Device for automatic drying of motor windings. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Electric motors

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Electric motors used in everyday life and industry are often operated and stored in conditions of high humidity. The motor housing is not hermetic, moisture inevitably penetrates inside, it is absorbed by the insulation of the windings. This leads to a decrease in insulation resistance, an increase in leakage currents and, ultimately, to breakdown. The proposed device constantly monitors the insulation resistance of a three-phase asynchronous electric motor and automatically maintains it at a given level, excluding engine failure as a result of waterlogging.

The device, which will be discussed, forms a single system with an electric motor, a power supply network and a starting device, the structure and principle of operation of which are protected by a copyright certificate [1]. The design was awarded the silver medal of VDNKh (VVTs). Insulation resistance is controlled and restored in the most dangerous, from the point of view of moisture condensation, time intervals - during breaks in the operation of the electric drive.

As shown in the figure, the asynchronous motor M1 is connected to a three-phase network through a switching device KM1. The drying device itself consists of power units (transformer T1, rectifiers on diode bridges VD1, VD3), insulation resistance control (microcircuit DA1, transistor VT2, relay K1) and control (microcircuit DD1, transistors VT1, VT3, relay K2). Triacs VS1 and VS2 serve as actuating elements.

(click to enlarge)

The drying device is switched on with switch SA1, the first group of contacts of which (SA1.1) closes the circuit of the primary winding of the transformer T1, and the second (SA1.2) connects the windings of the motor M1 to the input of the control unit. If the power contacts of the KM1 switch are closed and the engine is connected to the mains, the drying device does not function, since the circuit of the primary winding of the transformer T1 is open by the auxiliary contacts of the switch.

Zener diodes VD6 and VD7 stabilize the voltages needed to power the DA1 and DD1 microcircuits, and VD2 is a voltage of 130 V, which serves as a test to check the resistance between the windings and the motor housing M1. The test voltage is applied to the motor housing through a protective resistor R4.

Op-amp DA1 is covered by positive feedback through the resistor R21, which turns it into a Schmitt trigger. The voltage at the inverting input of the op-amp depends on the current flowing under the action of the test voltage through the insulation resistance between the housing and the motor windings, and on the position of the trimmer resistor R12, which regulates the response threshold. With the selected value of the test voltage, the leakage current of closed triacs VS1 and VS2, connected in parallel to the controlled circuit, is small and does not lead to a significant error. Due to the relatively small values ​​​​of resistors R11-R13, the sensitivity of the node to interference is low and the wires connecting it to the engine can be of considerable length.

While the insulation resistance is normal, the voltage at the inverting input of the op-amp DA1 is greater than at the non-inverting one. The voltage at the output of the op-amp is low, the transistor VT2 is closed, the relay winding K1 is de-energized. The signal lamp HL1 "Insulation control" is on. With the moistening of the windings, the insulation resistance drops, the voltage at the inverting input of the op-amp DA1 decreases (test voltage is negative). When the voltage reaches the trigger threshold, transistor VT2 opens, relay K1 is activated. Lamp HL1 goes out, HL2 "Insulation drying" lights up.

Through the closed contacts of relay K1.2, power is supplied to the DD1 microcircuit, on the elements of which and the transistor VT1 a multivibrator is assembled [2]. Independent adjustment of pulse duration and pauses between them is provided. The duration of the pulses can be changed with a variable resistor R20 within 0,3 ... 7 s, pauses - with a variable resistor R14 within 3 ... 16 s. The output signal of the multivibrator is fed to the transistor switch VT3, which controls relay K2. Contacts K2.1 and K2.2 are in the control electrode circuits of triacs VS1 and VS2. The switched on triacs supply phase mains voltage to two series-connected windings of the M1 electric motor. This is not enough to rotate the rotor, but the current flowing through the windings heats up and dries them.

At the time of drying, contacts K2.3 break the control circuit. Resistor R5 prevents false triggering of the Schmitt trigger by simulating insulation resistance reduced to 510 kOhm. With switch SA2, this resistor can be connected permanently, which will force the device to switch to drying mode. Capacitors C5, C6 keep the voltage at the trigger input unchanged during the "flight" and contact bounce K2.3. They also protect the input from interference.

In pauses between pulses, when relay K2 is de-energized, and triacs VS1, VS2 are closed, the control mode is temporarily restored. If the insulation resistance has already returned to normal, the trigger on the op-amp DA1 will change its state, de-energize relay K1 and stop drying. Otherwise, it will continue with the beginning of the next multivibrator pulse.

Alternating heating and insulation control is much more efficient than continuous drying [3]. Compared with previously known devices [4], the desired result is achieved at lower energy costs, which was the purpose of the invention [1]. Another advantage is the ability to start the electric motor regardless of the state of the drying device due to the fact that in the "Insulation Drying" mode, the auxiliary contacts of the switch KM1 break the control circuit of the triac VS2 before the main power contacts close. Even if the contacts of relay K2.2 were closed at that moment, the triac will have time to close without causing phase C to close to the neutral of the three-phase network.

The device uses fixed MLT resistors, variables - SPZ-16, non-polar capacitors - K73-17, with C1 for a voltage of 630 V, and C2 for at least 250 V. Oxide capacitors of any type. As DD1, the K155LAZ chip is suitable, DA2 - K140UD6. Transformer T1 with an overall power of at least 20 watts. The voltage on the winding II is 140 ... 150 V at a current of 10 mA, on the winding III - 16 ... 18 V at a current of 0,2 A. Relay K1 - RES-47 passport 4.500.408, K2 - RES-22 passport 4.500.131. Signal lamps HL1, HL2 - МН18-0,1. The permissible power of the electric motor M1 depends on the type of triacs used VS1, VS2. For those indicated in the diagram, it should not exceed 5 kW. The device is assembled in a housing with dimensions of 260x160x150 mm from a magnetic starter.

Check and adjust the drying device without connecting it to the electric motor. An alternating voltage of 1 V is supplied to the winding I of the transformer T220. Between the upper terminal of the resistor R4 according to the circuit and the normally closed contact of the relay K2.3, several series-connected resistors with a power of at least 0,5 W and a total resistance of 6,8 ... 10 MΩ are installed. Switch contacts SA2 must be open.

With a tuned resistor R12, they achieve that when the resistance of the set of resistors decreases to 4 MΩ, relay K1 works, and when the previous value is restored, it releases. The state of the relay can be judged by the ignition of the lamps HL1 and HL2. The operation of relay K1 must be accompanied by the generation of multivibrator pulses and characteristic clicks of relay K2. The ratio between the actuation and release thresholds of the control node depends on the value of the resistor R21. You can pick it up if needed.

Next, the device is installed in the place intended for it next to the M1 engine or the KM1 switch and connected to them according to the diagram. Naturally, at the time of installation, the entire system must be disconnected from the network.

To determine the optimal drying regime, the author has developed a special technique, the description of which is beyond the scope of a journal article. In practice, it is recommended that the switch SA2 forcibly turn on the dryer and set the variable resistors R14 and R20 to such durations of pulses and pauses that the temperature of the motor housing stabilizes in the range of 70 ... 75 ° С.

In conclusion, we note that the electric motor with the described device can only be connected according to the above scheme to an industrial three-phase electrical network with a "dead-earthed" neutral. It is impossible to connect the housings of electrical installations with the neutral wire of household electrical networks. In this case, the motor housing should be grounded with a separate wire, and the circuit connecting the housing with the VS2 triac output and the network neutral should be broken.

If the SA1 switch is left closed while the motor is running, the elements of the protection device are connected to one of the phases of the network and touching them is life-threatening.

Literature

1. Pakhomov A. Energy supply system. Description of the invention to the author's certificate No. 1585862. - Bulletin "Discoveries, inventions ...", 1990, No. 30.
2. Dyakonov V. Wide-range self-oscillating multivibrator based on integrated circuits of transistor-transistor logic. - Devices and experiment technique, 1976, No. 2, p. 103.
3. Vanurin V., Pakhomov A. Drying of electric motors by current pulses. - Technique in agriculture, 1986, No. 6, p. 28, 29.
4. Martynenko I., Korchemny M., Mashevsky V. A method for protecting the insulation of the motor windings from moisture condensation and a device for its implementation. Description of the invention to the author's certificate No. 680102. - Bulletin "Discoveries, inventions ...", 1979, No. 30.

Author: A.Pakhomov, Zernograd, Rostov region

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