Design And Construction Of An Electric Popcorn Frying Machine

Inverter Construction and design with AVR switching mode

 Design and Construction of Inverter with Automatic Voltage Regulator Using Switch Mode Square Wave Switching Scheme

Abstract

This write up investigates the use of switch mode square wave (SMSW) switching scheme to design an inverter system with automatic voltage regulator. The inverter is set to cut-off supply to the load below a threshold voltage of 9V and above a threshold of 13V during charging. The range of regulation of the AVR is between 170-250V. SMSW switching scheme uses an astable multivibrator to drive the semiconductor switches (MOSFET). For the automatic voltage regulator (AVR), an auto-transformer with taps at different voltage levels is used. These taps are selected by the comparison of the voltage from the AC mains to a reference voltage using an operational amplifier (LM324) set to differential mode. The inverter and AVR are connected by an AC relay, and this combination was tested for a load of 400W for which it worked accordingly.

Keywords: Inverter, switching, regulator, multivibrator, automatic and voltage.

Introduction

The availability and stability of electrical energy is necessary for domestic and industrial use. However, due to its insufficiency, hence, the need for alternative sources of electricity supply. In the event of power failure from public utility, motor generating sets and inverters could be used to power appliances and machines in homes, offices, and industries. Inverters are preferred to motor generating sets because it is noiseless, relatively small size, and pollution-free operating mode.

An inverter is an electrical device that converts direct current (DC) to alternating current (AC). The resulting AC can be at any required voltage and frequency with the use of appropriate transformers, switching, and control units (Mohan et al. 1989; Theraja and Theraja 2007). Stability of power systems is also important as it has an adverse effect on both domestic user and industrial production. Instability in power systems comes in form of either voltage surge caused by switching, power frequency and lightning or voltage dips caused by overloading. One solution to this is an Automatic Voltage Regulator (AVR), which monitors the input voltage constantly to deal with both surges and dips, unlike some surge protectors that only ground excess voltages.

The AVR uses an auto-transformer to maintain an AC output voltage that is as close to the standard mains supply (220V) as possible. This is achieved by using a series of comparators to monitor the voltage from the mains and then select the appropriate tap on the auto-transformer through a relay; the AVR is also set to automatically cut-off supply to the load if the voltage from the public utility is higher than 250V so as not to cause damage to the connected load.

Evolution of Inverter Systems

The earliest inverter was the motor-generator, which was developed to serve the needs of World War II. The motor generator was reliable and, at the time, was the only way to convert DC power to AC (Theraja and Theraja 2007). The output waveform fit a wide variety of applications but it was inefficient. In the early 1960s, solid state transistors replaced the mechanical vibrators. The first advantage of this type of inverter was that it was not a motor

generator. The unregulated square wave design could operate resistive loads. It was unable to operate reactive loads like compressors, icemakers, or microwave ovens. There were many compatibility problems including no frequency control, which was added later (Gurdjian and Maxwell 2000; Owen 1996; PowerStream 2006).

In the early twentieth century, vacuum tubes and gas filled tubes began to be used as switches in inverter circuits. The most widely used type of tube was the thyristor. Since early transistors were not available with sufficient voltage and current ratings for most inverter applications, it was the 1957 introduction of the Thyristor or Silicon-Controlled Rectifier (SCR) that initiated the transition to solid state inverter circuits.

Switch Mode Square Wave Inverter

Figure 1 shows a simple switch-mode inverter in an AC motor. In the square wave switching scheme, each switch of the inverter leg of Fig. 1 is ON for one- half cycle (180°) of the desired output frequency. A circuit representation of this is shown in Fig. 2. The resulting square wave is shown in Fig. 3.

Fig. 1. Switch-mode inverter in AC-motor drive.

Fig. 2. Schematic circuitry showing One-leg switch mode inverter.

Fig. 3. Square-wave switching.

The peak values of the fundamental frequency and the harmonic components in the inverter output waveform can be obtained from Fourier analysis. It should be noted that the square-wave switching is also a special case of Pulse Width Modulation (PWM) switching, where the output voltage is independent of the amplitude modulation ratio in the square-wave region.

Inverter Design theories and calculations

The SMSW design scheme is divided into two sections- inverter and AVR sections; all of which are made up of eight different modules as shown in Fig. 4 below. Interconnections between the two sections and various modules are also depicted in the block diagram.

Fig. 4. Block diagram of the inverter and automatic voltage regulator using SMSW scheme.

DC Power Supply

There are many different types of battery systems. The conventional lead-acid batteries are commonly used for the inverter applications. In the normal mode, when the line voltage is present, the battery is trickle charged to offset the slight self-discharge by the battery. This requires that a constant trickle charge voltage be applied across the battery and it continuously draws a small amount of current, thus maintaining itself in a fully charged state.

In the event of a power outage, the battery supplies the load. The battery voltage should not be allowed to fall below the final discharge level; otherwise the battery life is shortened. Discharge currents in the excess of 10-hour current cause the final discharge voltage to be reached sooner than their magnitude would suggest. Therefore, the higher discharge currents reduce the effective battery capacity.

It is possible to program the battery charging characteristic to bring it to a full charge state more quickly.

Frequency Generation

To generate the required frequency- 50Hz-60Hz, an oscillator is used. For the purpose of this design, astable multivibrator is used. It is also known as “free-running relaxation oscillator” and it has no stable state but two half-stable states between which it keeps oscillating continuously on its own without any external excitation. Figure 5 shows a symmetrically collector-coupled astable multivibrator.

In this circuit neither of the two transistors reaches a stable state. When one is ON, the other is OFF. They continuously switch back and forth depending on the RC time constant of the circuit.

The OFF time for Q1 is:

T1 = 0.69 R1C1. (1)

Also, the OFF time for Q2 is:

T2 = 0.69 R2C2. (2)

Therefore, the total period of the wave is:

T = T1 + T2 = 0.69 (R1C1 + R2C2). (3)

Fig. 5. Astable multivibrator.

Since the two stages are symmetrical (i.e., R1 = R2 = R and C1 = C2 = C), then:

T = 1.38 RC. (4)

The frequency of oscillation is given by:

f = 1 / T = 1 / (1.38 RC). (5)

The value of the common emitter DC forward transfer ratio (β) for which the transistor must saturate is given by:

β = R / RL. (6)

Therefore, the minimum value of β choosing RL = 1KΩ is:

β = 12.2 / 1 = 12.2.

The waveform generated is a square wave. Sine-wave voltage waveforms are not easy to generate, but has the advantage of a soft temporal rise of voltage and the absence of harmonic oscillations, which causes unwanted counter forces in engines, interferences on radio equipment and surge currents on condensers. On the other hand, square wave voltage waveforms can be generated very simply by switches. The efficiency of a square wave inverter is higher than the appropriate sine wave inverter, due to its simplicity.

Fig. 6 shows a sinusoidal voltage waveform superimposed over square voltage waveform. In each case, an r.m.s. of 230V. Also, since conduction only occurs at the peak, the ON and OFF times of the sine wave is shorter than that of the square wave.

Fig. 6. Sine wave voltage and conventional square voltage with both 230V r.m.s.

Switching and Rectification

Semi-conductor devices are preferred as switching components in inverter circuits as they can be turned ON or OFF by the use of control signals. They are of different types (Thorborg 1988).

For this work the semiconductor device used is the MOSFET. It is a voltage-controlled device, which is fully ON and approximates a closed switch when the gate-source voltage is sufficiently large. The MOSFET is OFF when the gate-source is below the threshold value, VGS(th). MOSFETs require the continuous application of a gate-source voltage of appropriate magnitude in order to be in the ON state. No gate current flows except during the transitions from ON to OFF or vice versa when the gate capacitance is being charged or discharged. The switching times are very short, being in the range of a few tens of nanoseconds to a few hundred nanoseconds depending on the device type.

MOSFETs are available in voltage ratings in excess of 1,000V but with small current ratings and with up to 100A at small voltage ratings. The maximum gate-source voltage is ±20V, although MOSFETs that can be controlled by 5V signal are available. MOSFETs can be paralleled easily as are the two shown in Fig. 7 because of the positive temperature coefficient of their on-state resistance. However, the gates of the MOSFETs cannot be directly connected together, but rather small resistance must be used in series with the individual gate connections as shown in Fig. 7.

For a 1KVA load the current drawn from the battery is:

P = I × V, and (7)

I = 1,000 / 12.5 = 80A.

A MOSFET of 110A maximum current rating was chosen so as to conveniently handle the current drawn. However, there is a heat build up on the MOSFET and transient currents during switching. Another MOSFET is connected in parallel to share the temperature coefficient and the increased current during switching. MOSFETs are bidirectional devices and can also be used for rectification when connected as shown in Fig. 8. The 12V battery is charged from AC mains through the arrangement of MOSFETs coupled to the transformer.

Whenever there is AC mains supply, the secondary coil of transformer turns to be the primary by stepping down the 220V to 12.5V. The 12.5V AC is fed into the MOSFETs through the arrangement as shown in Fig. 8, and the voltage is rectified to DC. The rectified DC voltage is then used to charge the 12V battery (Sittig and Roggwiller 1982).

Fig. 7. Parallel connection of MOSFETs.

Fig. 8. Rectification circuit.

Voltage Transformation

Voltage transformation can be achieved by the use of a transformer. A transformer is a machine that converts energy from one form to another at the same frequency. It operates on Faraday’s laws of electromagnetic induction. Mathematically, we have:

tNe12φφ−=tN12φφ−−=, (8)

tNe∂∂=φ. (9)

If φ is sinusoidal, then ftmπφφ2sin=:

tftNem∂∂=)2sin(πφ, (10)

ffNemπφπ2cos2=. (11)

Since the r.m.s. value of cosine = 1/20.5:

212mfNeφπ=, (12)

mfNeφ44.4=. (13)

Generally,

fNABE44.4=, (14)

which is known as the universal e.m.f. equation, where:

ABm=φ;

=mφmaximum flux;

A = cross-sectional area;

B = flux density;

f = frequency;

N = number of turns.

The voltage per turn in a transformer is given by:

QkEt=, (15)

where k = constant, and Q = rated output of transformer in KVA.

The number of turns can also be expressed as a function of the induced voltage and voltage per turn as:

N = E / Et. (16)

From Fig. 9, we have:

tNEpp∂∂=φ1, (17)

tNEss∂∂=φ2, (18)

ppssNttNEEφφ∂∂×∂∂=12, (19)

Fig. 9. A typical transformer.

E2 / E1 = Ns / Np = K, (20)

where K = transformation ratio.

Suppose a load is connected to the transformer, the primary current produces a positive magneto-motive force (magneto-motive force (m.m.f.) = NI = φR.):

Fp = Np I1, (21)

and the secondary current produces a negative magneto-motive force,

Fs = −Ns I2. (22)

The reluctance of well designed transformer is approximately zero (0), therefore:

0=+=spnetFFF, (23)

021=−ININsp, (24)

021=−ININsp, (25)

Ns / Np = I1 / I2. (26)

Comparing Eqs. 20 and 28, we have:

E2 / E1 = I1 / I2, (27)

so that, 2211IEIE=. (28)

This means that input power equals the output power for an ideal transformer.

Transformers can be classified by construction; shell type and core type, and further by mode of operation; step-up transformer and step down transformer. For a step-up transformer the secondary voltage is higher than the primary, while for a step-down transformer the secondary voltage is lower than the primary (Theraja and Theraja 2007).

Since a 12.5V DC battery is being used, the transformer will be a 12.5V-0V-12.5V to 220V center-tapped transformer.

For a core transformer choosing the value of k as 0.48 and output power of 1KVA, the volt per turn is:

Et = 0.48 × 10.5 = 0.48.

From Eq. 16, the number of turns at the primary conductor is:

Np = 12.5 / 0.48 = 26.04 ≈ 26.

From Eq. 7, the current flowing through the primary conductor is:

I1 = 1,000 / 12.5 = 80A.

Therefore the primary magneto-motive force from Eq. 21 is:

Fp = 26 × 80 = 2,080N.

Selecting a conductor of diameter d = 1.22mm, and current carrying capacity of 4.68A, the current density J is given as:

J = I / A, or (29)

J = (4I) / (πd2), so that (30)

J = [(4 × 4.68) / (3.142 × (1.22)2)] = 4A/mm2.

The number of turns at the secondary conductor is obtained from Eq. 20:

Ns = (26 × 220) / 12.5 = 457.6 ≈ 458.

Control Unit

Voltage cut-off can be achieved using a variable resistor, zener diode, transistor, capacitor and a relay as connected as shown in Fig. 10.

When a voltage (Vcc) is applied, the variable resistor is set so that until the voltage exceeds the value of the zener diode the relay would not be energized. The capacitor at the coil of the relay keeps it from flickering; this means the relay always gets the value of Vcc before it is triggered. With this circuit the battery of the inverter can be made to stop discharging at a set voltage and to stop charging at another.

A 6V zener diode was chosen for the cut-off since it is considerably smaller than Vcc (either 9V or 13V). The voltage cut-off variable resistor was set using the voltage divider rule:

V1 = (V × R1) / (R1 + R2). (31)

For the discharging cut-off, V = 9 Volts, V1 = 6 Volts and R1 + R2 = 100KΩ. R1 was then calculated:

R1 = (6 × 100) / 9 = 66.67KΩ.

For the charging cut-off, V = 13 Volts, V1 = 6 Volts and R1 + R2 = 100KΩ. R1 was then also calculated:

R1 = (6 × 100) / 13 = 46.15KΩ.

Fig. 10. Voltage cut-off circuit.

Control and Switching

An operational amplifier, usually referred to as an op-amp, is a DC coupled high-gain electronic voltage amplifier with differential inputs and, usually, a single output. In its typical usage, the output of the op-amp is controlled by a negative feedback which largely determines the magnitude of its output voltage gain, input impedance at one of its input terminals and output impedance.

The amplifier's differential inputs consist of an inverting input and a non-inverting input and ideally the op-amp amplifies only the difference in voltage between the two. This is called the "differential input voltage." In its most common use, the op-amp's output voltage is controlled by feeding a fraction of the output signal back to the inverting input. This is known as negative feedback. If that fraction is zero, i.e., there is no negative feedback, the amplifier is said to be running "open loop" and its output is the differential input voltage multiplied by the total gain of the amplifier, as shown by the following equation:

Vout = (V+ − V-) Gopen loop, (32)

where V+ is the voltage at the non-inverting terminal, V- is the voltage at the inverting terminal and G is the total open-loop gain of the amplifier. Because the magnitude of the open-loop gain is typically very large and not well controlled by the manufacturing process, op-amps are not usually used without negative

feedback. Unless the differential input voltage is extremely small, open-loop operation results in op-amp saturation.

The operational amplifier can be used as a differential amplifier as shown in Fig. 11. The circuit shown in Fig. 11 is used for finding the difference of two voltages each multiplied by some constant (determined by the resistors).

Whenever R1 = R2 and Rf = Rg:

(121VVRRVfout−=. (33)

When R1 = Rf = R2 = Rg:

12VVVout−=. (34)

For this design, a single IC chip (LM324) was used, as it has four operational amplifiers in it. These amplifiers have a gain of unity and follow Eq. 34 above, however, the output will only go high if Vout ≥ 0.2V and the voltages for comparison are less than the voltage used to power it.

The unregulated output from the 14V tap of the auto-transformer is fed to the non-inverting terminals of the IC through a 100KΩ variable resistor, and the voltage set to 8V using this variable resistor by the voltage divider rule. Knowing the value of the voltage, the resistance was then calculated: V = 14 Volts, V1 = 8 Volts, R1 + R2 = 100KΩ, and

R1 = (8 × 100) / 14 = 57.14KΩ.

The regulated 12V is fed to the inverting terminals using a variable resistor to set the voltages, the outputs from the LM324 are used to trigger four (4) relays on which the four (4) taps from the auto-transformer are connected and the fourth relay is used as the over-voltage protector. The over-voltage protector is set so that the relay will create an open circuit from 250V and above, at 220V the voltage at the non-inverting terminal is 8V so that at 250V the voltage there will be:

V = (250 × 8) / 220 = 9.09V.

Using the voltage divider rule the resistance of the variable resistor at the inverting terminal is then calculated: V = 12 Volts, V1 = 9.09 Volts, R1 + R2 = 100KΩ, and

R1 = (9.09 × 100) / 12 = 75.75KΩ.

Below 250V, the relay selects the 190V tap until the voltage gets to 220V for which it then selects the 220V tap, the voltage at the non-inverting terminal is 8V. Using the voltage divider rule the resistance of the variable resistor at the inverting terminal is then calculated: V = 12 Volts, V1 = 8 Volts, R1 + R2 = 100KΩ, and

R1 = (8 × 100) / 12 = 66.67KΩ.

At 200 V, the 250V tap is selected, and the voltage at the non-inverting terminal is calculated as:

V = (200 × 8) / 220 = 7.27V.

Using the voltage divider rule the resistance of the variable resistor at the inverting terminal is then calculated: V = 12 Volts, V1 = 7.27 Volts, R1 + R2 = 100KΩ, and

R1 = (7.27 × 100) / 12 = 60.58KΩ.

At 180V, the 265V tap is selected, and the voltage at the non-inverting terminal is calculated as:

V = (180 × 8) / 220 = 6.55V.

Using the voltage divider rule the resistance of the variable resistor at the inverting terminal is then calculated: V = 12 Volts, V1 = 6.55 Volts, R1 + R2 = 100KΩ, and

R1 = (6.55 × 100) / 12 = 54.58KΩ.

For voltages below 180V the 265V tap is selected until the power supply unit’s voltage cannot power the 12V voltage regulator.

The outputs of the IC are connected through 100KΩ resistors to trigger relays. The circuit diagram of the AVR is shown in Fig. 12. The complete circuit diagram of the inverter is shown in Fig. 13.

Fig. 11. Differential amplifier.

Fig.12. Circuit diagram of an AVR using LM324.

Fig.13. Complete circuit diagram of the inverter.

Performance Evaluation

Table 1 below shows various tests carried out and the results obtained. The voltage cut-off of the inverter protects it from damage to itself. As the voltage drops from the battery, collector to ground voltage used to drive the MOSFETs also drops and when this gets below 3V, it may overdrive a bank of the MOSFETs causing damage to it. If the inverter were a

take a longer period to charge when supply from the mains is below 220V.

T

obtained. Te was	V was measu
across the secondary of the transformer of the inverter. 2. 220V wa	across the primary.
across the neutral and the 220V tap of the auto-transformer.	was measured. Across the 190V tap, 197V was measured. Across the 250V tap, 239.5V was measured. Across the 265V tap, 258.7V was measured. The voltage
used to power the oscillator. 4. A fully	collector and ground was measured to be 3.7V. 245V was measu
12.5V battery was connected to the inverter. 5. The	across the terminals of the secondary of the transformer. The output voltage
continually loaded to 400W.	continually dropped as it was loaded to 180V, the laminations of the transformer and the MOSFET began heating gradually. The outp
on for a period of time without loading it. 7. The secondary of	gradually dropped.
the transformer of the inverter was connected to the AC mains. 8. The in	measured across the terminals of the battery was dependent on the supply voltage. At 9V the
on so the battery could completely discharge. 9. The automatic	automatically switched off. The
voltage regulator was tested for regulation. 10. The inverter and	constant 220V.
AVR combination was operated and loaded on the inverter mode. 11. While the inverter	constant 220V.
and AVR combination was on the inverter mode, the mains switch was turned on.	immediately cut-off and the battery began charging without interruption of supply to the load.