What devices can be operated by an inverter?

Sizing: Is the inverter big enough?

Simple: Look on the back of the device by the regulatory certification markings to see how many watts the device uses. If it only lists Amps, then multiply Amps by 120 to calculate watts. Use an inverter that is rated for at least this many watts. Motors use 3 to 7 times as many Amps during start up, so an inverter with a suitable surge rating may be needed. Power supplies like those on computers often have an Amp rating that is much higher than they really use.

Advanced: To put things in perspective, a standard 15 Amp circuit can handle a 90A surge current for 0.6 to 1.7 seconds and puts out 1800W continuously, so a 2000W inverter will run anything, except things like motors which have start up surges. To calculate a motor's wattage, multiply it's current by 120, this gives you VA (or Volt Amps, which a measure of watts used by the motor and energy in watts stored in the magnetic field and then re-injected back in to the supply unused). Multiply VA by the power factor if that is listed. Now you have the actual watts that the motor uses. A 3 step square wave inverter can usually handle a bad power factor (higher VA than watts), but a generator or pure sine wave inverter often cannot, so use VA instead of Watts where appropriate. Watts are equal to the motor's mechanical power output and inefficiency (heat generated).

A general purpose induction motor is made to only run at one speed, and puts out constant torque during start up but draws 6 times the normal full load current to do so. They make a single click noise when spinning down just before stopping and usually have a capacitor mounted on the side. Some examples of their use are: air compressors, machine tools, and farm equipment. Specialized induction motors which are found in many refrigeration compressors, vacuum cleaners, fans, and sump pumps do not draw as much start up current because full torque isn't needed. Lastly there are universal motors with brushes which are found in kitchen appliances, power tools and many other intermittently used tools. They put out torque which is proportional to the current drawn. They can put out very high power when overloaded. They draw a large current surge during start up, but start up very quickly. They don't need extra current during start up, but will use it.

For a general purpose induction motor, an inverter with a surge capacity of 6 times the motor wattage may be needed, and that surge time may need to be close to one second. For example, an air compressor with a full air tank is one of the most demanding tasks for an inverter. An 1800W 1.5HP air compressor would need an inverter with 12000W surge capacity for 0.5 to 1 seconds. A battery bank with over 2000 cranking amps is probably needed for a 12 Volt system. The soft start feature of an inverter wouldn't do any good for this, unless the tank was empty.

For smaller specialized induction motors, most inverters have enough surge capacity to start them. Else, the soft start feature will allow it to start. For brushed universal motors, the surge power and soft start is plenty to get it going so a high surge capacity inverter isn't needed.

Keep in mind whether the surge and soft start features of an inverter happen only when when the inverter is turned on, after an overload reset, or kick in dynamically while it is operating. Manufacturers usually don't give such details about their inverter's features. Some inverters put out the surge power for such a short time that it is useless. Others do surge only at turn on, and then just do soft start while operating. Good inverters will have surge power available at any time while operating. If surge and/or soft start is only available at turn on, the inverter will have to be power cycled each time the device is turned on or comes on. If soft start is needed to start a device while the inverter is operating, then voltage will be briefly reduced to any other devices plugged in to it. Refrigerator compressors stall very quickly, and TVs and other electronics may have to be turned back on.

What devices can be used on a "modified sine wave"?

The output of this type of inverter is a 3 step square wave, or "modified sine/square wave" instead of a pure sine wave. Modified square wave inverters are usually a bit more efficient than pure sine because of the lack of a final switching stage and coil. They also tend to tolerate poor power factors without shutting down (a 1000W inverter may handle 1500VA). The output voltage of a 3 step square wave inverter alternates quickly between about +145V, 0V, and -145V rather than a smooth transition. This is better than old inverters and many computer UPS units that put out 2 step square waves. A 3 step square wave can operate almost all devices that normally operate on normal sine wave AC power, but there are a couple of exceptions. All devices are designed to handle a sharp voltage spike when first plugged in or turned on, but the repeated spikes from the alternating square wave are what make certain devices unusable.

The following devices should not be used on this kind of output:

• Any device which uses a capacitor to limit current. These include:
• Some fluorescent lights and other special high brightness lighting with a capacitor ballast instead of an inductor.
• Battery chargers which having a warning saying that high voltage is present at the terminals.
• Small battery chargers such as plug in rechargeable flash lights, battery charges for razors and tooth brushes, and battery chargers for some power tool batteries. If the device becomes excessively hot when used, then disconnect it. These are the most frequently damaged devices. If the battery charger gets slightly warm or makes a faint hum when it is not charging a battery then it probably uses a transformer, and it is safe to use.
• Some induction motors will start oscillating, especially at small load. They start drawing high current spikes several times or more per second. This usually makes lights that are connected to the circuit flicker and the motor will jump from torque surges. This usually causes smaller inverters to shut down or burn out. If your inverter does run it, and the motor does not say "thermally protected", monitor its temperature for overheating. If the surges are severe don't use it. This problem is more likely with smaller inverters, and possibly motors with a run capacitor in addition to a start capacitor. Use a larger inverter or a possibly a pure sine inverter.
• Plug in capacitor line noise or "dirty electricity" filters.
• Newer electric blankets with adjustable power levels are known to fry. SoftHeat Low Voltage™ series from Perfect Fit Industries do work. Consider skipping the inverter and buying an electric blanket that plugs in to DC power directly. Switching to DC will also reduce or eliminate the potentially harmful exposure to alternating electrostatic and magnetic fields.

  These devices sometimes have problems:

• Laser printers often do not work properly.
• Computer UPS units often detect a square wave as bad power and switch to battery backup (which is hypocritical since they put out the same bad power).
• Induction motors (the ones without brushes) run about 20% hotter than normal. This is usually within the tolerance of the motor. Motors naturally run hotter during low voltage brown out conditions anyway. Cheap devices may not have much margin for error. If you live in a hot climate and intend to use an inverter for long term use, and pure sine inverter may be a better choice. Also, in long term use applications, if a significant portion of your load is from induction motors, the increased motor efficiency of a pure sine wave may be cost effective.
• Some devices which have active power factor correction (APFC) power supplies have been known to have problems, and are damaged in some rare cases. Most devices work fine. APFC is getting more and more common with switching power supplies (like TVs, projectors, game consoles, and desktop computers) and sometimes even smaller devices that use under 200W have it too. If the power supply has a 110/220 volt selector switch on the back then it is not APFC. There is a capacitor inside that charges and discharges with the AC input voltage and this can cause a buzzing sound.
• Light dimmers may not dim correctly.
• Some analog TVs, stereos, or amps with poor filtering will have a buzzing sound in the audio or visual interference.
• Microwave ovens usually don't put out as much power (or draw as much), but almost always otherwise work fine.
• Some clocks won't keep time correctly.
• Devices which generate a buzzing sound. This is usually just annoying but in some rare cases indicates harm. If the noise comes from a motor or transformer it's harmless vibration of the coils.
• Medical equipment like oxygen concentrators.

  These devices always work just fine:

• Plug in transformer DC power supplies (wall warts)
• Plug in switching DC power supplies (more advanced and efficient wall warts)
• Lights
• All electric heating devices, like space heaters, hair dryers, and toasters.
• Transformer based linear power supplies.
• All medium and small switching power supplies (without APFC). Laptop power supplies. All computer power supplies with a 110/220 volt selector switch on the back (these don't have APFC and are more efficient than APFC models).
• Nearly all TVs, VCRs, DVD players, screens, etc., (based on the previous two bullet points).

  These devices work better on a square wave:

• Brushed AC (universal) motors, such as angle grinders, and variable speed kitchen mixers.
• Cheap transformer based car battery chargers may be less likely to overcharge the battery.

  Filtering the output:

  It is possible to make a small filter to get rid of the voltage spikes that are causing buzzing or problems for a certain device. This is best left for an electronics hobbyist as the filter should be made specifically for the device. Don't just put a capacitor across the output, this tries to filter the entire output and makes the inverter create more heat and could harm it. Put a small high wattage resistor on the input before the capacitor. Even a 2 ohm resistor will draw 70 amp current spikes. Some people say to use a 1:1 transformer as a filter on problematic devices, but the resistor and capacitor method should accomplish the same thing. The transformer method may be more efficient. Be careful using inductors as filters. Although they can be great if done correctly, they can create high voltage spikes if the load suddenly changes, and high frequency ringing when combined with capacitors.

  Installing an inverter and connecting to your house power

  Always turn off the main circuit breaker to your house while the inverter is connected to a building's power (unless it's a grid tie unit). Backfeeding in to commercial power is a hazard for utility workers, especially in rural areas. If commercial or other power is turned on while the inverter is turned on (or even turned off) the inverter will likely be damaged. Do not parallel inverters which are not designed for such. They can't be synchronized like generators. Always install the inverter close to the batteries. If the inverter must be far from the batteries, then use thick enough wires so that the voltage loss does not exceed 0.5V at 12V. Aluminum wires will be cheaper and are a good choice if installed correctly. Also, switching to a 24V system from 12V will allow for four times the distance from the batteries with the same loss when using the same size cables. Use longer AC power cords and not DC cables. The DC power cables should connected to a marine ANL fuse before leaving the batteries and going to the inverter. Make sure the interrupting current of the fuse exceeds the short circuit current of the battery bank. When sizing the battery cables, check an ampacity chart for open air conductors to make sure the wires are sufficiently sized and have a high enough insulation temperature rating such as 105C. Paralleling two cables of 3 AWG sizes smaller may be cheaper and give a little bit higher ampacity rating. Correctly sized 120VAC power cords should be used. Use 12 AWG or bigger wires on a 4000W inverter.

  More potential hazards:

  When connecting an inverter output to a building's power, be aware of some things. Many people use male to male "suicide cords" to plug power in to a building. This is a hazardous connection and children should not be allowed to have access to it. If the inverter or generator puts out more than 1800W at 120VAC and doesn't have its own 15A breaker, the possibility of overloading a standard 15 Amp circuit exists. The circuit breaker in the fuse box can't protect against overloads from devices plugged in directly to the circuit being back fed. Having a fuse on the suicide cord is recommended. An 18 AWG cord in open air will start feeling hot around 15 Amps. Look out for air compressors, space heaters, and other high power devices that may be plugged in to the back fed circuit.

  If two houses are being powered from the same inverter or generator, remember that the neutral lines of both buildings are already connected. If the polarity of one of the back feed cords is reversed, a short circuit will exist from the hot wire of the back feed cord through the unfused neutral line of the building, which is connected to the neutral of the other building and back to the power source's neutral.

  In order to supply power to both sides of a split phase power system people sometimes hook the power in to a 220V socket and bridge the 220V hot wires together. This can be hazardous as it doubles the fusing capacity of the circuit and can result in doubling the maximum rated current going through the unfused neutral line (a 20A 220V outlet becomes 40A). It's better to connect through a regular 120VAC socket and then short out the terminals on one of the 220V sockets in the building to power the other phase.

  Do not connect 240V to a building's power unless the generator or inverter has a split phase 120V + 120V output and there is a secure neutral connection. If the neutral comes loose there will be 240V on the 120V circuits.

  Isolated output: Most inverters sold today feature an isolated output. If an inverter does not have this feature, and it uses a standard H bridge output design where both outputs alternate between 0V and 145VDC, then a shock hazard exists. The AC outputs are electrically connected to the DC input, which connects to your batteries, vehicle, or solar array, etc. When it's hooked to a building's power, that building's neutral line is hooked to earth ground. Because of this, there exists a 120VAC shock hazard from the DC side of the inverter to building and earth ground.

  Insure that the DC side connections are tight and that they do not become excessively warm during loaded operation. As the load doubles, the connections and wires will become four times as hot.

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