Power from an AC inverter

PART 2 - POWER INVERTERS

Combination Charging and Inversion Units

Many boats and barges will have these types installed and many of these units have a modified sine wave output. The chargers can be set for the various battery types such as the lead acid flooded cell, Gel or AGM batteries. Inverters from Heart use a 3-stage microprocessor charging process. An automatic transfer switch and relay transfers between charger mode and inversion mode when AC power is off. As a note of caution, always remember that combination units may automatically supply AC so proper isolation from the battery source is essential before opening or working with the unit. There have been fatalities due to this simple safety matter

DC INPUT CABLE INSTALLATION

Many units are installed with undersized cables supplying the inverter from the battery. This causes voltage drop under high current loads, and a significant drop in efficiency. Always install the maximum size cables possible as considerable volt drop problems are possible at peak loads. Another problem area is the use of under rated connections. Small portable units either have clips or they have a cigarette lighter plug. Always permanently install using the properly rated connections. Also make sure that the cable connections are tight. If they are not tight voltage drop problems will reduce efficiency. Most units have an under voltage cut-out that is typically set at around 10.5 Volts. Inverters also have overload protection and a high voltage cut-out, as well as thermal overload protection that will shut down the inverter if an over-temperature condition is reached. Other protection systems include reverse polarity indication and protection if the DC leads are reversed, short circuit protection of the output and some units have AC back-feed protection and a built in earth leakage protection device on the AC output circuit.

INVERTER AC GROUNDING

The power inverter must have be properly grounded. There are several documented cases of deaths where this has not been done. Grounding introduces a number of factors that require consideration. Some people do not ground as they believe it affects corrosion of the hull. This is not true and the corrosion reasons must never be put ahead of or compromise the safety ones. Most wiring standards or recommendations specify that the AC ground be connected to the DC negative. One recommendation is that the unit chassis should be bonded to the DC negative. This is actually doubling up the grounding, as the AC ground and DC are already bonded. Always follow the manual instructions.

FAULT PROTECTION ON INVERTERS

It is normal to install what is called a GFCI or RCD earth leakage protection device on the AC output. The device must be tested monthly. The device should be function tested when inverting or when transferring generator or the shore power supplies. Do not test when the unit is in idle mode, as the unit may not trip, and the protection device electronics circuits could be damaged by the idle mode sensing pulses. Intermittent tripping does happen on some units, and this is usually caused by neutral to ground leakages. This is usually caused by surge suppression circuits which contain capacitors connected across the active and ground or neutral and ground. This is generally blamed on the waveform harmonics on a modified sine wave outputs. Another cause of protection device tripping is caused by the improper connection of the AC output neutral to the main neutral bus. Follow the manual and if you don’t know then get expert AC marine electrical help.

CLICK HERE FOR USEFUL INFORMATION ON INSTALLING AND MAINTAINING INVERTERS

FAILURE TROUBLESHOOTING

The control electronics, such as the logic circuits and memory circuits can be corrupted. This may be from onboard electrical power system surges, voltage dips and spikes. In many cases the unit may simply require resetting if no signs of catastrophic failure have been detected such as smoke or burning smells. Try the following procedure first before taking the unit to your local technician.

1. Switch off power at the remote panels if installed

2. Switch off the main power switch.

3. Disconnect the AC input power source if it is a combination unit.

4. Disconnect the DC negative cable for at least 5 minutes.

5. Reconnect the DC negative cable, sometimes a small spark will be seen as filter capacitors start charging.

6. Switch on the power switch and switch on an AC load to ensure inverter has a load. Check that the inverter supplies AC power.

7. Reconnect the AC input power source and check that the automatic transfer functions properly.

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