DC -> AC inverter, UPS questions/discussion

I've been using a 1 kW (1.2 kW peak) modified sine-wave inverter with our NHW11 Prius in power outages since 2005. Loads have included:

• gas furnace - needed for winter storms when power lines were down for over a day
• 5,000 BTU window air conditioner - when summer storms took down power lines
• TVs, florescent lights, fans

I tested every load in nice weather, not waiting for the emergency, and found the following:

• 750 W gas furnace fan - ran about about 10 degrees warmer but speed was no problem.
• inrush Mac and TV power - both would trip the overload circuit, typically a 400 ms window

The modified sine-wave has a two-step voltage output, ~50 V followed by ~170 V then switching the polarity for the other half of the cycle. The sharp step means the extension cord wires look like a capacitor and there is a measurable voltage drop over the length of the extension cord. This reduces the effective voltage leading to larger currents.

To deal with "inrush" loads, start them first before the heavier loads. However, I also made a 'pig-tail', a wall outlet in a plastic wall box with a thermistor. A thermistor starts at room temperature as high-resistance resistor that fairly quickly heats up and goes to a low resistance. This hot part has to be in the box to prevent burns and shocks. Radio Shack carries thermistors and any hardware store has the parts needed for the 'pig-tail' box.

Now I am not a fan of trying to use 'clips' to the 12 V battery because they make lousy connections. They get hot and chew-up the battery cable/terminals. Instead, I prefer a heavy gauge cable bolted to the battery connectors. The B+ has a nice bolt under the plastic cover (disconnect the battery ground wire FIRST!) Then take pains to make sure no vibration or accidental movement can short it to the car frame. If you don't want to connect the inverter permanently, a short piece of garden hose that slips over the B+ cable can be used to keep it connected and stowed. The ground cable can be stowed without a cover just making sure it won't wander around the battery.

To connect the inverter, ALWAYS:

1. disconnect the ground, make sure switch is off
2. connect the B+, do not strike car body with wrench
3. make sure switch is off, connect ground
4. shake it to make sure nothing is loose
5. switch on, connect extension cords, add loads

After 10-15 minutes, visit every load and the inverter and inverter feed cables to feel the temperature. Warm is OK, hot is not.

Bob Wilson

 

IIRC I think the reason it's not hurting the laptops is because the charging power goes to the battery and the power to run the puter comes from the battery which outputs good clean power. IOW your not running the laptop on inverter power your running it off the battery which tolerates poor noisy charging power better than the electronics on of your computer. This does not explain your findings with the APC backup power units. Check this info as my info may be old and thus YMMV.

 

Bob,

Are you running those loads of the 12 vdc battery? The idea of running a window A\C unit on such a small battery is pretty scary. Say your 750 watt furnace load is ~60 amps. Asking the tiny 12vdc batter to deliver that much current sounds to me to be a recipe for killing it quickly. The starting load of the -A/C is likely even bigger,

Can you clarify your set up better?

Icarus

PS Motor loads really don't like MSW inverters. Try tend to run much hotter, and fil sooner. Also, some electronics can be killed instantly on MSW.

 

Here is the detailed write-up: Prius - UPS Project


I fabricated the plywood, hinged door to mount the inverter. In normal operation, the inverter is laid flat and the trunk raised to ensure adequate cooling.


I covered the plywood with a dark felt to minimize damage to the plywood and make it look similar to the interior. Because the hinge goes across the bottom, I don't have to worry about small parts falling in the battery well.


I'm running the inverter directly from the 12V buss, the car is running. In the above photo, I'm drawing just under 90 A into the portable heater, the load. But what I found is the voltage begins to drop off above 75 A, the practical limit which is also 1 kW.

That is why I tested the gas furnace motor before the emergency. I ran the furnace motor long enough for the temperature to stabilize to get a baseline on line power. Then I switched it to the inverter power and monitored the motor temperature. It only went up 10 degrees F.

This goes directly to my recommendation to test all loads 'in nice weather' when you can fix anything that might go bad. You want to know it works before the emergency.

There are two non-sine wave inverters, modified and square wave. I have not surveyed every modified, sine-wave inverter but throughly tested the one I use.

Bob Wilson

 

Good lord, you had a two day power outage in the Bay area? Where? What were the circumstances?

 

Eight years ago, I was in Costco and they had a 1kW (1.2kW surge) unit marked down to $90. The price was right so I bought it and used it once with my wife's Echo. We could only get 400 w because at idle the alternator could only put that amount of power. But when I bought our first Prius, I made one modification, a remote ON-OFF relay, and installed it in the 2003 Prius. I really didn't get into inverter specifications until I found it so useful but today we have choices:

Sine-wave Inverters

• Tend to solve a lot of electrical problems. Noise is reduced and the power flows down extension cords with much less loss. Sensitive devices don't see the spikes from the sharp dV changes of the modified and square wave inverters.
• They tend to cost ~2-3 times that of modified and square wave inverters.
• Efficiency used to be in the 82-85% range but today they are showing up in the 92-95% range. GO WITH THE HIGHER EFFICIENCY UNIT. You can't 'buy' power in a power outage so don't waste it as heat from the inverter.

Modified Sine-wave Inverters

• Every load needs to be tested and some will never work right. For example, the thermostat on our gas furnace refuses to follow the programming that controls the temperature. But manual operation works just fine.
• Affordable, $100 for a 1 kW unit or less. Not much different from what I paid eight years ago.
• Efficiency is in the 92-95% range but this does not account for potential losses on the load devices. For a resistance heater, no problem, but motors will run warming meaning a net loss of efficiency.

Square-wave Inverters

• Junk power to the loads and typically low power, 500 w or less.
• Cheap
• Efficiency doesn't matter, loads run hot, worst power losses through extension cords ... don't bother

Options

• Current read-out, very useful for load management.
• Remote ON-OFF, very useful if running in the car for laptop power. I'm not impressed with the wireless units because they need batteries for the remote.
• High-current, DC plug so the inverter is kept in the house and brought to the car during an emergency. Avoids having space taken by the inverter. Probably a good idea.

GOOD LUCK!
Bob Wilson

 

I've documented installing a 1.5kW inverter in my wife's ZVW30:
110 VAC inverter installation | PriusChat


I'm still testing but thought I'd share the initial setup.

Bob Wilson

 

I used this one for the past couple of days, it worked just fine.
12 Volt DC to AC 1500 Watt Power Inverter-PW1500-12 at The Home Depot

 

Ok, here are your choices:

• 1 kW modified sine wave with surge capability (~$100) - it was my first modification and worked in every power outage. Poorly documented, the surge capability is often only for a couple of hundred milliseconds, a fraction of a second. However, every load needs to be tested as soon as it is installed which will take the rest of the day and over night:
  • motors - measure their case temperature on line power after it reaches a constant temperature and then on the modified sine wave until it levels out. A few degrees, no problem but more than 10F/5C, it is not worth it. Run a fridge overnight and spot check the temperature for at least 3-4 hours.
  • TV, cable boxes, and computer supplies including bricks - they have serious inrush issues. These need to be tested after putting on the maximum base load. If it trips the inverter low-voltage, try bringing them online first and then the motor(s) and finally the lights.
  • avoid heavy loads on long extension cords - the modified sine wave has sharp dV/dt edges and there is a significant AC voltage loss.
  • effectively, a 9A, AC circuit with limited extension cord range
• 1.5 kW sine wave with surge capability (~$300) and high crank rated battery ($200) - this is my current effort. I have sustained a stable, 930 W, resistive load even though I was losing about 100 W in the Anderson connector and 120A 12V DC circuit breaker with the original 12V battery. When I rewire it directly to the Odyssey PC930, I'm expecting a full 1 kW load, about a minute at 1.5 kW and surge to 3 kW for a couple of hundred milliseconds:
  • 330 CCA (330A, 12.8 -> 7.20 V 30 sec, 4.2 kW -> 2,3 kW) - easily, it should handle 1.5 kW for more than 30 seconds which means a solid, 12A, AC circuit
  • ordinary extension cord limits because it is sine wave
  • no motor issues but there could be inrush issues

Bob Wilson

 

I would like to compare and contrast the difference between modified sine wave and sine wave inverter operation by discussing what happens in practice followed by a simplified description of why. I'll also include best practices versus 'good enough.'

Operation

We backup the Prius with the modified sine-wave inverter as close to the side, kitchen door as possible so a heavy duty, 15 A, contractor cord with three outlets on one end, can reach as far as possible into the house. Measuring the voltages without a load we find:

• 110VAC at the inverter power outlet
• 108VAC at the end of a 25 ft, heavy duty, 15 amp, three outlet extension cord, at kitchen/TV room doorway
  • Power strip for cable box and TV
  • Power strip for laptop(s) and light
  • 98VAC at the end of a 12 amp rated, 50 ft extension cord at the bathroom/bedroom/gas heater

Notice there is a 2 VAC drop on the 25 ft cord but a 10 VAC drop on the 50 ft cord. This voltage drop is without any loads due to the capacitance between the three conductors:

• 'hot' - the power voltage conductor, usually black
• 'neutral' - close to ground, the return path for 'hot' current, usually white
• 'safety ground' - a second path that keeps a defective short inside from electrifying the case, usually green

The 15 amp contractor cord has thicker insulation on the conductors which reduces capacitance per foot. The thinner insulation in the 12 amp extension cord increases capacitance and all capacitance 'leaks' AC voltage. The modified sine wave 'leaks' through the capacitance much worse than a pure sine wave.

Modified versus Pure Sine Wave

Modified sine waves can come in different styles, there is no one standard, but measurements show mine steps between roughly two voltages:

• +/- 45 VAC - applied to transition through 0 V by going from "+ to -" and "- to +" with a hefty current
• +/- 110 VAC - applied to increase voltage from "+45 V to +110 V" and "-45 V to -110 V"

Now rate of each of these steps happens in 10s of microseconds, a dV/dt rate of about:

• (+110V - (-45V)) / 10 usec ~= 15.5 million volts per second
• (-45V - (-110V)) / 10 usec ~= 6.5 million volts per second

It is the voltage, rate of change, that 'leaks' through the extension cord capacitance.

In contrast, a 60 Hz, sine wave changes polarity every 8.3 milliseconds between "+110 V to -110 V" and "-110 V to +120 V":

• (-110 V - (+110 V)) / 8.3 msec ~= 26.5 thousand volts per second

Notice the different rates are off by a factor of 1,000 and why the 'leak' voltage is so high for a modified sine wave inverter. This reduces the delivered voltage and causes bad things to happen especially with motors and electronic power supplies.

An AC, induction motor and an electronic power supply tries to draw the same load even if the AC voltage drifts up or down. When the AC voltage drifts down, these loads compensate by drawing more current to sustain their power. This increased current wastes energy by the square of the current!

• 1A @110 VAC -> 110 W, losing 2 W in the extension cord
• 1.2A @98 VAC -> 98 W, losing 2.9 W in the extension cord

It doesn't take long before we're talking about the difference between running all loads or having to disconnect some to run others.

In theory, the extension cord capacitance can be reduced by disconnecting the 'ground wire' using a 2-plug to 3-plug adapter at the inverter. In effect all ground wire distances double to the 'hot' and 'neutral' and it stops being an effective capacitor path. Since the Prius is typically resting on non-conductive rubber tires, the shock risk is somewhat tolerable because very little current would flow. I do not recommend this practice but if the alternative is the difference between having enough power and not:

Source: here is little Effie's head - Poem by E. E. Cummings

Summary

Engineers often have to choose between good, fast, cheap, pick two. The reason for having a Prius powered inverter is to preserve a quality of life when power is lost. But Prius power is limited to 1 kW. So the choices are between a cheap, adequte modified sine wave inverter versus a more efficient, expensive sine wave inverter.

We have 7 years of using a 1 kW, modified sine wave inverter including 4 days, 6 hours during the 2011 tornado outage. At two gallons of gas per day and starting with just half a tank of gas, I had to fetch more by driving out of our area to one that had working gas stations. The longer the outage, the more appreciation for getting all of the available AC power and minimizing the gasoline burned to produce it. So I decided our second, ZVW30 Prius would have the best possible, sine-wave inverter even though this inverter costs an extra $200 and I've had to re-integrate and test.

Now if you are expecting to go through just short outages, say a day, then a modified sine wave inverter is the way to go. Even a 120 W, cigarette outlet powered inverter brings lights or a TV or a computer online but you have to choose which load to use. But if you anticipate outages longer than a day, outages that require refueling the Prius, a pure sine wave inverter makes a lot of sense. As for size, 1 kW has worked well for us.

Bob Wilson

NOTE 1. We will have both the 1 kW, modified sine wave Prius, and add the 1 kW, pure sine wave Prius. The modified sine wave inverter has to use the shortest possible cords which means it is needs to be parked as close to the rear kitchen door as possible. In contrast, the 1 kW Prius can be parked in the next closest space, 15 ft further, yet lose less power.

NOTE 2. How much power depends upon what loads are needed. Living in Alabama, an air conditioner in the bedroom means we can get a restful sleep at night and survive heat stress days. The 9 amp, modified sine wave inverter has operated a box fan or a 5,000 BTU window air conditioner. But now we have larger pedestal units and 12 amps, sine wave power is needed.

 

It's a myth that modern electronics don't like square waves or modified sine waves. Actually, you'll probably find that many like square waves better than sine waves. There's some truth for motors, but keep in mind that the Prius motors run from square waves at high speeds since it actually nets a higher efficiency. The problem with long extension cords is definitely true (look up "inverter standing waves"), but just adding some series inductance (2-5mH or so in each line) close to the inverter is a cheap way to fix the problem.

As for fans, a good one can be obtained from an electronics or computer store for $10-20. They're much more efficient than common AC fans and have a built in inverter tuned just for the fan motor. One of those will easily run for hours or even days from a small lead acid battery.

 

I realize I am already the proud owner of several inverters.
I have a small Targus unit that I used to use on long airline flights (eg; United) to power personal electronics.
Trying to figure out if this is true sine wave. Perhaps not.
But I was trying to suggest small pure sine unit and bigger MSW unit.
Bob- You said TV...Do you think a modern LED/LCD 3D HDTV set (don't own one yet) runs off the MSW?

 

The 1500w inverter I used ran my freezer and fridge together with no problem.

 

The Targus airplane inverter is indeed MSW.
Here is what NOT to use it for:
This inverter is not recommended for use with
inductive loads, such as fluorescent lamps or
pumps and most heat generation equipment
such as hair dryers, coffee makers, irons, heaters
and toasters. Otherwise, permanent damage to
the unit and excessive overheating may result.