Probing the Mysteries of Power Inverters: Part I

Thanks to power inverters, generating your own electricity doesn't mean you have to give up the convenience of AC living.



power inverters - inverter control box
Power inverter control boxes look something like this.
Photo courtesy of Best Energy Systems for Tomorrow
power inverters - Fig 1 AC and DC voltage wave forms
AC current is (ideally) an even sine wave. DC current is in one direction at (ideally) a constant level.
MOTHER EARTH NEWS Staff
power inverters - Fig 2 inverter current flow
The first half of the AC cycle: In the inverter, the right transistor shuts off, forcing current to flow counterclockwise through the left transistor and back to the battery. The induced current in the load flows clockwise.
MOTHER EARTH NEWS Staff
power inverters - Fig 3 inverter current flow
The second half of the AC cycle: In the inverter, the left transistor shuts off, forcing current to flow clockwise through the right transistor and back to the battery. The induced current in the load flows counterclockwise.
MOTHER EARTH NEWS Staff
power inverters - Fig 4 power inverter efficiency vs. output
An inverter usually performs best when it's fully loaded, or close to it.
MOTHER EARTH NEWS Staff
power inverters - Fig 5 three wave forms
The three voltage wave forms you can expect as output from a power inverter. 5A is a standard square wave. 5B is a modified square wave, which somewhat reduces the distortion between a sinusoidal wave and a square wave. 5C is a stair-step wave form that more closely approximates a sine wave; however, inverters of this type are more expensive.
MOTHER EARTH NEWS Staff
power inverters - Fig 6 current and voltage out of phase
This diagram shows what happens as the voltage increases across a motor winding. The current must work harder to stay in step with it. However, despite such efforts, the current can't keep up and lags behind the voltage.
MOTHER EARTH NEWS Staff
power inverters - DC to AC converter motor
A DC to AC converter motor. Because of their bulk and poor efficiency, units like these have largely been replaced by inverters.
Honeywell Motor Products Division











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