
${\displaystyle W_{c}}$ is the core loss

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

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

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

See also[edit]

• Open-circuit test
• Circle diagram

References[edit]