●Requirement for the motor--Motor moment of inertia measurement
The rotor is mounted in the motor in the normal way. In order to ensure that the rotor of the motor is flexible, it is necessary to use rolling bearings and to perform the necessary lubrication. If the motor is a plain bearing, the rotor must be mounted separately on the balancing machine. For motors with brushes, all brushes must be lifted during testing.
● Requirements for the production and installation of the auxiliary pendulum
The auxiliary pendulum consists of a pendulum and a connecting rod. The pendulum is cylindrical, and its mass should be as light as possible under the premise of overcoming the moment of inertia of the rotor of the motor under test. Generally, its moment of inertia is not more than 10% of the moment of inertia of the rotor of the motor under test.
The quality of the connecting rod should be as light as possible, preferably made of wood. The length is selected according to the size of the motor. For motors above 10 kW, the swing period is controlled within 3-8 s. For motors of 1 to 10 kW, the swing period is controlled by 1~3 s.
The auxiliary pendulum is firmly mounted on the shaft extension so that the auxiliary pendulum naturally hangs down and its axis coincides with the vertical line.
This method is applicable to motors of 100 kW and above after assembly. The advantage is that no auxiliary device is needed, and no matter what kind of bearing the motor uses, it can be measured, so it is more suitable for a larger motor; the disadvantage is that the mechanical loss of the motor to be tested needs to be measured beforehand. Instrumentation and testing capabilities.
Before the test, the motor to be tested should be idling for a certain period of time, so that the mechanical loss reaches a steady state, and the mechanical loss Pm of the motor is measured.
●Test procedures and precautions
(1) Use the method of increasing the power frequency (for AC motor), increasing the voltage or reducing the excitation (for DC motor) or other mechanical drag, so that the speed of the motor under test exceeds 1.1 times its rated value, and then cut off the power or Leave the prime mover and let the motor slow down and stop.
(2) During the process of reducing the motor speed from 1.1 times to 0.98 times the rated speed, or from 1.05 times to 0.9 times the rated speed, accurately measure the falling speed difference Δn(r/nin) and the time Δt used to change Δn ( s).
In order to accurately read the speed of the motor, it is best to use a tachometer that can directly observe the continuous deceleration of the speed, such as a centrifugal pointer type tachometer.
(3) Using the measured data, obtain the moment of inertia J (kg•m2) of the rotor of the motor under test by equation (3).
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