SINUSOIDAL PWM OPERATION OF AN AC INDUCTION MOTOR CONTROLLER CONTENTS Abstract I Introduction II Design Overview A AC-to-DC Converter B PWM Generator C Gate Driver D DC-to-AC Inverter III Theory of AC Motor Controller Operation A Faraday’s Law B Torque-speed characteristics C Inductive reactance D Duty Cycle E Volts per Hertz Ratio F Synchronous PWM G Gate Driver H Fundamental and Harmonics J Switching Edges K Overshoot L Frequency-Speed Relationship IV Efficiency and Reliability Considerations 19 A Efficiency Considerations Zero-voltage switching DC-to-DC conversion Deadtime distortion B Reliability Considerations 1.Snubber circuits Electrical isolation Overcurrent protection AC MOTOR CONTROLLER PAGE APRIL 26, 2000 Source conversion Thermal protection Layout 25 V Results A Converter Module B PWM Module Variable Width Pulse Generation Switcher Pulse C Inverter Module Analog Switchers Gate Drivers AC Synthesis VI Conclusion 35 References 36 Appendix 37 A Specification B Assembly Test Form C Performance Test Form D Component Data Sheets AC MOTOR CONTROLLER PAGE APRIL 26, 2000 SINUSOIDAL PWM OPERATION OF AN AC INDUCTION MOTOR CONTROLLER Curtis Nelson, Kelly McKeithan, Tom McDonough, Mark Gelazela ABSTRACT A single-phase ac induction motor controller is presented and the PWM (pulsewidth-modulated) frequency control part of the operation is verified experimentally The application for this ac motor controller is existing single-phase ac induction motors less than ½ hp The ac motor controller is a VFD (variable frequency drive) Control is by a voltage source PWM inverter that uses IGBTs for power transistors The IGBTs are configured in a full H-bridge with unipolar voltage switching A variable frequency (45 Hz to 75 Hz) output waveform is generated by the inverter to run a motor at variable speeds that is directly proportional to this range of frequencies The objective of this project is to construct an ac motor controller from scratch with as many base components as possible, and to demonstrate the sinusoidal PWM operation of the inverter part of the design that runs an ac induction motor at variable speeds I INTRODUCTION Numerous motor driven appliances operate in our homes and businesses today (refrigerators, air-conditioners, washers, dryers, basement water pumps etc.) Most of these appliances run on single-phase ac induction motors less than ½ horsepower And most of those motors lack a proper motor controller in order to run the motor more efficiently Motors can run more efficiently by varying the speed of the motor to match the load Motors are rated to operate best at full load A motor controller that can vary the speed of the motor automatically as the load changes will save energy Fifty percent of electrical energy is consumed by motors An estimated 10% of this is wasted at idle and an additional 5% to 10% is wasted when the motor operates at less than full load Therefore, the purpose of this paper is to describe an ac motor controller that can be applied to existing single-phase ac induction motor appliances, and to demonstrate sinusoidal PWM operation of the converter-inverter phase of the design The prototype of this design experimentally verifies the key concept of pulse waveform generation to produce the switching action of an inverter in order to form a synthesized sine wave that runs an ac induction motor The motor controller of this design is a converter-inverter configuration The converter supplies dc voltage to the inverter by rectifying the 120V, 60 Hz incoming ac signal from the wall outlet to a dc voltage The ac voltage is also stepped down for the low-voltage control circuits The PWM drives the gates of the power transistors in the inverter The inverter synthesizes an ac sine wave from the dc voltage by the switching intervals of its power AC MOTOR CONTROLLER PAGE APRIL 26, 2000 transistors The switching intervals are determined from the duration of the pulse from the PWM The ac induction motor then synthesizes this chopped signal from the inverter into an ac sine wave, because the motor’s inductance smoothes out the “notches” in the waveform The motor will then run at a speed proportional to the frequency of this signal This design is intended to be an interface between the ac source and the motor in order to regulate the motor speed of an appliance to match the load efficiently The load requirements are determined by feedback from sensors to the PWM waveform generators The prototype demonstrates waveform generation of the switching action of the inverter to run an ac induction motor The small-signal waveforms from the PWM shape the high voltage sinusoidal output waveform that runs the motor The sine wave determines the frequency of the output waveform applied to the load The triangle wave determines the switching frequency by the power transistors (IGBTs) of the inverter The resulting pulses from the comparison of the sine and triangle waves turns on the switches The output waveform varies from 45 Hz to 75 Hz The speed of the motor varies in direct relationship to the frequency II DESIGN OVERVIEW This design overview section will briefly review the relevant issues of motor controller design Fig.1 below is a block diagram of the ac motor controller A section covering theory of ac motor controller operation, then a section on efficiency and reliability considerations, and then the results of the experiment with the prototype will follow this Fig.1 Block diagram of the ac motor controller showing the ac-to-dc conversion, the PWM pulse, and the dc-to-ac inverter AC MOTOR CONTROLLER PAGE APRIL 26, 2000 The motor controller shown in Fig.1 operates by converting the ac input signal into a dc signal through the converter It then converts the dc signal back into a controllable ac signal through the inverter in order to run the ac induction motor at a variable speed Part of the dc voltage from the converter operates wave generators in the PWM that then send pulses in the form of square waves through the gate driver and into the inverter There the pulses turn on and off transistor switches at a designed rate in order to synthesize an ac sine wave at a variable frequency A AC-TO-DC CONVERTER The converter section of the motor controller converts ac to dc Both high and low voltage dc are required The converter rectifies the incoming ac signal for the high voltage requirement The converter also supplies the low voltage requirements A transformer steps down the incoming ac signal and then this low ac voltage is rectified into a dc signal in order to power the low voltage PWM generators and control circuits The purpose of the rectifier is to convert an ac signal into a dc voltage However, unlike voltages cannot be directly connected or else KVL problems will result If the diode bridge rectifier connects an ac voltage with a dc voltage, the input current can be extreme The ratio between the peak input current and the average output current would be high in this arrangement The higher the capacitance, the briefer the total on-time, and the higher the current spike will need to be to transfer the necessary energy So a smoothing inductor acting as a current transfer source should be added to the output of the rectifier in order to improve its performance B PWM GENERATOR The purpose of the PWM component of the controller is to generate pulses that trigger the transistor switches of the inverter The pulse-width modulated signal is created by comparing a fundamental sine wave from a sine-wave generator with a carrier triangle wave from a triangle wave generator as shown in Fig.2 below The variable width pulses from the PWM drives the gates of the switching transistors in the inverter and controls the duration and frequency that these switches turn on and off The frequency of the fundamental sine wave of the PWM determines the frequency of the output voltage of the inverter The frequency of the carrier triangle wave of the PWM determines the frequency of the transistor switches and the resulting number of square notches in the output waveform of the inverter AC MOTOR CONTROLLER PAGE APRIL 26, 2000 Fig.2 PWM operation V1 is compared to Vcarrier For each time period, T, a square pulse operates the switch of the inverter to output the fundamental waveform Vo1 [3, p.212] A figure of the PWM waveforms together with the resulting pulse is shown below in Fig.3 Fig.3 PWM operation The square pulse from the PWM is superimposed on the sine and triangle waves as shown in this figure The pulse is high during the interval when the sine wave is greater than the triangle wave [2, p.223] The square pulse waveform that is formed from the sine and triangle waves drives the gates of the transistor switches in the inverter and controls the duration and frequency that these switches turn on and off The dotted line sine wave in Fig.3 represents both the low voltage PWM generated sine wave, and also the high voltage output waveform from the inverter that drives the motor C GATE DRIVER The gate driver receives the logic-level control signal generated by the PWM and then conditions this signal to drive the gates of the power transistors of the inverter The gate drivers provide a floating ground for the high-side switching of the IGBTs in a full H-bridge configuration The gate voltage must be higher than the emitter voltage for high-side switching The input components have to be level-shifted from common to the emitter voltage This is accomplished by charging a bootstrap capacitor, which is composed of a capacitor and diode network, that gates the high-side IGBTs The gate AC MOTOR CONTROLLER PAGE APRIL 26, 2000 driver also provides under-voltage protection to ensure the gate stays on for the duration of the turn-on period The gate driver also provides fast switching edges, by minimizing turn-on and off times, in order to prevent the IGBTs from operating in a high dissipation mode and overheating D DC-TO-AC INVERTER The purpose of the inverter is to convert the dc signal into an ac signal with a variable frequency The output waveform from the inverter is a series of square waves that the motor ‘sees’ as a sine wave because the inductance of the motor smoothes out this “chopped” waveform The amplitude of the synthesized sine wave is determined by the widths of these square waves The relative widths of these square waves represent the applied voltage The wider the widths and the narrower the notches between the widths, the higher the amplitude of the synthesized sine wave because more voltage is being applied H-BRIDGE The inverter consists of an H-bridge, which is a configuration of power transistors A full H-bridge for single-phase application using IGBTs (in the positions where switches are shown) for the power transistors is shown in Fig.4 below Fig.4 Single-phase full H-bridge inverter The switches are IGBTs The topology is for bipolar voltage switching [3, p.212] TA and TB are the IGBT switching transistors D A and DB are free-wheeling diodes Free-wheeling diodes are used to clamp the motor's kickback voltage, as well as to steer the motor's current during normal PWM operation SWITCH The symbol for the transistor switch in the inverter that the PWM controls is shown in Fig.5 below: AC MOTOR CONTROLLER PAGE APRIL 26, 2000 Fig.5 Power transistor (IGBT) that does the switching in the inverter The gate is turned on by applying 15 Vdc to G Current then conducts from C to E The high-voltage dc signal from the rectifier is applied to the collector (C) When the pulse from the PWM arrives at the gate (G), the switch is turned on and the transistor conducts for the duration of the pulse AC MOTOR CONTROLLER PAGE APRIL 26, 2000 III THEORY OF AC MOTOR CONTROLLER OPERATION Faraday's Law sets the saturation flux for maximum energy conversion The torque-speed characteristics of a motor are affected by the applied voltage and frequency Inductive reactance can increase current and heat up the motor The duty cycle is a measure of the ratio of the applied voltage and frequency to the motor The volts-perHertz ratio is set by the sine and triangle waves of the PWM and controlled by a PLL Synchronous PWM reduces harmonics The gate driver conditions the pulse that drives the IGBT gates Harmonics are a function of the frequency modulation index Transistor switching edges dissipate power Overshoot occurs at switching The synchronous speed of the motor is directly related to the applied frequency A FARADAY'S LAW The ac induction motor requires a constant volts per Hertz ratio in the sinusoidal signal that it receives in order to operate at saturation flux as reported in [1, 2, 3] Saturation flux represents the highest value for a machine to maximize the energy conversion process, so the motor can supply its rated torque The constant volts per Hertz ratio is explained by Faraday’s Law: d ∫ E • dS = − dt ∫ B • da c s The line integral of the electric field intensity, E, around a closed contour is equal to the time rate of change of the magnetic flux, B, linking that contour In magnetic structures with windings, like a motor, the E field in the wire is extremely small and can be neglected, so that the first term reduces to the negative of the induced voltage, e, at the winding terminals The flux in the second term is dominated by the core flux φ Since the winding (and hence the contour C) links the core flux N times, Faraday’s Law reduces to [1, p.10]: e=N dφ dt since λ = Nφ dλ e= dt and e = V0 cos ωt AC MOTOR CONTROLLER PAGE APRIL 26, 2000 dλ = V0 cos ωt dt integrating : V λ = sin ωt ω so Therefore, the saturation flux is : λmax = V0 2πf Where φ is the magnetic flux in Webers, and λ is the flux linkage in Weber-turns So in order to achieve maximum flux linkage, a constant volts per Hertz ratio must be maintained B TORQUE-SPEED CHARACTERISTICS The final speed of the motor is determined by the point in which the load torque equals the generated torque of the motor as shown in Fig.6 below Fig.6 Torque-speed characteristics of a motor operating at saturation flux from stall to slip for operation with no change in voltage, frequency, or speed [4 p.266] From the figure it can be seen that the final speed of the motor occurs at a low slip of around 0.1 EFFECTS OF CONSTANT VOLTAGE If the motor does not maintain a constant volts per Hertz ratio, the torque speed curve will not maintain that straight line characteristic around slip shown in Fig.6 that is required for saturation flux The following figure (Fig.7) illustrates maintaining constant voltage while varying the frequency AC MOTOR CONTROLLER PAGE 10 APRIL 26, 2000 Fig.21 I-V characteristics of a semiconductor device to determine overcurrent protection [3, p 718] A safe value to detect overcurrent would be about twice the continuous current rating of the transistor A shunt resistor in the ground path can detect overcurrent conditions From the International Rectifier product document, one of the traditional protection networks is described: "One can detect the line-to-line short and shoot through currents by inserting a Hall-effect sensor or a linear opto-isolator across the shunt resistor The device should be in series with the negative dc bus line For ground fault protection, an additional Halleffect leakage current sensor could be placed either on the ac line input or on the dc bus The protection circuit is then implemented by using fast comparators The output of these comparators is 'Or'd' with the …PWM generator to initiate the shutdown of the gate signals" [13] The IR2137 integrated monolithic IC device is available to perform this same function SOURCE CONVERSION An ac motor controller performs source conversion The input ac voltage is converted to a dc voltage through the rectifier However, this may cause KVL (Kirchoff’s voltage law) problems since unlike voltages appear to be connected Therefore, a transfer source in the form of a smoothing inductor should be placed between the diode bridge rectifier and the rectifier capacitor as was shown in Fig.19 Since the inductor represents a current source, the incoming voltage converts to a current and then this current converts to a voltage for the inverter Krein describes the process this way, “The source conversion concept is a fundamental of power electronics In a well designed power converter, both the input and output ought to have the characteristics of an ideal source If the input is a voltage source, then the output should resemble a current source If the input is a current source, then the output should have properties of a voltage source" [2, p.87] THERMAL PROTECTION One of the most critical reliability design criteria for a controller is to reduce the junction temperature of the power transistor switches, as reported in [2, 4] This will help lower the mechanical stress level and prolong the life of the transistor For each 10°C rise in the junction temperature, the long-term reliability of the transistor is reduced by 50% Thermal resistance, Rθ , is a measure of the temperature change of a semiconductor material per the applied power level The case-to-heatsink thermal resistance is AC MOTOR CONTROLLER PAGE 23 APRIL 26, 2000 the thermal resistance of the interface material times the average material thickness divided by the area of the device mounted on the heatsink: t RθCS = ρ A The junction-to-case thermal resistance, RθJC is equal to the change in temperature divided by the power dissipation: T toT RθJC = J C PD The power dissipation, PD, is equal to the current, I C , times the on voltage of the IGBT, VCE ,ON PD = I C VCE , ON The current draw for calculating the thermal resistance occurs at locked rotor condition and 100% duty cycle This represents the worst case for IGBT power dissipation Silicone grease is used to ensure a contact surface for heat conduction but may eventually dry out, so thermal conduction interface pads may be used instead A temperature sensor may be added to shut down the power stage at critical temperatures A natural air-convection heatsink is used to dissipate the unwanted heat from the power stage LAYOUT As reported in [3, 4], proper circuit layout is critical to the total design of a motor controller Valentine suggests the following rules for layout: Minimize loop areas because loops are antennas that can pick up noise and affect the power stage Run the signal and return close together in order to cancel the electromagnetic fields in the wires Avoid 90° angles on wires that carry high-speed signals because this discontinuity will produce unwanted reflections And connect the control signal ground from the PWM to a single point on the power stage of the inverter because transient voltage drops can be substantial along power grounds due to the high values of di/dt that flows through a finite inductance [4, p.233] Inductive voltage kickback, VPK , is caused by conductors with excessive length and is calculated by the following equation, di VPK = L dt where the change of current with time is determined by the controller’s switching speed And the inductance of the conductors is calculated by  4l  L = (.002 × l )(2.3026 × log − 75  ) µH d  where l is the length of the conductor, and d is the diameter Therefore, increasing the diameter of the conductor has a minimum effect on stray inductance compared to decreasing the length In general, leads should be kept under an inch if possible AC MOTOR CONTROLLER PAGE 24 APRIL 26, 2000 V RESULTS This results section analyzes the three modules that comprise the ac motor controller: the converter module, the PWM module, and the inverter module The converter module consists of a step-down transformer and two diode bridge rectifiers The PWM module consists of a triangle wave generator, a Wien bridge sine wave generator and a comparator The inverter module consists of a full H-bridge with IGBTs, two analog switchers, and two gate drivers A CONVERTER MODULE The circuit diagram for the converter module is shown in Fig.22 below The incoming ac signal is sent to both the high and low-voltage sections of the converter The diode bridge rectifier of the high-voltage section uses diodes that are rated for 15 A, 250 V operation The diodes of the rectifier form a dc voltage across the resistor and capacitor The smoothing inductor, L1, is a transfer current source between the ac input and the dc output to overcome source conversion The PTCs are positive temperature coefficient thermistors They dampen the inrush current at turn-on When the current and resulting temperature rises, the resistance increases When the current and resulting temperature falls off with motor operation, the resistance falls off with it for normal operation The temperature rises within microseconds of the current rise An 11kΩ power resistor is required on the output of the high-voltage section due to the high currents CONVERTER MODULE PTC therm L1 S1 V1 HIGH VOLTAGE SECTION D7 400uH D9 PTC therm + D10 D8 C1 C2 C4 1.2mF 1.2mF C3 1.2mF 1.2mF R2 11k 170 Vdc - Dbreak D7 R3 D10 + 15 Vdc C1 D9 D8 TX1 R4 LOW VOLTAGE SECTION - 15 Vdc Fig.22 This is the converter module of the ac motor controller The high-voltage section supplies the 170 V dc signal for the inverter The low-voltage section supplies the +15 Vdc and –15 Vdc signals to run the op-amps of the PWM module and the control circuits of the inverter module AC MOTOR CONTROLLER PAGE 25 APRIL 26, 2000 The low-voltage section of the converter supplies the small signal voltages for the PWM and the control circuits of the inverter This section consists of a step-down transformer, a diode bridge rectifier, a capacitor, and two resistors The negative 15 Vdc is obtained by placing a ground between the output resistors as shown in the figure B PWM MODULE The PWM module is an analog, sinusoidal pulse generator This module consists of op-amps that generate the necessary waveforms and pulses to operate switching transistor gates A schematic of the PWM module is shown in Fig 23 below PWM MODULE C1 COMPARATOR PWM PULSE R1 10k TRIANGLE WAVE - BUFFER + PULSE INVERTER SINE WAVE C2 C3 R3a SWITCHER PULSE R2a R2b R3b BUFFER Fig.23 The PWM module of the ac motor controller showing the triangle wave generator, the variable frequency Wien bridge sine wave generator, the comparator, the switcher pulse generator, and the PWM pulse inverter The triangle wave generator provides a high frequency triangle wave whose amplitude is independent of the frequency The generator consists of an integrator as a ramp generator and a threshold detector with hysteresis as a reset circuit The threshold detector is implemented by using positive feedback around an op-amp Frequency is determined from the capacitor and resistor circuit, R1 and C1 shown in Fig.23, AC MOTOR CONTROLLER PAGE 26 APRIL 26, 2000 f=1/(2πR1C1), as well as from the positive and negative saturation voltages of the amplifier The frequency of the triangle wave is 2.1kHz The Wien bridge sine wave generator provides variable frequency pulses to operate the motor at variable speeds The frequency of the sine wave is determined from the resistor-capacitor networks shown in Fig.23 R2a and R3a are NTC (negative temperature coefficient) thermistors for heat sensing R2b and R3b are floatation switch resistors for water level sensing These resistors vary with sensory input The frequency of the sine wave then varies with the change in resistance from the relationship, f=1/(2πRC) For heat sensing, the resistor-capacitor networks of R2a and C2, and R3a and C3 vary the frequency of the sine wave For water level sensing, the resistorcapacitor networks of R2b and C2, and R3b and C3 vary the frequency of the sine wave The frequency and speed of the motor varies with sensory input to these resistors The fundamental sine wave and the carrier triangle wave are both generated and compared in the PWM section of the circuit The two resistor-capacitor networks shown in lower left of the above figure determine the frequency of the fundamental sine wave; therefore, in order to meet the specification that the frequency of the inverter vary, one of the resistor-capacitor parameters in each of the networks has to vary For this purpose, the resistors were chosen to be thermistors in combination with floatation switch resistors The thermistors allow that the frequency of the wave applied to the motor will increase or decrease with a temperature change The floatation switch resistors allow the frequency applied to the motor to increase or decrease with an increase or decrease in surrounding water height for a water pump application These components allow for automatic sensory feedback to the PWM section in order to speed the motor up or down if, for instance, the temperature in the room rises and the VFD is hooked to a fan or if the water in the basement rises and the VFD is hooked to a water pump The buffer amplifiers are used to connect the source resistance to the load resistance in order to prevent significant signal attenuation The comparator op-amp receives both the sine wave and the triangle wave signals as inputs The comparator then outputs a variable width, square wave pulse, generated by the intersection of these two waveforms that is called a PWM pulse This PWM pulse operates the gates of selected switching transistors in the inverter to conduct current to the motor in one direction An inverting op-amp inverts this PWM pulse which then operates the diagonally opposite gates of the inverter to conduct current in the other direction Comparing the sine wave to ground generates a switcher pulse The switcher pulse outputs a square wave pulse with a 50 % duty cycle This pulse has the same frequency as the sine wave and is used to direct the PWM pulse and the inverted PWM pulse through the motor VARIABLE WIDTH PULSE GENERATION One of the functions of the PWM module is to generate a variable width pulse (called a PWM pulse) that gates the switching transistors of the inverter in order to synthesize an ac signal to run the motor The mechanism for producing this pulse is shown in Fig.24 below AC MOTOR CONTROLLER PAGE 27 APRIL 26, 2000 SINE W AVE TRIANGLE W AVE PW M PULSE Fig.24 Variable width pulse generation with a sine wave and a triangle wave (PWM pulse) A low frequency sine wave is compared to a high frequency triangle wave to generate a variable width pulse From Fig.24 it can be seen that the pulse is high (on) when the amplitude of the sine wave is greater than the amplitude of the triangle wave, and low (off) when the sine wave is less than the triangle wave The width of the pulse is determined by the successive intersections of the waveforms as shown by the dashed lines in the figure The pulses are wider when the sine wave is high (indicating a longer duration that the pulse is on), and narrower when the sine wave is low (indicating a shorter duration that the pulse is on) The variable width pulses from the PWM control the duration and frequency that the switching transistors turn on and off The frequency of the fundamental sine wave of the PWM determines the frequency of the output waveform to the motor The frequency of the carrier triangle wave of the PWM determines the frequency of the transistor switches in the inverter and the resulting number of square notches in the output waveform SWITCHER PULSE In addition to the PWM pulse, the PWM module also generates a switcher pulse shown in Fig 25 below The switcher pulse operates at the same frequency as the output waveform to the motor, and is the mechanism that alternates the current to the motor AC MOTOR CONTROLLER PAGE 28 APRIL 26, 2000 PWM OUTPUT PULSES SWITCHER PWM SINE WAVE PULSE PWM ON OFF PASSED PULSE SWITCHER PULSE INVERTED ON OFF PWM PASSED PULSE Fig.25 The graphs on the left show the mechanism for generating the switcher pulse When the sine wave is high, the switcher pulse is on (logic 1) When the sine wave is low, the switcher pulse is off (logic 0) The graphs on the right show the three pulses generated by the PWM module to operate the inverter module Only the wide pulses (representing maximum voltage output) are passed to the inverter, as indicated in the figure The switcher pulse is constructed by comparing the sine wave generated by the PWM with ground; and thus the switcher pulse has the same frequency as this sine wave as well as the same frequency as the output waveform The switcher pulse determines which gate driver is on Notice from the graphs on the right of the figure above that the PWM pulses to the left of the red dashed line (over the period that the switcher pulse is on) are identical to the inverted PWM pulses to the right of the red dashed line (over the period that the switcher pulse is off) These three pulses are the pulses that form the output waveform C INVERTER MODULE The function of the inverter module is to control the gating of the switching transistors in the proper sequence in order to synthesize an ac signal from a dc input The schematic of the inverter module is shown below in Fig.26 The gate drivers condition the pulses from the PWM to provide a floating ground for high-side switching of the IGBTs in a full H-bridge configuration Gate voltage must be 10-15 volts higher than the emitter voltage to fully utilize the full switching capabilities Being a high side switch, the gate voltage would have to be higher than the rail voltage, which is frequently the highest voltage available in the system The gate voltage must be controllable from the logic, which is normally referenced to ground Thus, the control signals have to be level-shifted to the emitter of the high side power devices AC MOTOR CONTROLLER PAGE 29 APRIL 26, 2000 INVERTER MODULE GATE DRIVERS 100 V+ V+ D2 IN VB 12 SD 100 NC D1 170 VDC HO NC 14 VB IN D1 D2 SD 12 VS DT S1 S2 NC1 VSS 10 NC2 COM IN NC3 Z3 V+ HO Z1 V+ V- GND MOTOR VS S1 S2 DT 10 VSS NC1 IN COM NC2 GND V- LO 1u 1u Z2 Z4 NC3 LO 100 DT1 200K H-BRIDGE 100 DT2 200K SWITCHERS Fig.26 The inverter module of the ac motor controller consists of an H-bridge with four IGBT power switching transistors, two analog switchers, and two gate drivers Under these constraints, several techniques are presently used to perform these functions The design presented here utilizes a bootstrap circuit, which is the most simple and economical approach The gate driver chosen was an International Rectifier control IC, IR21094, for high voltage, high speed power MOSFET and IGBT drivers with dependent high and low side referenced output channels The logic inputs are fully compatible with standard CMOS or LSTTL output The output driver features a high pulse current buffer stage designed for minimum cross-conduction The floating channel drives an N-channel IGBT in the high side configuration, which operates up to 600 volts The operation of the bootstrap circuit utilizes a capacitor and diode network to gate the high IGBT The Vbs voltage (the voltage difference between the Vb and Vs pins on the control IC) provides the supply to the high side driver circuitry of the control IC that in most cases is a high frequency square wave The supply needs to be in the range of 10-20 volts to ensure that the control IC can fully gate the IGBT Under-voltage protection circuitry ensures that the IC does not drive the IGBT below a certain voltage This prevents the IGBT from operating in a high dissipation mode When Vs is pulled down to ground (either through the low side field effect transistor or the load), the bootstrap capacitor (Cbs) charges through the bootstrap diode (Dds) from the 15-Volt Vcc supply Thus providing a supply to Vbs, as the figure above illustrates (Fig.26) There are five factors which contribute to the supply requirements from the Vbs capacitor These are: Gate Charge required to enhance the FET AC MOTOR CONTROLLER PAGE 30 APRIL 26, 2000 Iqbs-queiscent current for the high side driver circuitry Currents within the level shifter of the control IC FET gate-emitter forward leakage current Bootstrap capacitor leakage current Note: Factor is only relevant if the bootstrap capacitor is an electrolytic capacitor, and can be ignored if other types of capacitor are used The following equation details the minimum charge, which needs to be supplied by the bootstrap capacitor: Qbs = 2*Qg + Iqbs(max)/f + Qls + Icbs(leak)/f (1) Where: Qg = gate charge of high side FET Icbs(leak) = Bootstrap capacitor leakage current Qls = level shift charge required per cycle = 5nC(500V/600V IC’s) or 20 nC (1200V IC’s) f = frequency of operation The bootstrap capacitor must be able to supply this charge, and retain its full voltage, otherwise there will be a significant amount of ripple on the Vbs voltage which could fall below the Vbsuv undervoltage lockout and cause the HO output to stop functioning The charge in the capacitor must be a minimum of twice the above value The minimum capacitance value can be calculated from the equation below: C > (2*(2*Qg + Iqbs(max)/f + Qls + Icbs(leak)/f)) (2) Vcc – Vf - Vls Where: Vf = Forward voltage drop across the bootstrap diode Vls = Voltage drop across the low side FET Note: The capacitance above is the absolute minimum required, however, a general rule should apply by multiplying the above factor by 15 to ensure proper charging of the capacitor The bootstrap diode (Dbs) needs to be able to block the full power rail voltage, which is seen when the high side device is switched on It must be a fast recovery device to minimize the amount of charge fed back from the bootstrap capacitor into the Vcc supply Therefore: Diode characteristics: Vrrm = Power rail voltage Max Irr =100ns If =Qbs * f Resistor-diode networks are on the input of each gate of the H-bridge These networks serve two purposes, they prevent conduction overlap of switching intervals by adding dead-time, and reduce the peaking of the current spike during reverse recovery time of the free-wheeling diodes (not shown) across the IGBTs Conduction overlap occurs when the switches that conduct in one direction through the motor not completely turn off before the switches that conduct in the opposite direction turn on A shoot-through current would result and the switches would short out Deadtime delays turn-on to prevent this condition Deadtime is also inserted into the PWM signal by resistors DT1 and DT2 on the DT posts of the gate drivers AC MOTOR CONTROLLER PAGE 31 APRIL 26, 2000 The following analysis of ac synthesis divides the inverter into the left and right-side switchers and gate drivers The left-side operates during the first half-cycle of the switcher pulse The right-side operates during the second half-cycle of the switcher pulse ANALOG SWITCHERS The analog switchers provide the mechanism for directing the current to the motor Diagrams of the analog switchers and their operation are shown below in figures 27 and 28 a FIRST HALF-CYCLE The first half-cycle of the switcher pulse, when the switcher pulse is a logic one (on) passes the PWM pulse, as shown below in Fig.27 Since the first half-cycle of the switcher pulse corresponds to the first half-cycle of the PWM sine wave, the PWM pulses are widest Therefore, the gates of the IGBTs will turn on for a longer period of time to supply the maximum applied voltage to the motor ANALOG SWITCHERS LEFT-SIDE RIGHT-SIDE PWM PULSE S1 D1 D1 S1 INVERTED PWM PULSE S2 D2 D2 S2 SWITCHER PULSE SWITCHER PULSE IN IN First half-cycle Fig.27 During the first half-cycle of the switcher pulse, S1, of the left-side switcher, is switched on when a logic (on) pulse arrives at the IN post from the PWM module The right-side switcher does not conduct Ground is provided to the non-conducting S2 of the left-side switcher and S1 of the right-side switcher to reduce noise b SECOND HALF-CYCLE The second half-cycle of the switcher pulse, when the switcher pulse is a logic zero (off) passes the inverted PWM pulse, as shown below in Fig.28 Since the second half-cycle of the switcher pulse corresponds to the second half-cycle of the PWM sine wave, the inverted PWM pulses are also at their widest over this period to maximize the applied voltage to the motor AC MOTOR CONTROLLER PAGE 32 APRIL 26, 2000 ANALOG SWITCHERS LEFT-SIDE RIGHT-SIDE PWM PULSE S1 D1 D1 S1 INVERTED PWM PULSE S2 D2 D2 S2 SWITCHER PULSE SWITCHER PULSE IN IN Second half-cycle Fig.28 During the second half-cycle of the switcher pulse, S2, of the right-side switcher, is switched on when a logic (off) pulse arrives at the IN post from the PWM module The leftside switcher does not conduct GATE DRIVERS The gate drivers provide the mechanism for turning on the switching transistors (IGBTs) of the inverter Diagrams of the gate drivers and their operation are shown below in figures 29 and 30 a FIRST HALF-CYCLE The first half-cycle of the output waveform is synthesized by the H-bridge from the left-side gate driver (as viewed from the previous Fig.26) shown in Fig.29 below Left-Side Gate Driver +15 Vdc OUTPUT WAVEFORM V+ Vb H-Bridge IN Ho Vdc DT Vs PWM PULSE Z1 First half of Switcher cycle MOTOR Vss Z4 Lo GRD Comm IR21094 First half-cycle Fig 29 The left-side gate driver (in this analysis) forms the first half-cycle of the output waveform from the first half-cycle of the switcher pulse and the PWM pulse During the first half-cycle of the switcher pulse, the left-side gate driver receives the PWM pulse from the left-side switcher The high output, Ho, conducts when the PWM pulse AC MOTOR CONTROLLER PAGE 33 APRIL 26, 2000 is on (logic 1) The low output, Lo, is open when Ho conducts Lo defaults to normally closed when there is no pulse, or a logic applied to the IN post Ho is attached to the high-side switching transistors (170 Vdc) Lo is attached to the low-side switching transistors (ground) From the figure it can be seen that Z1 will be gated on and off by the PWM pulse from Ho Z4 will stay gated on over this half-cycle interval because it is attached to the low output (Lo) of the right-side gate driver that is not receiving a pulse at this time So the right-side Lo defaults to normally closed and the transistor, Z4, is gated on The result is that current will conduct from Z1 to Z4 through the motor The output waveform shown in the figure above is identical to the PWM pulse but at a higher voltage b SECOND HALF-CYCLE The second half-cycle of the output waveform is synthesized by the H-bridge from the right-side gate driver (as viewed from the previous Fig.26) shown in Fig.30 below During the second half-cycle of the switcher pulse, the right-side gate driver receives the inverted PWM pulse from the right-side switcher The pattern of the inverted PWM pulse over this half-cycle interval is identical to the PWM pulse over the first half-cycle interval From Fig.30 it can be seen that Ho conducts pulses which gate Z3 on and off Z2 stays gated on over this half-cycle interval because the low output, Lo, in the left-side gate driver is normally closed over this interval Right-Side Gate Driver OUTPUT WAVEFORM H-Bridge Vb Ho INVERTED PWM PULSE PWM PULSE DT Lo +15 Vdc IN Vs Vdc V+ Vss Z3 MOTOR Z2 Second half of Switcher cycle GRD Comm IR21094 Second half-cycle Fig.30 The right-side gate driver (in this analysis) forms the second half-cycle of the output waveform from the second half-cycle of the switcher pulse and the inverted PWM pulse The result is that current will conduct in the opposite direction through the motor from Z3 to Z2 The output waveform shown in the figure is identical to the inverted PWM pulse but at a higher voltage Therefore, the conduction of current alternates direction through the motor every half cycle to synthesize an ac signal AC MOTOR CONTROLLER PAGE 34 APRIL 26, 2000 AC SYNTHESIS The ac output waveform that is synthesized by the inverter module is shown below in Fig.31 OUTPUT WAVEFORM Fig.31 Synthesized output waveform that runs the ac induction motor The full cycle of the output waveform to the motor looks like the figure above An alternating current waveform is synthesized by the switching action of the IGBTs This output waveform is a series of square waves that the motor ‘sees’ as a sine wave because the inductance of the motor smoothes out this “chopped” waveform The amplitude of the synthesized sine wave is determined by the widths of these square waves The relative widths of these square waves represent the applied voltage Increasing the duty cycle results in a higher synthesized sine wave amplitude because the average applied voltage is higher CONCLUSION This design proves the principle of sinusoidal PWM operation of an IGBT based inverter, as it applies to existing single-phase ac induction motor appliances Construction of the prototype conforms to the design principles The rectifier circuit develops a dc voltage within the range required by the inverter The PWM circuit generates a variable width pulse that turns on the switches in the inverter The gate driver circuit conditions the PWM pulse waveform for proper operation of the inverter gates The inverter circuit utilizes the switching action of the IGBTs to synthesize a sine wave The resulting synthesized waveform runs the motor over the specified range of frequencies and speeds This design demonstrates variable speed control of single-phase ac induction motors in order to match the load requirements for efficient operation The prototype controller operated only a small fractional AC MOTOR CONTROLLER PAGE 35 APRIL 26, 2000 horsepower motor, but the design could be theoretically applied to larger motors with the addition of the efficiency and reliability considerations to the circuit REFERENCES A.E Fitzgerald, Electric Machinery, 5th., McGraw-Hill, NY, 1990 Philip T Krein, Elements of Power Electronics, Oxford, NY, 1998 Ned Mohan, Torre M Undeland, William P Robbins, Power Electronics, Converters, Applications, and Design, 2nd., Wiley, NY, 1995 Richard Valentine, Motor Control Electronics Handbook, McGraw-Hill, NY, 1998 Adel S Sedra and Kenneth C Smith, Microelectronic Circuits, Oxford University Press, NY, 1998 Masayuki Morimoto, Katsumi Oshitani, Kiyotaka Sumito, Shinji Sato, Muneaki Ishida, and Shigeru Okuma, “New single-phase unity power factor PWM converter-inverter system”, IEEE Power Electronics Specialists Conference, pp 585-589, 1989 Michael A Boost, Phoivos D Ziogas, “State-of-the-art carrier PWM techniques: a critical evaluation”, IEEE Trans Ind Applicat., vol 24, no 2, pp 271-280, March/April 1988 J F Bangura, N A Demerdash, “Simulation of inverter-fed induction motor drives with pulse-width modulation by a time-stepping coupled finite element-flux linkage-based state space model”, IEEE Transactions on Energy Conversion, Vol 14, No 3, September, 1999 Omar Stihl, Boon-Tech Ool, “A single-phase controlled-current PWM rectifier”, IEEE Transactions on Power Electronics, vol 3, No 4, October, 1988 10 Ken Berringer, “AC motor drive using integrated power stage”, Motorola semiconductor application note AN1524/D, Motorola, Inc., 1996 11 Norbert R Malik, Electronic Circuits, Analysis, Simulation, and Design, Prentice Hall, Englewood Cliffs, NJ, 1995 12 International Rectifier, "HV Floating MOS-Gate Driver Ics", Document INT978, International Rectifier 13 International Rectifier, "Solving IGBT Protection in AC or BLDC Motor Drive", Document address: www.irf.com/product-info/motor/igbtprotect.pdf, International Rectifier AC MOTOR CONTROLLER PAGE 36 APRIL 26, 2000 AC MOTOR CONTROLLER PAGE 37 APRIL 26, 2000 ... of these appliances run on single-phase ac induction motors less than ½ horsepower And most of those motors lack a proper motor controller in order to run the motor more efficiently Motors can... the purpose of this paper is to describe an ac motor controller that can be applied to existing single-phase ac induction motor appliances, and to demonstrate sinusoidal PWM operation of the converter-inverter... Form C Performance Test Form D Component Data Sheets AC MOTOR CONTROLLER PAGE APRIL 26, 2000 SINUSOIDAL PWM OPERATION OF AN AC INDUCTION MOTOR CONTROLLER Curtis Nelson, Kelly McKeithan, Tom McDonough,