SU1663728A1 - Device for automatic control of excitation current of synchronous motor - Google Patents

Abstract

The invention relates to electrical engineering and can be used in main electric drives of rolling mills. The purpose of the invention is to reduce the loss of electrical energy. For this purpose, in the device for automatic regulation of the excitation current of the synchronous motor, the nonlinear element 13 is made with a "hysteresis" characteristic. A summing amplifier 14 and two diodes 15, 16 are inserted into the device. The cathodes of the diodes 15, 16 are connected to the second input of the comparison unit 7. The anode of the diode 15 is connected to the output of the rectifying bridge 12, and the anode of the diode 16 to the output of the nonlinear element, the input of which is combined with one input of summing amplifier 14 and connected to the secondary winding of the voltage transformer 11. The other input of the summing amplifier is connected to the secondary winding of the current transformer 10. The device provides optimal operation of the synchronous motor when its load changes. The nonlinear element 13 ensures stable operation of the synchronous motor, since with a significant reduction in the mains voltage it is foreseen to force the excitation with the limitation of the forced value of the excitation current. 3 il.

Description

The invention relates to electrical engineering and can be used to automatically control the excitation of synchronous motors, for example, main electric drives of rolling mills,

The purpose of the invention is to reduce the loss of electrical energy.

FIG. 1 shows a functional diagram of an apparatus for automatically controlling the excitation current of a synchronous motor; in fig. 2 - U-shaped dependences of the stator current I on the excitation current IB, taken at different powers Pi - P4 on the shaft, and the curve of the adjusting characteristic of the synchronous motor with the power factor cos p 1; neither FIG. 3 - voltage vectors IJ2 - U4 of the phases of the stator winding of the synchronous motor and the linear voltage Uz-4 between the two phases of the stator winding of the synchronous motor.

The device for automatic regulation of the excitation current of the synchronous motor 1, the phases 2-4 of the stator winding of which are connected to the network, contains a thyristor exciter 5 for connection to the excitation winding (rotor winding) of the synchronous motor 1. The control input of the thyristor exciter 5 is connected to the output of the regulator field current torus 6, the input of which is connected to the output of comparator block 7. The first (inverting) input of the comparator unit 7 is connected to the output of the current sensor 8, the input connected to shunt 9 connected to the power supply circuit of the excitation winding of the synchronous motor 1.

The device also contains a transformer 10 with primary winding for connecting a synchronous motor to the stator winding phase 2, a primary winding voltage transformer 11 for connecting to the stator winding phases 3 and 4, a rectifying bridge 12 and a nonlinear element 13, performed with a type characteristic hysteresis. A summing amplifier 14 and two diodes 15 and 16 are introduced into the device.

The secondary winding of the current transformer is shunted by a resistor 17 and connected to the first input of summing amplifier 14 formed by resistor 18. The secondary winding of a voltage transformer is connected to the second input of summing amplifier 14 formed by resistor 19, the resistor 20 is connected to the feedback circuit of the amplifier,

The output of the summing amplifier is connected to the input of the rectifying bridge 12, the output of which through the first diode 15 connected in the forward direction is connected to the second input of the comparator unit 7 and the cathode of the second diode 16 whose anode is connected to the output of the nonlinear element 13.

the last is connected to the second input of the summing amplifier 14.

Curves 21-24 (FIG. 2) are dependencies of stator current I on excitation current 1c; curve 25 - adjusting

characteristic of a synchronous motor,

FIG. 3 denotes: pos, 26–28 are the stress vector U2 - U4; 29 is the linear voltage vector U3-4J.30 and 31 is the active component vector | Va and reactive

component (8p of a given current in the excitation, the vector of which corresponds to a vector 32.

The device works as follows

The first and second inputs of summing amplifier 14 from the secondary windings of transformers 10 and 11 receive sinusoidal signals proportional to the current and (respectively, in complex form

recording a sinusoidal signal I) phase 2 of the stator winding and linear voltage U3-4 (Oz-4) between phases 3 and 4 of the stator winding of the synchronous motor 1. The output of summing amplifier 14 is formed

a voltage proportional to the sum of the two sinusoidal signals mentioned above, which is rectified by rectifying bridge 12 and is fed to one of the inputs of comparator block 7 as a driver

signal to the excitation current of the synchronous motor 1. The other (inverting) input of the block 7 in comparison with the output of the excitation current sensor 8 receives a signal proportional to the excitation current.

The optimum (from the point of view of electric energy loss) operating mode of a synchronous motor is provided at cos p 1, i.e. when the engine is running at the lower extremum of the U-shaped characteristic (Fig. 2), for example, at point A of characteristic 24, taken at Р4 Рн (Рн - rated power) on the shaft. As can be seen, in this case (Fig. 2), the current I I minimum in the stator winding circuit flows for a given load power. When the load power changes from Pi to Pi P4, the point A corresponding to the minimum loss of electrical energy moves along curve 25.

The optimal adjustment characteristic when the load on the shaft changes and the deviation of the mains voltage is described by the expression

1in2 isp2 + lea2 ieo2 (y + a Xd sin) 2 +

+ g 1van2, G

where bp is the reactive component of the excitation current IB

lep Boft + aXciSinpH), (2)

lea - active current component)

about va - 77 per (3)

Y (

providing optimal (from the point of view of electric energy loss) regulation of the excitation current;

IBO is the excitation current of the motor at a voltage across the stator winding equal to its nominal value;

van is the nominal value of the active component of the excitation current;

ha Xd ,, - - relative resolution

rotor resistance;

1H is the rated current of the stator;

ifn is the nominal voltage across the stator winding;

Xc) 0m is the rotor reactance along the longitudinal axis;

ph is the angle of shear between currents and voltages on the stator winding in nominal mode of operation;

about. Sed / He is the relative reactive power of a synchronous motor;

Ul / il - relative nominal line voltage of the network;

Р Рсд / Рн - the power consumed by the motor from the network is relatively active.

With the law of regulation called (1) Osd 0, therefore, a 0 and

vr ieo U.

(four)

The equation of the optimal law for the regulation of the excitation current can be conveniently represented as

Pv1 UPva12 + G | vr12,

where Willow, Pvr are the moduli of the quantities described respectively by dependencies (3) and

(four).

Thus, while ensuring the regulation of the excitation current in accordance with dependencies (4) and (5), the Synchronous motor is optimally controlled (from the point of view of the loss of electrical energy) when the load on the shaft changes and the deviations of the mains voltage. However, the direct implementation of dependencies (3) - (5) is difficult, since it requires filling in squaring operations and extracting the square root.

five

ten

 .., -

20

25

zo

35

40

45

50 55

In this device, the squaring and square root extraction operations are replaced by vector, summing, orthogonal active 6a and reactive BP components of the excitation current IB, i.e.

. (6)

As a master signal, proportional to the active component Ba, the device uses the current i2 of phase 2 of the stator winding (r-v, respectively 2 IBS). With optimal regulation, the condition cos 1 is satisfied, therefore, the current in the phases of the stator winding is purely active. As a consequence, vectors 26 and 30

12 Ј va (7)

on the vector diagram are out of phase.

In a three-phase symmetric voltage system, the vector 29 of the linear voltage IJ3-4 lags behind the vector 26 (U2) and. respectively, from the vector 30 (la) to 90 el. hail. In addition, it follows from expression (5) that, with optimal control, the value of the reactive component of the field current is unambiguously determined by the voltage level of the network. Consequently, the line voltage Us-4, measured by the voltage transformer 11, can be used as a driving signal for setting the reactive component of the excitation current iep.

РЗ-4-, 1вр .., (8)

respectively Oz-4 - iep. Vectors 12 and Oz-4 (Fig. 3) are orthogonal and, accordingly, the driving sine-wave signals i and Uz-4 are shifted one relative to the other by 90 el. hail. Consequently, in accordance with (7) and (8), performed the summation of signals proportional to r and 4, perform the vector addition of the active 1 Va and reactive 1p components of the excitation current 1B, i.e. optimal regulation of excitation current 1B.

In the device, the addition of sinusoidal signals is carried out using a summing amplifier 14. The control signal is generated using a rectifying bridge 12.

However, with a significant decrease in the mains voltage (below 0.85 - 0.9UH) and optimal regulation of the excitation current (1), the stability of the synchronous motor may decrease and, as a result, out of sync. In addition, with optimal regulation of the excitation current and with a significant reduction in mains voltage,

increase in drive current above nominal level. To prevent the named undesirable modes, this device provides forcing the excitation by means of a nonlinear element 13 with a hysteresis characteristic.

When the voltage drops to a level below 0.85UH, the voltage at the output of the nonlinear element 13 becomes higher than the voltage at the output of the rectifying bridge 12. Due to the counter-switching of the diodes 15 and 16 to the input of the comparison unit 7, the larger of the two signals fed to the anodes of the diodes 15 passes and 16. As a result, when the network voltage drops below 0.851) n, the force is turned on, and the excitation current rises to the nominal level. Since the nonlinear element 13 has a characteristic of the hysteresis type, the force is disconnected at a network voltage of 0.9UH. Such a characteristic of the nonlinear element is necessary to prevent frequent switching of the reference to the excitation current when the supply voltage is close to 0.85 UH. In addition, the forced excitation current is limited at a predetermined nominal level.

The device can be implemented using the elements of the analog block system of regulators.

This device is simple and provides higher speed when controlling the excitation current of a synchronous motor in comparison with the known ones. This is due to the fact that it has one control loop applied, which improves the response speed by at least three times. As a result, the switching stability and, accordingly, the reliability of the synchronous motor increase.

Thus, the device provides a reduction in electrical energy losses when the load on the shaft of a synchronous motor changes and the deviations of the mains voltage in the operating range (10 - 15% downwards relative to the nominal value)

Claims (1)

Apparatus of the Invention A device for automatically controlling the excitation current of a synchronous motor, comprising a thyristor exciter for connection to the excitation winding of a synchronous motor, an excitation current regulator, a shunt for switching on the excitation winding power supply circuit, the current sensor connected to the shunt and the output to the first input of the comparator unit, the output of which is connected to the input of the current regulator, the output connected to the control input of the thyristor exciter, the current transformer with the primary winding to be included in one of the the phases of the stator winding of a synchronous motor, the voltage transformer with the primary winding for connecting to the two phases of the stator winding of a synchronous motor, a rectifying bridge and a non-linear element that differs from the fact that sheni loss of electric power, the nonlinear characteristic element is adapted to the type of hysteresis and introduced two diodes and a summing amplifier, the first and second inputs connected respectively to secondary windings of transformers current and voltage, and the output of the summing amplifier through a rectifying bridge and the first diode connected in the forward direction is connected to the second input of the reference unit and the cathode of the second diode, the anode which is connected to the output of the nonlinear element, the input connected to the second input of the summing amplifier. , IS V / L 26 fa), & (ig) Y /