Why do we need to find the zero position when starting a synchronous motor?

Why do we need to find the zero position when starting a synchronous motor?

When starting a synchronous motor, it is necessary to find the zero phase position of the motor. This is because the rotor of the synchronous motor has no starting capability and must be synchronized with the power grid at a common intersection before it can be started. If the zero position is not found, the motor cannot be synchronized with the power grid and cannot be started.

Moreover, when the synchronous motor is running, the input current phase and the back electromotive force phase must meet certain requirements. The purpose of finding the zero position is to know the electrical angle of the rotor at that time.

Normally, the zero position of a synchronous motor is determined by detecting changes in the grid voltage over a certain period of time when the motor is stationary.

Therefore, it is very important to find the zero position when the synchronous motor starts, otherwise the motor cannot start normally.

Why can't the synchronous motor be started directly, but needs auxiliary starting?

The operation of synchronous motors requires a specific speed and phase, which requires synchronization between the motor and the power grid. When the motor starts without load, it needs to reach the same speed and phase as the power grid, but since it is an automatic synchronous motor, it can only achieve synchronization when the frequency of the phase electromotive force of the power supply matches the rotor speed in its coupling.

Therefore, synchronous motors require auxiliary starting during the starting process to provide starting torque and matching phase. This auxiliary starting is usually achieved through three methods: using an external starter, changing the characteristics of the synchronous motor by changing the stator winding, or using an electronic starter to provide initial current.