Shore power and air conditioning

Shore power, once common only on the largest yachts in the form of a substantial supply of alternating current, is now commonplace on even quite small craft. The motives for providing shore power extend to meeting the boat owner’s desire for the comfort provided by air conditioning on hot days and heat for cold ones. In some installations, shore power provides energy for galley appliances: refrigeration, microwave cookers, etc. Battery charging is often a high priority, as well.

While many of the shore power arrangements simply bring the dock-side power on board through a suitable multi-pole connector and circuit breaker, some incorporate inverters, powered from the boat’s battery bank, making the availability of shore power somewhat independent of the actual availability of AC power at the slip. These systems, usually equipped with automatic load sensing and the ability to substitute inverter output for shore power can be a great convenience, however, they must be treated with respect since potentially lethal AC power may be available at all times.

A typical shore power connection will connect the two “live” shore power conductors (l-1 and l-2) and in addition the “green” safety ground wire to the boat’s main electrical panel via a dual pole circuit breaker. A “reverse” polarity indicator is often a part of the shore panel wiring and will illuminate if the polarity of the shore power is reversed from normal.

An inverter-based shore power system may be able to provide some worthwhile advantages, especially when trying to obtain power from dockside supplies that are poorly wired or habitually provide lower than normal voltage. The inverter shore power module will use the available AC energy to recharge the boat’s battery bank, in turn using the energy from the battery to power an AC-output inverter.

Assume circuits are “hot”
The electrical system on any boat equipped with shore power must always be treated with respect and caution. The AC output from the shore power inverter is designed to be present and depending on the specifics of the installation may be “on” even when some indicators may suggest otherwise. The best advice is to always assume the shore power circuits are “hot.” One of the main uses many power voyagers have for shore power is to run their air conditioning unit when they are at the dock. Regardless of where you voyage, there will be a day or night when having a cool boat will make a world of difference. The intense discomfort created by the heat and humidity we often encounter when voyaging during hot weather has made onboard air conditioning a popular option on virtually every power boat with sleeping accommodations. If your boat is already equipped with an air conditioning unit and especially if you are considering installing a system, you will want to know about a new “black box” (that is actually off-white in color) that can significantly improve the occasionally troublesome starting performance of the AC system.

Today’s marine air conditioning equipment is easy to install, reasonably energy efficient, quiet and fairly priced. The energy efficiency of today’s equipment will allow a one-ton (12,000 BTU/hour) cooling capacity unit (8.7-amp cooling current draw) to be powered from a 15-amp AC shore power line or from the boat’s genset. (It’s also possible to power the AC unit from an inverter supported by a suitable battery bank.)

Start-up surge
While the operating power demand of the AC unit is relatively modest, it becomes a power hog on start-up. For example, an AC unit can demand a momentary starting or in-rush current as high as 59 amps for the 12,000 BTU unit, six times the steady-state running current. Depending on the quality of the boat’s AC power system (proper voltage, steady-state current capability and impedance) the start-up surge demand can delay or prevent the air conditioner from starting or restarting as it cycles on and off to maintain the desired temperature.

The AC unit’s high in-rush current demand is an inherent characteristic of the induction motor that powers the refrigeration compressor. This challenge to the power supply system, also present in a wide variety of other induction motor applications, is generally met with the use of a current-limiting, soft-start circuit. The typical marine air conditioning unit’s soft-start circuit may, however, not be sufficiently effective. It may be unable to assure consistent starting.

The problem is often most apparent on a hot night, when AC power is being supplied from a heavily-loaded, often marginal-voltage shore power system. Under the best of circumstance, the AC unit may have to make repeated start attempts. Under less-favorable conditions the sustained high in-rush current will trip a circuit breaker — on the boat’s electrical panel if you are lucky, or, if it’s the middle of the night, on the dock’s shore power pedestal.

Reducing in-rush current

The new SmartStart module from Dometic (Cruisair and MarineAir) uses a microprocessor and custom software to precisely manage each air conditioner starting cycle. The device can reduce the in-rush current by up to 65 percent, limiting the current demand of the one-ton AC unit to about 20 to 22 amps. The SmartStart product will also protect the AC unit by preventing it from operating if the AC line voltage decreases to the point where the compressor motor may become overloaded. The unit’s software “learns” the characteristics of the compressor motor, using the data it collects from successive start cycles to optimize starting performance.

The SmartStart achieves its in-rush current reduction performance by monitoring and precisely controlling the amplitude and waveform of the AC current applied to the air conditioner’s compressor motor, feeding it just enough energy to initiate rotation and subsequent acceleration. The reduction in in-rush current achieved lessens the strain on the AC power supply and at the same time reduces the mechanical stress on the motor and compressor.

Housed in a sealed 5 inch x 3 inch x 2 inch, 15-ounce plastic enclosure, the four-wire SmartStart is easy to retrofit to virtually any marine air conditioning unit.

Contributing editor Chuck Husick is based in Tierra Verde, Fla.

By Ocean Navigator