If you are intent on taking your band or set to the streets or you feel like setting up an out door gig where power is an issue, then there are a few good ways of getting round the problem. Generators and battery operated systems provide good alternatives to consumer mains, but if you haven’t got the cash to splash out on a decent generator you may well consider opting for the cheaper and noiseless battery, charger and inverter set up.
Here we learn the differences between types of dc – ac inverter, how they work and how to select the right inverter for your rig.
Sine Wave and Modified Sine Wave Inverters
DC – AC Sine wave inverters convert the DC (direct current) supply from a power source such as a car or deep cycle battery, into an AC (alternating current) supply which can be used to power regular household equipment such as fx racks, amplifiers, stereo’s, tv’s, hairdryers, microwaves and computers etc.
300 w Inverter
There are basically two types of sine wave inverter.
Pure or True Sine Wave Inverters
A pure or true sine wave inverter converts the dc supply into a near perfect or pure sine wave, replicating the supply attained from a domestic ac power source such as a plug socket. The sine wave has very little harmonic distortion resulting in a very ‘clean’ supply and makes it ideal for running electronic systems such as computers, digital fx racks and other sensitive equipment without causing problems or noise. Things like mains battery chargers also run better on pure sine wave converters.
Ideal for all applications, the pure sine wave inverter is a must for anyone needing to convert power from a dc source to a universally useable ac supply. Unfortunately they are very expensive compared to the modified alternative.
Modified (Quasi) Sine Wave Inverters
Modified sine wave inverters are a much cheaper and somewhat rougher alternative to the pure SWI. Instead of the output being a pure sine wave, the cheaper circuitry in the MSI outputs a rough sine wave. This means equipment with circuitry that relies on the smooth oscillation of a true sine wave, like dimmer switches, PC power supplies, variable speed motors and scientific equipment like oscilloscopes etc. may not work properly or as efficiently as they would otherwise.
Comparison of Sine, Square and Modified Sine Waves
The cheapest power supplies generate square waves, which as you can see from the diagram above doesn’t really follow the arc of the sine wave to any degree. A more expensive power supply producing a modified sine wave provides a more closely matched signal to the pure sine wave, but is still not ideal.
Drills and dimmer switches produce a variable output depending on the position they are in, so it’s a gamble as to how well they will perform using a modified wave generator. You may get away with it, or you could well experience problems with reliability, noise and motor irregularity.
3000 w Inverter
As for running musical equipment such as fx racks, keyboards, amps and guitars etc. with the modified wave inverter, the advice is generally the same. Depending on your set up, and how robust your equipment is you may find you get away with it, and your equipment runs fine, but you could experience noise, equipment buzz, overheating and reliability issues. It could also affect the life span of some of your more delicate gear.
A better quality supply producing a true sine wave will certainly run 99% of your digital fx equipment, amps, synths and laptops etc. with no hassle at all, just as you would expect if you were plugging in at home.
The best advise if you are unsure as whether to take the risk and go for a cheaper MSI would be to test it out before you buy. Any respectable dealer will allow you to test the inverter on your set up and give you the opportunity to return it, if it’s not right for your system.
Choosing the Correct Inverter Power Rating
Choosing the right inverter for your set up is vital.
The efficiency of an inverter varies greatly depending on the amount of power being drawn through it. It can range from around 90% when being used at it’s full rating, to around 50% when being used with light loads. In general an inverter is at its most efficient when being used at around 1/3 to 3/4 of it’s full rating. When used at optimum levels an efficiency of around 95% is attainable.
A ‘load’ is generally what ever is connected to the output of the circuit.
Inverters with Resistive and Inductive Loads
Another factor to be taken into consideration is what you are running with your inverter.
Most inverter efficiencies are rated using a resistive load.
A resistive load is generally when current is converted into something else along the lines of a lighting or heating system. So if you were powering a resistive system such as a bunch of floodlights for your gig, then the efficiency percentage given by the manufacturers should be reasonably accurate. However, if you are using the inverter to power an inductive load, ie. something that uses magnetic fields such as motors, solenoids, compressors, pumps or relays etc. then you have to take into consideration the way the motors efficiency works with the wave that is powering it.
An inductive load such as a motor works most efficiently when powered by a true sine wave, but looses a great deal of its efficiency when powered using a modified sine wave. A motor may easily use 10 – 20% more power than it would otherwise when powered with a less than perfect source. Bear this in mind when you are doing your calculations and think about the inefficiencies of everything you are intent on powering in the circuit.
To cut a long story short, a true sine wave inverter is best for every occasion, but not totally essential if you haven’t got much cash and can get away with it.
To choose the right inverter, you need to know it’s 3 ratings and the ratings of the equipment you intend to power.
1. Surge Rating – Starting Load – or Peak Load . This is how much the inverter can handle for literally a few seconds while it deals with power spikes caused by the switching on of equipment such as amps, fans, motors, tv’s etc.
2. Continuous Rating – Continuous Load – This is the load the inverter can handle for as long as it likes. Generally, this is what the inverter’s advertised rating would be.
3. Limited time rating. This is how much power the inverter can handle for short periods of time when an excess load is placed on the system. This time period can be anything from a couple of minutes up to ten or twenty.
Obviously the peak and continuous power consumption levels drawn by the equipment you are using with your inverter i.e amps, tv’s lights etc. should in total be less than your inverters capabilities. You will normally find power ratings marked on the equipment or in the instructions.
If you are worried about whether or not your equipment, be it a light, amp, compressor or fridge etc. has a starting load to worry about, most inverters have a maximum peak load rating of 3 or more times their continuous power rating. This should probably do you in most cases but check your inverter to make sure it does. Also check the specs of the equipment you are running. Some types of machinery require starting loads many more times that of it’s normal running load.
Here’s a rough example of an inverter rating calculation. Obviously you can add as many amps, mics mixers, monitors and a P.A to match your needs. I’ve just thrown in some equipment to give you an idea of how to proceed with your own set up.
Say you wanted to do a small set in town with a couple of mates with combo amps and an unamplified drum kit.
You might have a guitar fx unit, bass fx, a keyboard, a 30 watt combo amp, a 25 watt combo amp, a 50 watt bass amp, a reverb rack and couple of mics fed into the combo’s. (Don’t write in correcting the set up, it’s just an example)
We deal with rms values in all instances. As an amplifiers rating is generally stated in watts rms and standard Ac electricity supply is 220 volts rms (in the UK).
Work out the individual power consumption of all your fx units, amps, mixers, lights and anything else you are going to use. The power consumption of each unit should be written on the side or back of the unit or will be stated somewhere in the specification sheet in the instruction manual. If not, you can use the equation below to work it out.
P = V I
Power (watts) = Voltage (V) x Current drawn in Amps (I)
For example, my guitar fx unit says it uses 9 volts at 2 A
So using P=VI my guitar rack uses 18 watts.
Keyboard 12 w
Reverb rack 16 w
Bass pedal uses 15 watts
Combo amps together use 25+30+50=105 watts.
Add them all together
Total power needed = 166 watts continuous.
For obvious reasons, I would always advise to buy equipment that can cope with work loads larger than what you initially need as your demands are bound to increase as your needs expand. Also you might want to take into consideration other losses that are not so obvious such as converting ac power back into dc for your equipment, ambient temperature, and general losses due to mechanics, age and wear and tear of all the gear involved.
I reckon an extra 25 % is a large enough excess.
That would give us… 0.25 x 166 = 41.5
166 + 41.5 = 207.5
So we would need a continuous supply of 207.5 watts RMS
Bearing in mind an inverter will have an average efficiency rating of between 85 and 95 %, we can take the value of 90% and say the 207.5 watts we need to get out of the inverter on the other side will only be 90% of the power we need to put in to achieve that.
So we have to multiply our 90% figure by some factor to work out the 100% figure we originally fed into the inverter to attain our 90% out.
90% x 1.1 = 99 %
If we multiply 207.5 watts by a factor of 1.1 we get
207.5 x 1.1 = 228.25 w
This means in theory, if we input around 228.25 watts in one end, we’ll get 207.5 out the other.
So if we went out and bought a true sine wave inverter that had a constant power rating of 250 watts we should be fine and have plenty of spare room to play with.
The Easy Way
There’s always an easier way of doing things. If you go out and buy a clamp on ammeter, you can set your equipment up in your own home and measure the actual current draw (I) on a normal domestic supply. It’s always better to have a real result rather than a calculated one. Then use P=VI to calculate the correct sized inverter for the job.
You should be able to buy a decent clip on ammeter for around £20.
Remember to check your inverter has a good peak power rating, incase you need to use it for other things like inductive loads in future.
One last thing to be aware of is that cheap or incorrectly rated connection cables between your battery and inverter can also cause noise, overheating and more efficiency problems. The distance between connections is also an issue an can create voltage drops which will affect the output of your inverter. You should refer to the manufacturer’s recommendations to find the correct cable lengths and ratings instead of using just any old cable you find in the shed.
Now you know all about inverters, their uses and power ratings, check out my other guides to help you decide what’s best for your outdoor set up.
For time and legal reasons I don’t answer reader’s questions on calculations or individual set up’s, but if you need to know anything about creating your own power set up or need advice on calculations, make sure you fully read through my battery guide and posts on inverters, battery calculations and battery connections.
There you should find all you need to know to help you get started.
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