High-frequency Transistor Induction Heating Power Supply Type HFG-4-500
SHORT TECHNICAL DESCRIPTION
I. Purpose The high-frequency transistor power supply HFG-4-500 is designed for supplying of induction heating installations using for fast heating and melting of small quantities nonmagnetic metals and alloys. This power supply is specially developed for operation with loads which worth very low power factor values and need to be induction heated at frequencies more than 500 kHz. An induction furnace with ceramic crucible is used as a power supply load. A specific metal quantity in the form of grains mixed with insulating material in a specific proportion is put inside the crucible. Induction heating and melting may be done in inert atmosphere as well.
II. Basic technical parameters of the power supply Installed (maximal) DC power Operating frequency range Supply voltage Power regulation range Maximum cable length from power supply main cabinet to load unit Maximum cables length from power supply main cabinet to remote cabinet Overall dimensions of power supply main cabinet Overall dimensions of power supply remote cabinet Cooling Communication interface Weight of power supply main cabinet Weight of power supply remote cabinet
– 4000 W – 500 ÷ 600 kHz – 380 VAC / 50 Hz – 0 ÷ 100 % –5m –4m – 800 x 600 x 265 mm – 300 x 250 x 165 mm – water and air – MODBUS over TCP/IP; RS-485 – 35 kg – 6 kg
III. Functional blocks. Description The basic structural diagram of power supply HFG-4-500 is shown on Fig.1.
The power supply consists of three separate units: 1. Main cabinet. 2. Remote control unit. 3. Load circuit.
3.1. Main cabinet The power supply main cabinet including load circuit realizes in full account the obtaining of required high-frequency power for induction heating of the metal. It realizes as well all functions of automatic control, regulation, protection and diagnostics of the installation. The power supply main cabinet consists of the following units: - Power supply unit; - High-frequency transistor inverter; - Main controller; - Block of operative circuits and elements. The appearance of the main cabinet front panel is shown on Fig.2, its lateral view â€“ on Fig.3 and relative position of its main units â€“ on Fig.4 and Fig.5.
The main cabinet front panel has the following elements: - LCD display and belonging to it setting push-buttons.
The display provides information on the current state of the power supply – „Idle”, „Preparing”, „Unpreparing”, „Ready”, „Running”, „Discharging” and „Error”. It visualizes as well the current values of power supply basic parameters. The setting push-buttons are used to adjust the operating power level, to establish the type of errors, to change some of the operating parameters of the main controller and etc; - Operative lamps: – OP (Operative circuits and voltages) – white – verifies the successful passing of the power supply initial test and availability of all operative voltages; – RDY (Ready) – orange – verifies the readiness of the power supply to start generation of high-frequency energy; – HF (High Frequency) – green – verifies that a generation of high-frequency energy at a specified level is taking place; – ERR (Error) – red – shows that any error is detected and the power supply operation is stopped. - Operative push-buttons: – RDY (Ready) – after pressing of the button part „I” (Start) begins a process of setting of the power supply to mode of readiness for generation of highfrequency energy. After pressing of the button part „O” (Stop) the power supply returns to state „Idle”; – HF (High Frequency) – after pressing of the button part „I” (Start) the power supply starts to generate a high-frequency energy at a specified level. After pressing of the button part „O” (Stop) the power supply returns to state „Ready”; – Emergency Stop – this button is used for the power supply stopping in emergency conditions. - Operative switch „LOCAL / REMOTE” – the switch is used for selecting the unit which will be used as operational device. If a position „LOCAL” is selected, the main cabinet of the power supply can be used as operational device. If a position „REMOTE” is selected, the remote control unit of the power supply can be used as operational device. - two locks for unauthorized access control prevention The main cabinet lateral panel from the side of electrical power supply (fig.3) has the following elements: Input circuit-breaker – connects and disconnects the power supply and incoming network feeders. It is located at the bottom of the lateral panel; Automatic circuit-breaker – it used for the power supply switching on and switching off in normal and emergency conditions. It is located above the input circuit-breaker. Additional box – it is located at the top of the power supply lateral panel. A high-frequency choke (from the alternating circuit of the inverter) is accommodated in the box. The choke is connected by means of high-frequency cable to the output transformer primary. 5
3.1.1. Power supply unit This unit converts the AC supply network voltage 380 V / 50 Hz into DC voltage for supplying of the high-frequency inverter. An output power regulation of the power supply is realized by means of this unit. The power regulation level can be set between 0 to 100 % from the installed (maximal) DC power (4000 W). The power supply unit is located in the lower half of main power supply plate (see fig.4) and has the following blocks: - Power block – includes uncontrolled rectifier and DC/DC converter; - Control system block – includes control system, regulation system and protection system of the DC/DC converter; - Cooling system of the DC/DC converter. The circuit diagram of the power block is shown on fig.6.
Fig.6 DC/DC converter represents a half-bridge pulse converter. Operating frequency is 40 kHz. The required AC output voltage is obtained on the secondary of the matching transformer T2. Then after rectifying and filtering the obtained DC voltage supplies the high-frequency inverter. The converter control system realizes the operating algorithm of the power supply unit. Its regulation part accomplishes the change of the output voltage by means of changing of the pulses mark-to-space ratio of the converter operating transistors. The main controller sets this changing of the pulses mark-to-space ratio on the basis of desired power level of the power supply. By means of a pulse-width modulator (PWM) and PIDregulator of the main controller so called “soft” start of the power supply is realized. The protection part of the converter control system includes an over-current protection, an over-voltage protection and temperature protection. The cooling system of the power supply unit is realized as air-forced. All power devices have separate heat-sinks with mounted on them fans for cooling. A large fan located on the top of the main cabinet is used for taking away of the total internal air. 6
3.1.2. High-frequency transistor inverter This unit is the "heart" of the power supply HFG-4-500 since exactly it generates high-frequency energy for supplying of the induction furnace. Actually, this is a resonant inverter which is performed on classic half-bridge circuit (fig.7). MOSFET transistors operate in a mode of forced commutation. The inverter includes the following blocks: - Power circuit; - Control and protection system; - Cooling system.
Fig.7 The control system of the inverter realizes it operating mode of forced switching off of the power devices close to the end of their operating half-period. Since the operating frequency is too high – more than 500 kHz, this inverter operating mode is chosen in such a manner to ensure the transistors switching off quite near to the zero value of the alternating current pulse. The transistors switching on takes place in the zero value of the current pulse and the peak amplitude of the current through them is not rather high. Main element of inverter control system is PLL regulator. This regulator decides the most important task of the control system – maintaining of the inverter operating point near to the resonance of inverter equivalent circuit during the generation of high-frequency oscillations. Power supply HFG-4-500 is developed for heating of small quantities non-magnetic metals and alloys in a ceramic crucible. An additional feature of the load is the poor magnetic coupling of the system inductor coil–load by reason of the large technology distance between the inductor coil and the crucible. As a result, the load has too low value of the power factor cosφ which reaches a value of 0.05 or less. This circumstance, in addition to the high frequency, makes a significant impact on the operation of the PLL regulator since a small change in the load parameters leads to a large shift in the location of the operating point in the inverter resonance characteristics. The regulator must quickly respond to the feedback signal from the current transformer in order to change adequately the operating frequency of the transistors and to set the inverter operation to the optimum operating point. 7
This load feature of the power supply HFG-4-500 is at the root of the decision for an operating start-up sequence of the power supply to generate high-frequency oscillations. After connecting to the electrical network and verifying availability of all operating voltages starting procedure continues with pressing of part „I” (Start) of the „RDY” (Ready) pushbutton. In this case, DC/DC converter strats to run but deep regulated by the main controller and on the inverter input applies DC voltage less than 15 V. Inverter starts to generate high-frequency oscillations and sets its optimum operating point. Only when this state of the inverter takes place the main controller indicates that the power supply is ready to generate a high-frequency power at a high level – switches on a lamp „RDY” (Ready) and the state „Ready” and setted operating frequency are indicated on the display. The protection part of the inverter control system includes over-current protection and temperature protection. The cooling system of the inverter is realized as mixed and has two parts – water part and air-forced part. Two large copper heat-sinks are used for each pair of power devices (a MOSFET transistor and a diod). Two additional aluminium heat-sinks with fans for air-forced cooling are located on each copper heat-sink. Another fan is used for cooling of the choke. It is located on the top of the additional box.
3.1.3. Main controller The main controller is designed for controlling the overall operation of the power supply HFG-4-500. This the most important for the power supply control block carries out the complete interrelation between main units of the power supply and between main blocks of these units. It processes information coming from the blocks and control sensors, evaluates it and provides a direction for the corresponding continuation of the power supply operation. The structural circuit diagram of the main controller including all external units, elements and connections is presented on fig.8.
The structure of the main controller includes: Microcontroller, LCD display and setting push-buttons, Input-output subsystem, Control subsystem, Analog subsystem, Communication subsystem, Power supply subsystem. A microcontroller is the main block of the main controller. The overall control is carried out by means of the software especially developed for using in the power supply HFG-4-500 for realizing of the specified technology process. A part of the main functions which the software executes are as follows: - Availability of all operative voltages in the main and remote units after switching on of the electrical power supply network; - The process of reaching to state „Ready” for starting of the induction heating; - A possibility for changing of the power level from the panel of selected operating unit during the generation of the high-frequency oscillations (in state “Running”); - An indicating of the operating states and patameters values simultaneously on the displays of main and remote units; - An indicating of the operating states by means of operative lamps simultaneously on the displays of main and remote units; - A possibility for „soft” starting of the power supply; - A possibility for maintaining of the specified power level; - A possibility for emergency stopping of the power supply operation from the panel of main unit and from the panel of remote unit as well; - A possibility for authentic reflection of the error state on the display – showing and description of the error; - A possibility for showing and storing of the errors; - A possibility for editing of threshold values of the main parameters and ranges of the power supply; - A realizing of „MODBUS over TCP” connection with external computer (or Ethernet network) and sending information about main parameters of the power supply during the generation of the high-frequency energy. LCD display provides visualization of the main parameters current values of the power supply – DC power PS in percents, DC power P in watts, inverter input DC voltage in volts, DC current consumed from the inverter in amperes, operating frequency in kilohertz. A current operating unit (Local or Remote) and a current state of the power supply are presented on the display as well – fig.9.
Setting push-buttons are located next to the LCD display and have the functions as follows: 9
- F1 and F2 – the simultaneously pressing of these bush-buttons is used for moving to mode of review and editing (fig.10) by means of several screens of threshold values of the following parameters: analog channels for measuring of DC voltage and DC current, operating frequency range, maximum DC voltage and minimum DC current in state „Ready”, parameters of the PID-regulator. - ◄ and ► – these push-buttons are used in editing mode for reaching to the desired parameter and when the power supply is in state „Error” – for reaching to the corresponding error state; - + and – – these push-buttons are used for setting of the necessary power level for heating in percents as well as for correction of the threshold values of the parameters in editing mode; - and × – these push-buttons correspond to buttons „Enter” and „Cancel”. They are used in „Error” state for reaching to the errors description screen or for leaving it. In editing mode these buttons are used for confirmation or canceling of the corrections. Input-output subsystem of the main controller includes two parts: digital inputs and digital outputs. Most of input signals enter to inputs located on the main controller PCB and some of them – to inputs of the input-output expander. The control system of the main controller has a purpose to fulfil several functions. First – measuring of the operating frequency in all power supply states except in a state „Idle”. Second – realizing of a „soft” start of the power supply after starting of highfrequency energy generation. Third – maintaining of the specified operating power. The control subsystem performs these functions with the assistance of main controller analog subsystem where the values of inverter DC voltage and DC current are measured. The communication subsystem of the main controller ensures the connection with power supply remote unit, with an interface converter RS422/MODBUS to Ethernet network and with an external programming device. This is very important feature since it ensures a possibility for a comfort distance operating or with the power supply HFG-4-500 or monitoring of the heating process from external network device – PLC or PC.
3.1.4. Block of operative circuits and elements This block combines all the electrical components (circuit-breakers, contactors, transformers, lamps, switches and push-buttons) and all the electrical connections between them used for realizing the logics of switchind on, diagnostics, starting, stopping, visualizing and control of heating process parameters. This block includes also all the components and connections of power supply water cooling system, system for temperature control of power devices operation and leakage protection.
3.2. Remote control unit The second basic unit of the power supply HFG-4-500 set is a remote control unit. This device allows the personnel either to do all the steps of carring the heating process into effect or for distance monitoring of power supply operation. The front panel of remote unit cabinet is presented on fig.11 and includes the following blocks: Control block and Block of operative circuits and elements.
Fig.11 Remote unit control block may consider as an element (distance part) of the structure of the power supply main controller. When the operating switch „LOCAL / REMOTE” is switched over position „REMOTE” the possibility of operation with main cabinet front panel is eliminated. Then, going hand in hand with main controller, the remote unit is transmuted from monitoring unit into unit for real control of the installation heating process. The structural circuit diagram of the remote unit control block is designed in the same manner as the main controller one. The main differences in comparison with the main controller structural diagram are the less number of subsystems, elements in their structure and operating possibilities. When the remote unit is used for monitoring its control block only visualizes operating states and parameters of the heating process. There is also a possibility for emergency stopping of the power supply, but setting push-buttons and input-output subsystem are inactive. When the remote unit is used as operating unit all control block subsystems operate together with main controller control and analog subsystems.
3.3. Load circuit The third basic unit of the power supply HFG-4-500 set is a load circuit. This unit is a basic element of matching between the source of the high-frequency oscillations in the power supply – the transistor inverter and the load (induction furnace). The appearance of the load circuit is presented on fig.12 and fig.13. It is accomodated in a cylindrical box.
On one side the load circuit is connected in parallel with the inductor coil, on the other side, by means of a power cable in series with the high-frequency choke as an element of the alternating inverter circuit. The power cable is performed by a pair Litz conductors placed in common insulation tubing. The load circuit is designed to ensure the following most important requirements: - A capacitive detuning in the equivalent alternating circuit of the inverter as a condition for realizing of stable oscillating process; - A high-frequency voltage with necessary amplitude across the induction coil. The load circuit includes: - High-frequency transformer – it is designed as a step-up transformer since the load of this type (inductor coil with poor magnetic coupling with the metal put in a ceramic crucible) needs a high supply voltage – some kilovolts. The transformer’s primary is water cooled; - Power capacitor – it is realized as a water cooled battery with special construction; - Inductor coil – the whole construction of the inductor coil including its terminals has been assigned from the customer.
Table of contents: Technical description …............................................................................................ 2 I. Purpose …….......................................................................................................... 2 II. Basic technical parameters of the power supply ................................................. 2 III. Functional blocks. Description ……...……………………………………………….. 3 3.1. Main cabinet ……………………………….……………………...………. 4 3.1.1. Power supply unit................................................................................. 6 3.1.2. High-frequency transistor inverter ……………...................................... 7 3.1.3. Main controller ……….......................................................................... 8 3.1.4. Block of operative circuits and elements ........................................... 10 3.2. Remote control unit ………………………………................................... 11 3.3. Load circuit …………............................................................................ 12 Table of contents.................................................................................................... 13