"AC motor" is a machine used to realize the mutual conversion of mechanical energy and AC energy. Due to the great development of AC power system, AC motor has become the most commonly used motor. Compared with DC motor, AC motor has no commutator (see commutation of DC motor), so it has the advantages of simple structure, convenient manufacturing, relatively firm, easy to make high speed, high voltage, large current and large capacity motor. AC motor power covers a wide range, from a few watts to tens of thousands of kilowatts, or even millions of kilowatts. In the early 1980s, the largest steam turbine generator reached 1.5 million kilowatts.
classification
According to statistics, AC motors can be classified in the following two ways:
By function
AC motors are usually divided into alternator, AC motor and synchronous condenser according to their functions. Because of the reversibility of the working state of the motor, the same motor can be used as both generator and motor.
It is not very accurate to divide the motor into generator and motor, but some motors are mainly used for generator operation, and some motors are mainly used for motor operation.
By variety
AC motor can be divided into synchronous motor and asynchronous motor. Synchronous motor rotor speed ns and rotating magnetic field speed is the same, called synchronous speed. There is a strict relationship between ns and the frequency of AC (f) and the number of pole pairs (P).
ns=60f/P
In China, the power frequency is 50 Hz, so the synchronous speed of one pair of pole motors in three-phase AC motors is 3000 rpm, and that of two pairs of pole motors in three-phase AC motors is 1500 rpm, and so on. Asynchronous motor rotor speed is always lower than or higher than the speed of its rotating magnetic field, hence the name of asynchronous. The difference between rotor speed and rotating magnetic field speed (called slip) is usually less than 10%.
Spindle introduction
AC motor is a very broad classification, the type of motor itself is very large, AC motor itself can be divided into low-speed and high-speed, low-speed can be called low-speed motor, AC low-speed motor, high-speed can be called high-speed motor, high-speed motor, AC high-speed motor, and high-speed motor can be called motorized spindle, high-speed motorized spindle and so on.
Motorized spindle:
Motorized spindle is a new technology in the field of numerical control machine tools in recent years, which integrates the machine tool spindle and spindle motor. Together with linear motor technology and high-speed tool technology, it will push high-speed machining to a new era. Motorized spindle is a set of components, which includes motorized spindle itself and its accessories: motorized spindle, high frequency frequency converter, oil mist lubricator, cooling device, built-in encoder, tool changing device.
Technology of motorized spindle:
High speed bearing technology: the motorized spindle usually adopts composite ceramic bearing, which is wear-resistant and heat-resistant, and its service life is several times longer than that of traditional bearing; sometimes it also adopts electromagnetic suspension bearing or hydrostatic bearing, whose inner and outer rings are not in contact, so its service life is unlimited in theory;
High speed motor technology: motorized spindle is the product of motor and spindle. The rotor of motor is the rotating part of spindle. Theoretically, motorized spindle can be regarded as a high-speed motor. The key technology is dynamic balance at high speed;
Lubrication: the lubrication of motorized spindle generally adopts timing and quantitative oil air lubrication; grease lubrication can also be used, but the corresponding speed should be discounted. The so-called timing is to inject oil at regular intervals. The so-called quantitative, is through a device called quantitative valve, accurately control the amount of oil each time. Oil air lubrication means that the lubricating oil is blown into the ceramic bearing with the compressed air. The control of oil quantity is very important. Too little oil can't play the role of lubrication. Too much oil will generate heat due to the resistance of oil when the bearing rotates at high speed. Cooling device: in order to heat the high-speed motorized spindle as soon as possible, the outer wall of the motorized spindle is usually filled with circulating coolant. The function of the cooling device is to maintain the temperature of the coolant.
Built in pulse encoder: in order to realize automatic tool change and rigid tapping, a pulse encoder is built in the motorized spindle to realize accurate phase angle control and coordination with feed.
Automatic tool changing device: in order to apply to machining center, motorized spindle is equipped with automatic tool changing device, including disc spring, broach cylinder, etc;
The clamping mode of high-speed tools: BT and ISO tools, which are widely known, have been proved not suitable for high-speed machining. In this case, HSK, ski and other high-speed tools appear.
High frequency frequency conversion device: in order to achieve the speed of tens of thousands or even hundreds of thousands of revolutions per minute of the motorized spindle, a high frequency frequency conversion device must be used to drive the built-in high-speed motor of the motorized spindle, and the output frequency of the frequency converter must reach thousands or thousands of Hertz.
Principle of motor
The single-phase capacitor motor has two windings, namely starting winding and running winding. The two windings are 90 degrees apart in space. A capacitor with large capacity is connected in series on the starting winding. When the running winding and the starting winding pass through single-phase alternating current, the current in the starting winding is 90 degrees ahead of the current in the running winding due to the action of the capacitor, and reaches the maximum value first. Two same pulse magnetic fields are formed in time and space, which makes a rotating magnetic field in the air gap between the stator and rotor. Under the action of the rotating magnetic field, the induction current is generated in the rotor of the motor. The current interacts with the rotating magnetic field to generate electromagnetic torque, which makes the motor rotate.
Maintenance
1、 Preparation and inspection before startup
1. Check whether the grounding of electric and starting equipment is reliable and complete, and whether the wiring is correct and good.
2. Check whether the voltage and frequency shown on the motor nameplate are consistent with the power supply voltage and frequency.
3. The phase to phase and phase to ground insulation resistance of winding shall be checked before starting the motor newly installed or stopped for a long time. The insulation group should be greater than 0.5 megohm. If it is lower than this value, the winding must be dried.
4. For wound rotor, check whether the brush device on the collector ring can work normally and whether the brush pressure meets the requirements.
5. Check whether the motor rotates flexibly and whether the oil in the sliding bearing reaches the specified oil level.
6. Check whether the rated current of motor fuse meets the requirements.
7. Check whether the fastening bolts and mounting bolts of the motor are tightened. The motor can be started after all the above inspections meet the requirements. After the motor is started, it will run for about 30 minutes without load. Pay attention to observe whether the motor has any abnormal phenomenon. If abnormal conditions such as noise, vibration and heating are found, measures should be taken. After the situation is eliminated, it can be put into operation. When starting wound motor, the starting rheostat should be connected to the rotor circuit. For the motor with brush lifting mechanism, put down the brush, disconnect the short circuit device, close the stator circuit switch and pull the rheostat. When the motor is close to the rated speed, lift the brush, close the short circuit device, and the motor is started.
2、 Maintenance in line
1. The motor should always be kept clean, and no foreign matters are allowed to enter the motor; the air inlet and outlet must be kept unblocked.
2. Monitor power supply voltage, frequency and load current of motor with instrument. The voltage and frequency of the power supply shall conform to the data of the motor's nameplate, and the load current of the motor shall not exceed the specified value on the nameplate, otherwise, the cause shall be found out and measures shall be taken, and the motor can continue to operate after the bad situation is eliminated.
3. Take necessary measures to detect the temperature rise of each part of the motor.
4. For phase wound rotor motor, attention should be paid to the contact pressure, wear and spark between brush and collector ring. When the motor stops, the switch in the stator circuit should be disconnected, and then the brush lifting mechanism should be pulled to the start position to disconnect the short circuit device.
5. Regular maintenance of motor after operation is generally divided into minor repair and major repair. Minor overhaul belongs to general overhaul. The starting equipment and the whole of the motor should not be disassembled once a quarter. For major overhaul, all the transmission devices and all parts of the motor should be disassembled, and the disassembled parts should be inspected and cleaned comprehensively, generally once a year.