Circuit Working Of Pwm Induction Motor

Bascically we have divide our circuit in 5 parts.

1. Power Supply2. Zero Crossing3. Pic Micro4. Dimming Status Read Via Pot5. Traic Driving(look attached pdf)

Power Supply

Power supply is the main important part of any circuit . Power supply dc should be clean for free running of any circuit. If there is any noise in power supply then circuit will not work as you are expecting.

In our case we are using full wave rectifier using 4 diodes.

There is lots of other methed’s we can make our power supply.

Look Here.

The Full Wave Rectifier

In the previous Power Diodes tutorial we discussed ways of reducing the ripple or voltage variations on a direct DC voltage by connecting capacitors across the load resistance. While this method may be suitable for low power applications it is unsuitable to applications which need a "steady and smooth" DC supply voltage. One method to improve on this is to use every half-cycle of the input voltage instead of every other half-cycle. The circuit which allows us to do this is called a Full Wave Rectifier.

Like the half wave circuit, a full wave rectifier circuit produces an output voltage or current which is purely DC or has some specified DC component. Full wave rectifiers have some fundamental advantages over their half wave rectifier counterparts. The average (DC) output voltage is higher than for half wave, the output of the full wave rectifier has much less ripple than that of the half wave rectifier producing a smoother output waveform.

In a Full Wave Rectifier circuit two diodes are now used, one for each half of the cycle. A transformer is used whose secondary winding is split equally into two halves with a

common centre tapped connection, (C). This configuration results in each diode conducting in turn when its anode terminal is positive with respect to the transformer centre point C producing an output during both half-cycles, twice that for the half wave rectifier so it is 100% efficient as shown below.

Full Wave Rectifier Circuit

The full wave rectifier circuit consists of two power diodes connected to a single load resistance (RL) with each diode taking it in turn to supply current to the load. When point A of the transformer is positive with respect to point C, diode D1 conducts in the forward direction as indicated by the arrows. When point B is positive (in the negative half of the cycle) with respect to point C, diode D2 conducts in the forward direction and the current flowing through resistor R is in the same direction for both half-cycles. As the output voltage across the resistor R is the phasor sum of the two waveforms combined, this type of full wave rectifier circuit is also known as a "bi-phase" circuit.

As the spaces between each half-wave developed by each diode is now being filled in by the other diode the average DC output voltage across the load resistor is now double that of the single half-wave rectifier circuit and is about  0.637Vmax  of the peak voltage, assuming no losses.

Where: VMAX is the maximum peak value in one half of the secondary winding and VRMS is the rms value.

The peak voltage of the output waveform is the same as before for the half-wave rectifier provided each half of the transformer windings have the same rms voltage value. To obtain a different DC voltage output different transformer ratios can be used. The main disadvantage of this type of full wave rectifier circuit is that a larger transformer for a given power output is required with two separate but identical secondary windings making this type of full wave rectifying circuit costly compared to the "Full Wave Bridge Rectifier" circuit equivalent.

The Full Wave Bridge Rectifier

Another type of circuit that produces the same output waveform as the full wave rectifier circuit above, is that of the Full Wave Bridge Rectifier. This type of single phase rectifier uses four individual rectifying diodes connected in a closed loop "bridge" configuration to produce the desired output. The main advantage of this bridge circuit is that it does not require a special centre tapped transformer, thereby reducing its size and cost. The single secondary winding is connected to one side of the diode bridge network and the load to the other side as shown below.

The Diode Bridge Rectifier

The four diodes labelled D1 to D4 are arranged in "series pairs" with only two diodes conducting current during each half cycle. During the positive half cycle of the supply, diodes D1 and D2 conduct in series while diodes D3 and D4 are reverse biased and the current flows through the load as shown below.

The Positive Half-cy cle

During the negative half cycle of the supply, diodes D3 and D4 conduct in series, but diodes D1 and D2 switch "OFF" as they are now reverse biased. The current flowing through the load is the same direction as before.

The Negative Half-cycle

As the current flowing through the load is unidirectional, so the voltage developed across the load is also unidirectional the same as for the previous two diode full-wave rectifier, therefore the average DC voltage across the load is 0.637Vmax. However in reality, during each half cycle the current flows through two diodes instead of just one so the amplitude of the output voltage is two voltage drops ( 2 x 0.7 = 1.4V ) less than the input VMAX amplitude. The ripple frequency is now twice the supply frequency (e.g. 100Hz for a 50Hz supply)

Typical Bridge Rectifier

Although we can use four individual power diodes to make a full wave bridge rectifier, pre-made bridge rectifier components are available "off-the-shelf" in a range of different voltage and current sizes that can be soldered directly into a PCB circuit board or be connected by spade connectors. The image to the right shows a typical single phase bridge rectifier with one corner cut off. This cut-off corner indicates that the terminal nearest to the corner is the positive or +ve output terminal or lead with the opposite (diagonal) lead being the negative or -ve output lead. The other two connecting leads are for the input alternating voltage from a transformer secondary winding.

The Smoothing Capacitor

We saw in the previous section that the single phase half-wave rectifier produces an output wave every half cycle and that it was not practical to use this type of circuit to produce a steady DC supply. The full-wave bridge rectifier however, gives us a greater mean DC value (0.637 Vmax) with less superimposed ripple while the output waveform is twice that of the frequency of the input supply frequency. We can therefore increase its average DC output level even higher by connecting a suitable smoothing capacitor across the output of the bridge circuit as shown below.

Full-wave Rectifier with Smoothing Capacitor

The smoothing capacitor converts the full-wave rippled output of the rectifier into a smooth DC output voltage. Generally for DC power supply circuits the smoothing capacitor is an Aluminium Electrolytic type that has a capacitance value of 100uF or more with repeated DC voltage pulses from the rectifier charging up the capacitor to peak voltage. However, their are two important parameters to consider when choosing a suitable smoothing capacitor and these are its Working Voltage, which must be higher than the no-load output value of the rectifier and its Capacitance Value, which determines the amount of ripple that will appear superimposed on top of the DC voltage. Too low a value and the capacitor has little effect but if the smoothing capacitor is large enough (parallel capacitors can be used) and the load current is not too large, the output voltage will be almost as smooth as pure DC. As a general rule of thumb, we are looking to have a ripple voltage of less than 100mV peak to peak.

The maximum ripple voltage present for a Full Wave Rectifier circuit is not only determined by the value of the smoothing capacitor but by the frequency and load current, and is calculated as:

Bridge Rectifier Ripple Voltage

Where: I is the DC load current in amps, ƒ is the frequency of the ripple or twice the input frequency in Hertz, and C is the capacitance in Farads.

The main advantages of a full-wave bridge rectifier is that it has a smaller AC ripple value for a given load and a smaller reservoir or smoothing capacitor than an equivalent half-wave rectifier. Therefore, the fundamental frequency of the ripple voltage is twice that of the AC supply frequency (100Hz) where for the half-wave rectifier it is exactly equal to the supply frequency (50Hz).

The amount of ripple voltage that is superimposed on top of the DC supply voltage by the diodes can be virtually eliminated by adding a much improved π-filter (pi-filter) to the output terminals of the bridge rectifier. This type of low-pass filter consists of two smoothing capacitors, usually of the same value and a choke or inductance across them to introduce a high impedance path to the alternating ripple component. Another more practical and cheaper alternative is to use a 3-terminal voltage regulator IC, such as a LM78xx for a positive output voltage or the LM79xx for a negative output voltage which can reduce the ripple by more than 70dB (Datasheet) while delivering a constant output current of over 1 amp.

In the next tutorial about diodes, we will look at the Zener Diode which takes advantage of its reverse breakdown voltage characteristic to produce a constant and fixed output voltage across itself.

Zero Crossing Detectors and Comparators(The Unsung Heroes of Modern Electronics Design)

Rod Elliott (ESP)

Introduction

Zero crossing detectors as a group are not a well-understood application, although they are essential elements in a wide range of products. It has probably escaped the notice of readers who have looked at the lighting controller and the Linkwitz Cosine Burst Generator, but both of these rely on a zero crossing detector for their operation.

A zero crossing detector literally detects the transition of a signal waveform from positive and negative, ideally providing a narrow pulse that coincides exactly with the zero

voltage condition. At first glance, this would appear to be an easy enough task, but in fact it is quite complex, especially where high frequencies are involved. In this instance, even 1kHz starts to present a real challenge if extreme accuracy is needed.

The not so humble comparator plays a vital role - without it, most precision zero crossing detectors would not work, and we'd be without digital audio, PWM and a multitude of other applications taken for granted.

Basic Low Frequency Circuit

Figure 1 shows the zero crossing detector as used for the dimmer ramp generator in Project 62. This circuit has been around (almost) forever, and it does work reasonably well. Although it has almost zero phase inaccuracy, that is largely because the pulse is so broad that any inaccuracy is completely swamped. The comparator function is handled by transistor Q1 - very basic, but adequate for the job.

The circuit is also sensitive to level, and for acceptable performance the AC waveform needs to be of reasonably high amplitude. 12-15V AC is typical. If the voltage is too low, the pulse width will increase. The arrangement shown actually gives better performance than the version shown in Project 62 and elsewhere on the Net. In case you were wondering, R1 is there to ensure that the voltage falls to zero - stray capacitance is sufficient to stop the circuit from working without it.

Figure 1 - Basic 50/60Hz Zero Crossing Detector

The pulse width of this circuit (at 50Hz) is typically around 600us (0.6ms) which sounds fast enough. The problem is that at 50Hz each half cycle takes only 10ms (8.33ms at 60Hz), so the pulse width is over 5% of the total period. This is why most dimmers can only claim a range of 10%-90% - the zero crossing pulse lasts too long to allow more range.

While this is not a problem with the average dimmer, it is not acceptable for precision applications. For a tone burst generator (either the cosine burst or a 'conventional' tone burst generator), any inaccuracy will cause the switched waveform to contain glitches. The seriousness of this depends on the application.

Precision zero crossing detectors come in a fairly wide range of topologies, some interesting, others not. One of the most common is shown in Project 58, and is commonly used for this application. The exclusive OR (or XOR) gate makes an excellent edge detector, as shown in Figure 2.

Figure 2 - Exclusive OR Gate Edge Detector

There is no doubt that the circuit shown above is more than capable of excellent results up to quite respectable frequencies. The upper frequency is limited only by the speed of the device used, and with a 74HC86 it has a propagation delay of only 11ns [1], so operation at 100kHz or above is achievable.

The XOR gate is a special case in logic. It will output a 1 only when the inputs are different (i.e. one input must be at logic high (1) and the other at logic low (0v). The resistor and cap form a delay so that when an edge is presented (either rising or falling), the delayed input holds its previous value for a short time. In the example shown, the pulse width is 50ns. The signal is delayed by the propagation time of the device itself (around 11ns), so a small phase error has been introduced. The rise and fall time of the squarewave signal applied was 50ns, and this adds some more phase shift.

There is a pattern emerging in this article - the biggest limitation is speed, even for relatively slow signals. While digital logic can operate at very high speeds, we have well reached the point where the signals can no longer be referred to as '1' and '0' - digital signals are back into the analogue domain, specifically RF technology.

The next challenge we face is converting the input waveform (we will assume a sinewave) into sharply defined edges so the XOR can work its magic. Another terribly under-rated building block is the comparator. While opamps can be used for low speed

operation (and depending on the application), extreme speed is needed for accurate digitisation of an analogue signal. It may not appear so at first glance, but a zero crossing detector is a special purpose analogue to digital converter (ADC).

Comparators

The comparator used for a high speed zero crossing detector, a PWM converter or conventional ADC is critical. Low propagation delay and extremely fast operation are not only desirable, they are essential.

Comparators may be the most underrated and under utilised monolithic linear component. This is unfortunate because comparators are one of the most flexible and universally applicable components available. In large measure the lack of recognition is due to the IC opamp, whose versatility allows it to dominate the analog design world. Comparators are frequently perceived as devices that crudely express analog signals in digital form - a 1-bit A/D converter. Strictly speaking, this viewpoint is correct. It is also wastefully constrictive in its outlook. Comparators don't "just compare" in the same way that opamps don't "just amplify". [2]

The above quote was so perfect that I just had to include it. Comparators are indeed underrated as a building block, and they have two chief requirements ... low input offset and speed. For the application at hand (a zero crossing detector), both of these factors will determine the final accuracy of the circuit. The XOR has been demonstrated to give a precise and repeatable pulse, but its accuracy depends upon the exact time it 'sees' the transition of the AC waveform across zero. This task belongs to the comparator.

Figure 3 - Comparator Zero Crossing Detector

In Figure 3 we see a typical comparator used for this application. The output is a square wave, which is then sent to a circuit such as that in Figure 2. This will create a single pulse for each squarewave transition, and this equates to the zero crossings of the input

signal. It is assumed for this application that the input waveform is referenced to zero volts, so swings equally above and below zero.

Figure 4 - Comparator Timing Error

Figure 4 shows how the comparator can mess with our signal, causing the transition to be displaced in time, thereby causing an error. The significance of the error depends entirely on our expectations - there is no point trying to get an error of less than 10ns for a dimmer, for example.

The LM339 comparator that was used for the simulation is a very basic type indeed, and with a quoted response time of 300ns it is much too slow to be usable in this application. This is made a great deal worse by the propagation delay, which (as simulated) is 1.5us. In general, the lower the power dissipation of a comparator, the slower it will be, although modern IC techniques have overcome this to some extent.

You can see that the zero crossing of the sinewave (shown in green) occurs well before the output (red) transition - the cursor positions are set for the exact zero crossing of each signal. The output transition starts as the input passes through zero, but because of device delays, the output transition is almost 5us later. Most of this delay is caused by the rather leisurely pace at which the output changes - in this case, about 5us for the total 7V peak to peak swing. That gives us a slew rate of 1.4V/us which is useless for anything above 100Hz or so.

One of the critical factors with the comparator is its supply voltage. Ideally, this should be as low as possible, typically with no more than ±5V. The higher the supply voltage, the further the output voltage has to swing to get from maximum negative to maximum positive and vice versa. While a slew rate of 100V/us may seem high, that is much too slow for an accurate ADC, pulse width modulator or zero crossing detector.

At 100V/us and a total supply voltage of 10V (±5V), it will take 0.1us (100ns) for the output to swing from one extreme to the other. To get that into the realm of what we need, the slew rate would need to be 1kV/us, giving a 10ns transition time. Working from

Figure 3, you can see that even then there is an additional timing error of 5ns - not large, and in reality probably as good as we can expect.

The problem is that the output doesn't even start to change until the input voltage passes through the reference point (usually ground). If there is any delay caused by slew rate limiting, by the time the output voltage passes through zero volts, it is already many nanoseconds late. Extremely high slew rates are possible, and Reference 2 has details of a comparator that is faster than a TTL inverter! Very careful board layout and attention to bypassing is essential at such speeds, or the performance will be worse than woeful.

Using A Differential Line Receiver

This version is contributed by John Rowland [3] and is a very clever use of an existing IC for a completely new purpose. The DS3486 is a quad RS-422/ RS-423 differential line receiver. Although it only operates from a single 5V supply, the IC can accept an input signal of up to ±25V without damage. It is also fairly fast, with a typical quoted propagation time of 19ns and internal hysteresis of 140mV.

Figure 5 - Basic Zero Crossing Detector Using DS3486

The general scheme is shown in Figure 5. Two of the comparators in the IC are used - one detects when the input voltage is positive and the other detects negative (with respect to earth/ ground). The NOR gate can only produce an output during the brief period when both comparator outputs are low (i.e. close to earth potential). However, tests show that the two differential receiver channels do not switch at exactly 0.00V. With a typical DS3486 device, the positive detector switches at about 0.015V and the negative detector switches at approximately -0.010V. This results in an asymmetrical dead band of 25mV around 0V. Adding resistors as shown in Figure 6 allows the dead band to be made smaller, and (perhaps more importantly for some applications), it can be made to be symmetrical.

Figure 6 - Modified Zero Crossing Detector To Obtain True 0V Detection

Although fixed resistors are shown, it will generally be necessary to use pots. This allows for the variations between individual comparators - even within the same package. This is necessary because the DS3486 is only specified to switch with voltages no greater than ±200mV. The typical voltage is specified to be 70mV (exactly half the hysteresis voltage), but this is not a guaranteed parameter.

Indeed, John Rowland (the original designer of the circuit) told me that only the National Semiconductor devices actually worked in the circuit - supposedly identical ICs from other manufacturers refused to function. I quote ...

We did some testing with "equivalent" parts made by other manufacturers, and found very different behavior in the near-zero region. Some parts have lots of hysteresis, some have none, detection thresholds vary from device to device, and in fact even in a quad part like the DS3486 they are different from channel to channel within the same package. Eventually we settled on the National DS3486 with some added resistors on its input pins as shown in Figure 6. The most recent version of the circuit uses trimpots, 100 ohm on the positive detector and 200 ohm on the negative detector. These values allow us to trim almost every DS3486 to balance the noise threshold in the +/-5mV to +/-15mV range. Occasionally we do get a DS3486 which will not detect in this range. Sometimes, we find that both the positive and negative detectors are tripping on the same side (polarity) of zero, if so we pull that chip and replace it.

The additional resistors allow the detection thresholds to be adjusted to balance the detection region around 0V. The resistor from pin 1 to earth makes the positive detector threshold more positive. The resistor from the input to pin 7 forces the negative detector threshold to become more negative. Typical values are shown for ±25mV detection using National's DS3486 parts. In reality, trimpots are essential to provide in-circuit adjustment.

References

1 - Quad 2-input EXCLUSIVE-OR gate 74HC/HCT86, Philips Semiconductors Data Sheet2 - A Seven-Nanosecond Comparator for Single Supply Operation, Linear Technology, Application Note 72, May 983 - Differential Line Receivers Function As Analog Zero-Crossing Detectors, John Rowland

Pic Micro

This the heart of the circuit. Basically on every zero crossing it turn off output which is driving to triac. In this position lamp or motor goes off.

St this position pic micro read pot value & store it in one variable. We are using TMR0 for timer purpose. Timer0 is 8 bit wide, so it’s value can be 0 to 255 according to count.

We set prescale value for this 64 step. On every 64 clock count program increment tmr0 by 1. & for 10 ms its value can not be greater then 156 count.

Our microcontroller run on 4 mhz internal oscillator & it divide it by 4, so our mcu clock running frequency is 1 mhz.

How does a potentiometer work?

Potentiometers Explored - Construction & Working PrinciplesWritten by:  Dr. Crystal Cooper • Edited by: KennethSleight Updated Sep 6, 2009 • Related Guides: Electronics

A potentiometer is an electrical component with a variable resistance. This article discusses how they work, and how to use a multimeter to read one.

Introduction

In electrical engineering parlance, the term "potentiometer" is used in either one of two ways. It may refer to an instrument that measures an unknown emf or voltage by comparing it to a standard emf. In this capacity, it is functioning as a null instrument; it permits precision measurement by adjusting a value of a circuit element until a meter reads zero. Alternatively, "potentiometer" may refer to an electronic component that is

used to vary resistance in a circuit. In this article, we discuss the construction and working principle of the resistive component.

A potentiometer is also referred to as a variable resistor or pot. They have three terminals, where the one in the middle is known as the wiper, and the other two are known as ends. The wiper is a movable contact where resistance is measured with respect to it and either one of the end terminals.

They are useful for circuits where the resistance needs to be dynamically changed to control the current. They are also popular as voltage dividers,

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The Basics

Potentiometers come in different types.

Rotary and trimpot potentiometers are wire wound, and are single or multi-turn, depending on the number of times the knob may be turned. The resistance is lowered or increased by turning the knob clock or counterclockwise, where the knob direction is dependent on the end that is used with the wiper.

Faders or sliders as they are also known, have a rectangular shape, and the wiper moves back and forth in a linear fashion.

How does a potentiometer work?

Potentiometers work by having a resistive element inside. Both end terminals are attached to it, and do not move. The wiper travels along the strip when the knob is turned. The closer the wiper is to the end terminal it is wired in conjunction with, the less the resistance, because the path of the current will be shorter. The further away it moves from the terminal, the greater the resistance will be.

The symbol for a potentiometer is the same one as a resistor, save for an arrow in the middle. In a circuit where they are used strictly as variable resistors or rheostats, only two

terminals are wired to the other components. All three terminals are wired separately when they function as voltage dividers. Light dimmers in houses and volume controls on electronics are two common applications. Others include switches and position sensors.

How to Read One

To read a potentiometer, you need a multimeter. Put in on the proper ohmmeter setting, which should be higher than that of the resistance rating of the potentiometer as noted on the package or the device. If you don't know the resistance rating, use the lowest setting of the multimeter and increase it until you get a reading.

When reading the potentiometer, one multimeter lead must always be on the wiper or middle part, and the other one can be on either of the remaining terminals. For example, place one multimeter lead on the wiper, and the other on the left terminal of the potentiometer. If you place one lead on the left terminal and the other on the right, you will read the entire value of the pot, and the resistance will not vary when you turn the knob.

Note that Inexpensive ones may approximate only fifty percent of their rated value when they are measured.