First, do not apply full voltage to a stator with no rotor installed. This will result in overcurrent and stator winding failure.
Wolf39's 21 Apr 11 13:19 post is, in my opinion, the closest to answering the question at hand. By 'closest correct answer', I am not implying that what he says is wrong. My opinion is quite the opposite. Wolf39's response contains detailed information that is absolutety (star quality) correct. However, zlatkodo is looking for an similar answer that is on a different level.
wolf39 said:
An adjustable three-phase voltage source of rated frequency is connected to the stator terminals. Then the voltage is raised until rated current is obtained. The respective voltage divided by the generator rated voltage is equivalent to the per unit (p.u.) value of the so-called sum of stator leakage plus bore reactance.
In a motor rewind shop this is called an 'open stator impedance test.' The reason that I say that wolf39's answer is on a different level is that in a motor rewind shop you are not normally concerned with design aspects of the winding such as 'synchronous reactances, transient reactances and subtransient reactances.'
The primary concern in a motor rewind shop is to successfully copy the existing winding or to make a redesign based on the existing winding.
In this case, the 'open circuit stator impedance test' is a test that is used to evaluate whether the winding design is valid. It is not an 'absolute' test with a definite answer. Instead, it is a general test that is similar to a 'no load run test.'
Specifically, as wolf39 describes, the variable source of three phase voltage is applied to the stator and the voltage is increased until the stator current is equal to nameplate current.
The ratio of applied voltage to nameplate voltage (applied volts/nameplate volts) is then calculated. The result is a comparison of the apparent impedance of an 'open stator' with the apparent impedance of a fully loaded motor.
The correct answer is a ratio in the range of 15-30%, much like a the way that a no load test run results in a 'no load amps/full load amps' ratio of similar values. Like a no load test run, a ratio in the desired range is not a guarantee of a proper winding but, it is one check among many that the winding is correct. Also, like a 'no load test run,' a ratio outside of the 15-30% range is not an indication of a guaranteed fault but it is an indication that you should take another look at the winding to make sure it is correct.
This test is performed after the winding is fully connected and tied down but
before the winding is varnish treated. This is an 'in process' test that is normally performed on a rewind but is not normally performed on an otherwise good winding that is being reconditioned (ie. varnish treated alone) unless a problem is indicated.
Other checks that are performed at the same time are the 'dummy rotor' test and the open stator amp balance check that others have mentioned. All three of these tests are equally important in determining the winding condition before proceeding with varnish treatment and motor assembly. The idea is that this combination of tests will give an indication of a problem before further work is performed.
The test sequence for a new winding would be: meggar test, high potential test, winding impedance test (bridge test, not open stator test), surge test, open stator impedance test, open stator amp balance test, and dummy rotor test. If these tests all give good indications then you would proceed with varnish treating of the winding. More tests follow the varnish treatment and then more tests are performed after motor assembly.
