How to measure the back electromotive force of permanent magnet synchronous servo motor?
According to the law of electromagnetics, when the magnetic field changes, the nearby conductor will generate an induced electromotive force whose direction conforms to Faraday's law and Lenz's law, which is the opposite of the voltage originally applied to the coil. This voltage is the counter electromotive force. In the permanent magnet synchronous servo motor, as long as the motor is rotating, there is bound to be a coil cutting magnetic line, so there is a counter electromotive force. The back electromotive force is expressed by E1, and its effective value is calculated as follows:
among them:
KE - is a proportional constant;
FN - the frequency of the stator current;
NL - the number of turns of the stator windings per phase;
ф - the amplitude value of the main flux.
First, the counter-electromotive force determinants
◆Rotor angular velocity
◆The magnetic field generated by the rotor magnet
◆Number of turns of the stator winding
When the motor is designed, the rotor magnetic field and the number of turns of the stator winding are determined. Therefore, the only factor determining the back electromotive force is the rotor angular velocity, or the rotor speed. As the rotor speed increases, the back electromotive force also increases.
Second, the back EMF test method
According to the general technical conditions of permanent magnet AC servo motor, we generally use the back electromotive force constant to evaluate the permanent magnet synchronous motor. The back EMF constant is a very important technical parameter of permanent magnet synchronous motor, which is related to the running performance of the motor and the design of the controller. Therefore, in the production of permanent magnet synchronous servo motor, this parameter must be accurately tested, and the standard test method is used to verify whether the design product meets the technical requirements.
GB/T50349 permanent magnet AC servo motor general technical conditions stipulate that the motor to be tested is first dragged to a certain speed n specified by the technical conditions, and then the waveform is observed with an oscilloscope, and the back electromotive force constant (Ke) is calculated by voltage and speed. And requires Ke to meet the technical requirements of permanent magnet synchronous servo motor.
Figure 1: Back EMF Test Platform
As shown in the above figure, the permanent magnet synchronous servo motor back electromotive force test platform, according to the test standard requirements, the test motor is used for electric operation, and the driven motor is driven to no-load running speed to n; the speed sensor is used to complete the speed test, WP4000 variable frequency power analyzer The tested motor line back EMF U test was completed.
During the test, the line back electromotive force U when the motor no-load speed is n is measured, and the back electromotive force constant is calculated by the following formula to verify whether the technical requirements are met.
In the formula:
Ke - the back electromotive force constant in volts per arc and minus one second (V/rad*-1^s);
U——the back electromotive force of the motor line, the unit is volt (V), the effective value of the line back electromotive force of the sine wave drive motor, and the back electromotive force amplitude of the square wave drive motor;
N——the speed of the measured point, the unit is the revolution per minute (r/min).
This scheme selects WP4000 variable frequency power analyzer to complete the back electromotive force and speed test, and adopts effective synchronization means to ensure the synchronous test of electrical parameters and non-point parameters. At the same time, the WP4000 variable frequency power analyzer opens the original data port, and the front-end acquisition of raw waveform data can be directly shared with the host computer through Ethernet, which can completely replace the requirements of the oscilloscope. The back electromotive force constant test can be directly completed by the custom analysis software, which is satisfied with the automatic test requirements of the servo motor test system.
