Introduction
today's industry, agriculture is undergoing tremendous changes. In the anti-show, the motor is essential, especially in three-phase asynchronous motor, has been an unprecedented development. Obtained under different load requirements in speed, so you have to speed. There are several speed control, speed control in different environments in different ways. This paper focuses on the slip speed change.
Chapter
a motor speed, AC drive system features
AC motor in 1885 years appears, because the governor has not been the ideal solution, can only be used in the constant drag area. Until the 1930s, many countries began to make a variety of AC motor speed of the original program.
emergence of thyristor AC motor speed control there is a leap forward, making use of semiconductor technology, communication speed can be achieved. .
AC speed control system, compared with the DC drive system, AC speed control system characterized by:
1, large capacity. This is the motor itself, the capacity of the decision, the DC motor unit capacity is generally achieved 12-14MW, while the AC motor capacity can be much higher than this value.
2, high speed high temperature. Determine the capacity of DC motor itself can only reach a maximum voltage of 1000V, the maximum voltage of AC motor is easy to reach 6-10KV or more. General DC motor can only reach a maximum speed of 3000r/min, the AC motor can turn up tens of thousands per minute, which makes the AC motor speed control system with high temperature and high speed.
3, AC motor size, weight, price is higher than the same volume of small DC motor, and motor construction is simple, rugged, economical and reliable, inertia, as a major advantage of AC speed control system.
4, AC motor, especially cage asynchronous motor speed control device environment adaptability.
5, communication device can significantly speed energy saving, widely used industrial fans, pumps, compressors class load is by AC motor to drag, these devices accounted for industrial consumption of electricity 50. None of the past, motor speed, while the anti-use baffles to control the air flow or throttle traffic, a lot of current is wasted; if the AC motor speed control system to change the air volume or flow, the efficiency will be greatly enhanced.
every point of view, changing the constant speed AC motor speed control for the AC motor, there is considerable energy efficiency.
Second, the motor speed
Figure 1 is a three-phase cage induction motor stator and rotor profiles. 6 small circles on the rotor shown in Table
into a closed loop from the rotor conductors. When the three-phase stator winding of three phase alternating current into, it will produce
a synchronous speed n, in the space of the magnetic field rotates clockwise. The beginning of the rotor does not move, so
rotor conductor lines on the back cutting magnetic force generated due to the rotor into a closed loop from the conductor, so there is current through the rotor conductor, the current direction can be right-hand rule to determine.
so-called load speed is the same in the case of employing the control method to change the motor speed to meet the needs of different production machinery.
three-phase asynchronous motor speed formula:
n = (1 s ) n1 = (1 s) 60f1 / p
where: f1 - three-phase AC frequency;
P - pole stator windings logarithmic;
n1 - rotating magnetic field speed, r / min;
s - slip.
be seen from the above equation, the following aspects can adjust its speed:
1, change the slip s speed, with lower voltage and winding the stator winding three-phase induction motor rotor circuit series resistance other method.
2, the motor stator winding pole pairs p speed.
3, change the motor power supply frequency f1 speed.
Example: Known Y2-112M-4 Three-phase asynchronous motor synchronous speed n1 = 1500r/min, rated slip sN = 0.04, find the motor rated speed.
solution: n = (1 s) n1 was:
nN = (1 s nN) = (1 0.04) × 1500r/min = 1400r/min
∴ the motor's rated speed of 1400 r / min.
three trends
present, a representative of AC thyristor speed control system with speed control system, cascade induction motor speed control system, frequency conversion adjustment speed system, no commutator motor speed control system, vector commutation control systems. With the emergence of a large number of new devices and calculator technology, AC variable speed system performance greatly improved, some problems have been more of this breakthrough. AC speed control system is widely used in the production of various fields.
Chapter
a slip speed, slip
three-phase asynchronous motor work, if the rotation speed increases the speed of the magnetic field, the rotor conductors and the relative motion between the rotating magnetic field disappears, the rotor of the electromagnetic torque is equal to zero. The actual speed of the rotor rotating magnetic field is less than the total n sync speed n1, there is a speed difference between them reflects the rotor conductors cut the line speed of magnetic induction. Therefore, commonly used in this speed difference n1-n n1 synchronous speed with the rotating magnetic field the ratio of said induction motor performance paul smith london, said the slip, with s, ie
; s = (n1 n) / n1
moment in the motor starter, n = 1, s = 1, turn maximum slip; as the speed increased, the slip decreases, when n = n1, when, s = 0. That is, s = 0 ~ 1 between changes. At rated load, small and medium sized asynchronous motor slip range is generally between 0.02 to 0.06.
Example: Y-160M-2-phase asynchronous motor rated speed nN = 2930r/min, f1 = 50Hz, p = 1, find the slip.
solution: the speed formula available
n1 = 60f1 / p
n1 = 60 × 50 / 1 = 3000r/min
by the s = (n1 n) / n1 available
s = (3000 2930) / 3000
; s ≈ 0.02
∴ The three-phase induction motor slip is about 0.02.
Second, the transformer speed
swing speed is by changing the voltage on the motor stator windings to achieve, the cage induction motor for speed control.
(a) of the stator windings
motor stator windings are part of the circuit. Its role is to pass into the three phase alternating current, a rotating magnetic field. Embedded in the stator windings of the coil on the stator core slot connection is made according to certain rules. Small three-phase asynchronous motor stator windings are usually of high-strength magnet wire wound on; and medium-sized induction motor is flat with enamelled copper wire or glass fiber package made of flat copper wire. Three-phase asynchronous motor stator windings of three phase windings, generally have six outlet end, placed outside of the junction box base, according to pick up a star (Y) or triangle (△) link, as shown in Figure 2 show.
three-phase stator windings different asynchronous motor main circuit. Asynchronous motor, such as electric power transmission from power after the stator windings in the way of electromagnetic induction transfer to the rotor, the mechanical power output by the rotor. Stator winding induction motor can also be considered the
(b) of the transformer speed
Figure 3, curve 1 is the three-phase induction motor stator rated voltage when the Ts curve. When the terminal voltage decreases the Ts curve in the curve in Figure 2 and 3. TL1 in the case of constant torque load, the lower the stator voltage, slip into the s1 s2 or s3. Slip over s3, the induction motor can not run stable. This shows a negative constant torque speed control method, the speed range is very small. However, the load on the fan (shown in dotted line TL2). The load torque and speed proportional to the square. From a wide speed range, speed and obvious, but the mechanical properties of very soft, for use with a load speed feedback control system to solve the speed stability problems. Note, however, the motor current issues that may arise.
speed regulator is a key device to provide power supply voltage changes, voltage regulation methods are currently used in series saturable reactor, autotransformer, and other types of thyristor regulator. The best way thyristor regulator.
(c) of the thyristor SCR is a silicon transistor
short, is the industry to achieve high-capacity power conversion and control of major power electronic devices. There are various types of thyristors, thyristor both the name of the family system on behalf of its general term, it represents a large and widely-used general thyristor . Since the power electronic technology developed, due to which almost no consumption of copper, iron materials, small size, light weight, inertia is small, easy to control. Therefore, the composition of the regulator with the thyristor automatic AC voltage regulator soon became the main form. Using three anti-parallel thyristors or three reverse thyristor motor stator voltage regulator, which is the thyristor AC voltage regulator.
1, thyristor thyristor structure
up points from the shape, bolt-shaped and flat-shaped with two structures, the thyristor rated current less than 200A only with a bolt-shaped, the adoption of greater than 200A flat shape. For Stud thyristor, thyristor anode bolt is A, it is closely connected with the radiator. Thick braid line is thyristor cathode K, thin braid line is the gate G. For the plate-shaped thyristor, it's two planes are the anode and cathode, while the thin braid line is the gate. Use two mutually insulated thyristor heat sink to tightly clamped together. Inside there is a thyristor semiconductor material made of silicon die. The die is a circular sheet, which is four (P, N, P, N) three-terminal (A, K, G) device (Figure 4), which determines the SCR performance.
2, thyristor thyristor works on and off
the law is as follows:
① When the thyristor withstand reverse anode voltage, regardless of what the gate withstand voltage thyristor are in the OFF state.
② When the thyristor forward voltage tolerance, the only gate to bear in the case of thyristor forward voltage to turn-on, pressure is sunny and positive gate voltage between the two are indispensable.
③ Once the thyristor conduction, the gate loses control action, regardless of the gate voltage is positive or negative, the thyristor remains on, turn the control signal so only the forward pulse voltage trigger pulse is called can.
④ To turn off thyristor, the forward voltage of the anode must be removed or added to the anode back pressure or reduce the positive anode voltage, the current through the thyristors to reduce to a certain value or less. Thyristor to maintain the minimum current, called the holding current.
⑤ not increase when the gate trigger voltage thyristor with a forward blocking capability, it is generally not available in the diode.
three-phase delta connected thyristor regulator circuit
The circuit uses only three thyristors burberry accessories, which makes the circuit structure and control is more simple and economical. Thyristors in the load back, it can reduce the impact of power surges. However, the circuit requires a star connected motor stator windings and the neutral point can be open (Figure 5). This circuit is the load on even harmonics, the same motor adverse thyristor high voltage requirements than the star join.
is characterized by its speed:
(1) regulator speed line is simple, easy to implement automatic control;
(2) regulating the process of heat transfer slip to the form of consumption in the rotor resistance, the transfer rate is low;
(3) speed regulator for 100kw following general production machinery.
Third, the variable resistance speed
rheostat speed control circuit by changing the external rotor resistance to achieve, it applies only to wound rotor motor.
one rotor winding rotor winding
role is to generate current, and under the action of rotating magnetic field leaving the rotor electromagnetic torque. According to the structure of the different cage rotor winding and the winding is divided into two major categories. Cage rotor structure can be divided into cast copper bar rotor and the rotor into two categories, of which the most frequently used cast aluminum rotor, it is to cast molten aluminum centrifugal method or pressure cast aluminum made the legal system, while the copper bar rotor is in each tank has a bare copper strips, the core out of the notch out both ends, two end ring all the copper strips are shorted together to form the structure shown in Figure 6.
increase can be seen from Figure 7, the rotor circuit resistance,
Ts curve down. If the load torque constant,
different series resistance in the rotor, its not the same slip,
in series resistance, slip increases accordingly, the speed drop
Low, to achieve speed purposes.
this speed control method is constant torque speed characteristics, the electromagnetic torque T is known
; T = CTФmI2cosφ2
when the supply voltage U1 is constant, the main flux Фm basically planting, the rotor current I2 can be maintained in the ratings work About cosφ2 derived as follows:
rotor current
< br> From the above equation shows, series resistance of the rotor speed control loop, if the rotor current is rated to keep planting that I2 = I2N, there must be
;
relationship. Where Rs is the series resistance of the rotor circuit.
rotor circuit series resistance Rs of the power factor considerations
rear
shows, series resistance of the rotor speed control loop is constant torque speed control method.
speed range of this method is not speed, maximum speed is the motor rated speed, the lower limit allowed by the static slip limit, the actual speed range can be up to (2-3) 1. Load hours, speed range even smaller.
As the rotor loop current, resistance boxes are bulky, not easy to tap, so the smoothness of bad timing.
speed when inefficient. In the constant torque speed, the electromagnetic power PM = TΩ1 if the constant, sPM power are consumed in the rotor circuit, very economic, the lower the speed, the situation is more serious.
this speed control method is used in the production of mechanical intermittent work, they run at low speed is not long paul smith usa, static gap is also less demanding, such as bridge cranes, rolling mill auxiliary machinery.
Conclusion
after learning of three-phase asynchronous motor, the continuously enrich their theoretical knowledge, graduates will be designed by the theoretical knowledge learned. First use and practice, not only to improve their ability to work, but also found his own shortcomings and to further their study in the future pointed out the direction, so that their exercise capacity, over time, constantly and assiduously learning, and constantly enrich and improve themselves. In the logic function and application of knowledge, the only way to make their work better in the future for the community, serving the people.
article from the three-phase induction motor slip speed aspect to introduce. The so-called load speed is the same in the case of employing the control method to change the motor speed to meet the needs of different production machinery. Now speed has been widely used, the AC motor better applied to various fields, is the various machinery industry than indispensable. But in the thesis, due to their own capacity limitations and less time in the design there are many gaps, there is still need for improvement still, hoping the teacher more suggestions, please exhibitions! I will try to improve their ability to make their own higher level.
Xieci
In the graduation project, I get the Tang Shengpei enthusiastic guidance and help teachers, guidance from graduation I began to access the relevant literature, have been concerned about the progress of the entire graduation, I made the difficult time to give help and guidance, so I was able to smoothly complete my graduation. I thank the school out of the university campus in the track by giving me a such a good training opportunities, so that my expertise has been consolidated and developed, but also enhance their own ability, which makes me even more for future work to full confidence. In this, the other teachers also received care and help, thank you for taking the time in his busy schedule to review my thesis. Sincerely say this: You have worked hard!
References
[1] Dude Chang editor of Edition)
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