I read somewhere, and I can not remember where, that to perform a locked rotor test on a motor you could simply drop out one phase to the motor in order to stall it, rather than physically lock the rotor. What I would like to know is: Is this acceptable and would it give an accurate reading?
I am also interested in any other tests which are performed in a modern motor workshop. I have a couple of motors i would like to test and experiment with.
------------------ Not a good plan- In addition to excessive currents, the readings will not show normal parameters. I suppose that with quite a few assumptions, and symmetrical component analysis, the data could be used to get a questionable estimate of motor parameters. You would not get a direct measurement of starting torque.
To do a locked rotor test, lock the rotor and apply a lower than normal voltage- at most sufficient to circulate rated current. Try this at lower voltages as well. Measure power, voltage and current. From these you can find the impedance of the motor. At a low voltage, the exciting current will be small and its effect, hopefully, can be neglected. It is equivalent to a short circuit test on a transformer but with a bit more error because of higher magnetising current. Take a no load test at rated voltage to determine the approximate exciting branch impedances/admittances. Better results if the motor could be driven at synchronous speed but error will be small at no load. From the two tests you can make a motor model. This is the same process as done with a transformer (an induction motor is modelled as a special case of a transformer )
if one phase in the motor is cut of ,the rotating magnetic flux is not at all created in the motor so there is no actual rotation of the rotor .but it may rotate like the single phase motors where secondary winding is introduced.
hence the actual characteristic of a three phase motor is lost,and the rated current will defenitely be different .
so i conclude that this process may not be accurate.
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