Chapter 16 Notes: Residential and Industrial Applications
16-1 INTRODUCTION
Power electronic converters are generally described in Chapters 1-9.
Applications in DC and AC power supplies are in Chapters 10 & 11.
Motor drives are covered in Chapters 12-15.
Chapter 16 objectives:
Overview of various residential power electronic applications.
Describe additional industrial applications like welding and induction heating.
16-2 RESIDENTIAL APPLICATIONS
Residential homes/buildings consume ~35% of total electricity in the US.
This corresponds to ~8.5% of total primary energy usage.
Applications include:
Space heating/air conditioning.
Refrigeration/freezing.
Water heating.
Lighting.
Cooking.
Television.
Clothes washer/dryer.
Miscellaneous appliances.
Role of power electronics:
Energy conservation.
Reduced operating cost.
Increased safety.
Greater comfort.
16-2-1 SPACE HEATING AND AIR CONDITIONING
Space heating/air conditioning uses ~25-30% of electric energy in all-electric homes.
Heat pumps are used in 1 out of 3 new homes.
Load-proportional capacity modulation can increase heat pump efficiency by up to 30% compared to conventional single-speed heat pumps.
Conventional heat pump: Compressor operates at a constant speed when running.
Compressor output is matched to building load by cycling on/off.
Load-proportional capacity-modulated heat pump (Fig. 16-1):
Compressor motor speed adjusted to match building load.
Eliminates on/off cycling.
Uses induction motor drive (Chapter 14) or self-synchronous motor drive (Chapter 15) to adjust compressor speed.
Benefits of eliminating on/off cycling (Fig. 16-2):
Conventional heat pump (cooling mode):
If sensed temperature exceeds thermostat upper limit, compressor turns on.
Motor draws max power almost immediately, but compressor output increases slowly.
Shaded area in compressor output plot represents energy efficiency loss.
When building temperature reaches thermostat lower limit, motor/compressor turns off.
On-off cycling matches average compressor output (dashed line in Fig. 16-2) to building load, maintaining temperature within a tolerance band.
Load-proportional capacity-modulated heat pump eliminates loss due to on/off cycling.
Compressor speed adjusted to equal building load.
Energy consumption reduced by up to 30% compared to single-speed heat pumps, despite losses in the power electronic converter.
Maintains building temperature in a narrower band, increasing comfort.
16-2-2 HIGH-FREQUENCY FLUORESCENT LIGHTING
Lighting consumes ~15% of energy in residential buildings, ~30% in commercial buildings.
Fluorescent lamps are 3-4 times more energy-efficient than incandescent lamps.
Operating fluorescent lamps at high frequency (>25 kHz) can further increase energy efficiency by 20-30% compared to conventional 60-Hz lamps.
Fluorescent lamps exhibit a negative resistance characteristic.
Requires an inductive ballast (choke) in series for stable operation (Fig. 16-3a).
Lamp impedance is essentially resistive.
Voltages related as: V{ballast} + V{lamp} = V_s
Lamp and ballast characteristics plotted in Fig. 16-3b (V-I).
Intersection provides a stable operating point.
Conventional 60-Hz rapid-start system (Fig. 16-4a):
Two lamps in series.
Cathodes continuously heated by heater windings A, B, and C.
Simplified schematic (Fig. 16-4b) without heating windings.
Input voltage boosted by autotransformer (primary in series with secondary).
Leakage inductances of transformer windings provide ballast inductance.
Starting capacitor: low impedance when lamp unignited, high impedance when lamp ignited.
At start-up, it provides a shunt across lamp B, causing nearly all input voltage to appear across lamp A, striking an arc.
Once arc discharge established in lamp A, a high voltage appears across lamp B, igniting an arc in lamp B.
Series combination of lamps A and B is in series with power factor correction capacitor C_{pf}.
High-frequency fluorescent lighting system (Fig. 16-5a):
High-frequency electronics ballast converts 60-Hz input to high-frequency output (25-40 kHz).
Block diagram of ballast (Fig. 16-5b):
Diode rectifier bridge (Chapter 5).
DC-to-high-frequency-AC inverter (e.g., class E resonant converter (Chapter 9) or switch-mode converter like half-bridge topology (Chapter 10) without isolation transformer/rectifying stage).
EMI filter before rectifier bridge to suppress conducted EMI.
Input current contains significant harmonics, leading to poor power factor.
Input current wave shaping circuit (Chapter 18) can remedy harmonics.
Electronic ballasts are generally more energy-efficient compared to standard ballasts because they don't require a large electromagnetic ballast.
Dimming control can be incorporated in 60-Hz and high-frequency systems to compensate for daylight.
Leads to energy savings: Lumen capacity of lamp diminishes with time, therefore new lamps have ~30% higher lumen capacity than nominal requirement, and dimming control allows operating new lamps at reduced power to deliver nominal requirement.
16-2-3 INDUCTION COOKING
Standard electric/gas cooking range: significant heat escapes, resulting in poor thermal efficiency.
Induction cooking avoids this (Fig. 16-6).
60-Hz AC input converted to high-frequency AC (25-40 kHz), supplied to induction coil.
Induces circulating currents in metal pan, directly heating the pan.
Similar circuits to high-frequency electronics ballasts can be used to convert 60-Hz AC input to high-frequency AC.
16-3 INDUSTRIAL APPLICATIONS
Induction heating and welding are discussed in terms of converters from previous chapters.
16-3-1 INDUCTION HEATING
Heat in electrically conducting workpiece produced by circulating currents caused by electromagnetic induction.
Clean, quick, efficient.
Allows defined section of workpiece to be heated accurately.
Magnitude of induced currents decreases exponentially with distance x from surface:
I(x) = I_0 e^{-x/\delta}I_0 is current at the surface.
\delta is penetration depth at which current is reduced to I_0/e (=0.368).
Penetration depth is inversely proportional to square root of frequency f and proportional to square root of workpiece resistivity ρ:
\delta = k \sqrt{\frac{\rho}{f}}
Where k is a constant.Induction frequency selected based on application.
Low frequency (utility frequency) for induction melting of large workpieces.
High frequencies (up to a few hundred kHz) for forging, soldering, hardening, and annealing.
Circulating currents caused by currents in induction coil.
Induction coil is inductive, and induction load can be represented by:
Equivalent resistance in series with coil inductance (Fig. 16-7a).
Equivalent parallel resistance (Fig. 16-7b).
Resonant capacitor used to supply sinusoidal current to induction coil and compensate for poor power factor due to coil inductance.
Two basic circuit configurations:
Voltage-source, series-resonant inverters (Fig. 16-7a).
Current-source, parallel-resonant inverters (Fig. 16-7b).
Voltage-source series-resonant inverter (Fig. 16-7a) is similar to series-loaded resonant (SLR) converters (Chapter 9).
Inverter input is DC voltage; output is square-wave voltage at desired frequency.
Operating frequency near resonant frequency results in essentially sinusoidal current due to impedance characteristic (Fig. 9-7).
Up to a few tens of kHz, thyristors can be used as switches.
Operating frequency must be below resonant frequency for capacitive circuit impedance and natural commutation of thyristor current.
Power to load controlled by controlling inverter frequency.
Current-source, parallel-resonant inverters (Fig. 16-7b) for induction heating were discussed in Chapter 9.
16-3-2 ELECTRIC WELDING
Melting energy provided by establishing an arc between two electrodes (one is metallic workpiece).
Welder's voltage-current characteristic depends on welding process.
Typical rated voltage/current: 50 V and 500 A DC.
Desirable to have very low current ripple once arc is established.
Output needs to be electrically isolated from utility input (provided by 60-Hz power transformer or high-frequency transformer).
Welders with a 60-Hz power transformer:
Input AC voltage stepped down to low voltage.
Converted to controlled DC by: (Fig. 16-8)
Full-bridge thyristor rectifier (Fig. 16-8a) with large inductor at input to limit current ripple.
Diode rectifier bridge providing uncontrolled DC, controlled by transistor series regulator (Fig. 16-8b).
Switch-mode, step-down DC-DC converter (Fig. 16-8c).
Drawbacks of 60-Hz transformer welders:
* Weight, size, and losses in 60-Hz power transformer.
* Low energy efficiency, particularly in series regulator scheme (Fig. 16-8b) due to power loss in transistor operating in active region.Switch-mode welder (Fig. 16-9):
* Electrical isolation provided by high-frequency transformer.
* Blocks similar to switching DC power supplies (Chapter 10).
* Resonant concepts (Chapter 9) can be used to invert DC into high-frequency AC.
* Small inductance needed at output to limit output current ripple at high frequencies.
* Efficiency in 85–90% range, with smaller weight and size compared to welders using 60-Hz power transformer.
16-3-3 INTEGRAL HALF-CYCLE CONTROLLERS
In industrial applications with resistive heating/melting where thermal time constants are much longer than 60-Hz time period, integral half-cycle control can be employed.
Shown in Fig. 16-10a for resistive Y-connected load supplied through three triacs or back-to-back connected thyristors.
If neutral wire is accessible, circuit can be analyzed on a per-phase basis (Fig. 16-10b).
Waveforms drawn in Fig. 16-10c.
Average power supplied to load controlled by controlling ratio n/m (keeping m constant).