ELEKTRO

• 1. DC-test

 

The goal of this test is to determine the stato resistance. For that we should:

• connect any two stator leads to a variable voltage DC power supply

• adjust the power supply to provide the rated stator current

• the current and the voltage can be read respectively from an amperemeter and a voltmeter

• with the equivalent voltage obtained, we can calculate the resistance.

 

The equivalent circuit of that measurement looks as follow:

 

 

 

 

 

 

 

 

 

 

 

The formula for the resistance calculation is:

 

 

 

 

 

The stator windings can be connected in two ways (Star- (Y-) and Triandle connection and the type of the connection plays a very importantrole in the calculation of the parameters:

• Y-Connection

 

 

 

 

 

 

 

 

 

 

 

 

In this way, the measured DC resistance is the resistance between two phases, but for the calculation of the parameters the resistance of the phases has to be considered instead. The formula for the conversion of the DC measured resistance into the phase resistance is:

 

 

 

 

 

 

 

 

 

 

• Triangle-connection:

 

 

 

 

 

 

 

 

 

But the resistance of one phase is parallel to the resistances of the two other phases. To get the exact value of the phase resistance we can apply the following formulas:

 

 

 

 

 

 

 

 

• 2. Short-cicuit test

The short circuit test is done to determine the rotor resistances and the leakage inductances in the stator and rotor.

During the test the rotor is blocked so, that it will not turn. Now we can connect the motor to a variable AC voltage supply and adjust untill the blocked-rotor current(short-circuit current) reaches the rated current. 

But in short-circuit modus the current flowing through the parallel branch is so small, that we can remove that branch, the slip s is equal one (s=1) and the circuit will be simplified to:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The parameters in the figure ca be determined with these formulas:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

• 3. Open-circuit test

The open-circuit test, also called No-load test enables the determination of the magnetizing reactance, iron resistance and combined core, friction and windage losses. To realize it, we connect the machine in the same way like for the short-circuit test. But this time the rotor is not blocked and allowed to run without being loaded at rated voltage and rated frequency.The equivalent circuit is:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Sometimes we can neglect the stator resistance R1, specially at high frequencies, because it will be too small compared to the magnetizing reactance. At no-load, the rotor speed is very close to the synchronous speed so, that they can be considered equal and the slip is then equal zero (s=0). The rotor resistance R2/s will be very big and the current though it very small so, that it will be neglected in the circuit.

 

The no-load parameters can be calculated with the following formulas: