Different test items are set for different series of motors due to the differences in performance requirements. For squirrel-cage three-phase asynchronous motors, one of the most important test items is stall test, which aims to test the motor's starting performance and quantitatively analyze the stator current and torque of the motor under rated voltage, rated frequency, and rotor stationary conditions.
By analyzing the magnitude of the stall current and its balance, it is possible to determine whether the squirrel-cage rotor of the motor has defects such as broken bars, thin bars, or excessive impurities in the squirrel-cage bars, whether the stator and rotor magnetic circuits are reasonable, and other quality defects that affect the normal use of the motor.
Both type testing and inspection testing of motors include locked-rotor tests. During type testing, the voltage applied to the motor stator starts at no less than 90% of the rated voltage and is gradually reduced until the stator current approaches the motor's rated current. Typically, 5 to 7 points are measured. During inspection testing, only the locked-rotor current at one voltage point is measured, and the result is compared to the type test data for qualification.
Controlling starting torque and starting current is one of the most challenging and critical technologies in motor design. Ideally, the starting torque should be large enough while the starting current should be as small as possible. For a finished motor, starting current and starting torque are positively correlated; that is, the larger the starting current, the larger the starting torque. The specific ratio is mainly determined by the magnetic circuit saturation, winding parameters, rotor structure, and the material of the squirrel cage bars.
What is a short-circuit test?
A short circuit occurs when a circuit or part of a circuit is shorted. For example, if a load and the power supply are connected by a wire, it is called a short circuit. During a short circuit, the current supplied by the power supply will be much greater than the current supplied during a continuous circuit. Short circuits are generally not allowed; if a short circuit occurs, it can severely damage the power supply or equipment.
In a power system, a "short circuit" refers to a connection between phases or between a phase and ground (or neutral line) outside of normal operating conditions. In a three-phase system, the basic forms of short circuits include: three-phase short circuit, two-phase short circuit, single-phase-to-ground short circuit, and two-phase-to-ground short circuit. When an abnormal connection (i.e., a short circuit) occurs between phases or between a phase and ground (or neutral line), a very large current flows. This current value is much greater than the rated current and depends on the electrical distance between the short circuit point and the power source. A short circuit is a low-resistance short circuit between conductive parts at different potentials, equivalent to the power source being directly connected to form a closed loop through a conductor without passing through a load. This is usually a serious circuit fault that should be avoided as much as possible, as it can lead to circuit burnout due to excessive current and even fire.
A power supply short circuit occurs when current flows directly across the positive and negative terminals of a power source without passing through the appliance. According to Ohm's Law (I=U/R), because the resistance of the wires is very small, the current in the circuit during a short circuit will be extremely large. Such a large current cannot be handled by batteries or other power sources, causing damage to the power supply. More seriously, the excessive current will cause the wires to overheat, potentially leading to a fire.
generator short circuit test
The generator short-circuit test refers to the relationship between the stator steady-state short-circuit current and the excitation current when the three-phase stator windings of the generator are short-circuited at rated speed. When performing the generator short-circuit test, the output terminals of the generator's three-phase windings must first be short-circuited. Then, while maintaining a constant speed, the excitation current and corresponding stator current values are read as the excitation current increases until the stator current reaches the rated current. It is particularly important to note that when adjusting the excitation current during the generator short-circuit test, it is strictly forbidden to adjust it back and forth.
The purpose of performing a generator short-circuit test is to check the symmetry of the three-phase current and, in conjunction with the no-load characteristics, to determine the generator's parameters. This can help determine if there is an inter-turn short circuit in the coil. Furthermore, calculating the generator's main parameters, such as synchronous reactance and short-circuit ratio, as well as voltage regulation calculations, also require attention to short-circuit characteristics.
● For three-phase short-circuit tests, copper or aluminum busbars should be used whenever possible, and the connections must be secure to prevent overheating.
● The excitation current should be adjusted slowly.
● In the experiment, when the current rises to 15~20%, the symmetry of the three-phase current should be checked and it should be qualified before continuing.
● The three-phase short circuit should be installed inside the output circuit breaker as much as possible to prevent the circuit breaker from tripping accidentally during the test and causing damage to the motor insulation.
Relevant data acquisition: Rotor excitation current and voltage can be directly read on the power cabinet or excitation regulating cabinet computer; stator current measurement and metering can be directly read on the unit power acquisition or monitoring computer; stator excitation current can be directly read on the excitation regulating cabinet computer; protection and differential currents can be read on the generator protection device, and attention should be paid to the correctness of their phases.
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