Inverters

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1. Q: My inverter seems to work properly, but when I check the output with a digital multimeter the reads very low -- about 102 Vac.  What's the problem?
   
   A: Most low cost inverter models put out a "modified sine wave" rather than a true sine wave.  The modified wave actually consists of a rectangular profile that approximates the same rms or effective value as a true sine wave.   However, unless you use a meter that measures true rms voltage, you will get a low reading.  The problem is that most meters are calibrated and designed for a true sine wave.
   
   
2. Q: Your website is very informative about my home electrical needs. I have a question. I am putting together a battery bank of 12 volt batteries and then hardwiring the output to an inverter and then hardwiring the inverter to my home electrical panel. For a typical household with its usual complement of electrical appliances, what wattage inverter would you recommend (1500 watts, 2500 watts ... assuming these are continuous loads). At 2500 watts continuous surge, would I be able to run my heat pump that draws 30 amps?
   
   A:There are a number of things you will want to consider as you design a battery/inverter system. Here is a listing of the major considerations:
   
   a) Make sure you use deep cycle batteries which will give you several hundred complete charge/discharge cycles. If you use the normal vehicle starting batteries they will wear out after about a dozen charge/discharge cycles.
   
   b) Determine how many batteries you will need based on the average electrical load total and duration. For example, if you want to supply 8 amps at 120VAC for 8 hours, you must convert this to amp-hours at 12VDC to size the batteries. First convert to watts:
   
       (8 amps) x (120 volts) = 960 watts
   
       (960 watts) / (12 volts) x (8 hours) = 640 amp-hours
   
   Then make corrections for inverter efficiency and the fact that batteries are usually rated at a 20 hour discharge and will produce fewer amp-hours if discharged faster than this (see plot).  We will assume an 80% efficiency and an adjustment of 85% for the 8 hour rate versus the 20 hour rating.
   
       (640 amp-hours) / (0.80 eff) / (0.85 adj) = 940 amp-hours
   
   This is the battery capacity required, which can be a combination of smaller cells wired in parallel to get this total.
   
   c) To convert the 12 volt dc to 120 volt AC select an inverter with a continuous rating greater than your running load that also has a surge rating greater than any inrush or starting load. So, for example, if you wanted to be able to power a freezer with a running load of 4 amps and a
   starting inrush of 12 amps:
   
       (4 amps) x (120 volts) = 480 watts continuous
   
       (12 amps) x (120 volts) = 1440 watts surge
   
   d) Pick a battery charger that will recharge your bank of batteries in a reasonable amount of time such as 12 hours:
   
       (640 amp-hour bank) / (12 hour recharge time) x 1.1 = 59 amps
   
   Note that the 1.1 factor here accounts for the losses during recharging.
   
   e) It normally is not economically feasible to power a heat pump using an inverter. First, it is probably 240VAC instead of 120VAC so a stepup transformer would be required unless two inverters are designed to operate in series to get 120/240V. Second, the inrush or starting current is so great you would need an extremely large and expensive inverter. And third, you would need a huge bank of batteries to run the heat pump for any length of time.
   
   As you can see, it can get pretty involved and costly to install a large battery backup power system. That is why we have described the alternative of using a running vehicle to power an inverter to provide 120VAC. We have found this sufficient for powering a refrigerator or freezer or microwave plus a few lights.
   
   
3. Q: When powering my refrigerator from an inverter, the compressor motor seems to make a louder hum than when using normal power.  Will my refrigerator be damaged?
   
   A: The hum is due to the modified sine wave supplied by the inverter.  This should not cause any problem with the refrigerator short term.  However, since the compressor motor may run a little hotter on a modified sine wave you would not want to run it this way continuously week after week.  Doing so might begin to shorten its life due to the prolonged higher running temperature.
   
   
4. Q: What is the difference between the Cherokee PI-XXXX series and the TS-XXXX series inverters?  I have seen the PI- series advertised at good prices.
   
   A: The new TS- series supersedes the former PI- series inverters.  The TS- series has higher surge ratings.  For a comparison, see table below:
   

   Model	Continuous Rating (watts)	Surge Rating (watts)
   TS-500 PI-500	500 500	1200 800
   TS-1000 PI-1000	1000 1000	2200 1800
   TS-1500 PI-1500	1500 1500	3000 2500
   TS-2500 PI-2500	2500 2500	4000 3000

    

5. For additional questions and answers, go to the Cherokee Electronics web site by clicking here.