Soft start of DC motor on ne555. Electrical diagrams for free

When studying the starting characteristics of starter electric motors, it was revealed that when voltage is applied to the electric motor, a reverse current pulse with a voltage of more than 2000 volts appears. The insulation of electric motor windings may fail and result in interturn breakdown. Sparking of the collector at high starting currents leads to burnout of the collector plates. You can avoid breakdown and an emergency when starting an electric motor by using the method of accelerating speed over time.

The starting current in this circuit is reduced to an acceptable value from 220 amperes to 20. Soft starting conditions are created by a double current level - the first is created by the control characteristic field effect transistor for a time of 0-10 ms, the second - with the contacts of the starting relay from 10 to 60 ms. The current during the starting mode increases almost linearly, which does not lead to destruction of the electrical part of the electric motor.

The circuit in the figure is a hybrid of a powerful field-effect transistor and a start relay.

After pressing the “Start” button, the field-effect transistor is opened by applying voltage from the battery GB1 to the gate through resistor R1. A circuit parallel to the gate of the transistor and the minus of the battery protects the transistor and slightly increases the turn-on time from 0.02 to 1 ms, depending on the values ​​of resistors R1, R2 and capacitor C1 - it supplies power to the starting motor M1 with increasing voltage. The electric motor will accelerate to rated speed, at the end of this process the powerful contacts K1.1 of relay K1 will close, the current through the field-effect transistor will stop, and the operating current of the electric motor will not create sparking of the contacts, since the acceleration mode has been completed.

Opening the “Start” circuit will lead to the opening of circuit K1.1 and de-energizing the electric motor, with the current decreasing exponentially.

A zener diode is introduced into the gate circuit of the field-effect transistor in the circuit to protect against exceeding the threshold voltage; in the source circuit of the transistor, a circuit for quenching is connected in parallel with the starting electric motor impulse voltage reverse polarity – diode VD2 and capacitor C2.

The winding of relay K1 is protected from reverse polarity pulses by a bipolar LED HL1 with a discharge resistor R4; resistor R3 limits the supply current to the winding circuit and reduces its heating during prolonged operation. Diode VD3 eliminates the penetration of impulse noise into the power circuit.

There are no scarce radio components in the circuit: field-effect transistors are installed for a total operating current of 212 amperes. Resistors type MLT-0.25, R3 for one watt. Diodes VD2, VD3 pulse type. Automotive relay - type MG16566DX for a contact current of 30 amperes and a voltage of 12 volts, the turn-on voltage of such a relay is 7 volts, the release voltage is 3.5 volts. We will replace the HL1 LED with KIPD 45B -2 or KIPD 23 A1-K, start button type KM 1-1. The design used an Italian-made starter motor; research was also carried out on other types of electric motors with power from 10 to 300 watts.

The structure is assembled in a housing measuring 110 * 35 * 55 and is fixed next to the starter, the start button is installed in a place convenient for turning on and is connected by a multi-core insulated wire with a cross-section of 0.5 mm. The field-effect transistors are secured with a common bolt to the radiator.

The LED can be used as a start indicator or left on the board.

The power supply circuits of the electric motor must be completed stranded wire with a cross-section of at least 10 mm and as short a length as possible to reduce voltage losses.

The circuit was tested on a bench with the specified 250-watt motor; for reliability, install two field switches in parallel, securing them on both sides of the radiator, the starting current can then reach 220 amperes. A current of 130 Amps is taken from the battery by the starter of the Zhiguli VAZ 2107.

List of radioelements

Soft starting of an electric motor has recently been used more and more often. Its areas of application are varied and numerous. These are industry, electric transport, utilities and agriculture. The use of such devices can significantly reduce starting loads on the electric motor and actuators, thereby extending their service life.

Starting currents

Starting currents reach values ​​7...10 times higher than in operating mode. This leads to a “sag” of voltage in the supply network, which negatively affects not only the operation of other consumers, but also the engine itself. The start-up time is delayed, which can lead to overheating of the windings and gradual destruction of their insulation. This contributes to premature failure of the electric motor.

Soft start devices can significantly reduce starting loads on the electric motor and the electrical network, which is especially important in rural areas or when the engine is powered from an autonomous power plant.

Overload of actuators

When the engine starts, the torque on its shaft is very unstable and exceeds the rated value by more than five times. Therefore, the starting loads of the actuators are also increased compared to operation in steady state and can reach up to 500 percent. Instability of the starting torque leads to shock loads on the gear teeth, shearing of keys and sometimes even twisting of the shafts.

Electric motor soft start devices significantly reduce starting loads on the mechanism: the gaps between the gear teeth are smoothly selected, which prevents their breakage. Belt drives also smoothly tension the drive belts, which reduces wear on the mechanisms.

In addition to a smooth start, the smooth braking mode has a beneficial effect on the operation of mechanisms. If the engine drives the pump, then smooth braking avoids water hammer when the unit is turned off.

Industrial soft starters

Currently produced by many companies, for example Siemens, Danfoss, Schneider Electric. Such devices have many functions that are user programmable. These are acceleration time, deceleration time, overload protection and many other additional functions.

Despite all the advantages, branded devices have one drawback - a fairly high price. At the same time, you can create similar device on one's own. At the same time, its cost will be small.

Soft start device based on KR1182PM1 microcircuit

The story was about specialized chip KR1182PM1, representing the phase power regulator. Were considered standard schemes its inclusion, devices for soft starting of incandescent lamps and simply power regulators in the load. Based on this microcircuit, it is possible to create quite simple device smooth start of a three-phase electric motor. The device diagram is shown in Figure 1.

Figure 1. Scheme of the motor soft start device.

Soft starting is carried out by gradually increasing the voltage on the motor windings from zero to the nominal value. This is achieved by increasing the opening angle of the thyristor switches over a time called the startup time.

Description of the scheme

The design uses a three-phase electric motor 50 Hz, 380 V. The star-connected motor windings are connected to the output circuits indicated in the diagram as L1, L2, L3. The center point of the star is connected to the network neutral (N).

The output switches are made on thyristors connected back-to-back - in parallel. The design uses imported 40TPS12 type thyristors. At a low cost, they have a fairly large current - up to 35 A, and their reverse voltage is 1200 V. In addition to them, the keys contain several more elements. Their purpose is as follows: damping RC circuits connected in parallel with the thyristors prevent false switching on of the latter (in the diagram these are R8C11, R9C12, R10C13), and with the help of varistors RU1...RU3 switching noise is absorbed, the amplitude of which exceeds 500 V.

DA1...DA3 microcircuits of type KR1182PM1 are used as control nodes for output switches. These microcircuits were discussed in some detail in. Capacitors C5...C10 inside the microcircuit form a sawtooth voltage, which is synchronized with the network voltage. The thyristor control signals in the microcircuit are generated by comparing the sawtooth voltage with the voltage between microcircuit pins 3 and 6.

To power relays K1...K3, the device has a power supply, which consists of only a few elements. This is transformer T1, rectifier bridge VD1, smoothing capacitor C4. At the output of the rectifier, an integrated stabilizer DA4 type 7812 is installed, providing an output voltage of 12 V, and protection against short circuits and overloads at the output.

Description of the operation of the soft starter for electric motors

Mains voltage is supplied to the circuit when power switch Q1 is closed. However, the engine does not start yet. This happens because the windings of relay K1...K3 are still de-energized, and their normally closed contacts bypass pins 3 and 6 of microcircuits DA1...DA3 through resistors R1...R3. This circumstance prevents capacitors C1...C3 from charging, so the microcircuit does not generate control pulses.

Putting the device into operation

When the toggle switch SA1 is closed, the 12 V voltage turns on relay K1…K3. Their normally closed contacts open, which makes it possible to charge capacitors C1...C3 from internal current generators. Along with the increase in voltage on these capacitors, the opening angle of the thyristors also increases. This achieves a smooth increase in voltage on the motor windings. When the capacitors are fully charged, the switching angle of the thyristors will reach its maximum value, and the rotation speed of the electric motor will reach the rated speed.

Engine shutdown, smooth braking

To turn off the engine, open switch SA1. This will turn off relay K1...K3. They are normal - the closed contacts will close, which will lead to the discharge of capacitors C1...C3 through resistors R1...R3. The discharge of the capacitors will last for several seconds, during which time the engine will stop.

When starting the engine, significant currents can flow in the neutral wire. This happens because during smooth acceleration the currents in the motor windings are non-sinusoidal, but there is no need to be particularly afraid of this: the starting process is quite short-lived. In steady-state mode, this current will be much less (no more than ten percent of the phase current in the nominal mode), which is due only to the technological dispersion of the winding parameters and the “misalignment” of the phases. It is no longer possible to get rid of these phenomena.

Details and design

To assemble the device, the following parts are required:

Transformer with a power of no more than 15 W, with an output winding voltage of 15...17 V.

Relays K1...K3 are suitable for any coil voltage of 12 V, having a normally closed or switching contact, for example TRU-12VDC-SB-SL.

Capacitors C11…C13 type K73-17 for an operating voltage of at least 600 V.

The device is made on a printed circuit board. The assembled device should be placed in a plastic case of suitable dimensions, on the front panel of which switch SA1 and LEDs HL1 and HL2 should be placed.

Motor connection

The connection between switch Q1 and the motor is made with wires whose cross-section corresponds to the power of the latter. The neutral wire is made of the same wire as the phase wires. With the component ratings indicated in the diagram, it is possible to connect motors with a power of up to four kilowatts.

If you plan to use a motor with a power of no more than one and a half kilowatts, and the start-up frequency will not exceed 10...15 per hour, then the power dissipated by the thyristor switches is insignificant, so radiators can not be installed.

If you plan to use a more powerful engine or the starts will be more frequent, you will need to install thyristors on radiators made of aluminum strip. If the radiator is supposed to be used as a common one, then the thyristors should be isolated from it using mica spacers. To improve cooling conditions, you can use heat-conducting paste KPT-8.

Checking and setting up the device

Before switching on, first of all, check the installation for compliance schematic diagram. This is the basic rule, and you cannot deviate from it. After all, neglecting this check can lead to a bunch of charred parts, and for a long time discourage you from doing “experiments with electricity.” The errors found should be eliminated, because after all, this circuit is powered from the network, and it is not to be trifled with. And even after this check, it is still too early to connect the engine.

First, instead of the engine, you should connect three identical incandescent lamps with a power of 60...100 W. During testing, it is necessary to ensure that the lamps “ignite” evenly.

The unevenness of the switching time is due to the scatter in the capacitances of capacitors C1...C3, which have a significant tolerance on capacitance. Therefore, it is better to immediately select them using the device before installation, at least with an accuracy of up to ten percent.

The shutdown time is also determined by the resistance of resistors R1…R3. With their help you can adjust the shutdown time. These settings should be made if the spread in the on-off time in different phases exceeds 30 percent.

The engine can be connected only after the above checks have passed normally, not to say even perfectly.

What else can be added to the design?

It has already been said above that such devices are currently produced by different companies. Of course, it’s impossible to replicate all the functions of branded devices in such a homemade device, but you can still probably copy one.

We are talking about the so-called. Its purpose is as follows: after the engine has reached its rated speed, the contactor simply bridges the thyristor switches with its contacts. The current flows through them, bypassing the thyristors. This design is often called a bypass (from the English bypass - bypass). For such an improvement it will be necessary to introduce additional elements to the control unit.

Boris Aladyshkin

Soft start asynchronous motor is always a difficult task because starting an induction motor requires a lot of current and torque, which can burn out the motor winding. Engineers constantly propose and implement interesting technical solutions to overcome this problem, such as using switching circuit, autotransformer, etc.

Currently, similar methods are used in various industrial installations for uninterrupted operation of electric motors.

The principle of operation of an induction electric motor is known from physics, the whole essence of which is to use the difference between the rotation frequencies of the magnetic fields of the stator and rotor. The magnetic field of the rotor, trying to catch up with the magnetic field of the stator, contributes to the excitation of a large starting current. The motor runs at full speed, and the torque value also increases along with the current. As a result, the winding of the unit may be damaged due to overheating.

Thus, it becomes necessary to install a soft starter. Soft starters for three-phase asynchronous motors allow you to protect units from the initial high current and torque that arise due to the sliding effect when operating an induction motor.

Advantages of using a circuit with a soft starter (SPD):

1. reduction of starting current;
2. reduction in energy costs;
3. increasing efficiency;
4. relatively low cost;
5. achieving maximum speed without damaging the unit.

How to start the engine smoothly?

There are five main soft starting methods.

• High torque can be created by adding an external resistance to the rotor circuit as shown in the figure.

• By including an automatic transformer in the circuit, the starting current and torque can be maintained by reducing the initial voltage. See the picture below.

• Direct starting is the simplest and cheapest method because the induction motor is connected directly to the power source.
• Connections using a special winding configuration - the method is applicable for motors intended for operation under normal conditions.

• Using SCP is the most advanced method of all the methods listed. Here, semiconductor devices such as thyristors or SCRs, which control the speed of an induction motor, successfully replace mechanical components.

Commutator motor speed controller

Most circuit diagrams of household appliances and electric tools created on the basis commutator motor 220 V. This demand is explained by its versatility. The units can be powered from direct or alternating voltage. The advantage of the circuit is due to the provision of effective starting torque.

To achieve a smoother start and have the ability to adjust the rotation speed, speed controllers are used.

You can start an electric motor with your own hands, for example, in this way.

Soft start

brushed DC motor

(DPT)

There may be a need to smoothly turn on a commutator motor, for example, to prevent current surges in power circuits. Or preventing sharp impacts on the drive transmission. It’s a good idea to set the headlights to turn on to increase the life of the lamps.

In my case it was necessary to submit maximum power to the running electric motor of an electric vehicle with the electronic control key removed from the PWM control mode to prevent it from overheating at maximum load.

In Fig. 1 and fig. 2 shows two implementation diagrams of such devices.

Design 1:

A simple circuit of a soft start circuit using an integrated timer KR1006VI1 (or imported 555 series)

Fig.1. Design 1

When a voltage of 12V is applied, the timer with trim elements (PWM) starts and begins to generate pulses at the output of IC 3 with a constant frequency and a pulse width that varies over time. The time is set by the capacitance of capacitor C1. Next, these pulses are fed to the gate of a powerful field-effect transistor, which controls the load at the output of the device. R3 is strictly 2Mohm. Operating voltage electrolytic capacitors 25 volts.
Note: This device is located as close to the fan as possible otherwise, interference may arise that will interfere with the normal operation of the car (of course, the Zhiguli is not an obstacle).

Design 2:

No less simple circuit on the same integral timer.

Fig.2 Design 2

Design 3:

Circuit applied to an electric car. The device is started using the "Start" button.

Fig.2 Design 3

The value of resistor R2 must be at least 2.2 mOhm, otherwise there will be no full (100%) opening of the transistors.
The power supply of the circuit is limited at 7.5V using a KS175Zh zener diode in order to limit the control voltage supplied to the gate of the transistors. Otherwise, the transistor bases go into saturation.
The device is turned on using the "On" button by applying power, while simultaneously unlocking the power transistors. When the device is turned off, linear mode is prevented when the power supply to the control circuits is reduced; the transistors close instantly.