RU2031509C1 - Inductive load with protective device - Google Patents

Abstract

FIELD: electrical engineering. SUBSTANCE: electric drive includes single-phase electric motor with working and starting windings and electron circuit of control over motor securing functions of starting of motor and of its protection against current overload. sensitive element is current transformer which secondary winding is connected to input of peak detector coupled with output to input of threshold device forming level of voltage of control over operation of logic NOT gate which in its turn controls key connecting starting winding to supply circuit. Starting winding is disconnected immediately after motor achieves rated working mode. In case of long rise of working current oscillations of blocking generator are terminated, keys cut off and both windings are de-energized after time set by counter. There is no mechanical contacts to connect and disconnect proper windings. EFFECT: expanded application field, increased reliability of protection. 2 cl, 2 dwg

Description

 The invention relates to electrical engineering and can be used in electric drives of general industrial mechanisms and household appliances to ensure the start-up and protection of single-phase asynchronous electric motors.

 To ensure reliable start-up of a single-phase asynchronous AC motor, an auxiliary starting winding is used, which is connected to the AC network at the time of engine start, and after the motor enters the operating mode, it is disconnected from the network.

 In particular, a device is known for starting a single-phase asynchronous electric motor, comprising a bipolar switch, an electromagnetic relay with a contact, a rectifying diode, a timing RC circuit, and a phase-shifting element.

 The disadvantage of this device is the presence of a mechanical contact in the power supply circuit of the starting winding, which can cause interference in the power supply circuit, sparking, reduced reliability due to oxidation of the contact, as well as a decrease in the reliability of starting when the load on the motor shaft increases.

 This drawback is deprived of the circuit for starting and controlling an AC electric motor, made on solid-state elements and containing an electronic sparkless circuit that generates a phase-shifted current to power the starting winding of the electric motor.

 The disadvantage of this scheme is the difficulty of implementation and the lack of protective equipment for the motor against overload.

 On the other hand, a number of devices are known which provide protection of a single-phase asynchronous electric motor against overload.

 In particular, it is known a device for protecting an electric motor with a working and starting windings, comprising an electromagnetic switch, the coil of which is connected to the working winding circuit, and a thermal relay with a bimetallic element included in the working winding circuit of the motor, and with a heating winding included in the starting winding circuit .

 The disadvantage of this device is the low reliability due to the presence of mechanical contacts and a bimetallic thermal element in the power circuit.

 The device for protecting the electric motor against overload is deprived of this drawback, comprising a current sensor, a rectifier, an ambient temperature sensor, a quadrator, an analog-to-frequency converter, an analog-to-digital converter, a reversible counter, a controlled frequency divider, a motor stop detection unit, included in the circuit of the protected electric motor engine start-up detection unit, engine current underload detection unit, engine overcurrent detection unit, adder, alarm unit, executive gan circuit breaker and the protected motor.

 The disadvantage of this device is the complexity and lack of means for starting an engine having an auxiliary winding.

 There are also devices that combine the functions of starting and protecting electric motors.

 In particular, it is known a device for starting and protecting electric motors having a working and starting winding, consisting of two thermal elements, one of which is connected to the starting winding and is characterized by a slow response, and the other is connected to the working winding and is characterized by a quick response.

 The disadvantage of this and similar devices is the presence of mechanical contacts in the power supply circuit of the windings, as well as a sufficiently high inertia of operation when an overload occurs, which can lead to the failure of the electric motor.

 The prototype is an AC drive containing an engine with a working and starting windings, the first terminals of which are interconnected and connected to the first power bus, and an electronic motor control circuit containing a rectifier and performing the function of starting the engine.

 The disadvantage of this device is the lack of protective equipment for the electric motor against overcurrents and insufficiently high starting reliability, since the starting winding is disconnected upon the expiration of a predetermined time, and not upon the establishment of a nominal operating mode.

 The purpose of the invention is to increase the reliability of starting the engine and ensuring its protection against overcurrent without the use of contact systems.

 The goal is achieved in that in an AC electric drive containing a motor with operating and starting windings, the first terminals of which are interconnected and connected to the first power bus, and an electronic motor control circuit comprising a rectifier, according to the invention, a current transformer is inserted into the electronic control circuit , a linear current-to-voltage converter, a peak detector, a pulse shaper, a threshold device, a pulse counter, a blocking generator, OR gates, "No", NOT, an amplifier, as well as first and second switches with bilateral conductivity; the primary winding of the current transformer is included in the power supply circuit of the working winding in series with the first key, the control input of which is connected to the output of the blocking generator and with the information input of the No element, the output of which through the amplifier is connected to the control input of the second key, the conclusions of which are through the start winding and the first key is connected to the power bus; the secondary winding of the current transformer is connected to the line converter and to the inputs of the peak detector, the output of which is connected to the input of the threshold device, the output connected to the control input of the element "No" and to the first input of the OR element, the second input of which through the element is NOT connected to the output of the pulse counter and the control input of the blocking generator, and the output is with the installation input of the counter, the counting input of which is connected to the output of the pulse shaper connected to the output of the rectifier connected by the inputs with power rails. In addition, a voltage regulator is connected to the output of the rectifier to power the nodes of the electronic control circuit.

 The achievement of the objective of the invention is ensured by the fact that a single electronic control circuit provides functions not only of starting, but also protecting the motor from overcurrent, while performing these functions is carried out without the use of mechanical contacts, i.e. sparkless method. In addition, the starting winding is disconnected upon the establishment of the rated current through the working winding of the motor, which prevents premature or delayed shutdown of the starting winding, ensuring a more reliable starting of the motor.

 Figure 1 shows a diagram of an electric drive; figure 2 - diagrams explaining his work.

 The electric drive contains an electric motor 1 with a working and starting windings 2, 3 and an electronic motor control circuit 4. The electronic circuit 4 contains a current transformer 6, a linear current-to-voltage converter 7, a peak detector 8, a pulse shaper 9, a threshold device 10, a pulse counter 11, an element HE 12, an OR element 13, a blocking generator 14, a No element 15, an amplifier 16, the first and second switches 17 and 18, a voltage stabilizer 19.

 The first conclusions of windings 2 and 3 are interconnected and connected to one of the power buses. The primary winding of the current transformer 6 is included in the power supply circuit of the working winding 2 in series with the key 18 and in the power supply circuit of the starting winding 3 in series with the keys 17 and 18, made, for example, on triacs. The secondary winding of the current transformer 6 is connected to a linear converter 7, made in the form of a reference resistor, and to the inputs of the peak detector 8. The output of the rectifier 5 is connected to the input of the pulse shaper 9, made, for example, in the form of a Schmidt trigger, whose inverse output is connected to the counting input counter 11. The input of the threshold device 10 is connected to the output of the peak detector 8, and its inverse output is connected to the first input of the OR 13 element and to the control input of the No element 15. The threshold device 10 can also be performed based on the Schmidt trigger (if necessary, unload the output circuit, two Schmidt triggers can be used). The output of the counter 11 through the element 12 is connected to another input of the OR element 13 and to the control input of the blocking generator 14, the output of which is connected to the other input of the element "No" 15 and to the control input of the key 18. The output of the element "No" 15 is connected through an amplifier 16 with the control input of the key 17. The installation input of the counter 11 is connected to the output of the OR element 13.

 The current transformer 6 is made on the core of a material with a rectangular hysteresis loop, which allows to reduce the dimensions of the transformer, as well as to transmit the amplitude of the measured current with a minimum error due to the insignificant magnitude of the magnetization current in the unsaturated state of the core.

 The AC drive operates as follows.

 Mains voltage is supplied to the input of the rectifier 5, and from its output to the input of the stabilizer 19 constant voltage. When the voltage at its output is established, the blocking generator 14 starts to generate bipolar current pulses (Fig. 2a), which are fed to the input of the No circuit 15 and to the control input of the key 18, unlocking the latter. When this occurs, the connection of the working winding 2 to the supply network (Fig.2b). When the working winding 2 of the motor 1 is connected, a jump in the consumed current is observed (Fig.2c), which is captured by the secondary winding of the transformer 6 and is supplied to the input of the peak detector 8 in the form of a voltage proportional to it at the input of the peak detector 8. A constant voltage is formed at the output capacitive filter of the peak detector 8 ( Fig. 2d), providing the triggering of the threshold device 10 (Fig. 2e). At the same time, a logic zero level is formed at its inverted output, which goes to the control input of the No logic element 15, ensuring the passage of pulses from the output of the blocking generator 14 to the output of the No element 15 and then through the amplifier 16 to the control input of the key 17 (Fig. .2e). In this case, the starting winding 3 through the public keys 17 and 19 is connected to the mains supply (Fig.2zh).

 The oscillation frequency of the blocking generator 14 is selected high enough that allows the inclusion of keys 17 and 18 in each half-cycle of the mains frequency near the transition of the load current through the zero level. This provides a lightweight switching mode and minimal distortion in the shape of the switching current.

 In the process of increasing the engine speed, the current through its windings, as measured by the transformer 6, will decrease (pigv). In this case, the voltage at the output of the peak detector 8 will also decrease to the value of the release threshold of the device 10 (Fig.2g). At its inverse output, the level of the logical unit (Fig. 2e) appears, which enters the inhibitory input of the logic element "No" 15, preventing the passage of pulses from the blocking generator to its output (Fig. 2e), which causes the key to be locked (Fig. 2g) . In this case, the starting winding 2 is disconnected from the network.

 In protection mode, the device operates as follows.

 With an increase in the current flowing through the working winding 2 of the motor, the voltage at the output of the peak detector 8 increases to the threshold value of the threshold device 10. The logical zero signal from its inverse output through the OR 13 element is supplied to the installation input of the counter 11 and transfers it from the blocking mode to account mode. The counting input of the counter 11 receives pulses of double the frequency of the supply voltage from the output of the shaper 9 pulses (fig.2z). If at the time when the output of the last digit of the counter displays a high level signal (Fig. 2i), the overload current and the corresponding voltage at the output of the peak detector 8 do not decrease to the release level of the threshold device 10, the specified signal, passing through the element NOT 12 and through the OR element 13, confirm the further operation of the counter 11 in the counting mode. At the same time, the signal from the output of the element NOT 12 enters the control input of the blocking generator 14 and transfers it from the generation mode to the silent mode. In this case, the key 18 is locked and the winding 2 is completely de-energized. The counter 11 continues to operate until the complete counting cycle determined by the capacity of the counter 11 is completed, after which it is set to its initial state, the counting mode is resumed, and the ban on the operation of the blocking generator is lifted. The operation of the device starts again.

 If the overload current has time to decrease until a signal appears at the output of the last discharge of the counter 11, a signal appears at the output of the threshold device 10, blocking the operation of the counter through the OR 13 element. When the engine starts, the time to establish the rated current through the motor windings is guaranteed to be shorter than the time from the beginning of the count to the appearance of a signal at its output, so the counter 11 is set in a timely manner by the signal from the output of the threshold device 10 to the count lock mode, ensuring the continuous operation of the blocking generator 13.

 Thus, the proposed device allows both the start-up and protection of an asynchronous single-phase electric motor, by disconnecting the corresponding windings without using mechanical contacts. Moreover, most of the nodes of the electronic circuit is used to implement both functions. Using a current transformer allows you to respond in a timely manner to changes in current in the load circuit and eliminate unnecessary energy overruns. The use of a blocking generator in the device makes it possible to obtain bipolar control pulses of sufficiently high power necessary to turn on the triacs at a low average power consumption, due to the large duty cycle of the generated pulses. This allows you to use a relatively low-power stabilized power source in the device. The compactness of the device allows its use in household electrical appliances.

Claims (2)

 1. AC electric drive, comprising a motor with operating and starting windings, the first terminals of which are connected to each other and connected to the first power bus, and an electronic motor control circuit comprising a rectifier, characterized in that a current transformer and a linear converter are introduced into the electronic control circuit current to voltage, peak detector, pulse shaper, threshold device, pulse counter, blocking generator, logical elements OR, NO, NOT, amplifier, first and second keys with two-way conductivity, while the primary winding of the current transformer is included in the power supply circuit of the working winding in series with the first key, the control input of which is connected to the output of the blocking generator and the information input of the NO element, the output of which through the amplifier is connected to the control input of the second key, the outputs of which are through the start the winding and the first key are connected to the power buses, the secondary winding of the current transformer is connected to the line converter and the inputs of the detector, the output of which is connected to the threshold input device connected to the output with the control input of the NO element and the first input of the OR element, the second input of which through the element is NOT connected to the output of the pulse counter and to the control input of the blocking generator, and the output to the installation input of the counter, the counting input of which is connected to the output of the driver pulses, connected by the input to the output of the rectifier, connected by inputs to the power buses.  2. The drive according to claim 1, characterized in that a voltage stabilizer is connected to the output of the rectifier to power the nodes of the electronic control circuit.

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