In some places people say they are fine to run pretty much anything, though expect 'noise' if using audio or televisual equipment.

Other places say they cannot be used with inductive loads. On your site you say to consider anything without an element as inductive. As you can see, I am a little confused. Any help gladly welcomed, and if I can get away with buying a modified one (for sander, drill, Hoover etc) I may well be able to make a donation to your excellent site with my saved cash.

Thanks a lot for you valuable time, and keep up the great work.

All the best,

Steve

ANSWER

Dear Dr M

As far as I can see drills, sanders, Hoovers, in fact any type of motor should be OK unless it has electronic speed control, then things start to get messy because some are OK with modified sine-wave and some are not. Also some AC motors that use brushes (universal motors) may be noisy and get hot when running on modified sine waves.

For instance microwaves with an electronic timer appear to need a pure sine wave inverter (as do some washing machines), but get one with a mechanical timer/programmer and they seem to work.

Once you start looking at consumer electronics (Audio, TV, computer etc) my information suggests that it all depends upon the design of the power supply. Switched mode may well work, but other types may not.

The usual advice is to get the supplier to let you test your doubtful equipment at their premises or to agree to a refund/exchange if it does not work.

My Maplins 150w cheep modified sine wave charges phones, batteries and a Dustet OK, however a Nokia digibox did not like it at all (and Nokia declined to give me any info about the power supply etc).

I also think it depends upon exactly how modified the square wave is and how inductive and capacative the load is.

To be honest, I think that I would be seeking a reliable contact in your University's Electronics Department (technicians are often very good) and seeing if they can help/advise.

If I was going to buy anything other than a "throw away" cheepy inverter I would go for a poure sine wave one to be sure for future use.

Sorry not to be much help.

Tony Brooks

Subject: Connecting up a 3 phase relay

Hi.

Having recently purchased a narrowboat, I wish to fit what is essentially a changeover relay between shore and narrowboat supply whereby the inverter supply provides a "hold off" supply for a relay which will isolate the shore supply until the inverter is switched off. I was going to use a small ac - dc transformer to energise the "hold off" relay.

What type of 3 phase relay would be suitable for a maximum load of 3KW(ac).

Regards Dave F

ANSWER

Dear Dave

I regret that I am unwilling to answer questions about mains electricity now in view of the danger of litigation if something goes wrong, however I make a few points that you must double check before you act on them.

First, it must be a hell of an electrical machine that you intend to install on the narrowboat to require a three-phase supply, and I have never heard of a three-phase inverter. I suspect you only need to deal with a single phase with two alternative supplies (three alternatives if you install a generator).

I get constant questions about inverters with auto-change over switches failing and my reading of them indicate that there may be a problem with such circuits where they make before they break for some reasons and connect both supplies, but out of phase with each other.

I know automating the process of switching supplies sounds good, but if I was installing mains on my boat I would use a manual change over system and a printed procedure to ensure that one supply is isolated before the other is brought into play. I am sure someone like Merlin (look in Waterways World) could supply a suitable switch. I am also sure that once you know a part number a mains rated, double pole change over switch would do if it had an adequate current capacity - it follows that a relay meeting the same criteria would also do, RS Ltd. come to mind a possible suppliers. However I would urge caution.

The simple and most expandable way of achieving what you want I have heard about is a bank of three (in this case because he also had an onboard generator) industrial blue sockets, each connected to one supply. The boat's mains circuits were terminated in a suitable plug. Thus it is impossible for two supplies to be online together.

You do not mention this, but as soon a shorelines become semi-permanently connected you would be advised to either use an isolating transformer or a galvanic isolator where the mains enters the boat.

I draw your attention to the disclaimer at the top of this reply.

Tony Brooks

ANSWER

Dear Martin

I am not qualified to give advice about mains equipment, so please check with experts before you take any action upon what I say. I also have no technical specs for equipment, so I do not know the details of your setup.

First of all is this inverter a modified sine wave or a pure sine wave one. If it only provides a modified sine wave you may find (nothing is certain in all of this) that a machine with an electronic timer may not work, so you might be better advised to look for one with a mechanical "knob" type control that "ticks" back - few and far between now.

Next, most washing machines incorporate a heater and that probably accounts for most of the power rating. If this was disconnected and you relied upon the boat's hot water supply you would only need to concern yourself with the motor rating. I would advise that you cultivate a washing machine engineer to discuss the ramifications of doing this.

Now you need to consider the method of motor speed control. Some are more tolerant of modified sine wave supplies than others. I fear you may be unable to get a definitive answer, so its "try it and see".

Finally you must recognise that motors draw a high surge current when starting up, so allow at least 25% more inverter capacity that the running load of the washing machine. However many inverters will allow this surge over and above their rating, but it is vital to check the technical specs first.

I fear no-one will give you a straight answer because there is too much chance of something not "matching" and you then seeking compensation for the cost of the washing machine or inverter.

If you have a pure sine inverter then things are easier once you ensure that the inverter is capable of dealing with the starting surge. I can not comment on the veracity of the inverter's rating, but please ensure that its surge capacity will cope with a 25% overload for at least a minute.

(Note - inductive loads like motors are usually wired for a 20% surge so 25% give a safety margin).

Please also take care that you have sufficient batteries in your battery bank. A high power inverter may well draw more current than a running starter motor and we accept a 2v voltdrop on starters. I doubt any inverter will take kindly to running on less than 11 volts. The "loss" of voltage is caused by the resistance inside the batteries, so you may need to add more batteries to the bank.

Tony Brooks