Study on Soft-start and Optimization Energy-saving Control Technology of AC Asynchronous Motor

At present, a large number of AC asynchronous motors are used in industrial and mining enterprises, including low-voltage motors and medium-voltage motors. A considerable number of asynchronous motors and their driving systems are still in a non-economical state of operation, wasting a large amount of electrical power in vain.

The reason for this is roughly caused by the following conditions: Since most motors use direct starting methods, in addition to causing shocks and accidents to the power grid and the drive system, double the starting current also causes huge energy losses (2 ) When carrying out motor capacity matching, one-sided pursuit of a large safety margin is often unavoidable, and the layers are overwritten. As a result, the motor capacity is too large, resulting in the phenomenon of large horse-drawn carts, causing the motor to deviate from the optimum operating point, and operating efficiency. Power factor reduction Considering the operating economy of the production machine driven by the electric motor, the electric drive system is often required to have a variable-speed variable-speed adjustment capability. If the constant-speed constant-speed drive is used, it will inevitably cause a large amount of extra power loss.

The non-economic operation of the motor has long attracted the attention of the relevant state departments, and has formulated and revised a mandatory national standard-three-phase asynchronous motor economic operation.

It is hoped that the economic operation of three-phase asynchronous motors will be regulated accordingly, and the publication of the national standard will greatly promote the economic operation of low-voltage motors, but it will have little effect on medium-voltage motors.

The reason is that the medium voltage motor generally has a large capacity. Once a failure occurs, its impact is also large. Therefore, the requirements for the reliability of power saving measures are even higher. The power saving measures for medium voltage motors are limited by the withstand voltage level of power electronic power devices. The development of electrical products is technically more difficult.

At present, there are no domestic fixed-voltage medium-voltage motor soft start and power-saving products available.

The soft start of the asynchronous motor has been increasingly demanding for the starting performance of the motor due to the constant updating and development of the industrial production machinery. The induction of the motor in the following aspects has been required to be large enough and can be steadily increased. Moment and compliance with the required mechanical characteristics The starting current of the starting device is as simple and economical as possible. The start-up operation is easy. The power loss during the start-up process should be as low as possible.

According to the above contradictory requirements and the actual situation of the power grid, there are two types of starting methods that are commonly used in the direct starting mode under the rated voltage, and the other is the step-down starting mode.

Direct start hazard The direct start is the simplest start method. When the starter, the motor is directly connected to the power grid through a guillotine blade or a fish.

The advantage of direct start is that the starting equipment is simple and the starting speed is fast, but the direct starting harm is very big. The net impact is too large starting current (the no-load starting current can reach twice the rated current, and can reach double or more when loaded and started. The conference caused the grid voltage to drop, affecting the normal operation of other electrical equipment, and may also cause the undervoltage protection to act, causing the equipment to trip.

At the same time excessive starting current will cause the motor windings to heat, thereby accelerating the insulation aging, affecting motor life.) Excessive mechanical impact torque often causes the motor rotor cage end ring to break and the stator end winding insulation wear, resulting in burn Machine, twisting of the shaft, damage to the transmission gear of the coupling, and tearing of the belt, etc. 3) Impact on the production machinery Pressure change in the starting process often results in damage to piping valves of the pump system, shortening the service life, affecting the transmission accuracy, and even affecting the normal process. control.

The safe and reliable operation of all these starting devices poses a threat, and at the same time it also causes excessive start-up energy losses, especially when frequent starts and stops.

Therefore, there are the following restrictions on the direct starting of the motor. The production machine is allowed to directly start the motor by dragging. This is the prerequisite that the capacity of the motive should not be greater than the voltage drop in the process of the power transformer and the voltage drop should not be greater than the rated voltage for medium and high power. Motors are generally not allowed to start directly, but require a certain starting equipment, before they can complete the normal starting work. The application of the old-style step-down starting method and performance comparison The purpose of step-down starting is to reduce the starting current, but it also makes The starting torque has dropped.

For heavy-duty start-ups, production machines with large peak loads cannot be started in this way.

Traditional step-down starting has the following methods. Star Delta Converter This method is suitable for motors with stator windings connected in normal operation.

There are six joints of the stator lead out, received the transfer switch, start with a star connection method, after the start of switching to connect.

The starting voltage is 2 operating voltages. The advantage of this starting device is that the starting device is simple and the energy consumed during start-up is low.

The disadvantage is that there is a secondary current impact, the equipment has a high failure rate, and it requires frequent maintenance, so it is not suitable for use in equipment that is frequently started.

During the conversion process, the potential generated by the transient potential and the remanence of the motor is often out of phase with the power supply voltage. In severe cases, a voltage is added, causing excessive inrush current and electromagnetic torque, thus greatly limiting it. usage of.

Since the starting voltage is the hall of the operating voltage, the starting torque for the rated torque can only be used in equipment with no-load or light-load loading ratio less than the starting.

In the light-load or no-load operation of the motor, the starting device can also be used for step-down operation to increase the power factor and efficiency of the motor.

The three-phase self-destructive transformer (also called the compensator) of the step-down transformer of step-down transformer is connected to the high-voltage side and the low-voltage side is connected to the motor. Generally, there are several taps, and different voltage ratios can be selected, relative to different starting torque loads.

Remove the motor after starting it.

The advantage is that the starting voltage can be selected, such as or to meet the requirements of different loads.

Disadvantages are large size, heavy weight, and high consumption of non-ferrous metals, high failure rates, and high maintenance costs.

Magnetron soft starter magnetron soft starter is the use of magnetic limiting voltage regulation principle, in the motor starting process, the voltage can be smoothly raised from a lower value to full pressure, so that the motor shaft torque uniform increase, Soft start characteristics and soft stop.

But its control voltage is in the left and right, the user can not adjust, there will be a larger current impact, and a larger size.

Series electro-hydraulic or water resistance For high-voltage motors, a step-down start can be achieved by connecting a reactor or a water resistor in series on the stator circuit. After the start is complete, remove it.

However, the cost of the reactor is high and the loss of the water resistance is large.

The series-connected frequency sensitive rheostat or the hydropower team can start the series-connected frequency-sensitive varistor or water resistance in series with the rotor-type asynchronous motor, and then remove it after the start is completed.

However, frequency-sensitive varistors have high costs, and water resistance losses are large.

There are other methods such as starting the Yanbian triangle and starting the stator string resistance.

It is worth pointing out that although various old-style step-down starting methods have their own advantages and disadvantages, they have a common advantage that there is no harmonic pollution.

New type of electronic soft starter With the development of power electronic technology and microcomputer control technology, a series of electronic start control devices have been developed at home and abroad for the start control of asynchronous motors to replace the traditional step-down starting devices.

The main circuit of the new type of electronic soft starter generally uses a thyristor voltage regulator circuit, and consists of six thyristor regulators, two or two antiparallel, and is connected in series to the three-phase power supply line of the motor.

When the microcomputer control system of the starter receives the start command, it carries out relevant calculations and outputs the trigger signal of the thyristor. By controlling the conduction angle of the thyristor, the starter adjusts the output voltage according to the designed mode to control the starting process of the motor. .

When the start-up process is completed, the starter normally pulls the bypass contactor and short-circuits all the thyristor main circuits so that the motor can directly enter the grid operation to avoid unnecessary power loss. The control block diagram of the soft starter is shown in the figure. Show.

The so-called soft start is actually a step-down starting process according to a preset control mode.

Current soft starters generally have the following types of starting methods. Current limiting soft start current limiting starter As the name suggests, during starting of the motor, the starting current is not limited to a certain set value. Next to the research on soft starting and optimization energy saving control technology of AC asynchronous motors. Road contactor (normal operation) two or two people r sweep into a few voltage detection drive circuit current detection computer fuzzy control system keyboard display power several slope starting ginseng Huajing II" a corpse less than 1112 Hucheng sulfur door Such as r to introduce the soft starter control diagram of Hutu soft starter).

Mainly used in step-down starting of light-load-started loads, its output voltage increases rapidly from zero until its output current reaches a preset current limit, and then gradually increases voltage while maintaining the output current until it reaches the rated value. Voltage, so that the motor speed gradually increased until the rated speed.

The advantage of this starting method is that the starting current is small and can be adjusted as required. The starting current limit must be set according to the starting torque of the motor. If the setting is too small, the starting will fail or the motor will be burned. ) The impact on the grid voltage is small.

The disadvantage is that it is difficult to know the starting pressure drop at the time of starting, the pressure drop space cannot be fully utilized, the starting torque is lost, and the starting time is relatively long.

The voltage dithering and starting output voltage rises linearly from small to large ramps. The traditional step-down starting is graded as stepless, mainly used in heavy-duty starting.

Its disadvantage is that the starting torque is small, and the parabolic type rise of the torque characteristic is disadvantageous to starting, and the starting time is long, which is unfavorable to the motor.

The improved method is to use double ramp to start the output voltage and quickly rise to the voltage value corresponding to the minimum torque required for starting the motor, then gradually increase the pressure at the set rate until the rated voltage is reached.

The initial voltage and voltage rise rate can be adjusted according to the load characteristics.

This starting method is characterized by a relatively large starting current, but a relatively short starting time, suitable for heavy-duty starting motor.

(3) Torque control starting is mainly used in heavy-duty starting. It controls the output voltage according to the law that the starting torque of the motor rises linearly. Its advantage is that the starting smooth and flexible is good, it is beneficial to the drive system, and at the same time reduces the power grid. Impact is the best way to start a heavy load.

Its disadvantage is that the starting time is longer.

(4) Torque plus sudden jump control Starting torque plus jump control start Same as torque control start is also used in heavy load start.

The difference is that with a sudden jump torque at the instant of starting, the static torque of the drag system is overcome, and then the torque rises smoothly and the start time can be shortened.

However, the jump will send a sharp pulse to the grid, interfere with other loads, and should be used with special care.

(5) Voltage control Starting voltage control start is used in the occasion of light load starting, the maximum starting torque of the motor is guaranteed on the premise of guaranteeing the starting pressure drop, and the starting time is shortened as much as possible. It is the best light load software. Flying diagram of various soft start waveform (a) current limit starting voltage ramp slope) torque control start (d) torque plus jump control start voltage control start method.

The corresponding starting curves of various soft-starting methods can be seen in the following three parking modes: free parking, soft parking, and brake parking.

The biggest advantages brought by the soft starter are soft stop and brake stop. The soft stop eliminates the anti-inertia impact of the drive system. For the water pump, it is the water hammer effect brake parking, which replaces the reverse brake parking function in certain situations.

Soft starter and traditional step-down starter comparison Soft starter and traditional step-down starter performance comparison table soft starter applicable equipment production equipment precision, does not allow start shock, otherwise it will cause adverse consequences of production equipment and products The power of occasional motive is relatively large, if it is directly started, occasions where the capacity of the main transformer is required to be increased) the voltage fluctuation of the power grid is strictly required, and the power supply system for the voltage drop requirement is not required for the starting torque, and the no-load or light load may be performed. Starting equipment.

Strictly speaking, the starting torque should be less than the rated torque of the drive system, it is suitable to use soft starter to solve the problem of starting shock.

For equipment requiring heavy load or full load start, if soft starter is used to start, it will not only fail to reduce the starting current, but will increase the capacity of the thyristor of the soft starter and increase the cost. If the operation is improper, the thyristor may be burned. .

Only frequency conversion soft start can be used at this time.

Because the soft starter is not frequency modulated and the slip power is always present, excessive starting current is unavoidable and the frequency converter adopts the frequency modulation and pressure adjustment method to achieve no-over-current soft start and can provide twice the rated torque. Moments, especially suitable for heavy-duty starting equipment.

But the price of the inverter is much higher than the price of the soft starter.

Asynchronous motor economic operation and optimization of power saving control technology Asynchronous motor step-down power-saving technology Overview For full-load or heavy-duty operation of the motor, reducing its terminal voltage will have serious consequences, as the terminal voltage decreases, the motor's magnetic flux and electromotive force With this reduction, iron consumption will undoubtedly decline.

At the same time, however, the torque of the motor that changes with the square of the voltage also decreases rapidly and is less than the load torque. The motor can only increase the slip rate and increase the electromagnetic torque so as to balance the load torque.

The increase in the slip rate causes the increase of the rotor current, and at the same time, the phase angle between the stator and the rotor voltage increases, resulting in an increase in the stator current, so that the increase in the copper loss of the stator and the rotor is greater than the decrease in the iron loss. The temperature rise of the motor windings will increase, the efficiency will drop, and even motor burns.

Therefore, the general rules stipulate that the voltage variation range during the normal operation of the motor must not exceed 95-1 of the rated voltage. However, for the light-load operation motor, the situation is very different, so that the supply voltage is properly reduced, which is economically advantageous.

This is because when the light-load operation, the actual slip of the motor is much less than the rated value, the rotor current is not large, the value of the increase of the rotor current is limited during the step-down operation.

On the other hand, due to the reduction of voltage, the comparison between the no-load power meter soft starter and the traditional voltage reducing starter is the main performance of the digital soft starter magnetron voltage reducing starter self-power reducing starter starting characteristic soft characteristics The user can adjust the starter power, which is harder to adjust than the hard characteristics cannot adjust the starting current characteristic curve.

Stream 2 is a lean current, not adjustable. N two: Ai Shaoyi's book Ren II under the bundle head, the starting voltage is arbitrarily adjustable left and right users can not adjust the left and right users can not adjust the number of starting impact current without a few times, about the rated current of the motor, about the rated current of the motor The doubled starting current can be viewed by the user more than one minus eight. It cannot be adjusted more than one. It can not adjust the load to adjust the torque characteristics of the motor without impact torque. After the constant torque of the moment, the torque is increased. , There is a second flush, a smooth ramp, a smooth ramp, and a steeper ramp. Knows the load. Stronger adaptability. Normal poor. Can it be frequently started. Can normally not generally. Cannot be started. Current limit Soft start or voltage ramp. Constant current soft start. Subsection constant voltage start. Components Power Electronics Magnetic Saturation Reactors (Magnetic Amplification Autotransformer Transformer Control Components and Control Positioning High-Performance Monolithic Computer Relays and Common Electronic Components Relay Type Fuzzy Control Relay Electronics Control Relay Control Machine Weight Weight Lighter Smaller Heavyer Larger The number of major external cables Root (or out) root or root (w out of the root) In and out) AC asynchronous motor soft start and excellent Energy-saving flow control technology research and core loss significantly reduced.

In this case, the total loss of the motor can be reduced, the stator temperature rises, and the operating efficiency and power factor can be improved at the same time.

From this, it can be seen that the economical efficiency of the motor and the load ratio of the motor are in great relation to the reasonable matching of the operating voltage.

The theoretical analysis shows that there is the following relationship between the power efficiency index of the motor and the power factor and the terminal voltage of the motor. Shanghai Jinyuan only climbs the Wing Shield Gas 3 1-2 B Condition and step-down running slip A and B are the power factor of the rated condition of the motor and the power factor of the step-down operation. Knife and knife are the rated condition of the motor and the efficiency of the depressurization operation is the regulating coefficient of the motor. The actual voltage during voltage and step-down operation is the no-load current factor of the motor, while the sum is the rated current and no-load current of the motor).

It is difficult to see that not all buck behaviors can achieve the purpose of power saving. Only when the voltage drop is greater than the slip rate and the increase of power factor can the operating efficiency be improved.

In fact, the relationship between motor efficiency and voltage change is a saddle-shaped curve. Corresponding to each output power (or load factor), there must be an optimal voltage regulation coefficient. When the second time, the motor has the lowest loss and the highest efficiency.

This is called the optimal voltage regulation factor for the motor.

The best voltage regulation coefficient under different load can be determined by the following formula according to the load coefficient of the motor knife: When the arts and craftsmanship of Chuanyi Art Corp. is the rated load of the motor, the active loss will be determined.

The no-load loss of the motor is calculated as the coefficient of the mechanical loss of the motor for the lack of dead bodies (monthly load factor of the motor, port (eight dead bodies for the output power of the motor, and the dead body for the motor rated power literature gives a light load After the motor adopts step-down power-saving measures, the formula for saving electric energy is to save the active power of the dead body (there is a saving power of reactive power for a body and a Q.) The dead body hits the mouth.

Bu Zhongkou is the reactive power when the motor is rated load, and the power saved by the no-load reactive power of the motor is the power of the second pass. It is the reactive economic equivalent when the motor is directly connected to the motor busbar. Transformer takes from two or three times the voltage change from one to the running time of the motor (Optimized power saving control based on the rate factor (liner control method) The earliest occurrence of the asynchronous motor optimization Power Saver is the Shanghai power factor controller, the principle is through detection The Shanghai value during the operation of the motor is compared with the preset reference value. When the actual value is lower than the set value, the motor is lightly loaded. The motor is increased by reducing the terminal voltage of the motor until it is actually measured. When the value reaches the set value, achieving a high power saving value indicates a heavy load, the motor terminal voltage is increased to ensure the output power on the shaft.

This is a kind of indirect power saving control object is the power factor of the motor, and the purpose is to save electricity.

Since the best power factor of the AC asynchronous motor changes in a curve over the entire working range, the power factor of an asynchronous motor of the same specification produced by a manufacturer is a certain degree of dispersion. The same motor is at different stages of its life, under the same conditions. The power factor also exhibits a certain degree of dispersion, which brings certain difficulties to the design and adjustment.

Therefore, this method cannot achieve the best power saving effect, and both theory and practice have proved that excessive power factor values ​​do not save power for asynchronous motors.

The minimum input power method When the AC asynchronous motor is working, part of the electric power input from the grid is converted into eight outputs of the mechanical power on the motor shaft, and the other part is its own loss corpse, including iron consumption and copper consumption.

The iron loss is proportional to the square of the input voltage, and the copper consumption is proportional to the square of the current. Only when the copper loss is equal to the iron loss, the motor has the highest efficiency and the loss is the lowest.

The principle of the minimum input power method is to reduce the loss of the motor by reducing the motor's terminal voltage under the premise of guaranteeing the mechanical power output of the shaft at any load point of the motor operation, thereby achieving the purpose of energy saving.

Although buck can reduce iron loss, when the voltage drops to a certain level, if it continues to decrease, the current will increase again, which will increase copper consumption.

Through the automatic optimization of the microcomputer, the iron consumption and the copper consumption are kept at the lowest level, that is, the product of the voltage and the current input the human's electric power to reach the minimum value, so as to achieve the goal of optimal power saving.

(3) Sudden load control When the load on the motor shaft rises sharply, it must be able to raise the voltage to the rated value in a very short period of time to ensure that the fishes in the shaft should bear the burden of the month. Turn over enough power output, otherwise the motor will stall.

Therefore, when the microprocessor performs the input power optimization control, it also monitors the rate of change of the load power. Once the rate of change of the load power exceeds the preset threshold, it is determined as a sudden increase in load and the motor terminal voltage is immediately increased. The ability of the motor to quickly respond to load changes.

The object of optimizing the energy-saving application is that there are no strict requirements on the motor speed and occasions where speed-regulating operation is not required, especially for loads that often change drastically, or for motors that are light or no-load for long periods of time, such as forging presses for rolling mills. Oil machines and other loads, using optimized power saving technologies, can receive significant power savings.

Its power consumption depends on the load factor of the motor and the length of time for light-load operation.

Step-Down Start-up Optimized Power-Saving Calculation Example An optimized control system for a type of motor with light-load operation is designed to calculate the start-up performance parameters and the power-saving effect.

The original data rated power of a type of motor Corps, two rated voltage two rated current people two rated speed maximum torque multiple two maximum torque rated torque starting current multiple locked rotor current rated current two starting torque multiple stall torque rated Torque two rated efficiency knife rated power factor Shanghai, two motor rated load active loss art, two motor no-load loss.

First of all, the external characteristics of the motor must be changed. The new external characteristics must enable the motor to have a wide and stable speed range.

In general, high slip motor, AC torque motor, or series resistor in the rotor winding of a wound motor must be used, and speed closed loop control must be added in order to achieve stable speed regulation.

Secondly, it is necessary to export the temperature rise of the rotor caused by the slip power during the speed regulation process, so as to achieve long-term stable operation.

Here, a rotating heat pipe structure may be adopted, and a special air duct cooling structure may also be adopted, which is an effective method.

In today's rapid development of power electronics technology, the price of frequency conversion speed control devices is no longer expensive, and then consider voltage regulation and speed regulation, which seems to have little practical significance.

With pump control function, it can avoid or reduce the flow surge and water hammer effect. It has phase balance and automatic compensation of power voltage. It has perfect protection alarm function. Starting mode Starting voltage Starting current Rated current and load type can be set With remote control and networking communication function self-diagnosis function.

After long-term use at different industrial sites, considerable economic benefits have been achieved.

Conclusion Based on the long-term in-depth research on AC induction motor soft-start and optimized power-saving technologies, the intelligent motor optimization controller series has successfully developed the intelligent motor optimization controller series. The adaptive motor power is applied to a single controller. The single-chip micro-controller for motor control has perfect detection and control functions. The main power device adopts a globally high-tech patented product integrated phase-shifting control thyristor module. This module breaks through the concept of conventional thyristor modules and introduces complex phase-shift control. The circuit and the thyristor die are creatively integrated to form a complete open-loop system with power phase-shift regulation.

The controller made of it not only greatly reduces the volume, but also increases the reliability and anti-jamming capability of the equipment.

In terms of technology, it is the longest of all public homes, and it greatly breaks the function of similar products at home and abroad. In addition to start protection and energy saving, it also increases the speed control function of fan and water pump load, and saves power during intermittent operation of pumping units. , Reactive power compensation in place.

In particular, the perfect protection function over-current over-voltage overload short-circuit grounding phase-to-phase imbalance and power module over-temperature and motor over-temperature protection and other functions, is the safe and economic operation of the protection of the god.

The controller has the following features: microcomputer intelligent control, keyboard setting, digital display, simple and intuitive start, soft stop function, effectively reduce the starting shock to optimize the motor operation mode, save power and improve the power factor fan and pump load adjustment Pressure-adjustable closed-loop control function (l) The electronic soft starter has a simple structure. Compared with the traditional starter, the self-destruction transformer starter has no contact, no noise, light weight, small size, and the starting current and starting time can be controlled. The start-up process is smooth, and the maintenance workload is small.

When the motor is unloaded or lightly loaded, the energy-saving effect is remarkable, and it is particularly suitable for short-term full-load, long-term no-load loads.

For high-slip motors, solid rotor motors, torque motors, etc., especially in the case of fans and pumps, there is a good speed control performance, but not for ordinary cage motor speed control.

Adopting intelligent controller, it has perfect motor protection function, convenient setting of protection setting value and reliable protection performance.

Its biggest shortcoming is that due to the use of thyristor phase-shifting control, harmonic disturbances to the power grid and the motor are present.

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