Asynchronous motor
Induction motor, also known as "asynchronous motor", that is, the rotor is placed in a rotating magnetic field, under the action of the rotating magnetic field, to obtain a rotating torque, and thus the rotor rotates.
The rotor is a rotating conductor, usually in the shape of a squirrel cage. The stator is the non-rotating part of the motor, whose main task is to generate a rotating magnetic field. The rotating magnetic field is not realized mechanically. Instead, alternating current is passed through several pairs of electromagnets, so that the magnetic poles of the nature of the cycle change, so it is equivalent to a rotating magnetic field. This motor is not like the DC motor brush or collector ring, according to the type of alternating current used in single-phase motors and three-phase motors, single-phase motors used in such as washing machines, electric fans, etc.; three-phase motors are used as power equipment in factories.
Braking
Three-phase induction motor electrical braking methods: energy braking, reverse braking, regenerative braking three.
(1) Energy braking cuts off the three-phase AC power supply of the motor and sends DC power into the stator winding. In the moment of cutting off the AC power supply, due to inertia, the motor is still rotating in the original direction, this way is characterized by smooth braking, but the need for DC power supply, high-power motors, the cost of the required DC equipment, low-speed braking power is small.
(2) reverse braking and load reverse braking and power reverse braking two.
1)Load reverse braking is also known as load reverse pull reverse braking. This torque causes the weight to fall slowly at a steady rate. This braking is characterized by: the power supply does not need to be reversed, do not need special braking equipment, and can also adjust the braking speed, but only for wire-wound motors, the rotor circuit needs to be strung into the large resistance, so that the rate of rotation is greater than 1.
2) power reverse braking when the motor needs to brake, as long as the arbitrary counter-adjustment of the two-phase power supply line, so that the rotating magnetic field opposite to the braking can be done very quickly. When the motor speed is equal to zero, immediately cut off the power supply. This kind of braking is characterized by: fast stopping, strong braking force, no braking equipment. But when braking due to the current is large, the impact force is also large, easy to make the motor overheating, or damage to the transmission parts of the components.
(3) regenerative braking, also known as feed-back braking, under the action of heavy loads (when the crane motor lowering heavy loads), the motor speed is higher than the synchronous speed of the rotating magnetic field. At this time, the rotor conductor produces induced current, under the action of the rotating magnetic field to produce anti-rotational direction torque, but the motor speed is high, the need to use a speed changer to slow down.
Synchronous motor
Synchronous motor (synchronous motor) is an alternating current (AC) motor that rotates at a synchronous speed due to the interaction of the DC-powered excitation magnetic field and the rotating magnetic field of the armature to produce torque.
A.C. motor whose rotor speed is the same as the rotating magnetic field of the stator. The rotor speed n and the magnetic pole number p, power supply frequency f meets n=60f/p. The rotational speed n is determined by the power supply frequency f, so when the power supply frequency is certain, the rotational speed is unchanged, and independent of the load. It is characterized by high operational stability and large overload capacity. Commonly used in multi-machine synchronous drive system, precision speed regulating and stabilizing system and large equipment (such as rolling mill).
Synchronous motor is an AC motor, the stator winding is the same as the asynchronous motor. Its rotor rotates at the same speed as the rotating magnetic field generated by the stator windings, so it is called a synchronous motor. Because of this, the current in a synchronous motor is ahead of the voltage in phase, i.e., the synchronous motor is a capacitive load. For this reason, in many cases, synchronous motors are used to improve the power factor of the power supply system.
Synchronous motor for motor operation. Because the synchronous motor can be adjusted by the excitation current so that it is running in the power factor in excess of the power factor, is conducive to improving the power factor of the power grid, therefore, large equipment, such as large blowers, pumps, ball mills, compressors, rolling mills, etc., commonly used synchronous motor drive. This advantage is especially prominent when synchronous motors are used for large equipment at low speeds. In addition, the speed of the synchronous motor is completely determined by the frequency of the power supply. When the frequency is certain, the rotational speed of the motor is also certain, and it does not change with the load. This feature is of great significance in certain transmission systems, especially multi-machine synchronous transmission systems and precision speed regulating and stabilizing systems. The operation stability of synchronous motor is also relatively high. Synchronous motors are generally operated under over-excitation, and their overload capacity is greater than that of the corresponding asynchronous motor. While the torque of an asynchronous motor is proportional to the square of the voltage, the torque of a synchronous motor is determined by the product of the voltage and the internal electromotive force generated by the motor excitation current, i.e., it is proportional only to the primary of the voltage. When the grid voltage suddenly drops to about 80% of the rated value, asynchronous motor torque tends to drop to about 64%, and stop running because it can't carry the load; while the synchronous motor torque but not much, but also through the forced excitation to ensure the stable operation of the motor.
Synchronous motor structure and synchronous generator is basically the same, the rotor is also divided into convex pole and hidden pole. But most synchronous motors are convex pole type. The installation form is also divided into horizontal and vertical. In order to solve the starting problem of synchronous motor, its rotor is generally equipped with a starting winding. It can also suppress oscillation during operation, so it is also called damping winding. In addition to the above traditional structure, there is also a claw-pole rotor structure without sliding contact. To 6-pole motor, for example, in the rotor shaft on the opposite side of the two sets of claw-shaped magnetic poles. A group in the claw disk along the axis to the right extends 3 pole body; another group of reverse installation in the right, so that the claw disk along the axis to the left extends 3 pole body. The polarity of the two groups of poles is reversed. Magnetic poles of the outer circumference surface assembly, no longer like the general convex pole motor as a round tile surface, but wedge-shaped tile surface, that is, one end of the pole arc longer than the other end, the entire rotor shape as shown in Figure. The excitation winding is assembled at the outer edge of the yoke on both sides. It produces a magnetic flux through the lateral main air gap gm between the N and S poles, the axial air gap between the rotor and stator g1 and g2, and then through the end cover and seat and closed, as shown by the dotted line in the figure. In order to prevent magnetic flux short-circuiting through the rotor shaft, the rotor shaft should be non-magnetic steel; or the rotor shaft is divided into three sections, the middle section is non-magnetic steel. The main advantage of this structure is that there is no winding in the rotating part, and there is no sliding contact between the collector ring and brushes, so the operation is reliable, the insulation structure is simple, and the maintenance is also convenient. But its main magnetic circuit is long and there are more air gap, so that the power required for excitation increases; motor shell has a strong magnetic, which causes bearing heat; and the shaft must also be used to isolate the magnetic measures. Therefore, this motor has not been widely promoted, only in some special occasions, the general capacity of not more than a few hundred kilowatts.