Thyristor is only open for current to flow when there is a positive voltage applied to the anode pole and there is a positive voltage pulse applied to the control pole, after the Thyristor is opened, the control pulse no longer works, current flows through the Thyristor due to the parameters of the dynamic circuit decided and the Thyristor will lock when the current flowing through it is 0, if we want to reopen, we must supply the control pulse again.
Therefore, with sinusoidal voltage, depending on the time of impulse control that we can control the current of Thyristor. To achieve these features we can use the following two principles:
- Principle of horizontal control.
- Principle of vertical control.
3.2. Control circuit structure.
3.3. synchronous
- Select the synchronous circuit two half cycle:
3.4. Stitch creating serrated.
3.5. The comparison stage.
Function: Compare the control voltage with the restraint voltage to determine the timing of the control pulse ⇒ Determine the control angle α
3.7. Stitch amplification creates pulses.
- Task: Create pulses to open Thyristor, pulses to open Thyristor require:
+ Enough capacity.
+ There is a vertical slope, usually rectangular pulse.
+ Isolation between the control circuit and the force circuit ⇒ Using a pulse transformer.
- Several ways to amplify pulses.
Direct: Isolating between the control circuit and the force circuit is not allowed.
Nội dung
The Topic: Design a rectifier to control speed of a separately excited DC motor TABLE OF CONTENTS THE PREFACE In the technological innovation and modernization of water, the problem of applying science and technology to regulated products is the most urgent issue Along with the development of a number of industries such as electronics, information technology Industry automation company has also developed dramatically Process production automation is very popular, can replace human labor, high productivity again, good product quality Project electric power The Topic: Design a rectifier to control speed of a separately excited DC motor In the technological innovation and modernization of water, the problem of applying science and technology to regulated products is the most urgent issue Along with the development of a number of industries such as electronics, information technology Industry automation company has also developed dramatically Process production automation is very popular, can replace human labor, high productivity again, good product quality Along with the development of the power electronics industry, the application of DC motors and industry is very important The use of 1-way motors for many purposes such as to ensure the technological requirements of the load To understand the role of electric drive system, power electronics and one-way electric motor through this subject project, under the guidance of Mr Nguyen Ngoc Khoat with the main content of the subject: Design a rectifier to control speed of a separately excited DC motor with the following parameters: 1) Half-wave three-phase controlled rectifier; 3x380V, 50Hz 2) DC motor: P = 10 kW, Uđm = 400V, nđm = 1480v/p, Iđm = 10A, Mc = 60%Mđm I sincerely thank Mr Nguyen Ngoc Khoat for his dedication and help for guiding, helping and creating favorable conditions for us to complete this topic We sincerely thank! Hà Nội, date 17, month 12, year 2020 Students do: Mai Văn Thịnh Đỗ Văn Trung Cao Xuân Tài Project electric power The Topic: Design a rectifier to control speed of a separately excited DC motor CHAPTER I: OVERVIEW OF DC MOTOR 1.1General structure 1.1.1Concept A DC motor is a DC machine that converts direct current into mechanical energy When a DC machine is operating in the motor mode, the input power is the electromechanical power and the output power is the mechanical power Figure 1: DC motor 1.1.2 Component of dc motor DC motors can be divided into two main components: the stationary and the dynamic part Figure 2: Construction of DC motors Project electric power The Topic: Design a rectifier to control speed of a separately excited DC motor 1-Plate, 2-Main pole with field coil, 3-Commutating with reel, 4-Ball bearing box, 5Laminated, 6- Armature roll, 7- Brush equipment, commutator, 9- Axis, 10- Terminal box cover 1.1.3 Classification of DC motors DC motors are classified according to excitation into the following categories: • Independent DC motor: The armature and the exciter are supplied from two separate sources • Parallel dc electric motor: The field coil is connected in parallel with the armature • Series magnetic dc motor: The exciter coil is connected in series with the armature • Combined d.c electric motor: Consists of two excitation windings, one connected parallel to the armature, one connected in series with the armature 1.1.4 Principle of DC electric motor DC motors operate based on the effect of a magnetic field on the wire frame with electric current flowing through the magnetic field When operating DC motors turn the electric current of direct current into mechanical energy 1.2 SPEED –TORQUE EQUATION OF DC MOTOR 1.2.1 Electrical motor characteristics The mechanical characteristic of electric motors is the linearity between the speed and the speed of the motor:M = f(ω) 1.2.2 Wiring schematic diagram of an independent dc electric motor Independent DC motor: DC power is supplied to the armature and supplied to the exciter independently Figure 1.3: Wiring schematic diagram of separatedly excited dc motor • Equation of voltage balance: Project electric power The Topic: Design a rectifier to control speed of a separately excited DC motor Uư = Eư +(Rư + Rf).Iư • Electromotive force of the engine armature: Eư = K • (1.2) The electromagnetic torque of the motor: M = KIư • Speed – Current characteristic: (1.4) Figure 4: Speed –Current • Speed – Torque characteristic: (1.5) Figure 5: Speed – Torque 1.2.3 Natural Speed -Torque characteristic Natural mechanical properties: = f (M) when parameters such as U, I, R of the motor are the rated parameters on the natural mechanical properties we have a rated working point is (; ) - Each motor has only natural mechanical property Project electric power The Topic: Design a rectifier to control speed of a separately excited DC motor • Natural Speed –Current characteristic: (1.6) • Natural Speed -Torque characteristic: (1.7) 1.2.4 Artificial Speed -Torque characteristic Artificial mechanical characteristics: = f (M) when the electrical parameters are not rated parameters or when the electric circuit has been added Rf, Lf - Each motor has many artificial mechanical properties • Speed -Torque characteristic: (1.8) 1.3 METHODS OF ADJUSTMENT CHANGE ENGINE SPEED DC 1.3.1Change the auxiliary resistance in the armature circuit Speed -Torque characteristic: (1.9) We see that when we change Rf, ω_o = const and change, so we will be adjusted by the same ω_o and steeper as the larger Rf, with the same load, the lower the speed Figure 6: Speed adjustment characteristic by Rfchange Adjustment characteristics: • • Ideal constant idle speed Only allows speed change adjustment on the downward side Project electric power The Topic: Design a rectifier to control speed of a separately excited DC motor • As Rf increases, the greater the slope of the mechanical properties, the softer the mechanical properties ⇒ the lower the speed stability, the greater the speed • error Power loss in the form of heat on the auxiliary resistor If we increase Rfto a certain value, it will make M ≤ Mc so that the motor will not spin and the motor is in short circuit mode (ω = 0).From now on, we can change the Rf and the speed will remain 0, which means the engine speed cannot be adjusted anymore.Therefore this adjustment method is not a radical adjustment method Advantages: The changing device is very simple, often used for crane motors, elevators, lifters, and excavators Disadvantages: The lower the adjustment speed, the greater the input resistance value, the softer the mechanical properties, the reduced stiffness leads to poor speed stability when the load changes poorly.The auxiliary losses are very large when adjusting, the lower the speed, the higher the auxiliary losses The Rfchange method is suitable only when starting the engine 1.3.2 Change of motor magnetic flux Speed -Torque characteristic: (1.10) We see that when changes, and Δω both change, so we will get the curves adjusted gradually and higher than the natural mechanical properties when ϕ is smaller, with the same load, the higher the speed when reducing the fluxϕ Figure 7: Speed adjustment characteristic by change ϕ Adjustment characteristics: Project electric power The Topic: Design a rectifier to control speed of a separately excited DC motor • Decreasing the flux results in inversely proportional change of speed The lower the flux, the more ideal idle speed increases, and the greater the motor speed • Constant short-circuit current • Mechanical property stiffness decreases with decreased flux If is too small, it may cause the motor speed to exceed the permissible limit, or make the switching condition worse due to the increased armature current.Thus, to ensure normal switching, it is necessary to reduce the armature current ⇒ the torque on the motor shaft rapidly decreases ⇒ the motor is overloaded Advantages: The speed adjustment method by varying the flux can be infinitely adjusted and gives the speed greater than the basic speed.The bouncing method is often used for machines such as: universal grinder, bed planer, The adjustment is done on the exciter circuit so the loss of energy is low, the equipment is simple so the price is low Disadvantages: Due to deep adjustment, β decreases, large static error, less stable with high speed.That means the deeper the adjustment, the larger Δω.So the more the characteristic is that the smaller the torque is until the smaller the load torque, the motor cannot run 1.3.3 Change of motor armature voltage Speed -Torque characteristic: (1.11) We see that when Uưchanges, changes and Δω = const, so we will be adjusted parallel by the property lines But if you want to change Uư, you must have a DC power supply that can change the output voltage, often using a converter Figure 8: Speed adjustment characteristic by Uư change Project electric power The Topic: Design a rectifier to control speed of a separately excited DC motor Adjustment characteristics: • The motor speed increases / decreases in the direction of increasing / decreasing the armature voltage • Variable both ideal no-load speed ω_o, and short-circuit current • Mechanical property hardness remains constant throughout the adjustment range • Speed can only be adjusted on the downward side because it can only be changed with UưUđm Advantages: The speed control method by varying the motor armature voltage will keep the characteristic line stiffness, so it is widely used in metal cutting machines.Ensuring economy, low energy loss, wide range of adjustment.If combined with the flux adjustment method, we can adjust the higher and smaller speeds than the rated speed Disadvantage: This method requires a power supply that can smoothly change voltage Project electric power The Topic: Design a rectifier to control speed of a separately excited DC motor CHAPTER II: STRUCTURE CALCULATION ANALYSIS 2.1 Choose the driving circuit As mentione20d in Chapter II, I chose the dynamic circuit as a controlled 3-phase ray rectifier Circuit diagram as shown below: Figure 2.1: Dynamic circuit Project electric power 10 The Topic: Design a rectifier to control speed of a separately excited DC motor Maximum forward voltage rise rate: du / dt Valve lock recovery time: tph Value of current growth rate: di / dt Forward pressure drop for valve: ∆U Control current: Iđk Minimum control voltage for valve opening: Uđk 2.3.3 Overvoltage protection for the valve Overvoltage protection due to the switching thyristor is protected by connecting RC in parallel with the thyristor When there is a switch, the electrolytes accumulated in the semiconductor layers emit outward currents for a short period of time, the rapid variation of the current produces a very large inductive electric charge in the inductors, make overvoltage between anode and katot on thyristor When there is R-C connected in parallel with the thyristor, it creates a circuit of charge room during the switching process, so the thyristor does not overvolt We choose parameters R1 and C1 as follows: R = – 30 () C1 = 0,25 - () 2.4 Determine the scope of the control angle - Select the minimum opening angle: α_min = 10 ° with this opening angle as the reserve angle we can compensate for the drop in grid voltage - When the opening angle is minimum then the voltage on the load is max - - When the opening angle is greatest then the voltage on the load is Since then we have: Project electric power 13 The Topic: Design a rectifier to control speed of a separately excited DC motor 2.5 Choose maneuver - We have the select engine available in the psim software, with the following information: + Power: P = 10 kW, Uđm = 400V, nđm = 1480v / p, Iđm = 10A, Mc = 60% Mđm + Voltage: Uđm = 400V + Speed rotation: nđm = 1480v / p + Rated current: Iđm = 10A CHAPTER 3: SELECTION AND ANALYSIS OF THE CONTROL NETWORK 3.1 Theoretical basis of the Thyristor control Thyristor is only open for current to flow when there is a positive voltage applied to the anode pole and there is a positive voltage pulse applied to the control pole, after the Thyristor is opened, the control pulse no longer works, current flows through the Thyristor due to the parameters of the dynamic circuit decided and the Thyristor will lock when the current flowing through it is 0, if we want to reopen, we must supply the control pulse again Therefore, with sinusoidal voltage, depending on the time of impulse control that we can control the current of Thyristor To achieve these features we can use the following two principles: - Principle of horizontal control - Principle of vertical control Currently, controlling Thyristor in rectifier diagram, people often use the principle of vertical control, so I use this method to design the control circuit Project electric power 14 The Topic: Design a rectifier to control speed of a separately excited DC motor The content of this method: Figure 3.1: Structure diagram of horizontal control principle Structure diagram and illustrative graph as shown Here, Uua generates a quasivoltage that has a fixed (usually serrated) form, which is cyclic due to Udb's synchronous rhythm The SS comparison stage determines the equilibrium of the two voltages U-U and U to initiate the TX pulse generation Thus, in this principle, the pulse timing or valve opening angle changes due to the change in the value Uđk Figure 3.2: For example control structure circuit graph Project electric power 15 The Topic: Design a rectifier to control speed of a separately excited DC motor 3.2 Control circuit structure The control circuit includes the following basic stages: Figure 3.3: Control circuit structure 3.3 synchronous - Select the synchronous circuit two half cycle: Figure 3.4: Synchronous circuit diagram The two half-cycle rectifier circuit has a midpoint using diodes D1, D2 and the rectifier load is resistor R0 The rectifier voltage Ucl after being created is brought to the (+) pole of the Opam to compare with (because the (-) pole of the opam is grounded) If Ucl> then Udb is equal to the saturation voltage (Ubh) Project electric power 16 The Topic: Design a rectifier to control speed of a separately excited DC motor If Ucl> then Udb is equal to the negative saturation voltage (-Ubh) The point of intersection of Ucl and is the transition point of the output voltage Figure 3.5: Oscillation graph synchronous circuit diagram 3.4 Stitch creating serrated Hình 3.6: Circuit Stitch creating serrated Activities: + When Udb < then D3 guide; so UR4 = Udb Udb = UC1 When the voltage reaches the threshold of Dz voltage diode, it will keep the output voltage at this voltage stabilizer (if there is no Dz ⇒ UC increased to + Udb) + When Udb > then D3 block Capacitor is launched UC dropped to and Dz keep UC in value - 0,7 Project electric power 17 The Topic: Design a rectifier to control speed of a separately excited DC motor - Calculation: Cycles: T = / f = 0,02 (s) = 20 (ms) select OA species TL082 The control angle range is 168 degrees Capacitor C launch time: = = 9,33 (ms) Choose the voltage regulator diode BZX79C has UDZ = 10 (V) Select capacitors C = 220 (nF) select R6 = 51k serial variable resistor P1 = 8k Time capacitor C loaded: tn = T/2 – = 10 – 9,33 = 0,67 (ms) The saturation voltage of the OA: Udb = E – 1,5 = 12 – 1,5 = 10,5 (V) So select R4 = (k) Figure 3.7: Oscillation graph Circuit Stitch creating serrated Project electric power 18 The Topic: Design a rectifier to control speed of a separately excited DC motor 3.5 The comparison stage Function: Compare the control voltage with the restraint voltage to determine the timing of the control pulse ⇒ Determine the control angle α The comparison stage can be done with an element such as a transistor, or an OA algorithm amplification - We use the OA element because it allows to ensure the highest accuracy is to use dedicated OA coparator, with low cost, without complicated adjustment - Comparison using two-door OA: Hình 3.8: Comparison circuit The two voltages to be compared are applied to two different poles of the OA In the above case Uđk = U +, Utua = U If UDC> Ura ⇒ Ura = + Ubh If Uđk