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the comparison of two synchronous motors
- Jan 12, 2019 -

the comparison of two synchronous motors

1. Brushless DC motor: The starting point is to replace the stator pole of the brushed DC motor with a rotor equipped with a permanent magnet, and change the rotor armature of the original DC motor into a stator. The brushed DC motor relies on a mechanical commutator to convert the DC current into an AC with an approximate trapezoidal wave, while the BDCM directly inputs the square wave current (actually a trapezoidal wave) into the stator, which has the advantage of eliminating the mechanical commutator and Brushes, also known as electronic commutation. In order to generate a constant electromagnetic torque, the system is required to input a three-phase symmetrical square wave current to the BDCM, and at the same time, the induced electromotive force of each phase of the BDCM is a trapezoidal wave, so BDCM is also called a square wave motor;

2. Permanent magnet synchronous motor (PMSM): The starting point is to replace the excitation winding on the rotor of the electric excitation synchronous motor with a permanent magnet to eliminate the excitation coil, slip ring and brush of the KAF87 reducer. The stator of the PMSM is basically the same as the electric excitation synchronous motor, and the current required to input the stator is still three-phase sinusoidal. In order to generate a constant electromagnetic torque, the system is required to input a three-phase symmetrical sinusoidal current to the PMSM, and at the same time, the induced electromotive force of each phase of the PMSM is a sine wave, so the PMSM is also called a sine wave motor.

The biggest advantage of synchronous motors is that adjusting the field current can change the power factor. At a certain active power, the U-shaped curve of the synchronous motor can be obtained. When over-excitation, the pre-reactive power is absorbed from the grid, and the lag-reactive power is absorbed from the grid when the excitation is under-excited.

When the exciting current of the synchronous motor is changed, the power factor of the synchronous motor can be changed, which is not available in the three-phase asynchronous motor. When the excitation current is changed, the law of the change of the power factor of the synchronous motor can be divided into three cases, namely, a normal excitation state, an underexcited state, and an overexcited state. When the synchronous motor is dragged and loaded, it is generally over-excited, at least in the normal excitation state, and should not be allowed to run in the under-excited state.


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