scap_ power inverter

Power Inverter

What is Power Inverter?

Power in the topic of this article is used to qualify the inverter so as to differentiate it from a logic invert

anywhere I write 'inverter' in this article, I mean 'power inverter'. An inverter is

system that produces AC power from DC. Inverters convert DC power stored in batteries

conventional appliances. Another application of inverter is in the case of uninterrup

aid of 12V DC battery is able to generate up to 110/220VAC (in this article, we shall

that can be used to power most house and office appliances depending on their power r

An inverter consists of the following: pulse generator (or oscillator), gate or base

circuit (optional), power switch (semiconductor switches) and step-up transformer. Th

diagram of an inverter is shown below.

(a)

(b)

Figure1 Block diagram of inverter

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Pulse generator: This is the signal processing and control circuit that generates the logic-level con

power switch (semiconductor switches) ON and OFF. There are many different circuits t

generator or oscillator, in fact many ICs that need few external components to be con

Such ICs include but not limited to NE555, CD4047, SG3524. The output of this circuit

or via the driver circuit for amplification before it is sent to the power switch as the case may be. Of cou

the design and/or transistors used as power switch.

Driver circuit: This circuit amplifies the signal from pulse generator to levels required by the powe

isolation when required between the power switch and the logic level signal processin

Power switch: Semiconductors like power transistors (Bipolar Junction Transistors or Metal-Oxide

Transistors) and thyristors are used here as switching devices. They should be sized

winding (low voltage side) of the transformer.

Transformer: Transformers are of various types: step up, step down, autotransformer etc. They

secondary windings which may or may not be isolated from each other. The windings are

magnetic circuit and operate based on the principle of electromagnetic induction. The

winding is related to their voltages and currents with the following equations.

Where,

= Number of turns of the primary

= Number of turns of the secondary

= Primary voltage

= Secondary voltage

= Primary current

= Secondary current

The size of transformer is proportional to its power. For an ideal (lossless) transformer, the input powe

but in practice, there is no lossless transformer.

Inverter Output Wave-form

One of the things one has to put into consideration when designing every components o

electronics system is the out. In the case of inverter, we have to put into considera

RMS values, and power output. For now, let us put power output aside as we shall disc

In conventional AC power system, the output wave-form is pure sine-wave as shown in f

the peak and RSM value of pure sine-wave is given by

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OR

Where,

= Peak voltage

= RMS or effective voltage

= Peak current

= RMS or effective current

RMS is the root mean square or effective value of an alternating current. It is equiv

same amount of heat when flows through a given circuit for a given time as thus AC.

The above equation was not brought from heaven but a derived equation from the interp

of the Square value) using standard equation of sinusoidal alternating current (AC),

OR

figure 2, pure sine-wave

Let us stop sine-wave at this junction since the inverter output is not sine-wave but

sine-wave from DC. We would talk more on square-wave. Wave-form shown in figure 3 was

designed, built and been using it since 2005 and still working perfectly. Nevertheles

problem is actually with the peak voltage of the output wave-form. The wave-form as s

equal to RMS value. As I designed it for RMS voltage of 220V, the peak voltage also e

operate on DC voltage from AC supply may not work. Check my 12V regulated DC power supply

voltage to determine the voltage applied to LM7812 (voltage regulator IC). This probl

with my desktop computer and it was not coming on. I sat back and checked my design v

until after some months. The problem was quite inexperience as I was so much in hurry

and by myself without putting into consideration all necessary things. As I said earl

desktop computer (other appliances I use at home work with it) that does not work wit

with it.


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figure 3, square-wave

This problem leads to introduction of what is called modified sine-wave as shown in f

designed to equal to the peak of sinusoidal voltage that will give the same RMS volta

you can see in figure 4, there is clearance in-between two half cycles. This is called duty cycle. Duty cy

RMS value that equal to that of sine-wave is 25% of period of a complete oscillatio

to show you how I came about this.

figure 4, modified sine-wave

figure 5, modified sine-wave showing duty cycle x, half

period (cycle) y and complete period (cycle) t, Peak voltage

and RMS voltage .


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From figure 5,

Therefore, pulse duration

I want you to follow how I will use 'square Root of Mean of the Square value'

Square value

Therefore, Mean of the Square value of a complete cycle (2 halve cycle)

square Root of Mean of the Square value

If we square both sides, the above equation becomes

By multiplying both sides by t, we are left with

Now let us divide both sides by

By collecting like terms

Therefore duty cycle


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complete cycle.

You don't have to be too worry if you don't understand that mathematical illustration

inverter as I only used it to show you how I arrived at 25% so that in future when I

significant.

Mode of operation

Figure 6, 7 and 8 bellow will be used to describe mode of operation of an inverter. W

of converting DC voltage to AC voltage. Let's start from figure 6 which is the first stage; switches SW1

SW2 and SW3 are opened. This makes current to flow in the direction shown with the ar

50Hz modified sine-wave inverter with duty cycle of 25% (discussed above) and 10ms in

shown in figure 3 above. I want you to take note of the direction through which curre

(A to B).

The second stage is only identified with modified sine-wave inverter. This is when al

when no current flows as all the switches are opened. This is called duty cycle and o

in the case of modified sine-wave inverter with duty cycle of 25% discussed earlier i

Third stage of the cycle occurs when switches SW2 and SW3 are closed while SW1 and SW

load from B to A (just opposite of what happen in the first stage) in the direction shown in figure 8 wit

for 5ms.

Last stage of the cycle is just the repetition of the second stage when all the switches are opened and

earlier, it only occurs in modified sine wave inverter. The stages are repeated continuously until the inve

The duration of each of the four stages is 5ms; this implies that a complete cycle will last for 20ms. That

Since period ; where f is the frequency of the AC voltage we want to achieve

Then . That is the frequency of the inverter.

If you are designing an inverter just like what I called my first inverter of the output wave-form as in fi

stages will not be there. However, first and third stages will have duration of 10ms

just like the modified sine-wave above.


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Figure 6, first stage of the process of converting DC voltage

to AC

Figure 7, second and last stage (in modified sine-wave) of

the process of converting DC voltage to AC

Figure 8, third stage of the process of converting DC voltage

to AC

The process discussed above is a bridge type inverter. AC voltage is achieved just li

transformer in the method depends on the battery voltage and desired AC voltage outpu

Another method which I will quickly discuss is the use of two switches and transforme

the method commonly found in inverter. Figure 9, 10 and 11 show the arrangement and t


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Figure 9, first stage of the process of converting DC

voltage to AC using center tapped primary winding

transformer

Figure 10, second and last stage (in modified sine-wave)

of the process of converting DC voltage to AC using

center tapped primary winding transformer

Figure 11, third stage of the process of converting DC

voltage to AC using center tapped primary winding

transformer

I have used switches to discuss process of converting DC voltage to AC in inverter t


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transpires. When I mentioned switches, I know many of you will probably think of thos

too wrong anyway because 'switch is switch', but different switches for different pur

that operated like a door bell were used for this purpose. Today, solid state electro

employed. The use of electronic switches eliminates the unpleasant noise generated by

days, and also makes control of switching easy.

Sizing of Various Components of Inverter

I said it earlier that when designing any electrical or electronics system, the outpu

will start my design from the outermost component.

Output socket/connector and Switch-over relay(optional)

Switch-over relay is used if you are designing your inverter to be interconnected wit

inverter output to utility, vice versa automatically as the case may be depending on

more on this in my inverter circuits.

Use the formula:

and

Where,

P is the power capacity of the inverter you are designing

V is the output voltage (the RMS voltage)

I is the output current (the RMS current)

Your output socket/connector and switch-over relay should be rated with current above

worried about RMS: this is the voltage or current your meter reads and displays when

socket or your current using clamp-on meter or ammeter. Next is the transformer.

Transformer

Primary and secondary winding current calculation

First, we assume the worst case of efficiency of 80%

Input power therefore equals

The secondary winding current,


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My preferred type of inverter is the one with center tapped primary winding transform

reasons are simple: simplicity in switches arrangement and reduced current in each ha

center tapped primary winding transformer, half cycle current will only flow in each

the winding is given by:

Primary winding for inverter with square-wave in figure 3,

Where

is the effective current flowing through the primary windings

is the total current delivered by the battery for a complete cycle.

Note: the use of lower case letter 'rms' is to differentiate primary rms values from secondary. Please le

in this article.

is the voltage of the battery for which you are designing your inverter. e.g. 12V, 24

Therefore,

For inverter with modified sine-wave in figure 4,

Therefore,


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Wire gauge selection

Wire gauge is chosen base on the chosen current density of your design. Current densi

insulated copper wire. It is chosen base on different conditions like: application (types of transformer), e

so on. For most transformer designed in conventional way, using the standard design r

efficiency and safe temperature rise, the wire is commonly run at current density in

circular-mils per ampere.

Now multiply your calculated currents (primary and secondary) above by the current de

circular-mils. Then check your wire table- published in many reference books and in m

appropriate wire AWG for your windings. One of such tables can be found at http://en.wikipedia.org

Core geometric

Figure 12, E-I type laminated iron core

Window (W) = i x j

Cross sectional area (a) = k x l

Silicon iron is the most common transformer core either as junks or new in the market

Power is related to Wa of the core by formula below.

Therefore,

in inch4; F = 1 for square waveform.


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F is the ratio of rms to average value. For modified sine-wave with duty cycle of 25%

in inch4

So while selecting your core in the market, look for one with core geometric (i.e, Wa

for your inverter.

Number of turns

Using the basic transformer design equation:

Primary turns (square-wave: )

Primary turns (modified sine-wave:

Each half of the primary windings is .

Secondary turns

Switches


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You may wonder why I keep referring those transistors as switches. Yeah they do exact

bipolar junction transistors (BJT) and thyristors (silicon control rectifier). Though thyristors can deliver

used for high power inverter, its switching circuitry is complex. MosFets and BJT are

but mostly MosFets. MosFets allow higher current than BJT. Unlike BJT which is curre

lesser power in the driver circuit.

The drain-source (MosFet) or collector-emitter (BJT) current is the effective current

with drain-source or collector-emitter current far above the effective current should

several hundreds of ampere and one cannot get a single transistor that can deliver th

type will be used. The transistors will be connected in parallel such that the current spread across them

current is 100ampere and the available transistor can deliver 30ampere, four or more

advisable to use transistor with drain-source or collector-emitter far above the effective current in applic

Oscillator/driver

I intentionally put the two together as there is no much as far as design of inverter

omitted if not needed. As I explained above driver is introduced when oscillator is n

to drive Mosfets or current that is enough to fire the BJTs to deliver required collector-emitter current. It

circuit.

Free multivibrator circuits are available online and in various electronic textbook t

multivibrators are those ICs; what I did was surfing internet for data sheets of different multivibrator IC

in the data sheet only for you to make little adjustment/modification that will make

most of the time is to calculate frequency determining components of the circuit as i

Battery

The common battery used in inverter is a lead-acid battery of the type used in automo

Automotive batteries are often used because they are relatively inexpensive. Ideally,

batteries that have thicker plates and more electrolyte reserves than automotive batt

seriously reducing the life of the battery or causing damage to it. In a well designe

ten years.

In a case where deep cycle battery is not available for use, truck batteries can be u

batteries, almost of the same thickness as deep cycle batteries. This will extend the

compared to a car battery.

Battery size calculation and specification

Batteries are rated in ampere-hour (Ah) and the sizing depends on your need: on how l

to the loads you place on it. The formula below gives you the required battery size.

Discharge capacity arise from the fact that one does not use complete battery capacit

capacity) of the battery would be used. A deep-cycle battery can be discharged up to

voltage disconnect) of its capacity.

Conclusion


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All you need to design and build your own working power inverter has been discussed i

other features that are not mentioned in this article that can be added to your inver

controller, low voltage disconnect circuitry, overload/short circuit protector, high temperature shutdow

be discuss as we come across them in my inverter circuits.

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scap_ power inverter

Documents

converting

bipolar junction

ac voltage

duty cycle

chosen base

output socketconnector

rms voltage

complete cycle