Search This Blog

Wednesday, November 25, 2015

Emergency Power - Portable Generators and Power Inverters.

After that last post about the wind storm, it got me thinking about alternate sources of electrical power.  There are two readily available ways to get emergency AC (alternating current) power.  You can use a power inverter, or you can use a generator.  


A power inverter uses solid state electronics to convert low voltage DC (like that of a 12 Volt automobile electrical system or solar installation) to 120 volts AC.   Below is a picture of a power inverter rated for 5000 watts peak, 2500 watts continuous duty.  This one is $189 at Wal Mart.
Theoretically, you could power your house by running jumper cables from your car battery over to the 12v DC to 120v AC inverter, right?  Not so fast...  Ohm's Law says that power = voltage x current.  The inverter may be able to put out 2500 watts, but to do that, we have to supply 2500 watts to it from a 12 volt car charging system.  That means we need 208 amps (208 amps x 12 volts = 2500 watts).  Odds are your car's electrical system cannot supply that many amps.  A typical car alternator puts out 40-50 amps, while a high output alternator might put out 130 amps, so your car battery will eventually discharge, even with the engine running full blast.  

Something that also should be considered is power quality.  Quality is the stability of the AC voltage and frequency that a generator provides.

Power inverters do not generate a true sine wave.  A true AC voltage sine wave looks like the one below:
The height of the wave is the voltage.  The speed at which it goes up and down is the frequency, which is controlled by the generator speed.  Power inverters don't deviate much in frequency, because that is electronically set in the device, and so frequency doesn't change when an electrical load is added or removed.  Where inverters have issues is providing a smooth output voltage.  

Below is a diagram of a very simple inverter.  This design switches the output from full positive voltage to full negative voltage.  12 volts is applied at the two right connections.  The charging of capacitors C1 and C2 alternately trigger the switches Q1 and Q2 to conduct, alternately routing current from the center of transformer T1 through the right or left switch, causing the transformer output to be either positive or negative.  The transformer also steps up the alternating 12 volts to 120 volts output. 


Below are three examples of power inverter output voltages with a true sine wave superimposed on them.  Note that because inverters are using switches, the voltage changes in discrete steps.  The left side image would be the waveform put out by the circuit above.

At the left, the inverter simply switches on for full positive voltage, then off for a moment, then goes to full negative voltage.  Not very close to a sine wave.  In the middle, there are more switches, with an intermediate step at half voltage.  At the right, there still more switches, and we see three different voltage steps.  At this point the inverter is getting closer to mimicking a sine wave.  The output can be smoothed further by adding an electromagnetic coil (sometimes called a reactor) on the output.  The magnetic field dampens the step-changes in the voltage, smoothing the output.

Below is an actual oscilloscope image of the output of a power inverter.  It still has distinct steps in the voltage, but the waveform is fairly close to that of a sine wave.  This would be a pretty complex and expensive inverter.

Would the stepped waveform in the image above damage your valuable electronics?  I don't know.  What I do know is that you don't plug delicate and expensive electronics directly into the wall anyway.  You plug a surge protector with AT LEAST a 500 Joule rating into the wall, and plug your expensive stuff into that. 

Below, a really good surge arrestor, with connections to protect phone, coaxial cable, and 120 volt power. 

Now let's talk about generators.  Generators are how most of the world's electrical power is produced.  They are essentially a bar magnet spinning inside a bunch of electrical windings.  Unlike power inverters, generators do have a perfect sine wave output.  

Below is a really cool animation of a three-phase generator.  

Generators can have variability problems with both voltage and frequency (engine speed).  In fact, a generator that is not connected to the grid will always sag in speed as electrical load is added.  Any field welder will tell you that his gas-powered welder bogs down at first when he starts welding...  The governor on the engine will bring the speed up after a couple of seconds, but in the meantime the frequency is lower than it should be.  The opposite situation occurs when electrical load is suddenly removed - the engine goes a little fast until the governor brings it back to the correct speed.

Generators also have a voltage regulator to control the output voltage.  Because the voltage regulator is electronic, it reacts much more quickly to sudden changes in electrical load than the engine does, so voltage sags and spikes are much less of an issue than speed (frequency).

Below, the waveform of a voltage sag lasting two cycles.
A key point for selecting a generator is that it must be sized correctly for the expected electrical load, so that the speed doesn't drop too much when a large electrical load is added.  The longer the generator is at reduced speed, the more likely damage will occur.  When frequency is too low, current goes up significantly.  Higher current generates more heat, which is what causes electronics to fail.

Below, a change in frequency for two cycles.  This is FM radio, but still applies to generator speed when a heavy load is added.


Some manufacturers make an excellent hybrid machine, called a generator-inverter.  These are pretty expensive machines for their output.  With this design, a gasoline engine spins an alternator.  The alternator output is converted to DC power, and then an inverter re-converts the DC to AC.  The advantage of this more complicated arrangement is that voltage and frequency don't sag when load is added, because the inverter has very a tight electronic control of the output.  The generator and motor will slow down when load is added, but the inverter output frequency will not.

Below, a generator inverter that costs $900.  It is rated for 1600 Watts continuous.  That's one hair-dryer's worth of power.

Below, a standard generator that also costs $900.  It is rated for 8000 watts continuous.  Five times the power for the same price, with some convenient features like low oil shutdown, 230 Volt output, and a large gas tank.  The power is dirtier, but there's a lot more of it for your money.


One of the ideas I had entertained was an installed whole-house emergency generator with automatic switch-over to generator power.  Below is one that can be set up to run on LPG or natural gas, so you don't have to worry about gasoline spoiling in the tank while the machine sits unused for long periods of time.

The cost alone made this idea impractical.  These installed generators cost thousands of dollars, and in ideal world, should rarely be used.  Furthermore, they can only be used for that one purpose. At least when you have a portable generator, you can put it to other uses, like providing electrical power in remote places, while camping, or you can allow others to use it for their own power loss emergencies.


No comments: