Estun servo motor - the structure and working principle of AC servo motor

structure

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 motor is working, neither the inner nor the outer stator moves, 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.

Estun servo motor - the structure and working principle of AC servo motor

The main components of the motor include: 1. a frame; 2. an iron core; 3. a winding; 4. an end cover; 5. a bearing; 6. Centrifugal switch or starting relay and PTC starter; 7. a nameplate.

1. a frame

The frame structure varies depending on the motor cooling method, protection type, installation method, and purpose. According to its material classification, there are several types of cast iron, cast aluminum and steel plate structures. Cast iron frame with ribs. The base is connected to the end cap and fastened with bolts. Cast aluminum machine bases generally do not have heat dissipation ribs. The steel plate structure base is made of thin steel plate with a thickness of 1.5-2.5 mm, which is rolled and welded, and then welded to the foot of the steel plate stamping part. Some special motors have a rather special base, such as the motor of a refrigerator, which is usually packed in a sealed jar with the compressor. The motor of the washing machine, including the motor of the dryer, has no base, and the end cover is directly fixed on the stator core.

2. Iron core

The core includes a stator core and a rotor core, which acts like a three-phase asynchronous motor and is used to form a magnetic circuit for the motor.

3. Winding

The stator winding of a single-phase asynchronous motor is often made into two phases: the main winding (working winding) and the secondary winding (starting winding). The central axis of the two windings is staggered by a certain electrical angle. The aim is to improve start-up performance and operational performance. The stator windings are mostly wound with high-strength polyester enameled wire.

The rotor winding generally adopts a cage winding. It is commonly used for aluminum die-casting.

4. an end cap

There are also iron castings, aluminum castings and steel plate stamping parts corresponding to different base materials and end covers.

5. a bearing

Bearings include ball bearings and oil-impregnated bearings.

6. Centrifugal switch or starting relay and PTC starter

(1) Centrifugal switch

In single-phase asynchronous motors, in addition to the capacitive running motor, in the starting process, when the rotor speed reaches about 70% of the synchronous speed, the starting windings of the single-phase resistive starting asynchronous motor and the capacitor starting asynchronous motor are often cut off with the help of centrifugal switches, or the starting capacitors of the capacitor starting and running asynchronous motor are cut off. Centrifugal switches are generally installed on the inside of the shaft extension end cover.

(2) Starting relay

Some motors, such as refrigerator motors, are replaced by starter relays because they are assembled with the compressor and placed in a sealed jar that is not convenient for installing centrifugal switches. The suction coil of the relay is connected in series in the main winding circuit, and when starting, the main winding current is very large, and the armature action makes the dynamic contact connected in series in the secondary winding circuit closed. So the secondary winding is turned on, and the motor is in the two-phase winding running state. As the rotor speed increases, the main winding current decreases and the suction power of the suction coil decreases. When a certain speed is reached, the suction force of the electromagnet is less than the tension of the reaction spring of the contact, the contact is opened, and the secondary winding is disengaged from the power supply.

(3) PTC starter

The latest actuating element is the "PTC", which is a thermistor that can be "turned on" or "off". PTC thermistors are a new type of semiconductor element that can be used as delay-type starter switches. When used, the PTC element is connected in series with the secondary windings of a capacitive-started or resistive-started motor. At the beginning of starting, because the PTC thermistor has not yet been heated, the resistance value is very low, the auxiliary winding is in the path state, and the motor starts to start. With the passage of time, the speed of the motor increases, and the temperature of the PTC element rises due to its own Joule heating, and when it exceeds the Curie point Tc (that is, the temperature point where the resistance increases sharply), the resistance increases dramatically, and the secondary winding circuit is equivalent to disconnecting, but there is a small maintenance current, and there is a loss of 2-3 watts, so that the temperature of the PTC element is maintained above the Curie point Tc value. When the motor stops running, the temperature of the PTC element continues to drop, and its resistance value drops below the Tc point for about 2-3 minutes, at which point it can be restarted, which is exactly the downtime between the refrigerator and the air conditioner.

Advantages of PTC starter: no contact, reliable operation, no noise and no electric spark, good fire and explosion resistance. And vibration resistance, shock resistance, small size, light weight, low price.

7. a nameplate

Including: motor name, model, standard number, manufacturer name, factory number, rated voltage, rated power, rated current, rated speed, winding connection, insulation grade, etc.

principle

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 a performance, that is, it can 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. the phase is shifted by 180o, the direction of the rotating magnetic field is reversed, and the resulting resulting moment is also in the opposite direction, and the servo motor will be 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 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 a forward rotating magnetic field and a reverse rotating magnetic field. Figure 3-13 plots the torque-speed characteristic curves of the rotor conductor generated by the forward and reverse rotating magnetic fields, and their resultant characteristic curves. Assuming that the motor is originally driven by a single positive rotating magnetic field, the load torque is . Once the control signal disappears, the air-gap magnetic field is converted into a pulsating magnetic field, which can be regarded as a synthesis of a forward rotating magnetic field and a reverse rotating magnetic field, and the motor operates according to the resultant characteristic curve 3. Due to the inertia of the rotor, the operating point is moved from point A to point B, and the motor generates a braking torque opposite to the original direction of rotation of the rotor. Under the action of load torque and braking torque, the rotor is stopped quickly.

It must be pointed out that 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 the operation of the AC servo motor and the ordinary asynchronous motor.

Estun servo motor - the structure and working principle of AC servo motor

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