MHD115C-058-NG1-AN Rexroth MHD同步电机

MHD115C-058-PG1-BA

Introduction to synchronous motor

As the name suggests, synchronous motors are able to operate at a constant speed regardless of the load acting on them. Unlike an induction motor, whose speed depends on the torque acting on it, a synchronous motor has a constant speed-torque characteristic. Synchronous motors have higher efficiency (electrical to mechanical power conversion ratio) than comparable motors. Efficiency range is 90-92%

Figure: 1 Synchronous motors are efficient, high-precision machines
Working principle -RMF constant magnetic field interaction
The constant velocity characteristic is achieved by the interaction between the constant magnetic field and the rotating magnetic field. The rotor of a synchronous motor generates a constant magnetic field, while the stator generates a rotating magnetic field.

Figure: 2 The interaction between the rotating magnetic field and the constant magnetic field contributes to the constant velocity characteristic
Stator-rotating magnetic field
The stator’s excitation coil is excited by a three-phase AC power supply. This creates a rotating magnetic field (RMF), which rotates at a synchronous speed. A way to generate RMF using a three-phase AC excitation is explained in another article. The RMF generated by the synchronous motor and its direction are shown in Figure 2.

Rotor – Constant magnetic field

The rotor is excited by a DC power supply, and the magnetic field generated by the DC excitation around the rotor coil is shown below. Apparently, because of this magnetic field, the rotor acts like a permanent magnet. The rotor can also be made of a permanent magnet system. The interaction between the rotor and RMF is interesting. Suppose you want to make the initial rotation of the rotor in the same direction as the RMF. You will see that the opposite poles of the RMF and Rotor will attract each other and will be magnetically locked. This means that the rotor will rotate at the same speed as the RMF, or the rotor will rotate at a synchronous speed.

Figure: 3 In the first figure, the opposite pole of the RMF and the rotor magnetic pole is attracted and the rotor has rotated: in the second figure, the magnetic pole is magnetically locked
Synchronizing speed
The speed at which RMF rotates or synchronizes can be easily derived.

N s = 120 f/P

It is obvious from the relationship that the speed Ns (rpm) of the synchronous motor is proportional to the electrical frequency f (Hz). P represents the number of poles of the rotor. This means that if the electrical frequency is controlled, the speed of the synchronous motor can be controlled very precisely. This is why they are suitable for high-precision applications.

Why can’t the synchronous motor start itself?
If there was no initial rotation of the rotor, the situation would be very different. The North Pole of the rotor will obviously be attracted to the South Pole of the RMF and will start moving in the same direction. However, because the rotor has a certain inertia, this starting speed will be very low. At that point, the RMF’s South Pole will be replaced by the North Pole. Therefore, it will produce a repulsive force. This will move the rotor backwards. Eventually, the rotor will not start. Therefore, it can be concluded that synchronous motors are not born with self-starting function.