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AC-to-DC PWM Converters Week 4

AC-to-DC PWM Converters Week 4. What is it?. PWM PULSE WIDTH MODULATION. Output signal alternates between on and off within specified period Controls power received by a device The voltage seen by the load is directly proportional to the source voltage.

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AC-to-DC PWM Converters Week 4

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  1. AC-to-DCPWM ConvertersWeek 4

  2. What is it? PWMPULSE WIDTH MODULATION • Output signal alternates between on and off within specified period • Controls power received by a device • The voltage seen by the load is directly proportional to the source voltage

  3. Switching pattern of a hypothetical four-state PWM converter

  4. Stator of a three-phase electric ac machine

  5. Generation of a space vector of the stator MMFs in a three-phase electric ac machine: (a) phasor diagram of stator currents, (b) vectors of MMFs

  6. MMF Magnetomotive force is a quantity appearing in the equation for the magnetic flux in a magnetic circuit, sometimes known as Hopkinson's law: where Φ is the magnetic flux and R is the reluctance of the circuit. Magnetic reluctance, or magnetic resistance, is a concept used in the analysis of magnetic circuits. It is analogous to resistance in an electrical circuit, but rather than dissipating electric energy it stores magnetic energy

  7. Magnetic flux in a magnetic circuit ℱ= NI where N is the number of turns in the coil and • I is the electric current through the circuit 2. ℱ= ΦR where Φ is the magnetic flux and • R is the reluctance 3. ℱ= HL where H is the magnetizing force (the strength of the magnetizing field) and • L is the mean length of a solenoid or the circumference of a toroid

  8. Space vector of stator MMFs and its components

  9. Introduction and definitions • Types of PWM • Methods of generation • Characteristics of PWM • Applications and examples

  10. Definitions • Duty Cycle: on-time / period • Vlowis often zero

  11. Types of Pulse Width • Pulse center fixed, edges modulated • Leading edge fixed, tailing edge modulated • Tailing edge fixed, leading edge modulated • Pulse Width constant, period modulated

  12. Types of Pulse Width

  13. Analog Generation of PWM Analog PWM signals can be made by combining a saw- tooth waveform and a sinusoid PWM output is formed by the intersection of the saw-tooth wave and sinusoid

  14. Digital Methods of Generating PWM • Digital: Counter used to handle transition • Delta : used to find the PWM at a certain limit • Delta Sigma: used to find the PWM but has advantage of reducing optimization noise

  15. Applications to DC Motors • The voltage supplied to a DC motor is proportional to the duty cycle • Both brushed and brushless motors can be used with PWM • Both analog and digital control techniques and components are available

  16. Three Phase AC motors with PWM • 3 different AC currents at different phases • Phase: 120 degrees apart • Creates constant power transfer • Rotating magnetic field • Pulses substitute for AC current

  17. Space Vector Modulation • Used for three-phase AC motors • Convert DC current to AC current • Gates turned on/off at different intervals • 3 PWM created

  18. Motor Control Diagrams

  19. Advantages of PWM • Average value proportional to duty cycle, D • Low power used in transistors used to switch the signal • Fast switching possible due to MOSFETS and power transistors at speeds in excess of 100 kHz • Digital signal is resistant to noise • Less heat dissipated versus using resistors for intermediate voltage values

  20. Cost Complexity of circuit Radio Frequency Interference Voltage spikes Electromagnetic noise Disadvantages of PWM

  21. Introduction and definitions • Types of PWM • Methods of generation • Characteristics of PWM • Applications and examples

  22. Applications of PWM • In the past, motors were controlled at intermediate speed by using resistors to lower delivered power • Electric stove heater • Lamp dimmers • Voltage regulation – convert 12 volts to 5 volts by having a 41.7% duty cycle • Sound production: PWM controlled signals give sound effects similar to a chorus • Power transfer: PWM used to reduce the total power given to a load without relying on resistive losses

  23. PWM used with D/A conversion • Commonly used in toys • Lowpass filter smooths out transients from harmonic effects • Frequency values of harmonics doesn’t change, but the amplitude does, which adjusts the analog output signal

  24. PWM used to transmit data in telecommunications • Clock signal is found “inside” PWM signal • More resistant to noise effects than binary data alone • Effective at data transmission over long distance transmission lines

  25. Frequency of the PWM Signal Upper Limits Lower Limits If too high the inductance of the motor causes the current drawn to be unstable MOSFET transistor generates heat during switching Limited by resolution of controller Eddy currents generated in electromagnetic coils which lead to adverse heating Heat losses in electromagnetic materials is proportional to frequency squared Must be at least 10 times higher than the control system frequency Higher than 20kHz – audible frequency of sounds to avoid annoying sound disturbances, caused by magnetostriction If too low the motor is pulsed, not continuous, because the motor’s inductance can not maintain the current Inverse of frequency should be much less than the motor/load time constant Higher error from ripple voltages

  26. Single-phase PWM rectifier with an LC input filter

  27. Example: PWM with 555 Timer Potentiometer is used to adjust the duty cycle

  28. Example: Specifying circuit elements Requirements • Maxon EC-16 brushless motor, • Time constant = 8.75 ms • 2. Want to avoid audible frequencies • f ≥ 20 kHz • 3. PID control loop running at 150 Hz • f ≥ 10 ∙ 150 Hz

  29. Example: Specifying circuit elements This circuit has a PWM frequency according to: Set f to 25 kHz to add in a factor of safety Choosing C1to be 100 nF, R1 is 576 Ω ~ 500 Ω Recalculating with these values f = 28.8 kHz Check constraints ≥ 117 Hz ≥ 20 kHz ≥ 1.5 kHz f

  30. Where can I buy a PWM controller? Texas Instruments Digikey Mouser Electronics Critical Velocity Motor Control HUGE BIGGER SMALL Texas Instruments TAS5508B 8-Channel Digital Audio PWM Processor 64 pin chip, max 192 kHz frequency $7.25 120 amps, used for hybrid vehicles $469.00 18 kHz frequency Continuous 28 amps $55.95

  31. SAMPLE PWM CIRCUITS

  32. Current-type PWM rectifier

  33. Reference current vector in the vector space of input currents of a current-type PWM rectifier

  34. Example waveforms of switching variables in one switching cycle of a current-type PWM rectifier

  35. Control scheme of a current-type PWM rectifier

  36. Waveforms of output voltage and current in a current-type PWM rectifier: (a) m = 0.75, (b) m = 0.35 (fsw/fo = 24, RLE load)

  37. Waveforms of the input current and its fundamental in a current-type PWM rectifier: (a) m = 0.75, (b) m = 0.35 (fsw/fo = 24, RLE load)

  38. Waveforms of (a) output voltage and current, (b) input current and its fundamental, in a current-type PWM rectifier: in the inverter

  39. Harmonic spectra of input current in a current-type PWM rectifier: (a) fsw/fo = 24, (b) fsw/fo = 48

  40. Voltage-type PWM rectifier

  41. Phase A branch of a voltage-type PWM rectifier

  42. Input-voltage space vectors of a voltage-type PWM rectifier: (a) line-to-line voltages, (b) line-to-neutral voltages

  43. Reference voltage vector in the vector space of line-to-neutral input voltages of a voltage-type PWM rectifier

  44. Principle of voltage-oriented control of a voltage-type PWM rectifier

  45. Control system of a voltage-type PWM rectifier using a rotating reference frame and Space Vector PWM (SVPWM)

  46. Direct power control system of a voltage-type PWM rectifier

  47. Waveforms of input voltage and current in a voltage-type PWM rectifier at unity power factor

  48. Waveforms of output voltage and current in a voltage-type PWM rectifier

  49. Electromechanical representation of a DC machine

  50. Plane of operation, operating area, and operating quadrants of a rotating electric machine

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