By the relative motion of the rotating magnetic field generated by the stator (the rotating speed is N1) and the rotor winding, the rotor winding cuts the magnetic induction line to generate the induced electromotive force. The induced current in the rotor winding acts on the magnetic field to produce electromagnetic torque, which causes the rotor to rotate. Because when the rotor speed is gradually close to synchronous speed, the induction current decreases, the electromagnetic torque generated is also reduced, when asynchronous motor works as a motor, the rotor speed is less than synchronous speed. In order to describe the difference between the rotor speed n and the synchronous speed n1, the slip ratio (slip) is introduced.
1 asynchronous motor starting mode
1.1 soft start
With the rapid development of microcomputer control technology, has successfully developed a number of electronic soft starting controller in control engineering field, is widely used in the starting process of the motor, the step-down starter will be replaced. Use the soft starting current electronic facilities are voltage circuit, thyristor regulating circuit, which is composed of the following description: thyristor six, 22 anti parallel connected to a three-phase power supply, the system sends the start signal, the microcomputer control system to calculate starter immediately, make the thyristor trigger delivery signal, the turn-on angle of thyristor control, according to the given output, adjust the output voltage, the motor control. The starting mode is suitable for three-phase AC asynchronous motors with various power values, including six and three connection modes.
1.2 direct start
This is the most simple way of starting the most basic way of starting motor, with the help of the first electric motors and are connected with a knife switch, the rated voltage of motor starting up and running, the features are: less investment, simple equipment, small number, although the starting time is short, but the torque is small, the starting current larger, more suitable for application in small capacity motor starting.
1.3 step-down start
Due to the disadvantages of direct starting, the start of the step-down start. For starting this starting environment for no load and light load of the two cases, the starting mode is also realized in step-down starting torque and starting current, so the starting after the end of the work needed to make the circuit back to the rated state.
There are three kinds of electric braking modes of three-phase induction motor: energy consumption braking, reverse braking and regenerative braking.
(1) the three-phase AC power supply is cut off when the motor is braking. To cut off the AC power of the moment, due to inertia, still according to the original direction of motor rotation, this method features are smooth braking, but high power DC power supply, DC motor, the required equipment cost, low speed small power system.
(2) the reverse connection is divided into two types: load reverse braking and power reverse braking.
(1) load reverse braking is called load reverse pull braking. This torque causes the weight to slow down at a steady rate. This is the reverse brake: no power, no need to brake equipment, but also can adjust the braking speed, but only suitable for winding motor, the rotor circuit on high resistance, the slip ratio is greater than 1.
2 when the motor needs to brake, as long as the switch between the two power lines, so that the rotation of the magnetic field can be very fast braking. When the motor speed is equal to zero, immediately cut off the power supply. The characteristics of this kind of brake are: fast parking, strong braking force, no braking equipment. However, due to the current braking, the impact is large, easy to make the motor overheating, or damage the parts of the transmission parts.
(3) regenerative braking, also known as regenerative braking, under the weight of the role (when the crane motor weight), the motor speed is higher than the rotational speed of the rotating magnetic field. At this time the rotor conductor induced current, the rotation of the magnetic field under the action of the counter rotating direction torque, but the motor speed is high, the need for speed reducer.