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12Vdc – 120Vac Emergency Power System PowerPoint PPT Presentation


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12Vdc – 120Vac Emergency Power System. Jim Mosley TA: Wayne Weaver. Introduction. ac power is taken for granted Most dc powered communication systems are charged by ac systems Back-up systems are rarely capable of extended operation

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12Vdc – 120Vac Emergency Power System

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12Vdc – 120Vac Emergency Power System

Jim Mosley

TA: Wayne Weaver


Introduction

  • ac power is taken for granted

  • Most dc powered communication systems are charged by ac systems

  • Back-up systems are rarely capable of extended operation

  • Economical alternative to stand-alone ac generation is needed


Batteries

  • First option to be suggested

  • Good source for clean dc power

  • Limited amount of energy storage

  • Usually charged by an ac source


Charging

  • Typical ac powered charger

  • Alternative power such as solar and wind

  • Alternators and generators


Power Source

  • All methods of recharging a battery require a power source

  • Source must be:

    Reliable

    Available at all times

    Maintenance free

    Not an expensive “just in case” item


People Power

  • People are always around

  • Reliable, although intermittent

  • Not sitting in storage waiting to be used

  • Don’t have a shelf life


Power Transfer

Next time I’m taking the bus!

  • Person + Bicycle + Alternator =

    Charged Battery + Tired Person


Muscle to Electrons

  • Modified commercial bike stands

  • Home-made stands


What is feasible?

  • Typical person has sustainable output of around 100 watts

  • Power directly from a human powered source is too intermittent to be reliable for most electrical devices

  • Main power source would be the battery with a person recharging the battery


Complexity of Simplicity

  • Design standardization is required for use by the general public

  • There are many different alternators, each with their own mounting and wiring quirks

  • There are just as many different bicycles, each with their own gearing, tire dimensions, and crank lengths


Determining the range of operation

  • Bench test several different alternators to find any similarities in operation

    • Efficiency

    • Minimum speed required to output at least 35 watts

    • Torque requirements at 35 watts

    • Calculate normal operating speeds from pulley dimension


Determining the range of operation

  • Record data from a variety of bicycles to find a common gear ratio

  • Perform tests to determine comfortable range of cycling

  • Perform tests to estimate the power a human can comfortably produce


Data

  • Only one suitable alternator was found

  • Machine shop was unable to complete mount in time

  • Bicycle data was collected, but not analyzed

  • 100-120 watts is practical


dc Voltage to ac Voltage

  • How to get the “readily available” dc source to power ac chargers and emergency communication equipment

  • Converter is necessary


Converter Components

  • Two components are needed

    • Push-pull forward converter to step up 13.4Vdc to 120Vdc

    • Inverter to produce 120V square wave


Push-Pull Forward Converter

  • To achieve a high gain necessary, the push-pull forward converter uses a dc bus with MOSFETs Q1 and Q2 switching at 50kHz to apply an ac current across the high frequency transformer T1

  • The diodes rectify the signal back to dc while L1 and C2 help to clean up the signal


Inverter

  • MOSFETs T1, T4 provide the positive pulse of the output while T2, T3 provide the negative pulse

  • Deadtime between the switching events eliminates the current spikes that would result from the short circuit


Unitrode UC2526 PWM Modulator


Factors in Choosing the Unitrode Chip

  • Low supply current

  • Soft-start

  • Over-current protection

  • Under-voltage protection

  • Thermal protection

  • Shut-down input for other external protective circuits


Testing the Unitrode 2526A

  • After several attempts to operate the converter with the Unitrode chip, it was replaced with the TL494 PWM modulator

  • TL494 has less features, but was chosen because of extensive use in the Dept

  • Lessons learned implementing the TL494 provided potential solutions for applying the UC2526A


Key Requirements for PWM control

  • Error amplifiers are non-inverting

  • Un-used amplifier inputs should not be left floating

  • Reference input should be kept 2V below Vref

  • Oscillator frequency easily adjusted with an RC circuit


TL494 Operation

  • 50 kHz oscillator signal used by comparitor


TL 494 Outputs at Different Feedback Voltages


MIC4424 MOSFET Driver

  • To protect the output of the TL494, a line driver was used

  • Higher current capacity

  • Cheaper and easier to replace

  • Two inputs and two outputs so only one chip is needed


Other Converter Components

  • MOSFETs were chosen to meet voltage and current requirements

  • Center-tapped transformer wound to provide the widest operating range

  • Large capacitor on supply to reduce switching noise

  • High current diodes to rectify the output

  • Large capacitor to smooth the output voltage


Output Waveforms

  • MOSFET gate signal and output voltage before the diodes

  • MOSFET gate signal and output voltage after the diodes and capacitor


Inverter Components

  • MC78L00 voltage regulator to provide 5Vdc control power

  • LM555 timer for 50% duty cycle 60Hz oscillator

  • SN74LS75 latch to provide complimentary outputs

  • IR2113 high/low side MOSFET driver


LM555

  • The versatile LM555 timer has been a reliable industry work-horse for many years

  • Simple RC circuit sets frequency


Difficulties Implementing LM555

  • Original design incorporated an 74LS14 Schmitt trigger inverter to provide complimentary inputs to the MOSFET driver

  • Unable to achieve dead-time at MOSFET driver due to only one rising edge from the LM555

  • SN74LS75 latch with complimentary outputs used to provide two outputs


Solving Dead-time Issues

  • IR2113 has Schmitt trigger input

  • Dead-time easily controlled by RC circuit


Difficulties in Inverter Operation

  • Dead-time

  • During testing, line driver was configured for low/low side operation

  • Jumper was not removed to allow high/low side operation

  • By-pass caps not installed to reduce switching noise


Tests Performed

  • Operation with varying input voltages

  • Effect on small ac charger output

  • Efficiency


Commercial “brick” with dc Output

  • Output waveform on inverter

  • Output waveform on commercial power

  • Voltage spikes are more pronounced


Commercial “brick” with ac Output

  • Output wave form on inverter

  • Output wave form on commercial power


Efficiency

  • Preliminary results show an overall efficiency of 3% at no load and 56% at full load

  • The converter is more efficient with a no load efficiency of 53% and 58% at full load

  • The main reason for the difference is that the converter requires a minimum load to operate, therefore the actual output is 0 at no load, but the converter is still consuming power


Future Modification

  • Safety features such as:

    • Floating the input and referencing the output to earth ground

    • Overcurrent protection

    • Short-circuit protection

    • Under-voltage shutdown


Credits

  • Professor Swenson

  • Professor Chapman

  • Wayne Weaver

  • Brett Nee

  • Jonathan Kimball

  • Dustin Kramer


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