Device for automatic restart of a three-phase electric motor. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Device for automatic restarting of a three-phase electric motor. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Electric motors

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In production and in everyday life, there are a number of units with electric motors that require continuous operation. This includes all kinds of pumping, ventilation installations operating in heat supply, water supply, water and air cooling systems for power electrical equipment (for example, thyristor voltage converters).

A sudden stop of such machines is undesirable, as it can lead to disruptions in the operation of these systems. A common cause of such shutdowns is short-term (from one to several seconds) voltage drops in the electrical network - a phenomenon for our power networks, unfortunately, is not uncommon.

Therefore, these units require the constant presence of service personnel who can perform a quick manual reset.

Another option is also possible - installation of devices for automatic reclosing of electric motors (AR). At the same time, the constant presence of people next to the unit becomes optional, and sometimes undesirable.

The schemes of several AR devices were once published in [1]. One of them was repeated, but worked satisfactorily. After making changes to the electrical circuit, the device began to operate successfully.

The proposed scheme is simple. In Fig. 1, it is shown in a practical version in the form of a block together with a circuit of a conventional non-reversing starter. The block is made as an addition to the starter circuit, which does not require breaking existing electrical connections.

(click to enlarge)

Turning the motor on and off is done using the buttons SB1, SB2. When the SB2 "Start" button is pressed and the starter K1 is turned on, the capacitor C1 is charged through the circuit R1, VD1, C1, SB2 (K1), SB1.

In the event of a power failure in the 380 V electrical network, the K1 starter is turned off. The capacitor is slowly discharged through resistor R5.

When the voltage is restored, the transistor VT1 opens, the capacitor is discharged through its K-E junction and the control transition of the thyristor VS1, which turns on during the control half-cycles, and itself turns on the motor starter K1.

The time delay of the automatic reclosure during a voltage drop is determined by the discharge time of the capacitor C1 through the resistor R5 and depends on the values of C1, R5 and the voltage on C1.

The voltage across the discharged capacitor is determined by the ratio of the resistance values of the resistors R1 and R5. It must not be greater than the rated voltage of the capacitor.

The zener diode VD2 prevents the rapid discharge of C1 during a smooth voltage drop.

With the ratings indicated in the scheme, the action time is about 15 s.

When you press the SB1 "Stop" button, VT1 opens and C1 quickly discharges through the transitions K-E VT1 and UK VS1.

The shutdown time should not be more than 0,5 s. During this time, the discharge current becomes less than the turn-on current of the thyristor.

For these reasons, the capacitance of the capacitor C1 and the charge voltage on it (determined mainly by the value of R5) should be as small as possible to ensure the necessary time for the APV to be ready for action.

This time should not be set excessively large, more than 15 s. It should be less than the time for which the worker servicing the installation will have time to approach it after a sudden stop of the electric motor. This complies with the requirements of labor protection standards.

Details. In addition to those indicated in the diagram, other widely used similar parts can be used. Thyristor VS1 can be replaced by KU202N. Transistor VT1 type KT602B, KT801A, KT630V. Diodes VD1, VD3 type KD209B. The zener diode VD2 can be of the KS650A type, but the value of the resistor R2 should be increased to 100 kOhm. Capacitors of types K50-35, K50-20, K50-12.

Resistors types MLT-0,25.

Design. For critical units, a backup installation is also installed next to the working one.

For these cases, the printed circuit board is doubled. One of its possible options is shown in Fig.2.

Three-phase motor automatic restart device

As the body of the AR unit, the body from the PE-21 or MKU-48 relay is used.

The diagram shows the numbers of the housing screw terminals to which the printed circuit board leads are connected.

The VS1 thyristor does not require a cooler.

Adjustment and operation. The AR block is connected to the starter circuit (without an electric motor) and by switching off and on the switch Q.1, the disappearance and appearance of voltage in the network are simulated.

If necessary, select the values of C1 and R5, taking into account the above requirements. In the same way, periodic checks of already installed and working units are carried out.

Several copies of the unit have been manufactured, installed and have been operating for many years in water cooling systems for power thyristors and water heating. 3a this time there was one failure due to the loss of capacitance of the electric capacitor. Therefore, it is advisable to change a "dried" capacitor occasionally - once every 5-10 years.

References:

Reference book on the power supply of industrial enterprises. Comp. T.V. Ancharova and others - M .: Energoizdat, 1981.

Author: A.V. Okatov

See other articles Section Electric motors.

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