Among single-phase asynchronous motors, special-purpose motors account for a large proportion, and their structures vary greatly. However, in common, the structure of an electric motor consists of three main parts: the stationary part—the stator, the rotating part—the rotor, and the supporting part—end covers and bearings.
structure
AC servo motors are typically single-phase asynchronous motors, with two structural forms: squirrel-cage rotor and cup-shaped rotor. Like ordinary motors, AC servo motors consist of a stator and a rotor. The stator has two windings: an excitation winding and a control winding, which are 90° out of phase. The frame that secures and protects the stator is generally made of hard aluminum or stainless steel. The rotor of a squirrel-cage AC servo motor is the same as that of a typical three-phase squirrel-cage motor. The structure of a cup-shaped AC servo motor consists of three parts: an outer stator, a cup-shaped rotor, and an inner stator. Its outer stator is the same as that of a squirrel-cage AC servo motor, while the rotor is made of a hollow cup shape from a non-magnetic conductive material (such as copper or aluminum), with the bottom of the cup fixed to the shaft 7. The walls of the hollow cup are very thin (less than 0.5mm), resulting in a very small moment of inertia. The inner stator is made of laminated silicon steel sheets and fixed to an end cap. The inner stator has no windings and serves only as a magnetic circuit. When the motor is working, neither the inner nor outer stator moves; only the cup-shaped rotor rotates in the air gap between the inner and outer stators. For AC servo motors with relatively low output power, the excitation winding and control winding are often placed in the slots of the inner and outer stator cores, respectively.
The main components of the motor include: 1. Frame; 2. Iron core; 3. Windings; 4. End cover; 5. Bearings; 6. Centrifugal switch or starting relay and PTC starter; 7. Nameplate.
1 base
The frame structure varies depending on the motor's cooling method, protection type, installation method, and application. Classified by material, there are several types, including cast iron, cast aluminum, and steel plate structures. Cast iron frames have cooling fins. The frame is connected to the end cover and secured with bolts. Cast aluminum frames generally do not have cooling fins. Steel plate frame structures are made by rolling and welding thin steel plates with a thickness of 1.5-2.5 mm, and then welding on stamped steel plate feet. Some specialized motors have quite unique frames; for example, refrigerator motors are usually housed in a sealed container with the compressor. Washing machine motors, including spin dryer motors, do not have a frame; the end cover is directly fixed to the stator core.
2 Iron Heart
The iron core includes the stator iron core and the rotor iron core. Its function is the same as that of a three-phase asynchronous motor, which is used to form the magnetic circuit of the motor.
3 windings
The stator windings of single-phase asynchronous motors are often made in two phases: a main winding (working winding) and an auxiliary winding (starting winding). The center axes of the two windings are offset by a certain electrical angle. This is to improve starting and running performance. The stator windings are mostly made of high-strength polyester enameled wire.
The rotor winding is generally a squirrel-cage winding. It is usually made of die-cast aluminum.
4 end caps
Corresponding to different base materials, end caps are also available in cast iron, cast aluminum, and stamped steel.
5 bearings
There are ball bearings and oil-impregnated bearings.
6. Centrifugal switch or starting relay and PTC starter
(1) Centrifugal switch
In single-phase asynchronous motors, except for capacitor-run motors, during the starting process, when the rotor speed reaches about 70% of the synchronous speed, a centrifugal switch is often used to disconnect the starting windings of single-phase resistance-start asynchronous motors and capacitor-start asynchronous motors, or to disconnect the starting capacitors of capacitor-start and capacitor-run asynchronous motors. The centrifugal switch is generally installed inside the shaft extension end cover.
(2) Starting relay
Some electric motors, such as refrigerator motors, are assembled with the compressor and housed in a sealed container, making it inconvenient to install a centrifugal switch. Therefore, a starting relay is used instead. The relay's electromagnet coil is connected in series in the main winding circuit. During startup, the main winding current is large, causing the armature to actuate and close the normally open contacts connected in series in the auxiliary winding circuit. This connects the auxiliary winding, and the motor operates in a two-phase winding state. As the rotor speed increases, the main winding current continuously decreases, and the attraction force of the electromagnet coil decreases. When a certain speed is reached, the attraction force of the electromagnet becomes less than the tension of the contact's reaction spring, the contacts open, and the auxiliary winding is disconnected from the power supply.
(3) PTC starter
The latest starting element is the "PTC," a thermistor that can "turn on" or "off." The PTC thermistor is a new type of semiconductor element that can be used as a time-delay starting switch. In use, the PTC element is connected in series with the secondary winding of a capacitor-start or resistor-start motor. Initially, because the PTC thermistor has not yet heated up, its resistance is very low, and the secondary winding is in a closed state, allowing the motor to start. As time progresses, the motor speed increases, and the temperature of the PTC element rises due to its Joule heating. When it exceeds the Curie point Tc (the temperature at which resistance increases sharply), the resistance increases dramatically, effectively disconnecting the secondary winding circuit. However, a small holding current, with a loss of 2-3 watts, keeps the PTC element's temperature above the Curie point Tc. When the motor stops running, the PTC element temperature gradually decreases, and its resistance drops below Tc in about 2-3 minutes, at which point it can be restarted. This timeframe coincides with the specified shutdown time between two start-ups for refrigerators and air conditioners.
Advantages of PTC starters: contactless operation, reliable operation, noiseless and spark-free, good fire and explosion-proof performance. They are also vibration-resistant, shock-resistant, small in size, lightweight, and inexpensive.
7 nameplates
This includes: motor name, model, standard number, manufacturer's name, serial number, rated voltage, rated power, rated current, rated speed, winding connection, insulation class, etc.
principle
The working principle of an AC servo motor is not fundamentally different from that of a single-phase induction motor. However, an AC servo motor must possess one crucial characteristic: it must overcome the so-called "self-rotation" phenomenon. This means it should not rotate without a control signal, and especially if it is already rotating, it should stop immediately if the control signal disappears. In contrast, a regular induction motor, once started, often continues to rotate even after the control signal is lost.
When the motor is initially stationary, if no control voltage is applied to the control winding, only the excitation winding is energized, generating a pulsating magnetic field. This pulsating magnetic field can be considered as two circular rotating magnetic fields. These two circular rotating magnetic fields rotate in opposite directions with the same magnitude and speed. The resulting forward and reverse rotating magnetic fields cut the cage winding (or cup-shaped wall) and induce electromotive forces and currents (or eddy currents) of the same magnitude but opposite phases. The torques generated by these currents interacting with their respective magnetic fields are also equal in magnitude and opposite in direction, resulting in a net torque of zero, preventing the servo motor rotor from rotating. Once the control system receives a deviation signal, the control winding must receive a corresponding control voltage. Under normal circumstances, the magnetic field generated inside the motor is an elliptical rotating magnetic field. An elliptical rotating magnetic field can be considered as the resultant of two circular rotating magnetic fields. These two circular rotating magnetic fields have unequal amplitudes (the forward rotating magnetic field, rotating in the same direction as the original elliptical rotating magnetic field, is larger, while the reverse rotating magnetic field, rotating in the opposite direction, is smaller), but they rotate in opposite directions at the same speed. The electromotive force and current induced by the cutting of the rotor windings, as well as the resulting electromagnetic torque, are in opposite directions and of unequal magnitude (larger for forward rotation, smaller for reverse rotation). The resultant torque is not zero, so the servo motor rotates in the direction of the forward magnetic field. As the signal strengthens, the magnetic field approaches a circle. At this point, the forward magnetic field and its torque increase, while the reverse magnetic field and its torque decrease, resulting in a larger resultant torque. If the load torque remains constant, the rotor speed increases. If the phase of the control voltage is changed, i.e., shifted by 180°, the direction of the rotating magnetic field reverses, and therefore the direction of the resulting torque also reverses, causing the servo motor to reverse. If the control signal disappears, and only current flows through the excitation winding, the magnetic field generated by the servo motor will be a pulsating magnetic field, and the rotor will quickly stop.
To enable the AC servo motor to stop rotating immediately upon the disappearance of the control signal, its rotor resistance is made exceptionally high, making its critical slip Sk greater than 1. During motor operation, if the control signal drops to "zero," the excitation current still exists, generating a pulsating magnetic field in the air gap. This pulsating magnetic field can be considered as a synthesis of the forward and reverse rotating magnetic fields. Figure 3-13 shows the torque-speed characteristic curves generated after the forward and reverse rotating magnetic fields cut the rotor conductors, as well as their composite characteristic curve. Assume the motor was initially running at point A under the drive of a single forward rotating magnetic field, at which point the load torque is [value missing]. Once the control signal disappears, the air gap magnetic field transforms into a pulsating magnetic field, which can be considered as a synthesis of the forward and reverse rotating magnetic fields, and the motor operates according to the composite characteristic curve 3. Due to the rotor's inertia, the operating point moves from point A to point B, at which point the motor generates a braking torque opposite to the original direction of rotor rotation. Under the action of the load torque and the braking torque, the rotor stops rapidly.
It must be pointed out that ordinary two-phase and three-phase asynchronous motors normally operate in a symmetrical state; asymmetrical operation is a fault condition. AC servo motors, however, can achieve control by operating with varying degrees of asymmetry. This is the fundamental difference between AC servo motors and ordinary asynchronous motors in terms of operation.
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