DC, AC, Modified Sine, Pure Sine, What are they?
Your RV uses electricity in two forms: DC (Direct Current) and AC (Alternating current). Direct Current is what comes from a battery bank and is usually a constant 11 to 14 volts. Alternating Current is what comes out of an electrical outlet and is produced by an inverter (drawing from a battery bank) or drawn from shore power (or an exterior power hook-up like a generator).
DC – Direct Current is produced by solar panels and stored in a battery bank. With DC, electrons flow in one direction at a fixed voltage. Appliances that run on Direct Current are very efficient because they do not rely on an inverter, which results in small energy conversion losses. Common DC loads include Lights, Fans, Water Pumps and some Refrigerators.
AC – Alternating Current in the United States cycles from about -170 volts to +170 volts 60 times per second (60hz). With AC, electrons flow in one direction for 1/60th of a second, then switch directions for 1/60th of a second at an RMS voltage ranging from 110 volts to 120 volts. Alternating Current in RVs is produced by an inverter or drawn from shore power or a generator. Alternating Current cannot be stored because of its constantly changing voltage. Alternating Current transmits over long distances much better than Direct Current, which is why it is so commonly used. AC current is also very efficient at turning motors, like blenders and refrigerator compressors.
RMS stands for Root Mean Square. To spell it out in math terms, the magnitude of the sine wave first gets squared. This means that all the negatives turn positive and you have half wave humps going from 0V to 28900V (170 x 170 = 28900). Then you take the square root of that to bring it back down to 0V to 170V. Then you take the average of the half wave humps and you get a flat line at around 120V. Simply put, the RMS is just an average assuming everything is positive.
It is important to note that an amperage measure in AC represents something very different from a similar amperage measure in DC because the voltage is different by a factor of ten. For example, an AC refrigerator that consumes 1 amp draws about the same amount of energy from your battery bank as a DC refrigerator that consumes 10 amps. Here’s the math: 1 amp x 120V AC = 120 watts and 10 amps x 12V DC = 120 watts.
In regard to RV applications, Alternating Current comes in two forms, Modified Sine and Pure Sine. A Sine wave is a mathematical function that alternates between positive and negative (-170 volts to + 170 volts, 60 times per second). The words “Modified” and “Pure” describe the path as the current alternates between positive and negative.
Modified Sine – Inverters that produce modified sine are usually less expensive than inverters that produce pure sine. Modified Sine inverters use a stair step pattern, going from -120 volts, to 0 volts, to +120 volts, resting at each stage for 1/240th of a second. Lights and heating coils work fine with a modified sine wave, but anything with moving parts is going to run roughly and have a hard time dealing with the rapid stair-step changes of voltage.
Pure Sine – Pure Sine is universal and works with anything. A pure sine wave is smooth and in a constant state of change. Microwaves, AC refrigerators, Blenders, etc. all work better with a pure sine inverter.
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