The Inverter is an electronic device that converts a DC voltage to AC voltage. Cruisers usually want to convert ships 12 volts DC battery power to 110 or 230 volts AC.
We love having an inverter on board and it gets used on a daily basis, we do use the inverter to power the washing machine, kitchen and workshop tools and it really makes our life on board easier .
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| A bare bones Pure Sine Wave inverter fitted in Matilda. |
This inverter is a combi unit and includes a built in battery charger for when we are connected to the shore power at the dock. By bare bones I mean it does what its meant to do very well, it outputs a single phase pure Sine Wave and charges the batteries when connected to the dock power. It doesn't have a lot of the bells and whistles that most cruisers dont really need. While they may be necessary for complex installations, for most of us they are unused and are just an added expense and that money could have been used elsewhere.
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Before we get started basic abbreviations and power rating.
A well used electrical calculation: Watts = Voltage multiplied by the current in Amps; W = V x A
AC is an abbreviation for alternating current: an electric current that reverses its direction many times a second at regular intervals, in Australia its 50 times per second and in the US its 60. also known as Hertz or Hz. Output is from a portable generator, inverter or mains (the big generator at the power station)
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| The AC wave form, output from a Sine Wave Inverter or generator. |
DC is an abbreviation for direct current: an electric current flowing in one direction only. Output is from solar cells, batteries.
RCD Safety switch: An RCD switch will operate when there is an imbalance in the current flow in the active and neutral conductors. Once this imbalance reaches the set threshold, usually 30mA, it opens the circuit, cutting the power output. A lot of people believe that an inverter is safe and will cause no harm because it runs off batteries. However this is a false sense of security, the power from even the smallest inverter can kill as quickly as a mains power point. So it is certainly recommended a RCD safety switch be fitted. For a discussion on safety switches
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| RCD Safety switch to reduce the chance of being electrocuted from the output of an Inverter |
Why have an Inverter when you go cruising?
In Australia we all have 230 volts AC as the primary mains voltage source at home, we already have many appliances that run off 230 volts, it may be the high powered microwave oven or the very low power charger for a laptop, tablet or camera. An Inverter allows you to use the appliances you already have when you are away from the shore power. The disclaimer here is that the inverter type and size has to be matched to the appliances you want to use.
Benefits of using an Inverter
Instant power with the flick of a switch, no turning on the gen set, or switching over the source of the power. The main reason for most people is that there is a wide variety of appliances. Using an Inverter can save you the cost of replacing the appliances you already have at home with 12 volt equivalents. In some cases 12 volt appliances are more expensive due to lower sales, or possibly they are not available for every application.
Why a lot of Cruisers buy an Inverter
A lot of cruisers like to take the convenience of household appliances out on to the water with them. There are so many appliances used in the kitchen or workshop that make quick work of tedious jobs. I use a powerful mixer on a daily basis to knead bread dough so we can have a fresh loaf of bread while we sail offshore miles from land. Using the appliance we reduce the kneading time from 20 min to just 5, not that we don't like the upper body workout, we like using that time to do other jobs. There is just about an appliance or power tool to help with most jobs, and we have met people who use some appliances we never thought of taking on board, it's only limited by imagination, storage and of course battery bank endurance.
During our travels we have met with dozens of cruisers who had planned fitment of their inverter during preparations before going cruising. The cruisers then executed the install and had the 12 volt side of the inverter connected with the right gauge wire, fuses and safety grounds. They can also have wired in power points outlets to make the inverter use convenient. By having the inverter properly installed they were able to install and configure a RCD safety switch and circuit breakers to protect the people and appliances. Add on kit for an RCD info here
There is also another good reason a lot of people buy an inverter, they do not know there are 12 volt alternatives for the most commonly used items. So what are these commonly used items? most of the time its battery chargers (phones laptops, cameras) and TV’s, and this is why so many people buy an inverter, they head off to the marine/camping/electronics/tech shop to get something to charge their laptop, phone or even the head torch, and really the list is almost endless these days. The sales person quickly takes the customer to the inverter section and gets a quick sale. The customer goes home not realising there are other products out there that can charge and run their small appliances and in nearly all cases are a lot safer and much more efficient at doing it than an inverter. But more on that in another post.
Being on a yacht we want products that make use of stored electricity in the most economical and safe way. Unfortunately for just charging laptops, phones and these types of appliance, a small inverter isn't always the best and or safest option. This is why we feel its a great idea to have a balance of electronics to do the jobs on board. This way should the inverter fail (happens from time to time) you can carry on with minimal disruption.
Please read on, for other uses an inverter can come into its own, if set up correctly.
Operation
A great reason for using an Inverter is, being much quieter than using a generator to produce the AC power, and when your running off batteries, there are no concerns about fumes or heat from the exhaust, and no flammable fuel to handle. If the generator isn't built in and running on the ships fuel, most of the small portable generators popular with cruisers run on petrol. This adds the complexity of firstly safely storing the generator and secondly the fuel to run it. The storage of petrol does have its complexities on a vessel designed and fitted out as a diesel fuel craft.
Versatile
An Inverter can be used whilst the vessel is running, and therefore not draining the house battery. Unless your a sailboat and having a good day out in a nice breeze. When motoring this has practical applications; ie arrive at the anchorage with fully charged batteries, camera batteries, laptop, and so on and so forth.
For safety an Inverter must never be switched on when you go to start the engine unless you have a totally isolated motor start battery. If you don’t the surge of current required to start a boat motor can drop the voltage down below 10V DC. After the motor is running and the alternator kicks in, it will surge back up to 12 V DC, the combination of surges can damage the inverter, the appliance running on the inverter at the time or of course both.
Interpreting the numbers written on the inverter
An Inverter changes the voltage and current, the power it puts out is always less than the input power, due to inverter efficiency being less than 100%. Most, but not all inverters list the operating power in watts, a big inverter may output 2500 watts so if it really is outputting 2500 watts at 230 volts that will be 10.86 Amps output, just over the maximum of a standard power outlet in Australia. This large inverter is capable of running most appliances easily. However keep in mind most of the numbers on the case i.e. 2500, the inverter will not really output 2500 watts. One that I know of will output 2300 W and another that has 3000 on the case outputs 2200W, so the reality is that you need to read the spec sheet to see the real out put value.
On the other end of the inverter scale is small inverters with an output of, for example 300 watts, this will be 230 V at 1.3 amps, and would be enough to run most laptop chargers.
An inverter is usually between 80% and 90% efficient. The 10 – 20 % of power that is lost as electrical energy escapes as heat energy in the Inverter. While this doesn't sound too bad if you crunch some numbers when operating at higher power it can be a significant current (in Amps) drawn from the batteries that is wasted energy.
So as an example of power loss, if an inverter is drawing 100 Amps and is 80% efficient then 20 amps will be wasted power and converted into heat.
Electrical Power
Electrical power is usually measured in Watts (1000 W = 1 kilowatts, 1 kW). So to keep it simple an inverter that outputs 240 volts at 10 Amps is 2400 Watts (240V x 10A =2400W)
Some Inverters output are rated in VA (Volt-Amps) rather than Watts and for many appliances this will equate to the number of Watts with a power factor of blah blah blah, well beyond our ability to calculate the actual value. The simple truth of the matter is VA means nothing. If you want to know how much power your inverter is going to give you then ask for the rating in watts at 40 deg C, all the other ratings should be kept for the fairy tale books where they belong. Some brands of inverters have a lower value in the model name but, in fact, are the more powerful of the bunch when apples are compared with apples (watts with watts). So shop around for a unit where you get what you think you should be getting, and it does what it says on the box it does. Do the comparison in Watts at 40 deg C apples with apples, (watts compared with watts). I do know some manufactures use lower temp rating to give better figures, however unless your operating your vessel in the lab then the 25 deg C isn't really achievable. Its all in the advertised wording.
Peak and Continuous
Two numbers are used to describe the output of an Inverter, Peak and Continuous.
Peak (or surge) describes the output deliverable for several seconds the Inverter components won't blow up at that amount of load, but they will heat up really quickly. Continuous ratings are usually limited by the heat that needs to be removed when running continuously. Ideally you would find out the Peak and the instantaneous ratings of your appliances and check that they were less than the Inverter output, but you can't rely on this because most appliances don't state the start-up or peak power and the length of the peak really needs to be known too. Unfortunately, the only way to be sure, is to test the appliance with the inverter.
For safety never run the inverter continuously at full power for long times, I have a safety margin in the order of 10% factored in to our calculations for long run times as this allows for the odd surge.
Size of Inverter Needed
The following table is only a rough guide as there is a lot of variation in startup surge load and continuous load for similar appliances between makes and different models.
Size of Inverter Required Type of Appliance
Types of Inverters the all important “Wave” output
When generating AC from a spinning generator, it’s very easy to generate the sine wave that AC has, but when inverting DC to AC, the easiest wave shape to output is a square wave. Unfortunately a square wave has more energy for longer at frequencies other than the 50Hz or 60Hz (Hertz or Cycles per second), and this can cause problems of overheating in transformers and some motors.
A sine wave, this is the same power wave output at the mains power point on shore and is the same as output by a rotating AC generator, or a Pure Sine Wave Inverter.
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| A comparison of the three waveforms laid on top each other |
Today Square Wave Inverters are rare except in the smaller sizes or the real cheap ones and most output either a Modified Square Wave (MSW) or a Sine Wave/Pure Sine Wave. Inverters advertised as Modified Sine Wave are just a marketing department creation and are really only a Modified Square Wave. Nevertheless modified sine wave (MSW) does have a good ring to it when you have been told that the sine wave unit is the best one to get.
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A square wave display, as can be seen this wave form quickly switches between positive and negative and stays positive or negative at full power for longer, this quick switching can overwhelm some electronic appliances connected to the inverter. |
Modified Square Wave Inverter (MSW) A.K.A Modified Sine Wave Inverter (MSW)
Most appliances will work from these basic Inverters, but unfortunately unless you can look at the internal electronic design of the appliance, you can’t be sure if you will really need a Sine Wave Inverter. The squarish wave shape can also confuse the timing circuits in some appliances that use Frequency as a timing control. If the appliance doesn’t work normally or makes unusual noises, then disconnect it or you may have expensive damage.
These Inverters also generate more interference to TV's, AM and HF radio reception than Sine wave Inverters and can cause buzzing in CD players and stereos.
When using an inverter in the marine environment tests need to be carried out to make sure the inverters electrical noise isn't enough to compromise reception on VHF frequencies used for radiotelephone, Digital Select Calling,(DSC) and Automatic Identification System (AIS).
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| Modified Square Wave AKA Modified Sine Wave, as can be seen this wave form is certainly has a lot quicker notchy rise and fall times when compared to the Sine Wave gentle undulation of rise and fall. Also keep in mind the number of additional steps in the modified square wave will influence how the inverter and connected appliances will perform. |
Electric motors for an example in washing machines, bread makers, power tools, microwaves, vacuum cleaners may give trouble if used with a modified square wave inverter.
Can I run computers with my modified square wave inverter? Most laptop/notebook computer AC power adaptors work perfectly well with the Modified Square Wave Inverter. Desktop computers and some laptops/notebooks may be more sensitive so they may not function correctly or cause a humming noise, in which case a pure sine wave inverter is recommended.
Mac/Apple products are extremely sensitive in the way they use their power, and will require a Pure Sine Wave Inverter to run successfully. We also found a good power filter was also required when using apple products.
You may have trouble using an AA battery charger with any inverter due to the way the battery chargers draw energy. You will also find that a pure sine wave inverter is a better option for battery chargers due to the fact that there's much less ‘electrical noise’ interference compared to a modified square wave inverter. This ‘electrical noise’ can often wear away at the sensitive components in lower quality battery chargers. The rechargeable batteries, recharging transformer and Inverter may be damaged when trying to recharge devices of 10 volts or higher.
However, it is always a good idea to double check your appliance's manufacturer's guidelines first. If they can't be charged via a modified sine wave inverter, they will mention something similar to "Charge via Mains power only" or not as we have found out.
Sine Wave Inverter (marketed as Pure Sine Wave or Pure Sine or even the S model)
Sine Wave Inverters are more expensive than Modified Square Wave, some times up to twice as much because more components are needed to electronically generate the Sine Wave. You may consider that If the appliance has a Transformer or an Induction Motor think washing machines and appliances listed earlier, then it most probably needs a Sine Wave Inverter to prevent overheating. Now this is not always true, since some fridges and freezers can work fine off Modified Square Wave inverters, and in fact you shouldn't really notice any difference powering a fridge from a Modified Square Wave inverter, however the notchy waveform can interfere with timing circuits.
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| The wave form output by the an inverter, same shape as the mains power from shore. |
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A list of optional features that are listed as inverter options/specifications.
Electrocution I am lead to believe from years of health and safety briefings it only takes 35 milliAmps (that's 0.035 Amps) in the right conditions to kill a person. The 230 volts from an Inverter can kill you instantly, just like the 230 volt at home, so you need to take precautions. So if you do purchase an inverter that doesn't have total electrical isolation between the DC input and AC output there are several things to do for your safety.
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| A RCD safety switch, while we will never know for sure, we believe it saved us from a nasty accident when our washing machine developed a fault, and another time when I accidently dropped the hand mixer into the sink full of water and with out thinking tried to stop it sinking. |
This feature is also very important if the inverter is also a battery charger when plugged in to shore power. Most are marketed as combi units to designate an inverter and battery charger combination. When the unit is plugged into mains power the connection from earth to neutral is turned off to make use of the earth to neutral link from the shore power receptacle/power box, this enables the shore RCD safety switch's to work.
Not all manufacturers include an Auto start feature especially when it comes to Pure Sine Wave inverters, some have a hard wired remote and the more modern units Bluetooth. The major drawback to this auto start feature is the power used while in standby. This can be significant and should be included in the planned daily power usage.
Some units also switch the Earth Neutral connections so RCD switches work as intended, very important for crew safety.
Twelve volt inverters, can draw high current from the battery banks, the general rule is to divide the output power by 10 to understand how much current will need to be supplied. So a 1000 watt inverter will draw 100 amps from the battery and this means using battery cables which can supply this level of current without overheating, while also keeping total voltage-drop in the positive and earth lead to less than 1.5% in each - i.e. less than 0.2 volts.
Inverter Grounding
I am sure most of us know the high voltage of the AC system presents a shock hazard and can certainly be lethal. The DC system is not normally an electrocution hazard but can provide a lot of current, and so is potentially a source of fire. A suitable ground must be installed between the AC and DC system. During my inverter fitment I noticed in the operating instructions the requirement for grounding connections on both the AC side and DC side of the unit’s case. This grounding can prevent shocks from AC, and fire hazard from DC.
My previous unit only had the smaller AC grounding connection made. A fault in the DC side of the system could provide enough current to overheat the AC grounding conductor (think enough heat to melt insulation and start a fire) without blowing the large DC fuse. Keep in mind the fact that the AC ground is not fused and current will flow through the conductor until it burns out. A high amperage capacity DC grounding path back to the DC system should be made. The conductor size for the ground needs to be of sufficient cross sectional area (bigger is better) to be able to sustain enough current to blow the supply fuse.
The installation instructions for older inverters (as I had previously) typically do not include this precaution, but most newly designed units do. Though looking into some different manufactures install instructions this connection is not addressed in any depth, most only state or draw the connection in on the installation schematic. Due to the fuse size in my inverter we needed a 70 mm2 ground wire for the DC ground.
Limitations
Battery capacity
All the power for the load has to come from the battery, so with high power Inverters or appliances you want to run for a long time, you need large battery banks. If you wanted to run a small Air Conditioner that used 1000 watts on average, it would draw 84 amps, so you need to have a battery bank capable of supplying 2000 amp hours in a day. However we are not done yet, we still need to factor in the losses incurred (the 10-20% mentioned earlier) by running the 12 V DC through the inverter to output 230V AC the figure would be closer to 2400 amp hours draw on the batteries. That means big, heavy and expensive batteries, more importantly a way of recharging them or keeping them charged.
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Alternatives to an inverter
Use 12 volt appliances
Many appliances are available as both 12 volt DC or 230 volt AC versions, but the 12 volt version uses less power and is always more expensive. A 12 volt fridge will mostly have thicker insulation and use a more efficient motor/compressor. The designers of mains powered appliances don't factor in the same level of efficiency as they do for 12 volt powered appliances, due to the design spec being for an unlimited mains power source.
A 12 volt electric kettle will use less power because there are no losses in converting the 12 volts to 240 volts first, how ever a lot of people don't actually realise how much power an electric kettle draws and the install of the unit can be more expensive due to power cable size. Using only 12 volt appliances, may reduce your power needs, so you won’t need a generator or run the motor to recharge your batteries on shorter outings. However you need to ensure you use heavy enough cable to run the appliances connected to them.
Generator
These can easily generate the high power level often needed and they produce a 230 volt output directly. However an inverter is totally silent and much smaller than a generator and doesn't need regular servicing. Keep in mind that most portable generators exhaust can output deadly carbon monoxide gas and in the right conditions this lethal gas can find its way below and slowly kill you while you cook dinner or have a few beers.
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