SEMINAR PRESENTATION ON ANALYSIS OF INVERTER FOR AN INDUCTION MOTOR TRAINER

CHAPTER ONE

INTRODUCTION

Direct current is always higher than Alternating current and most of our home appliances make use of alternating current, therefore an Inverter is an electronic device that is used to change or convert direct current (DC) to Alternating Current (AC).

The inverter does not produce any power on its own, the power is usually provided by the DC source. A power inverter can be entirely electronics or maybe a combination of mechanical effects (such as a rotary apparatus) and electronic circuitry.

An inverter Drive (VFD) works by taking AC mains (single or three phases) and first rectifying them into DC, The DC is usually smoothed with capacitors and often a DC choke before it is connected to a network of power Transistors to turn it into three phases for the motor.

The network of power transistors of a small inverter drive is actually one “Intelligent Power Module” (known as an IPM) and includes its own protection and basic control circuits. The IPM inverts the DC into AC. hence the term “inverter”. The control method is known as “PWM” for “pulse width modulation”. This means the DC is switched on and off very quickly (chopped) by the transistor switches.

A sine wave of motor current is made by a series of DC pulses where the first has a very short “on” period, followed by a longer on period, then longer until the widest pulse appears in the center of the positive sine wave, then smaller until the DC is inverted and the same pattern of pulses generate the negative part of the sine wave since the transistor can be controlled to any time base the other phases are controlled by more transistors, displaced by the time necessary to equally space the phases at 120 degrees. The frequency of the pulses being turned on is known as the “switching frequency”.

The switching frequency is usually around 3kHz to 4kHz, so the pulses it makes for 50Hz will be 3000/50 or 60 pulses per full sine wave or each phase. When the fixed voltage pulses are presented to the inductance of the motor, the result is control of both voltage (by the width of the fixed voltage pulses) and frequency (by spreading the progression and regression of the pulse widths over more of the switching frequency base pulses).

From the above you can see the IPM in the inverter derive will control voltage and frequency over virtually any range the parameter settings in the VFD tell it to.

This means when setting up an inverter derive we can choose to run a small “delta connection 230v motor from a 230v single phase supply with a base frequency set at 50Hz, a 400v star connected small motor from a 400v three phase supply or any other arrangement of voltage and frequency we choose that will correctly flux the motor.

The motor will be correctly fluxed when its voltage curve rises from zero x 0Hz to its base frequency x normal voltage. Base frequency and voltage are what are shown on the nameplate. This also means we can correctly flux other motors such as a 400v x 50Hz motor from 230V supply at 230V three phases by setting the base frequency to 29Hz (at reduced speed) or run a 230V connected motor from 400V by setting the base frequency to 87Hz (at increased speed and power). Electrical Braking is applied to the motor shaft, via a resistor (DBR) is present.

The input stage of the inverter drive is a one-way power device, while the output stage allows power to flow in both directions. It follows that the inertia of a load will return its stored energy to the Inverter drive when an attempt is made to slow its speed at a greater rate than it would achieve for natural deceleration or coast down.

In this case, the Bus Voltage will rise unless there is provision to hold it down. The smoothing capacitors will be charged during an increase in Bus Voltage and this will apply a small amount of braking to the motor shaft. Usually, this is around 10% but is dependent on the smoothing capacitor size.

A Brake switch or “Chopper” needs to be present to divert the braking energy into a braking resistor. The resistor is usually external and is sized to pass enough current to match the current rating of the brake switch, not so high as to be ineffective and of physical size (watts) such that it is not overheated.

A motor trainer is a system that is intended for training students in the basic principles of control of the electric motor. It can also be said that it is an automatic system that is used for learning the control principles of electric motors.

1.2. PROBLEM STATEMENT

Students lack the ability to relate the theoretical knowledge obtain in the DC-AC converter to practical know-how and also the inability of the University to provide such a machine trainer in the workshop for the student to use.

1.3. AIM OF THE WORK

The aim of this work is to analyze DC-AC (INVERTER) for motor trainer

1.4. OBJECTIVES OF THE WORK ARE:

1.5. THE MOTIVATION FOR THE WORK

I was motivated to carry out research and construction on INVERTER because when I visited some industries in part of the eastern part of Nigeria, I found out most of these industries lack a steady power supply, and due to that most of their products spoil. Most of the time I was with them, they pay much on the energy being used even when the work done on the electric motor is less due to the same energy or power the electric motor use to work on a much load will be used to work on a little load.

1.6: OUTLINE OF THE WORK

The work has been arranged into four chapters. Chapter one discussed the introduction, chapter two deals with the literature review, chapter three discussed the methodology, and chapter four is the conclusion and recommendation.

Power inverters are used to convert a fixed DC supply to a variable AC Supply, they find application in AC motor control
Its’ also applicable in powering a car radio and backing up a building in case of a power outage
Inverters are used primarily in UPS or renewable energy systems or emergency lighting systems. Mains power charges the DC battery. If the mains fails an inverter produces AC electricity at mains voltage from the DC battery

INVERTER

To enable the students to get acquainted with the DC-AC converter
To bring the student's awareness to the working operation of inverters with respect to the induction motor.
To enable the students to determine the output voltages and current....

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