RU2537955C1 - Device to start and compensate for reactive power of induction motor - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: device to start and to compensate for the reactive power limits the start current during start-up and optimal compensation of the reactive power of the induction motor under work mode. The device contains stator windings with two connected in parallel in each phase capacitors, that during start-up at the specified transient point are automatically switched from parallel to series connection of the capacitors and in parallel with network phases.

EFFECT: losses decreasing under work mode due to exclusion of the induction motor overcompensation and improved reliability.

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Description

The invention relates to electrical engineering and can be used to start and compensate for the reactive power of induction motors.

A device for starting and compensating for the reactive power of an induction motor by sequentially connecting capacitors to the stator windings, which are shunted by keys after starting [see Petrov I.I., Meistel A.M. Special modes of operation of an asynchronous electric drive. M .: Energy, 1968, (Fig. 2-8, a)]. The disadvantage of this device is the danger of resonance phenomena due to the lack of control over the moment the capacitors are turned off, as well as the low power factor in the operating mode associated with the disconnection of the capacitors after start-up.

A device is also known for starting and compensating for the reactive power of an induction motor, in which at the start of starting the stator windings are connected in series with capacitors, which limits the starting current. At the end of the start, the capacitors are connected to the network in parallel [Pat. RF №2262179, class H02P 1/26, 2005]. A disadvantage of the known device is the danger of resonance phenomena due to the lack of control over the moment of switching capacitors from serial to parallel windings, as well as overcompensation of reactive power in the operating mode.

The closest set of essential features to the claimed invention is a device for starting and compensating for the reactive power of an induction motor, containing three phases of the network, a stator winding and capacitors sequentially connected in each phase of the network, a magnetic starter containing normally open and switching normally closed contacts, a magnetic coil starter together with a series-connected resistor is connected in parallel to one of the stator windings, and the resistor is shunted normal but with an open contact, the ends of the stator windings are connected to the first clamps of the capacitors, the second clamps of which are connected to a point, the second contact of the magnetic starter is connected between the beginning of the first stator winding and the end of the second winding, the third contact is connected between the beginning of the second and the end of the third winding, and the fourth contact is connected between the beginning of the third and the end of the first stator winding [Pat. RF №2444837, cl. H02P 1/16, 2012].

The prototype and the claimed invention have the following similar essential features: three phases of the network, a stator winding and capacitors connected in series in each phase of the network, a magnetic starter containing normally open and switching normally closed contacts, a magnetic starter coil together with a series-connected resistor connected in parallel with one of stator windings, and the resistor is shunted by a normally open contact.

The disadvantages of the prototype are the large losses of the device in the operating mode, which is explained by the need to have a large capacitance for the soft start of the induction motor and, as a result, the use of excess capacitance of the capacitors and overcompensation of the reactive power of the asynchronous motor in the operating mode.

EFFECT: reduced losses in the operating mode due to the elimination of overcompensation of the induction motor and increased reliability of the device due to the operation of capacitors in the operating mode at low voltage.

To achieve a technical result, the claimed invention — a device for starting and compensating the reactive power of an induction motor — contains three phases of the network, a stator winding and capacitors connected in series in each phase of the network, a magnetic starter containing normally open and switching normally closed contacts, a magnetic starter coil together with a series-connected resistor is connected in parallel to one of the stator windings, and the resistor is bridged by a normally open contact, each each phase of the network is connected to the first conclusions of normally open contacts, the second conclusions of which are connected to the beginnings of the stator windings, the ends of which are connected to a point, in addition, each phase of the network is connected to the first conclusions of two capacitors, the second conclusions of which, by means of switching normally closed contacts, are connected to the beginnings of the stator windings, the conclusions of the switching normally open contacts in each phase of the network are connected to the conclusions of the switching normally open contacts of other phases of the network.

The invention is illustrated in the drawing, which shows an embodiment of a device for starting and compensating for the reactive power of an induction motor.

The device contains three phases of the network 1, 2, 3, in each phase of the network stator windings 4, 5, 6 and capacitors 7, 8, 9, 10, 11, 12 are connected in series, the magnetic starter contains a coil 13 and normally open contacts 14, 15 , 16, 17 and switching normally closed contacts 18, 19, 20, 21, 22, 23, the coil of the magnetic starter 13 together with a series-connected resistor 24 is connected in parallel to one of the stator windings, in this embodiment, the winding 6, and the resistor is shunted by a normally open contact 14, each phase of the network is connected to the first terminals but open contacts, the second terminals of which are connected to the beginnings of the stator windings, namely, the phase of the network 1 is connected to the first terminal of the normally open contact 15, the second terminal of which is connected to the beginning of the stator winding 4, the phase of the network 2 is connected to the first terminal of the normally open contact 16, the second the output of which is connected to the beginning of the stator winding 5, the phase of the network 3 is connected to the first output of the normally open contact 17, the second output of which is connected to the beginning of the stator winding 6, in addition, each phase of the network is connected and with the first leads of two capacitors, the second leads of which are connected through the switching normally closed contacts to the beginnings of the stator windings, namely the phase of the network 1 is connected to the first leads of the capacitors 7 and 8, the second leads of which are connected through the switching normally closed contacts 18 and 19 to the beginning of the stator winding 4, the phase of the network 2 is connected to the first terminals of the capacitors 9 and 10, the second terminals of which, by means of switching normally closed contacts 20 and 21 are connected to the beginning of the stator winding 5, and the network 3 is connected to the first terminals of the capacitors 11 and 12, the second terminals of which are connected via the switching normally closed contacts 22 and 23 to the beginning of the stator winding 6, the terminals of the switching normally open contacts in each phase of the network are connected to the terminals of the switching normally open contacts of other phases of the network, namely, the output of the switching normally open contact 26 in the phase of the network 1 is connected to the output of the switching normally open contact 27 in the phase of the network 2, the output of the switching normally open contact 28 in the phase of the network 2 is connected to the output of the switching normally open contact 29 in the phase of the network 3, the output of the switching normally open contact 30 in the phase of the network 3 is connected to the output of the switching normally open contact 25 in the phase of the network 1. The ends of the stator windings 4, 5, 6 combined to a point.

The device operates as follows. To start the electric motor, a three-phase switch is turned on (not shown in the drawing), the contacts 14, 15, 16, 17 are open at the same time, and the switching normally closed contacts 18, 19, 20, 21, 22, 23 are closed, switching the normally open contacts 25, 26 , 27, 28, 29, 30 are open, the capacitors turn out to be connected in parallel, in pairs 7 and 8, 9 and 10, 11 and 12 and at the same time in series with the stator windings 4, 5, 6. respectively. The voltage applied to the stator windings 4, 5, 6, while reduced by the magnitude of the voltage drop across the resistances condensers 7, 8, 9, 10, 11, 12 which, in turn, limits the inrush current. As the engine speed increases, the voltage applied to the stator windings 4, 5, 6 increases and when it reaches a certain value, the magnetic starter coil 13 is triggered. As a result of this, a resistor 24 is shunted via pin 14, which increases the voltage across the coil 13 to the nominal one and ensures reliable operation of the magnetic starter. In addition, the closure of pin 14 eliminates losses on the resistor 24 in the operating mode. At the same time, the contacts of the magnetic starter 15, 16, 17 are closed, including the stator windings 4, 5, 6 directly to the phases of the network 1, 2, 3, and the switching contacts switch to normally open contacts 25, 26, 27, 28, 29, 30, at this capacitors are turned on in pairs in series and parallel to the phases of the network. So, the capacitor 8 is connected in series with the capacitor 9 and parallel to the phases of the network 1 and 2, the capacitor 10 is connected in series with the capacitor 11 and parallel to the phases of the network 2 and 3, and the capacitor 12 is connected in series with the capacitor 7 and parallel to the phases of the network 3 and 1, providing compensation engine reactive power in operating mode.

As the calculations and tests of the device showed, the most favorable turn-on time of the coil 13 is the moment when the voltage on the stator windings 4, 5, 6 reaches the rated value, which corresponds to the rated current in these windings. In this case, the transition of the engine to the operating mode occurs most smoothly. However, the coils of 14 serial magnetic starters have a trip voltage of 20-30% below the nominal. By setting the resistor 24 can be achieved for each specific magnetic actuator, when it will occur when the rated or other specified voltage value is reached.

Here is the rationale for the need for a smaller capacitance in the operating mode compared to the starting mode.

Knowing the voltage - U, a given value of the initial starting current is determined by the expression

, $$In=\frac{U}{Z_P+X_{KP}}$$

,

where Z _P - resistance to blood pressure at startup,

X _KP - capacitor resistance at start-up.

Since with a direct start-up, the starting current HELL is 4-6 of the rated current, the starting resistance HELL Z _P is 0.17-0.25 of the nominal resistance Zн. Hence, to reduce the starting current, for example, to the nominal value, the resistance of the starting capacitors X _KP is required, equal to (1-1.5) Zн in each phase.

The value of the capacitance required to obtain the set value of the power factor in the working compensated mode can be obtained by knowing the resistance of the blood pressure in the nominal mode Zн and the nominal cosφ _H1 . Then the capacitor resistances X _KH necessary to obtain a new specified power factor in the operating compensated mode cosφ _H2 , when connecting capacitors to a triangle for linear voltages, can be obtained from the formula

. $$X_{KH}=\frac{3Zn}{\cos {\varphi }_{H\mathrm{one}}(tg{\varphi }_{H\mathrm{one}}-tg{\varphi }_{H2})}$$

.

In the particular case cosφ _H2 = 1, the last formula has the form

. $$X_{KH}=\frac{3Zn}{\sin {\varphi }_{H\mathrm{one}}}$$

.

Those. in operating mode, capacitor resistance X is required_Kh, equal to (4-6) Zн, which corresponds to the need to reduce the capacitance of capacitors by (4-6) times in comparison with the starting mode, which ensures the proposed device.

Thus, the proposed device provides a limitation of the starting current and compensation of the reactive power of the induction motor in the operating mode, while in comparison with the prototype it provides a reduction in losses in the nominal mode by eliminating overcompensation of the asynchronous motor and increasing the reliability of the device due to the operation of capacitors in the operating mode at low voltage.

Claims (1)

A device for starting and compensating the reactive power of an induction motor containing three phases of the network, a stator winding and capacitors connected in series in each phase of the network, a magnetic starter containing normally open and switching normally closed contacts, a magnetic starter coil together with a series-connected resistor connected in parallel with one of stator windings, and the resistor is shunted by a normally open contact, characterized in that each phase of the network is connected to the first conclusions and normally open contacts, the second conclusions of which are connected to the beginnings of the stator windings, the ends of which are connected to a point, in addition, each phase of the network is connected to the first conclusions of two capacitors, the second conclusions of which are connected via the switching normally closed contacts to the beginnings of the stator windings, the conclusions of switching normally open contacts in each phase of the network are connected to the terminals of the switching normally open contacts of other phases of the network.