Soft start dpt. Soft starting a DC motor using timers

When controlling DC motors, sometimes there is a need for a sudden change in speed (for example, starting from 0% to 100% power or changing speed to the opposite). But this mode of engine operation requires very high currents - several times more than simple movement. If, for example, when rotating at a constant speed, the motor consumes a current of about 500 mA, then at the moment of starting this value can reach 2-3 A. Because of this, it is necessary to use a more powerful power supply subsystem and controller.

The problem of inrush currents can be solved by gradually increasing the speed. Those. Instead of instantaneous acceleration, the motor will accelerate gradually, while smoothing out the peak current consumption at the moment of starting.

Let's connect the motor to the motor-shield on the meringue L298P, as in the previous example:

Do not forget that the motor does not have a feedback connection, so to control the current speed we use the additional variable motorPower

unsigned long StartTimer; // Timer for soft start

pinMode(I1, OUTPUT);

for (motorPower=0;motorPower (

delay(StartTimeStep);

The engine now accelerates more smoothly. Accelerating from 0 to 255 will take almost half a second, and setting the change interval to 1 ms will generally take a quarter of a second. The difference is not very noticeable to the naked eye. But such overclocking is much more gentle on the power unit. In addition, we can adjust the acceleration speed to achieve the desired acceleration.

But the use of delay() does not allow parallel use

no other actions, so we implement a soft start using timers, as with.

byte E1=5; // Motor speed control - connection to output 5

byte I1=4; // Control the direction of rotation - connect to output 4

unsigned long StartTimer; // time counter for soft start

int StartTimeStep=2; // Engine power change interval, in ms

int StartPowerStep=1; // One step change in engine power

int motorPower; // Engine power

pinMode(E1, OUTPUT); // Set the operation of the corresponding pins as outputs

pinMode(I1, OUTPUT);

motorPower=0; // Initial power - 0

digitalWrite(I1, HIGH); // Pin I1 is set to a high logic level, the motor rotates in one direction

if (motorPower if ((millis()-StartTimer)>= StartTimeStep) // Check how much has passed since the last speed change

// if more than the specified interval, increase the speed by one more step

motorPower+= StartPowerStep; // increase speed

analogWrite(E1, motorPower); // At the ENABLE pin a control signal with a new speed

StartTimer=millis(); // Start of a new step

Now the engine accelerates smoothly, and in parallel with acceleration, you can perform any other actions

Starting an induction motor smoothly 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 are constantly proposing and implementing interesting technical solutions to overcome this problem, for example, using a switching circuit, autotransformer, etc.

Currently, similar methods are used in various industrial installations for the 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 circuits for household appliances and electrical tools are based on a 220 V commutator motor. 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.

MINISTRY OF EDUCATION AND SCIENCE OF UKRAINE

DEPARTMENT OF AUTOMATIC CONTROL SYSTEMS I

ELECTRIC DRIVE

COURSE PROJECT

DISCIPLINE: “ELECTRIC DRIVE THEORY”

ON THE TOPIC: “SOFT START OF A CONTINUOUS STREAM ENGINE

BY SYSTEM “PULSE WIDTH CONVERTER – MOTOR”

POSITIONAL STRUM“

Rozrobiv:

Kerivnyk:

CALENDAR PLAN

Student _____________

Kerivnyk _____________

“_______”______________________200 RUR

PERELIK SMALL POZNACEN

SHIP - pulse width converter

DPT - stationary engine

AD - asynchronous motor

IP - impulse converter

EOM – electronic computing machine

IDK - vimi-diagnostic complex

SD - stepper motor

VFD - variable frequency drive

Efficiency - coefficient of corysmic action

GPI - sawtooth generator

ZAVDANNYA

for a student's course project

____________________________________

1. Topic of work: Soft start of a stationary jet motor using the system “Pulse width reversal – stationary jet motor”. The main part is the design of a soft start system for a stationary jet engine based on a PIC 16F 877 microcontroller

2. Line of student’s completed work 01/28/03

3. Output data before operation, technical characteristics of the engine, technical characteristics of other systems of pulse width modulators

4. Substitution of an explanatory note, analysis of the main pulse converters and selection of the most optimal one, development of technical documentation for the stand, development of the principle and functional circuits, selection of power elements iv.

5. Date of publication 200 RUR

CALENDAR PLAN.. 2

THE OVERLINK OF THE MENTAL POSITIONS. 3

ZAVDANNYA.. 4

Introduction. 6

1. Advantages and disadvantages of the SHIP - DPT system. 8

1.1 Switching DC-DC converters (general information) 8

1.2 Analysis of existing pulse converters. 8

2. Functional diagram of the laboratory stand. eleven

3. Development of technical documentation for the laboratory bench of the SHIP - DPT system. 13

3.1 General view of the laboratory stand. 13

3.2 Schematic diagram of the stand after modification. 15

3.3 List of functional capabilities of the laboratory stand. 16

3.4 Control system based on microcontroller PIC 16F 877. 17

4. Calculation of equivalent circuit. 24

5. Static characteristics of the SHIP - DPT system. 26

6. Selection of power elements. 31

6.1 Selecting a power transformer. 31

6.2 Selecting a power transistor. 32

6.3 Selecting a reverse diode. 33

7. Calculation of the converter. 35

8. Calculation of energy characteristics. 42

9. Mathematical model of the SHIP – DPT system. 45

Introduction

Electrical energy conservation is becoming an important part of the overall trend towards environmental protection. Electric motors that drive systems in everyday life and in industry consume a significant portion of the energy produced. Most of these motors operate in unregulated mode and therefore with low efficiency. Recent advances in the semiconductor industry, especially in power electronics and microcontrollers, have made variable speed drives more practical and significantly less expensive. Today, variable speed drives are required not only in high-end and heavy-duty industrial applications such as processing machines or cranes, but increasingly in home appliances such as washing machines, compressors, small pumps, air conditioners, etc. These drives, controlled by advanced algorithms using microcontrollers, have a number of advantages:

increasing the energy efficiency of the system (speed regulation reduces power losses in engines)

improved performance (digital control can add features such as intelligent closed loops, changing frequency properties, controllable fault range, and the ability to interface with other systems)

simplification of electromechanical energy conversion (variable drives eliminate the need for transmissions, gearboxes, gearboxes) ease of software updates; systems based on microcontrollers with flash memory can be quickly changed if necessary. The main condition for their use is to maintain the total cost of the system within reasonable limits. For a number of systems, especially in the home, the total cost should be equivalent to the cost of the unregulated option.

1. Advantages and disadvantages of the SHIP - DPT system

1.1 Switching DC-DC converters (general information)

Changing the consumer voltage value using pulse converters (IP) is called pulse regulation.

Using a pulse converter, the voltage source is periodically connected to the load. As a result, voltage pulses are formed at the output of the converter. Load voltage regulation can be done in three ways:

changing the conductivity interval of the switch at a constant switching frequency (pulse width)

changing the switching frequency at a constant interval of switch conductivity (frequency-pulse)

changing the switching frequency and the conduction interval of the switch (time-pulse)

In this case, the relative conduction time of the switch is regulated, which leads to a smooth change in the average voltage value at the load (in our case, at the DPT armature)

1.2 Analysis of existing pulse converters

The PWB circuit with parallel capacitive switching is shown in Figure 1.1.

Figure 1.1. PWB with parallel capacitive switching

The disadvantage of PSG with parallel capacitive switching is that during the switching process, the voltage at the load reaches double the supply voltage. Another disadvantage is the difficulty of setting up a resonant circuit with capacitor ‘C’ and inductor ‘Dr’.

Figure 1.2 shows a PWB circuit with an additional switching thyristor and a linear choke in the switching unit.

The disadvantage of the circuit is the connection of the switching circuit with the load circuit. This feature complicates switching in light load modes and makes it impossible for the device to operate at idle.

Figure 1.3 shows a diagram of a non-reversible power supply with a sequential key element.

Figure 1.3. Irreversible SPIKE

This circuit is the most suitable for our purpose, since it is characterized by a small number of elements, simplicity of design, fairly high speed and reliability.

Operating principle:

When the VT transistor is turned off from the power supply, energy is consumed. When the transistor VT is turned off, the load current due to E.M.F. self-induction retains its previous direction, closing through the reverse diode VD. Due to the fact that the power source, as a rule, has an inductance, to protect the transistor from overvoltages that occur when the power supply circuit is interrupted, a low-pass filter is installed at the input of the power supply, the output link of which is the capacitor Swx.

2. Functional diagram of the laboratory stand

The functional diagram of an existing laboratory stand is shown in Figure 2.1

Figure 2.1 Functional diagram of the stand

The functional diagram shows the main elements of the stand and the functional interactions between them.

The main element of the stand is the ACS 300 frequency converter. Through it, power is supplied to the asynchronous motor with a squirrel-cage rotor M1 - AOL2-21-4. The stand provides the ability to operate asynchronous dynamic braking mode. It is also possible to control the speed of an asynchronous motor, currents and voltages of both IM and DPT.

In the power circuit of the IM there are a three-phase current sensor and a three-phase voltage sensor, the data from which is supplied through the communication unit to the EOM. The communication unit and the EOM form a measuring and diagnostic complex (IDC). The IDK also receives signals from other sensors and control elements

3. Development of technical documentation for the laboratory bench of the SHIP - DPT system

3.1 General view of the laboratory stand

The appearance of the designed stand is shown in the figure 3.1

1. Load resistor knob

2. Button SB2 “Stop blood pressure”

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20. Methods of starting a DC motor.

There are three possible ways to start the engine:

1) direct start, when the armature circuit is connected directly to the network at its full voltage;

2) starting using a starting rheostat or starting resistances connected in series to the armature circuit;

3) starting at low armature circuit voltage.

Direct starting is used only for engines with a power of up to several hundred watts, for which Ra is relatively large and therefore, when starting, the starting process lasts no more than 1-2 seconds.

The most common is starting using a starting rheostat or starting resistances

Methods for starting a DC motor

1. Direct start- the armature winding is connected directly to the network.

The motor armature current is determined by the formula. (4.1) If we assume that during direct starting the values ​​of the supply voltage U and the resistance of the armature winding R I remain unchanged, then the armature current depends on the back EMF E. At the initial moment of armature launch, the engine is stationary (  =0) and in its winding E=0.Therefore, when connected to the network, a starting current appears in the winding
. (4.2) Usually resistance  R I not much, especially for high-power motors, therefore the value of the starting current reaches 20 times the rated current of the motor. Unacceptably large values, 10 This creates a danger of breaking the machine shaft and strong sparking appears under the commutator brushes. For this reason, such a start is used only for low-power engines with  R I relatively large.

2)Rheostat start- a starting rheostat is included in the armature circuit to limit the current. At the initial moment of start-up at  =0 And R P =max The armature current will be equal

. (4.3) The maximum value of R p is selected so that for machines of high and medium power the armature current at start-up
, and for low-power machines
. Let's consider the process of rheostatic starting using the example of a motor with parallel excitation (Fig. 4.1). At the initial moment, the start-up is carried out according to the rheostatic characteristic 4, corresponding to the maximum resistance value R P, while the engine develops maximum starting torque M nmax.Adjustment rheostat R R is output so that I V And F were maximum. As the engine accelerates, the engine torque decreases, since as the rotor speed increases, the EMF also increases E, and as a result, the armature current, which determines its value, decreases. Upon reaching a certain value M pmin piece of resistance R P is output, as a result of which the torque increases again to M nmax, the engine switches to operation according to rheostatic characteristic 3 and accelerates to the value M pmin. Thus, gradually reducing the resistance of the starting rheostat, the engine is accelerated along individual segments of the rheostatic characteristic until it reaches natural characteristic 1. The average starting torque is determined from the expression
. (4.4) the engine accelerates with some constant acceleration.

A similar start is possible for series-excited motors. The number of starting stages depends on the rigidity of the natural characteristic and the requirements for smooth starting. Starting rheostats are designed for short-term operation under current.

In real devices, start-up is automatic. Microcontroller, according to given the algorithm, controls the switching elements (relay control), turning off sections of the starting rheostat and practically implementing the process described above.

The control algorithm can be constructed using three basic principles:

1) EMF principle

2) Current principle

3) The principle of time.

The idea of ​​implementing these principles can be explained using a starting circuit based on electromagnetic relays (which was practically used before the widespread introduction of microprocessor control systems) Figure 4.3. A series of relays are connected in parallel to the armature of the machine, which, with an increase in the rotation speed, and therefore the EMF, are sequentially activated and, with their contacts, remove the starting rheostat sections from operation, gradually reducing the resistance of the armature circuit.

When using the current principle, series-connected current relays are used, which give a command through their normally closed contacts to sequentially switch on the corresponding contactors K i when the current drops to a given level.

The time principle involves the use of time relays, which, through calculated time settings, give a command to bypass the rheostat sections.

4)Start by smoothly increasing the supply voltage - starting is carried out from a separate regulated power source. It is used for high-power engines, where it is impractical to use bulky rheostats due to significant energy losses.