All about the DC to AC inverter

PART 2 - POWER INVERTER

Combination Charging 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. The unit 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

Installing the DC Input Cables

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 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.

Connecting the AC Grounding

Inverters must be properly grounded. And there are documented cases of fatalities where this has not been done. Grounding introduces a number of factors that require consideration. The corrosion aspects must never be put ahead of, or compromise the safety ones.

Most standards or recommendations (ISO, ABYC, USCG) specify that the AC ground be connected to the DC negative. This requirement has raised considerable controversy in shore power installations. Many do not do this to reduce the risks of electrolytic corrosion. The ABYC recommends that the chassis be bonded to the DC negative, although this is doubling up the grounding as the AC ground and DC are already bonded. Always follow the manual instructions

Installing Ground Fault Protection

It is normal to install a GFCI on the AC output, and Heart interface use the Pass & Seymour/Legrand unit, although any make will be satisfactory. As indicated the GFCI must be tested regularly. The GFCI should be function tested when inverting or when transferring generator or shore power supplies. Do not test when in idle mode, as the unit may not trip, and the GFCI electronics circuits could be damaged by the inverter idle mode sense pulses.

Nuisance tripping does occur and this is generally caused by neutral to ground leakages, usually from surge suppression circuits which contain capacitors connected across active and ground or neutral and ground. This is attributed to waveform harmonics on modified sine wave outputs. Another cause of GFCI tripping is due to the improper connection of the AC output neutral to the main neutral bus.

Inverters such these from Heart ground the output neutral when in the OFF mode or in inverting mode. If the output neutral is connected to the main neutral, the main neutral will also be grounded, the GFCI will detect this and trip out before the inverter is able to disconnect the ground from the output neutral during shore power and generator transfers. Follow the manual and if you don’t know then get expert AC marine electrical help.

Troubleshooting Failure

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

7. Reconnect the AC input power source and check that the inverter automatic transfer functions properly. More on the inverter and other marine electrical systems