Introduction

A company’s 2500t/d new dry clinker production line was put into operation in 2004, and the equipment operation has been relatively stable. However, in July 2014, the reducer of the raw meal mill and separator was mechanically damaged. After replacing the new reducer, the motor of the separator was burnt out several times in a row. Only after the motor of the powder separator is re-selected can the safe use effect be achieved.

01 Problems

The motor power of the raw mill separator is 110kW, rated speed is 1450r/min, rated current is 198A, and the spindle speed of the separator is 48 ~ 190r/min. The matching reducer is FLENDER, the model is B2SV7, and the speed ratio is 7.88. In July 2014, the reducer housing was broken and damaged, so it was replaced. But because FLENDER reducer is imported from abroad, supply cycle is long, domestic and no spot, in order to reduce production loss, decided to use domestic reducer to replace, the replacement of domestic reducer in addition to reduce the speed ratio, shape size and input and output drive shaft position and the original reducer is exactly the same. After replacement, due to the reduced speed ratio, in order to achieve the same product quality index, the rotor speed of the powder sorter under the original working condition remains unchanged at 128r/min, and the working speed of the variable frequency speed regulating motor of the powder sorter is adjusted from 34.8Hz before the replacement of the reducer to 27.9Hz. At this time, the current of the separator is as high as 160A, although it does not exceed the rated current of the motor, but the motor has heating phenomenon, and after running for a period of time, the burning accident of the driving motor appears. Even if the replacement of the motor with more power, there is still the phenomenon of heating and burning of the motor, less than 2 months of continuous burning of 5 sets of sorting machine drive motor.

02 Analysis of problems

2.1 Frequency reduction operation analysis of the separator motor

Classifier rotor rotational speed n = n zero/I (type: n zero – drag the motor speed, I – reducer ratio), the classifier drive motor is ac frequency conversion motor, under the condition of without considering slip, n zero = 60 f/p (type: f – motor operating frequency, the logarithmic p – poles), p is a constant, so the motor output speed only associated with the motor operating frequency, lower frequency, the output speed is reduced.

After replacing the reducer, the speed ratio is 6.32, which is lower than the original reducer speed ratio. In order to obtain the same product quality indicators, it is necessary to change the rotor speed of the powder selection machine, that is, the output speed of the reducer is the same. When the input end is driven by a variable frequency motor, it is necessary to reduce the frequency of the variable frequency motor in order to obtain a lower input shaft speed.

2.2 Heating and burn down analysis of the motor of the separator

(1) Power selection machine is determined. The operation power of the powder sorting machine under stable state includes three parts: one is the work required for the material feeding into the powder sorting machine to be spread out by the material withdrawing plate; The second is the work consumed by the rotor blade to overcome the dust-containing gas for rotary motion; Third, the rotor overcomes mechanical friction resistance. Rotor sowing disk sowing material work done: P₁=A• VT2/7200, rotor blade cutting dust gas consumption work: P₂=0.18(CA +ρ)S• VT3/2000 type, A- powder sorting machine feeding amount; VT – rotor speed; S- Total area of rotor blades; CA – feeding concentration of sorting machine; ρ- gas density. Ignore the friction resistance factor, selecting power P=P₁+P₂. Replace the reducer, the feeding rate A and rotor speed V have not changed, so the separator motor is constant power operation, the power required did not decrease with the motor frequency. However, using the output torque ₀ T=9550P/ N ₀, the output shaft torque of the motor must be increased as the output speed N ₀ decreases with the same P.

(2) Working characteristics of variable frequency motor. Any form of frequency converter, in operation will produce different extent of harmonic voltage and current, high order harmonic and the corresponding time the presence of the magnetic flux leakage, the increase of magnetic flux leakage magnetic circuit saturation degree, and exciting current increases, thus make the fundamental composition of current, the stator winding current increases, the rotor ac resistance increased. The rotor resistance is much greater than the DC resistance, because of the existence of high-order time harmonics caused by the motor and stator copper consumption, rotor copper consumption, iron consumption and skin effect caused by the additional loss increase, these losses will make the motor extra heat, efficiency reduction, output power reduction. Because the impedance of the asynchronous motor is not ideal, when the power frequency is low, the loss caused by the higher time harmonics in the power supply is greater, which is likely to cause abnormal heating and even burn down of the motor.

(3) The influence of frequency and voltage of frequency converter and output torque. For variable frequency speed regulation, the speed decreases, the frequency decreases, if the voltage is constant, the magnetic flux X will increase, which will make the magnetic circuit saturated, the excitation current rises, so that the iron loss increases sharply, leading to the motor overheating damage. Therefore, it is required to change the stator voltage at the same time of frequency modulation to maintain the magnetic flux close to the same. For asynchronous motor, motor output torque T=K•I•X(where, K-coefficient; I- motor current; X- flux), flux X is a constant, the output current of the frequency converter depends on the load torque, and the size of I is independent of the speed. When the load torque is constant, according to T=9550P/ N formula, the output power of the motor will decrease, and the input current of the frequency converter will decrease with the decrease of frequency. For the powder selection machine, in the case of grinding conditions are not changed, the rotor speed of the powder selection machine is the same, the work done by the motor is the same, when the motor speed is reduced, in order to maintain the same output power, the required load torque increases, will lead to the motor current of the powder selection machine increases. The more the motor speed decreases, the greater the current will be. In the case of low frequency, the influence of high harmonic will make the magnetic flux leakage increase; The increase of current will also increase the harmonic current, the reactive component of current increases, the increase of excitation current, so that the magnetic circuit is saturated, resulting in overheating and burning of the motor.

(4) Cooling of frequency conversion motor. Classifier motor built-in fan cooling ways, the speed decreases, the effects of cooling air volume is not enough to satisfy the motor temperature rise, is required for an additional 2 sets of axial flow fan cooling, due to the load change under working conditions classifier at low speed and low frequency high order harmonic, the influence of the motor is difficult to effectively reduce the temperature caused by abnormal fever, which can lead to burning electric machine long time running overheating.

03 Solutions

Due to the influence of the high frequency harmonics generated by the frequency converter at a lower frequency and the load characteristic factors, the motor current increased and the loss increased after the reducer of the separator was replaced, the motor got hot, and the insufficient cooling caused the motor to burn out. The following corresponding measures were taken.

(1) Consider the suppression of harmonics. Reactor, filter or multiphase pulse rectifier are usually used to reduce or weaken harmonic influence and reduce line interference through reasonable wiring and shielding measures, so as to ensure the stable operation of variable frequency motor. However, this method cannot change the motor heating phenomenon caused by the increase of current due to the influence of load characteristics.

(2) Make full use of the functions of the frequency converter itself. Combined with frequency conversion motor characteristic curve changes, modify working parameters of frequency converter and to improve the V/f control values, optimizing the allocation of motor current, increase torque of the motor current component and reduce other numerical current component, without any increase in current situation, improve the motor output torque, to improve the motor at low speed when the temperature rise has a certain effect, but it is still not ideal.

(3) Improve motor characteristic curve and load characteristic curve. Combined with the balance point, improve the working frequency of the motor, reduce the influence of low frequency and high frequency time harmonics and the decline of the load capacity of the motor, improve the effective output power of the motor, reduce the working current of the motor, the reducer can be replaced to improve the speed ratio or select the appropriate powder separator to drag the motor.

Comprehensive consideration of various regulating methods and replace reducer procurement costs, think to change the rated speed is more operable in the lower motor, combining before replacing classifier the working speed of the motor and the existing reducer ratio, to check the motor output shaft torque, replaced a power of 132 kw, the speed of 1150 r/min, the motor rated current is 215 a. After the replacement, the motor current was reduced to less than 100A, and the frequency converter worked around 34.8Hz. There was no burning accident of the motor of the separator.

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