The main characteristic of a wound-rotor three-phase asynchronous motor is that its rotor windings can be connected in series with an external resistor. By changing the value of the external resistor, the total resistance of the rotor winding circuit can be altered, thereby changing its starting current and starting torque. A larger external resistance results in a smaller starting current and a larger starting torque. Given this characteristic, this type of motor is mainly used in applications with limited power capacity and requiring high starting torque. Because adjusting the rotor's external resistance allows for speed regulation within a certain range, it also finds wide application in equipment with limited speed control. Ms. will participate in a brief discussion regarding the motor's testing procedures.
Introduction to Test Items for Wound Rotor Three-Phase AC Asynchronous Motors
● Stator-rotor voltage ratio determination test. The stator-rotor voltage ratio is a performance indicator specified in the technical specifications of wound-rotor three-phase asynchronous motors. The test to determine the stator-rotor voltage ratio of a wound-rotor three-phase asynchronous motor is also known as the rotor open-circuit voltage measurement test.
Before energizing the test, to prevent the rotor from rotating due to the torque generated by the induced current in the rotor core, the rotor should be blocked with a device beforehand. When the three-phase stator voltage is balanced, the unbalance value of the three line voltages of the rotor should not exceed 5%.
● Measurement of DC resistance of rotor windings. The three-phase rotor windings of a three-phase wound-rotor asynchronous motor are generally connected in a star configuration and are sealed inside the motor, with no neutral wire brought out. Therefore, only the resistance of three lines can be measured externally.
●Determination of rotor winding temperature rise. The test method and related calculations are the same as those for the stator winding thermal test.
It should be noted that during the process of measuring the hot DC resistance after the temperature rise stabilizes and the machine is shut down, the two electrical measuring instruments connected to the stator and rotor windings should not be repeatedly switched on and off, otherwise the measurement results will be affected by the induced electromotive force generated in the windings.
●Inspect the pressure on the brushes and the contact area with the slip rings. This inspection mainly includes the pressure on the brushes and the contact area between the brushes and the slip rings.
● The low-voltage locked-rotor test is an inspection test item, with the same purpose as for a standard squirrel-cage motor. The applied voltage and the data to be measured are the same as those for the locked-rotor test in the inspection tests of a standard squirrel-cage motor. During the test, the rotor should be locked. For motors without a brush lifting device, the rotor windings should be short-circuited on the slip rings or their three-phase leads should be short-circuited together; for motors with a brush lifting device, the brush lifting device should be placed in the "running" position.
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