Structure
Explosion-proof AC servo motors are typically single-phase asynchronous motors, available in 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, spaced 90° out of electrical angle. 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 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; it 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. 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 device 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.
