SU1642548A1 - Method for overload protection of asynchronous motor - Google Patents

Description

(21) 4619025/07

(22) 12/13/88

(46) 04/15/91. Bksh. № 14 (72) A.V. Bulychev, E.V. Kulakov and V.L. Tretkov (53) 621.316.925 (088.8) (56) Geiler A.B. The basics of the drive. Minsk, Higher School, 1972, pp.240-247,

Coughlin R., Driskol F. Operational Amplifiers and Linear Integrated Circuits. M .: Mir, 1979.

USSR Author's Certificate No. 1001294, cl. H 02 H 7/085, 1981.

(54) METHOD FOR PROTECTING AN ASYNCHRONOUS MOTOR FROM OVERLOADING (57) The invention relates to electrical engineering and is intended to protect an induction motor against overload. The purpose of the invention is to increase the sensitivity of the motor protection to overloads caused by mechanical defects that occur when bearings are damaged and the motor shaft and mechanism are not aligned. The goal is achieved by the fact that in the motor overload protection method, in which the motor temperature is controlled by the stator current using a thermal analogue of the motor to be protected, components with frequencies other than the frequency of the power supply are also separated from the stator current. The magnitude of these components is measured and it reveals mechanical defects in the initial stage of development. 5 il.

§

(L

-t ft1

The invention is relative to electrical engineering and is intended to protect an asynchronous motor against overload,

The purpose of the invention is to increase the sensitivity of protection of an asynchronous electric motor to overloads caused by mechanical defects that occur when bearings are damaged and the shaft motor and mechanism are not aligned.

FIG. Figure 1 shows the mechanical characteristics of the asynchronous motor and the change in slip with a periodic change in the moment of resistance; Fig. 2 shows an L-shaped replacement circuit for an asynchronous electric motor, all of whose parameters are for the stator winding; in fig. 3 is a block diagram of a device for protecting an asynchronous motor against overload, which implements the proposed method; in fig. 4 shows an example of the implementation of a rejection filter; in fig. 5 - amplitude-frequency characteristic of the filter.

When a bearing is damaged or the shaft alignment of an electric motor and mechanism is damaged, the moment of resistance applied to the electric motor shaft changes periodically with a frequency proportional to the shaft rotation frequency. Periodic change of the moment of resistance causes the appearance in the stator current of the motor components with a frequency proportional to the frequency of rotation of the shaft. Separating these components of the current from the components of the main-frequency current and monitoring their level allows one to detect mechanical defects, the development of which can lead to an accident, at an early stage and thus prevent possible extensive damage to the protected motor.

The relationship between the values of the individual components of the stator of an asynchronous electric motor and the nature of the overload in the event of mechanical defects can be established using the conventional equivalent replacement circuit of an asynchronous electric motor shown in Fig. 2.

The stator phase current of the asynchronous motor in accordance with the replacement circuit with sinusoidal

The supply voltage is defined as follows:

i - i0 ×

lo +

Uim

J (,

 sin (0) "t -tf),

4mnct lltll / f

(one)

magnetizing current and

electric motor rotor current; voltage amplitude

nutrition;

inductive resistances of the stator and rotor windings

active resistances of the stator and rotor windings;

glide; supply voltage frequency.

the stator current is determined by the voltage of the motor power, but also

The slip of an asynchronous motor depends on the torque, and therefore on the moment of resistance. In the linear region of the mechanical characteristic of an electric motor, the bond of slip and moment can be expressed as

-% "

where p0 and M0 are the mean values of the COL and moment of the electric motor M and the torque of the electric motor.

The moment of resistance when a mechanical defect occurs in the bearing or the shaft alignment is disturbed depends on the angle of rotation of the motor shaft and periodically changes with the frequency of rotation of the shaft L. Then for the torque of the electric motor, you can write the following relationship:

M

M & + MasinLt,

where M „- the amplitude of the torque

that

Hence it is clear that the slip of an electric motor having a mechanical defect is variable, varying with frequency L L

 l inLt The periodic variation of the slip, as can be seen from expression (1), causes a periodic change in the equivalent resistance of the motor. A change in this resistance in turn causes a change in the stator current with a frequency L. However, the power source has a frequency W ,, different from L, and therefore components appear not only with frequency sd, but also with frequency L

 -It) i, - i, + -:: ---,

f. + -p; irrr v.

Pa -irV The ratio of motor parameters is usually such that

, +, v where

X

about p

Therefore, in order to obtain the inverse dependencies with an acceptable accuracy of description, it can be assumed that X l + X2 Rf O, then for the stator current we will have

sin (CO, t -if) + g

. baЈ9sinLt8in (6, kg

Substituting the value of the magnetizing current into this equation and decomposing the product of sinusoidal functions into two components, we obtain that h Ion, sin (W, t-4) +

Uimpo

H

sin (W, t-lf) +

(2)

Gsoz ((NM) - cos (y t-lf),

where G) is the amplitude and phase of the magnetizing current,

P CO, - L; y + l

As can be seen, the stator current in case of mechanical damage to the motor contains components with often

 ,

ten

15

20

25

thirty

2

35

equal to the frequency of the power source G) (, the difference in the frequency of the power source and the rotor rotation 0 С0 (- L and the sum of G), + L.,

The magnitudes of the components of the stator current with frequencies other than the frequency of the power source are mainly determined, as follows from (2), by the amplitude of the variable component of the slip, as well as by the power supply voltage and the parameters of the electric motor.

In normal operating conditions, the moment of resistance, and consequently, at any moment, the electric motors do not depend on the angle of rotation of the shaft and do not contain a variable component of the moment. Therefore, the slip does not change periodically and there are no components with frequencies and and Y that are different from & in the stator current) (v

From the level of the stator current components with frequencies other than C0, the level of the periodic component of the motor torque is estimated and, therefore, mechanical defects that cause the appearance of these periodic components of the resistance moment are detected.

The method is implemented with the aid of an apparatus, the scheme of which is shown in FIG.

The device contains a protected electric motor 1, a current sensor 2 included in the electric stator circuit. motor, quad 3, thermal analogue 4 of the protected motor and 40 threshold element 5, connected in cascade, logical element OR 6, the first input of which is connected to the output of the threshold element, the actuator 7, the input of which is connected to the 45 output of the logic element, A lateral filter 8 designed to suppress signals with a frequency equal to the frequency of the power source, the input connected to the output of current sensor 5Q, the driver module 9, the low-pass filter tO and the second threshold element 11 connected in cascade. The output of the second threshold element subclaunal to the second input of the logic element,

The device works as follows.

To the input of the quadrant 3 from the output of the sensor 2, a signal is given proportional to

the current of the stator winding of the protected motor 1. Quad 3 forms a signal proportional to the square of the current of the stator, which is fed to the input of the thermal analogue 4. It simulates the thermal processes occurring in the protected motor and generates a signal proportional to the temperature of the electric motor, which is fed to the input of the first motor threshold element 5. When the engine load increases, the level of this signal rises and, when it reaches the maximum permissible level, threshold element 5 produces a signal that Ory through the logical element 6 is fed to the input of the executive body 7. The executive body generates a signal that the engine is discharged or disconnected from the network.

If the bearings are damaged or the shaft alignment is disturbed, the load of the electric motor increases, but the output level of the thermal analogue may remain within acceptable limits. To prevent significant damage to the motor during overloads caused by the development of these defects, it is advisable to turn it off without preventing the development of defects.

The proposed device contains components that can detect minor mechanical overloads of a periodic nature with respect to the stator current of the protected motor. Using a notch filter 8 connected to the output of the current sensor 2, the components of the stator current are suppressed with a frequency equal to the frequency of the power source. The output of the notch filter 8 is rectified by the modulator driver 9 and smoothed by the low pass filter 10. The second threshold element 11 compares the output of the low pass filter 10 with the maximum allowable value. If defects occur, causing an increase in the periodic component of the moment, the input signal of the threshold element 11 rises, and when it reaches the maximum permissible value, the threshold element forms a second signal informing about the occurrence of a mechanical defect, which through logical element 6 enters

0

five

0

five

0

five

at the entrance of the executive body 7.

Separation of signals with frequencies other than the frequency of the power supply from the stator current is performed with a notch filter that suppresses signals with a frequency equal to Q, and passes signals with frequencies other than GOj,

A notch filter with a maximum attenuation of signals at a frequency of CO 314.6 s with more than 100 times can be performed according to a known scheme (L2) shown in FIG. 4.

Figure 5 shows the amplitude-frequency characteristic of this filter, obtained with the following filter parameters: C1 C2 0.047 μF, Rq 1000 Ω, RB 50 kΩ, R, 67.725 kΩ, RI, 677.25 kΩ.

The application of the proposed method of protecting an electric motor ensures detection of mechanical defects at an early stage of their development and prevents extensive damage to the protected electric machine in emergency situations. This makes it possible to significantly reduce production damage from switching off the electric motor and reduce repair and maintenance costs, since in order to eliminate the minor defects revealed by the proposed method and device, large volumes of repair work are not required.

Claims (1)

Invention Formula 40 50 The method of protection of an asynchronous electric motor against overload, at which the motor phase current is measured, a signal proportional to the square of this current is used, a signal proportional to the square of the current of the stator phase of the electric motor is used, and when it exceeds the critical value form the first signal informing about the overload of the electric motor, according to which the electric motor is disconnected from the network, characterized in that, in order to increase the sensitivity motor protection to overloads caused by mechanical defects, in addition, all current components with a frequency of 55 are separated from the stator current. different from the frequency of the power source, the magnitude of these components of the current is measured, and when this value exceeds the critical value, a second signal is generated informing the motor overload caused by mechanical defects and when any of the above signals informing about the overload occurs is turned off electric motor. FIG. 2 4 20 30 BUT 50 60 5 70 80 Hz