Principle and application of soft starter for the

2022-07-27
  • Detail

Principle and application of brushless automatic control motor soft starter

with the development of national economy, the production equipment is becoming larger and larger. The motor capacity of ball mill, crusher, fan, water pump, beater, rolling mill and other equipment in the industrial field is becoming larger and larger. Three phase wound rotor AC asynchronous motor is widely used in motors with large capacity and torque because its rotor can be connected with external resistance and frequency sensitive rheostat, and has high starting torque, starting power factor and small starting current

1 common starting methods and main problems of wound motor

common starting methods of wound motor include:

a. frequency sensitive rheostat starting cabinet: it introduces the rotor circuit of wound motor into the frequency sensitive rheostat starting cabinet on the ground through slip rings and carbon brushes. When the motor is started, the frequency sensitive rheostat is connected in series. With the increase of motor speed, the frequency of rotor current decreases, and the frequency sensitive rheostat impedance gradually decreases, The purpose of continuously limiting the starting current of the motor is achieved. After the motor is started, the frequency sensitive variable resistance shall be short circuited through the contactor or short circuit ring (some small and medium-sized motors are equipped with short circuit ring on the rotor). Since the frequency sensitive rheostat is actually an inductive element, it is thirsty for knowledge. It reduces the power factor when starting the motor. That is, to generate the starting torque of the rated torque of the motor, the starting current of the motor must be more than 2.5 times of the rated current of the motor. Therefore, it is generally used in fans, water pumps, air compressors and ball mill motors that do not require high starting torque. Its deformation product: brushless and ring less starter is to directly install the frequency sensitive rheostat on the motor rotor to realize the brushless operation of the motor. The brushless and endless starter has the following problems: (1) once the starting current is delivered, it cannot be adjusted. (2) The weight is more than 1.8 times of the soft starter of brushless automatic control motor, and the maximum power of the starter cannot exceed 500kW. (3) When the motor operates normally, the frequency sensitive rheostat is not short circuited, which will cause certain power loss

b. series resistance multistage starting cabinet of wound asynchronous motor, which also introduces the motor rotor current into the starting cabinet, and changes the resistance through time, current or cam controller, so as to increase the motor starting torque and reduce the starting current. However, due to the complex starting control device and the frequent switching of starting resistance during the starting process, resulting in multiple impulse currents, it can only be used on cranes, rolling mills and other equipment that require very high starting torque. Its variant product is: liquid resistance starting cabinet of wound asynchronous motor. It introduces the rotor circuit of wound asynchronous motor to the ground through slip ring and carbon brush, strings the liquid resistance into the rotor circuit of motor, changes the size of liquid resistance through servo motor, and achieves the purpose of stepless and continuous adjustment of motor starting torque and starting current. Compared with the cam controller, it has the following advantages: stepless and continuous adjustment of motor current; Disadvantages: the common liquid resistance starting cabinet of wound asynchronous motor does not consider the corrosive effect of liquid resistance on the pole plate and its transmission equipment, the sealing and explosion-proof of the device, and the impact of ambient temperature on the starting device, resulting in low service life, small application range, and high cost. It can not be installed in places with vibration (such as driving) and outdoors in the north

the cabinet starting equipment shall lead the rotor current to the ground control cabinet through brushes, slip rings, cables, etc. the carbon powder worn between the brushes and slip rings will reduce the motor insulation and cause motor failure. The slip ring and brush pressure spring are rusted, causing ignition between the brush and the slip controller ring. Because of higher requirements for motor maintenance. At the same time, the starting process of the motor is set artificially, which can not adapt to different loads; There are many components of the starting device, the secondary circuit is complex, the failure rate is high, the technical requirements for maintenance are high, and the comprehensive price of the driving system is high

due to some insurmountable defects of the above starting device, a brushless automatic control motor soft starter is developed to meet the requirements of users

2 working principle and structural characteristics of brushless automatic control motor soft starter

2.1 introduction to brushless automatic control motor soft starter

brushless automatic control motor soft starter is a device that directly installs the starting resistance on the motor shaft, uses the centrifugal force generated during motor rotation as the power, controls the resistance, reduces the motor starting current, increases the starting torque, and enables the wound asynchronous motor to realize brushless automatic control operation. The utility model is mainly composed of a casing, an electrolyte, a moving electrode plate, a spring, a terminal, a safety valve, an exhaust valve, etc. Its internal principle structure diagram is shown in Figure 1. When the motor is started, on the one hand, with the increase of the motor speed, the moving plate gradually approaches the housing under the action of centrifugal force, and the resistance in series into the motor rotor gradually decreases. When the rated speed is reached, it is short circuited with the housing, and the resistance drops to zero; On the other hand, the water resistance between the moving plate and the casing is heated by the current, and the resistance will gradually decrease under the action of the negative temperature characteristic of the water resistance. Under the action of the above two aspects, the motor is started with constant current and constant torque

2.2 working principle of soft starter of brushless automatic control motor

the control principle of soft starter of brushless automatic control motor is shown in Figure 2, which is equivalent to the variable resistance connected in series with the rotor of wound AC asynchronous motor. During the starting process of the motor, the water resistance can be adjusted through the following methods: the flame height can be easily adjusted:

a) change the distance between the electrode plates: with the increase of the motor speed, the centrifugal force increases, and the distance between the water and electricity resistance plates gradually decreases. After a period of delay when the motor speed reaches the rated motor speed, the distance between the water and electricity resistance plates will be reduced to zero

b) change the temperature of the water resistance: the temperature of the water resistance rises automatically after passing the current, and the resistance gradually decreases under the effect of the negative temperature characteristic of the water resistance

2.3 structural characteristics of the brushless automatic control motor soft starter

in order to ensure the long-term maintenance free operation of the water resistance on the motor rotor, the following technologies are applied in the overall design of the brushless automatic control motor soft starter:

1. The electrolyte with the following characteristics is selected as the water resistance: a. it has rust prevention effect on metals (copper and steel); B. After passing a large current, the electrolyte will not deteriorate, the electrode plate will corrode, and the chemical reaction of gas will not occur; C. The freezing point of the electrolyte is -25 ℃ and the boiling point is 120 ℃, so as to meet the requirements of safe work in different environments. 2. The starter adopts metal full sealing structure to minimize the number of sealing surfaces. 3. Set the safety exhaust valve. After the motor speed reaches the rated speed, open the exhaust valve automatically to ensure that the pressure inside the starter is consistent with the atmospheric pressure during normal operation; At the same time, in order to avoid long-term locked rotor of the motor

Copyright © 2011 JIN SHI