Výroba cementu
SLANVERT's Solution to Main Drive in Cement Rotary Kilns
1. System Driving Scheme:
The inertia of the rotary kiln is very large and the start-up current is large. During start-up, the rotary kiln starts rotate slowly and the material inside the kiln is brought to the other side with the rotation of the kiln. Under the effect of gravity, relative motion happens between the material and kiln wall and friction force is generated. With the increase of the rotational speed before it reaches the working speed, the amplitude of the relative motion between the material and the kiln wall increases and the generated friction force increases accordingly. At the time, the motor current increases considerably; when the inverter is adjusted to 10~13Hz, the current is twice the rated current of the motor. The rotational speed of the kiln keeps increasing. With the increasing centrifugal force, the material is brought to the highest point within the kiln. Under the effect of gravity, the material “pours and falls” from the highest point and the motor current decreases to a value less than the rated current. Considering that the inverter has an overload ratio of 150%, it is recommended that SLANVERT SB70G90KW Inverter be selected instead of an inverter with twice the rated current of the motor. This can both ensure normal start-up and reduce investment.
2. Working Status and Energy-Saving Performance:
After the main drive of the rotary kiln is altered to inverter drive, the start-up has been very easy and smooth and the kiln can operate safely when it is adjusted to a speed as low as 80r/min. Moreover, the energy-saving performance is outstanding. The energy-saving ratio can be calculated using formula (2) ~ (7), i.e.:
The output power of the main motor shaft of the electromagnetic motor:
P0=T0•n0 (2)
Where
T0—shaft output torque of the motor
n0—motor output shaft rotational speed
Output power of the slip clutch:
P1=T1•n1 (3)
Where
T1—output torque of the slip clutch
n1—the output shaft rotational speed of the slip clutch
The shaft output power of the motor is the shaft input power of the slip clutch. For constant torque load, T=T0=T1=constant, thus the efficiency of the slip clutch:
(4)
The slip ratio can be calculated using the following formula:
(5)
It is obtained that 
(6)
From this we can see that for a constant torque load, the efficiency of the slip clutch is proportional to the output speed. When the rotational speed decreases, the output power decreases proportionally and the input power remains unchanged. At the same time, the loss power Ph increases in proportion to the slip loss, i.e.:
(7)
The electromagnetic speed regulation motor originally has a rated rotational speed of 1,440r/min, works at a speed around 1000r/min and low-speed operates at a speed more than 200r/min. If the rotational speed is too low, the electromagnetic speed regulation torque will be insufficient. Using formula (7), we can calculate the loss power of the electromagnetic speed regulation motor at a rotational speed of 1,000r/min and further calculate the energy-saving ratio, which is 44%. Actually, according to the measurement and calculation of the cement plant, the energy-saving ratio is as high as 50%.
Frequency Conversion Solution Using SLANVERT High-voltage Inverters for Kiln Outlet Exhaust Fans
1. Characteristics and Performance of High-voltage Inverters
1.1 Design features SLANVERT high voltage frequency conversion speed regulation equipment adopts the advanced non-trip high-performance VVVF control or vector control technologies and has the following characteristics and functions:
(1) In design, the actually situation of grid supply has been taken into consideration and the range of fluctuation of input voltage has been increased by around ±15%, which effectively solved the reliability problem caused by voltage fluctuation and grid input harmonic.
(2) The individual bypasses of unit modules are designed and realized in such a manner that if one module fails, the unit bypass will exit the whole system. By reducing the rated rotational speed, the uninterrupted running of loads such as fans and pumps can be ensured.
(3) Complete machine bypass design. In case of a complete machine failure, the inverter can, within milliseconds, automatically switch from the frequency conversion status to the main frequency status (50Hz) or can be switched to the main frequency status manually.
(4) It has various types of protection at electric and electronic levels, such as overload, over-current, short-circuit, unbalanced three-phase and overheat protection, etc. In particular it has the “high-voltage power loss protection and re-start-up and running in 3 seconds” function to meet the special requirements of users such as power plants for high-voltage inverters.
(5) Standardized unit module design, which significantly facilitates maintenance and replacement, enables the replacement of failed power modules in several minutes and thus significantly increase equipment efficiency.
1.2 Product performance Low-voltage IGBT is used as the major power device. It is a high-voltage frequency conversion device based on the multilevel series connection technique. In the technical scheme, low and medium voltage IGBT power elements and devices (1200-3300V) are adopted as mature products. Judging from the technical performances, parameters of the elements and devices and from the actual application situations, they can meet the various requirements for functions and environment of the product and ensure the reliability of the product as required by its user.
The power units of the high-voltage frequency conversion speed regulation system are identity in structure and are interchangeable. It is the basic AC-DC-AC single-phase inversion circuit. The rectifier side is diode three-phase full bridge, which carries out PWM control for the IGBT inversion bridge.
On the input side, phase-shifting transformer unit is used to feed each unit. The secondary windings of the phase-shifting transformer are divided into 3 groups for each phase, forming an 18-impulse rectifier system. The rectifier system of multilevel phase-shifting superimposing can greatly improve the current waveform on the grid side and make the grid side power factor under its load close to 1.
Moreover, due to the independence of the secondary windings of the transformer, the main circuits are independent in relation to each other, which are similar to conventional low-voltage inverters and facilitate the adoption of existing mature techniques.
2. Analysis of Operational Characteristics of Frequency Conversion for Kiln Outlet Exhaust Fans in Cement Plants
(1) In cement production process, the air flow of the kiln outlet exhaust fan, the pre-heater high temperature fan and the kiln inlet exhaust fan frequently needs to be adjusted to meet the requirements of the production process. With variations in kiln conditions and in the production process, the air flow needs to be adjusted. The air flow of the kiln outlet exhaust fan is adjusted by adjusting the size of the throttle, wasting a lot of energy on the air damper.
(2) The method of adjusting the kiln outlet fan throttle controls the variation of the amount of induced air, controls the micro-negative pressure of the kiln outlet and the adjustment precision is poor. The degree of openness is not proportional to the flow and cannot fulfill the requirement of the process. In additional, the start-up current and noise are large and the working efficiency is thus low.
3. Example of Transformation Effect
The rated motor parameters of the kiln outlet exhaust fan on the production line are as follows: the rated power is 280kW; the rated voltage is 6,000V; the rated current is 36A; the rated rotational speed is 583rpm and the power factor is 0.814.
That means the investment on equipment can be recovered in around 25 months and the energy saving is 55%.
SLANVERT's Solution for Cement Pipe Manufacturing
1. Project Overview
Pipe Factory, has 4 centrifugal fillers. There are two 75KW motors and two 55KW motors, all of which adopts electromagnetic speed regulation. In order to change to frequency conversion speed regulation, the electromagnetic speed regulation motor is replaced with regular Y series three-phase squirrel-cage motor. Two SLANVERT SB70G75KW inverters and two SLANVERT SB70G55KW 2 inverters are used. The inverter is set to externally controlled terminal operation and the potentiometer is used to adjust the frequency of the inverter. Long accelerating and deceleration periods are set, which are both 120s. Start DC braking when the speed is decreased to the output frequency 5Hz of inverters. The operation is very simple. After using the inverters, the rotational speed becomes stable and the forming quality of the pipe piles is greatly improved.
2. Analysis of Energy Saving Effect
The cement pipe manufacturing machine uses 55kW or 75kW electromagnetic speed regulation motor with a rotational speed normally restricted to 300-1200r/min. This is a case that the motor speed is controlled according to the process of centrifugal pipe manufacturing. Practical experiences show that by replacing electromagnetic speed regulation with frequency conversion speed regulation, an average energy saving ratio of around 50% can be achieved. The output power of the main motor shaft of the electromagnetic motor is as indicated in formula (2):
P0=T0•n0 (1)
Where:
T0—shaft output torque of the motor
n0—motor output shaft rotational speed
Output power of the slip clutch:
P1=T1•n1 (2)
Where:
T1—output torque of the slip clutch
n1—the output shaft rotational speed of the slip clutch
The shaft output power of the motor is the shaft input power of the slip clutch. For constant torque load, T=T0=T1=constant, thus the efficiency of the slip clutch:
(3)
The slip ratio can be calculated using the following formula:
(4)
It is obtained that (5)
From this we can see that for a constant torque load, the efficiency of the slip clutch is proportional to the output speed. When the rotational speed decreases, the output power decreases proportionally and the input power remains unchanged. At the same time, the loss power Ph increases in proportion to the slip loss, i.e.:
(6)
From the loss power formula (6) we can see clearly that the lower the rotational speed of the electromagnetic speed regulation motor is, the larger the wasted energy would be. The electromagnetic speed regulation method is simple and the cost for equipment is low. However, this speed regulation method consumes a lot of energy. The low-speed operation period of a cement pipe manufacturing machine accounts for 50%, i.e. it is operating at a speed around 300r/min. Therefore, after changing to frequency conversion speed regulation, the theoretical energy saving will be:
Considering the efficiency of the inverter is 95%, the energy saving ratio will be 74%. Further considering that the low-speed operation period accounts for 50%, the energy saving ratio will be around 37%. According to the manufacturer, the electric power charges for the four pipe manufacturing machines are around 20,000 yuan per month and the total investment can be recovered in one year. The economic return is very outstanding.