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Green Collar Careers - Inverters

Whether electricity is generated from a solar source or wind source it will need an inverter to make the electricity compatible for residential or commercial use.  Module 305 Basic AC-DC Electronics introduced the concepts of an inverter.

Most residences will require an inverter that provides 240 VAC at 60 cycles.  Often you'll hear 110/220 VAC but in reality when you measure the wall socket voltage it's between 115 and 119 VAC.  We'll refer to 240 VAC to remain consistent. 

Electrical Power panels have three main wires to feed them from the utility:

• Leg 1 - 120 VAC at 0°
• Leg 2 - 120 VAC at 180°
• Neutral - 0 VAC at anytime and very little voltage difference from Ground
• Voltage potential between Leg 1 and Leg 2 is 240 VAC
• Voltage potential between Leg 1 and Neutral is 120 VAC
• Voltage potential between Leg 2 and Neutral is 120 VAC

Also connected to the frame of the Power Panel box is an Earth Ground.  It is very important that a good Earth Ground is established because it will be your best friend in a lightning storm.

Choosing an Inverter
Grid Tied or Off Grid - In most applications the inverter will be connected to a grid-tied system that takes special requirements from the utility company.  Not only will it need to be protected from surges from the utility connections, it will need to synchronize the AC cycle waveform to the utility company.

Power Rating - Inverter power output is rated in watts (watts = amps x volts). There are three levels of power rating-a continuous rating, a limited-time rating, and a surge rating. Continuous means the amount of power the inverter can handle for an indefinite period of hours. When an inverter is rated at a certain number of watts, that number generally refers to its continuous rating.  Limited-time is generally given as a set period of time like 5 minutes of operation at the limited-time rating.  Surge is an instantaneous surge that may come from a lightning strike or utility power line and is usually only given in volts.  Surge may also refer to switching on of high current devices like motors for HVAC systems.

Power and Waveform Quality - Some inverters produce "cleaner" power than others. Simply stated, "sine wave" is clean; anything else is dirty. A sine wave has a naturally smooth waveform, like the track of a oscillation of a string. It is the ideal form of AC power. The utility grid produces sine wave power in its generators and (normally) delivers it to the customer relatively free of distortion. A sine wave inverter can deliver cleaner, more stable power than most grid connections.

How clean is a "sine wave"? The manufacturer may use the terms "pure" or "true" to imply a low degree of distortion. The facts are included in the inverter's specifications. Total harmonic distortion (THD) lower than 6 percent should satisfy normal home requirements. Look for less than 3 percent if you have unusually critical electronics, as in a recording studio for example.

Other specs are important too. RMS voltage regulation keeps your lights steady. It should be plus or minus 5 percent or less. Peak voltage (Vp) regulation needs to be plus or minus 10 percent or less.

A stepped sine wave inverter is less expensive, but it produces a distorted square waveform that appears as a series of steps.

Stepped sine waves have detrimental effects on many electrical loads. It reduces the energy efficiency of motors and transformers by 10 to 20 percent. Wasted energy causes additional heat which reduces the reliability and longevity of motors and transformers and other devices, including some appliances and computers.

Some household appliances simply won't work on stepped sine wave power. A buzz will be heard from the speakers of nearly every audio device. An annoying buzz will also be emitted by some fluorescent lights, ceiling fans, and transformers. Some microwave ovens buzz or produce less heat. TVs and computers often show rolling lines on the screen. Surge protectors may overheat and should not be used.

Efficiency - Efficiency is the ratio of power out to power in, expressed as a percentage. If the efficiency is 90 percent, 10 percent of the power is lost in the inverter. The efficiency of an inverter varies with the load. Typically, it will be highest at about two thirds of the inverter's capacity. This is called its "peak efficiency." The inverter requires some power just to run itself, so the efficiency of a large inverter will be low when running very small loads.

Safety and Internal Protection - An inverter's sensitive components must be well protected against surges from nearby lightning and static, and from surges that bounce back from motors under overload conditions. It must also be protected from overloads. Overloads can be caused by a faulty appliance, a wiring fault, or simply too much load running at one time.

Underwriters Laboratories UL1741 is the most common safety standard applied to inverters used for commercial AC power distribution.

Idle Power - Even with no load, the inverter will be "idling" and consuming power and should be as low as possible. Typical idle power ranges from 15 watts to 50 watts for a home-size inverter. An inverter's spec sheet may describe the inverter's "idle current" in amps. To get watts, just multiply the amps times the DC voltage of the system.