Estun Servo Motor – The working principle of AC servo motor

AC servo motors are usually single-phase asynchronous motors, with two structural forms: squirrel cage rotor and cup rotor. Like ordinary motors, AC servo motors are constructed of a stator and a rotor. There are two windings on the stator, namely the excitation winding and the control winding, and the two windings are 90° different in space at an electrical angle. The frame that holds and protects the stator is generally made of duralumin or stainless steel.

The rotor of the cage rotor AC servo motor is the same as that of an ordinary three-phase cage motor. The structure of the cup rotor AC servo motor is composed of three parts: the outer stator, the cup rotor and the inner stator. Its outer stator is the same as that of a cage rotor AC servo motor, and the rotor is made of a non-magnetic conductive material (such as copper or aluminum) in the shape of a coreless, and the bottom of the cup is fixed on the rotating shaft 7. The walls of the coreless are thin (less than 0.5mm), so the moment of inertia is small. The inner stator is made of silicon steel sheets, fixed on an end cap, and there is no winding on the inner stator, which is only used for magnetic circuits. When the AC servo motor is working, the inner and outer stators do not move, only the cup-shaped rotor rotates in the air gap between the inner and outer stator. For AC servo motors with small output power, the excitation winding and control winding are often placed in the slots of the inner and outer stator cores respectively.

The working principle of an AC servo motor is not fundamentally different from that of a single-phase induction motor. However, the AC servo motor must have the ability to overcome the so-called "rotation" phenomenon of the AC servo motor, that is, when there is no control signal, it should not rotate, especially when it is already rotating, if the control signal disappears, it should be able to stop rotating immediately. After the ordinary induction motor is rotated, if the control signal disappears, it often continues to rotate.

When the motor is originally in a stationary state, if the control winding does not add control voltage, only the excitation winding is energized to generate a pulsating magnetic field. A pulsating magnetic field can be thought of as two circular rotating magnetic fields. These two circular rotating magnetic fields rotate in opposite directions with the same size and speed, and the established forward and reverse rotating magnetic fields cut the cage winding (or cup-shaped wall) respectively and induce the same size, opposite phase electromotive force and current (or eddy current), these currents are equal to the torque generated by their respective magnetic fields, and the direction is opposite, the resultant torque is zero, and the servo motor rotor cannot rotate. As soon as there is a deviation signal in the control system, the control winding has to accept the corresponding control voltage.

In general, the magnetic field generated inside the motor is an elliptical rotating magnetic field. An elliptical rotating magnetic field can be seen as a synthesis of two circular rotating magnetic fields. The amplitude of these two circular rotating magnetic fields is unequal (the forward magnetic field is larger than the original elliptical rotating magnetic field, and the reverse magnetic field opposite to the original rotation is smaller), but at the same speed, rotates in opposite directions. The electric potential and current induced by the rotor winding and the electromagnetic torque generated by them are also in opposite directions, the size is unequal (the forward rotation is large, the reverse rotation is small) and the resultant torque is not zero, so the servo motor rotates in the direction of the forward rotating magnetic field, with the enhancement of the signal, the magnetic field is close to the circle, at this time, the forward rotating magnetic field and its torque increase, the reverse magnetic field and its torque decrease, and the resultant torque becomes larger, if the load torque remains unchanged, the speed of the rotor increases. If the phase of the control voltage is changed, i.e. 180°, the rotation of the magnetic field is reversed and the resulting resultant torque is reversed. If the control signal disappears, and only the current is passed through the excitation winding, the magnetic field generated by the servo motor will be a pulsating magnetic field, and the rotor will stop quickly. In order to make the AC servo motor have the function of disappearing the control signal and stopping the rotation immediately, the rotor resistance is made particularly large, so that its critical slip rate Sk is greater than 1.

During the operation of the AC servo motor, if the control signal drops to "zero", the excitation current still exists, and a pulsating magnetic field is generated in the air gap, which can be regarded as a combination of the forward and reverse rotating magnetic fields. Once the control signal disappears, the air-gap magnetic field is converted into a pulsating magnetic field, which can be seen as a combination of a forward-rotating magnetic field and a counter-rotating magnetic field. Due to the inertia of the rotor, the AC servo motor generates a braking moment that is opposite to the original direction of rotation of the rotor. Under the action of load torque and braking torque, the rotor is stopped quickly.

Ordinary two-phase and three-phase asynchronous motors work in a symmetrical state under normal circumstances, and asymmetrical operation belongs to the fault state. The AC servo motor can rely on different degrees of asymmetrical operation to achieve the control purpose. This is the fundamental difference between AC servo motors and ordinary asynchronous motors in operation.

Estun Servo Motor – The working principle of AC servo motor

Estun Servo Motor – The working principle of AC servo motor

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