Blocked rotor test

A blocked rotor test is conducted on an induction motor. It is also known as short-circuit test (because it is the mechanical analogy of a transformer short-circuit test), ^[1] locked rotor test or stalled torque test. ^[2] From this test, short-circuit current at normal voltage, power factor on short circuit, total leakage reactance, and starting torque of the motor can be found. It is very important to know a motor's starting torque since if it is not enough to overcome the initial friction of its intended load then it will remain stationary while drawing an excessive current and rapidly overheat. ^[3] The test may be conducted at lower voltage because at the normal voltage the current through the windings would be high enough to rapidly overheat and damage them. The test may still be conducted at full voltage if it is brief enough to avoid overheating the windings or overloading the starting circuits, but requires much more care to be taken while performing the test. ^[4] The blocked rotor torque test is less significant on wound-rotor motors because the starting torque can be varied as desired although it may still be used to characterise the motor. ^[3]

Method

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

Short-circuit current at normal voltage

${\displaystyle I_{S}}$ 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

${\displaystyle W_{S}}$ 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

${\displaystyle Z_{01}}$ 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

• Open-circuit test
• Circle diagram

References

1. U.A.Bakshi, M.V.Bakshi, Electrical Machines - II, p. 6-7, Technical Publications, 2009 ISBN   8184315236.
2. M.V. Deshpande, Electrical Machines, p. 254, PHI Learning, 2011 ISBN   8120340264.
3. Industrial Power Engineering and Applications Handbook. Newnes. 8 October 2001. ISBN   9780080508634.
4. "Motor testing methods". Electronic Systems of Wisconsin, Inc. Archived from the original on 2012-12-31.
5. "Blocked rotor test". Sakshat virtual labs.
6. B.L. Theraja, A.K. Theraja (2010). Electrical Technology volume 2 (Twenty third revised multicolour ed.). S. Chand. p. 1317. ISBN   978-81-219-2437-5.
7. Knight, Dr Andy. "Electrical Machines". Department of Electrical and Computer engineering, University of Alberta. Archived from the original on 2012-11-29.
8. Pitis, CD. "femco squirrel.doc". Femco mining motors.
9. Deshpande, M.V. (30 May 2011). Electrical Machines. PHI Learning Pvt. Ltd. ISBN   9788120340268.