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How Induction Furnaces Work

How Induction Furnaces Work is an interactive computer program that will give you an intuitive understanding of induction furnaces through the use of text, graphics, and animation.  No prior knowledge of electricity is assumed or required.  It will give the user:
       1)  a general understanding of what an induction furnace does and how it does it,
       2)  a knowledge of the terminology that accompanies the field of induction furnaces, and
       3)  enough knowledge to pursue additional study if desired.

This program explains:  

Here is the Table of Contents of the program...

How Induction Furnaces Work

Table of Contents

           Chapter

            1.  Introduction
                        What is an Induction Furnace?
                        Why is it called a Coreless Induction Furnace?
                        Where do I go from here?
 
              2.  How are Induction Furnaces Built?
                        The Main Components of an Induction Furnace
                        Construction of an Induction Furnace
                        Pictures of Induction Furnaces

              3. Why Does the Metal Melt?
                       Magnetic Fields
                       Oersted's Discovery
                       Electromagnets
                       A.C. Source
                       Faraday's Law

             4.   The Power Factor Problem
                        Introduction
                        The Power Factor Problem
                        Why Current Lags Voltage in an Inductor
           
             5.  How Capacitors Help the Power Factor Problem
                        Introduction
                        What is a Capacitor?
                        Why Current Leads Voltage in a Capacitor
                        Summary of Resistor, Inductor, and Capacitor Phase Shift

             6.  Capacitors in Parallel with an Induction Furnace
                        Introduction
                        Resistance vs Reactance 
                        Connecting Capacitors in Parallel with an Inductor
                        Induction Furnace with Parallel Capacitors
                        Summary
                    
             7.  Capacitors in Series with an Induction Furnace
                        Introduction
                        Connecting Capacitors in Series with an Inductor
                        Induction furnace with Series Capacitors
                        Summary
           
             8.  How Power Supply Frequency Affects Furnaces with Parallel Capacitors
                        Introduction
                        Total Current vs Frequency
                        Furnace with Parallel Capacitors and Variable Frequency Power Supply
                        Summary
               
             9.  How Power Supply Frequency Affects Furnaces with Series Capacitors
                        Introduction
                        Total Current vs Frequency
                        Furnace with Series Capacitors and Variable Frequency Power Supply
                        Summary

                 Index 

                About the Author


Here is an excerpt from Chapter 4 explaining the Power Factor problem ...

The Power Factor problem:
Figure 4-2 below shows the A.C. voltage and current of an inductor.  Both the voltage and current are sine waves with a frequency of 60 Hz (60 cycles per second), so the period or time it takes for each sine wave of the voltage or current to occur is 1/60th of a second (1/50th of a second in Europe).  One cycle of the sine wave is created when an electrical generator makes one complete revolution of 360 degrees, so the horizontal axis of the sine wave can be marked off in degrees as well as units of time.

For a perfect inductor, one that has no resistance, the current through the inductor lags the voltage across the inductor by 90 degrees.  The next section explains why this occurs.  To recognize this lag on Figure 4-2, compare any two corresponding points on the voltage and current sine waves.  For instance, look at where the voltage sine wave crosses the horizontal axis on its way up; then look at where the current sine wave crosses the horizontal axis on its way up.  These two points are 90 degrees apart and the current sine wave crosses the horizontal axis after the voltage sine wave, so we say the current lags the voltage.  The same will be true for any other two corresponding points; like where the two waveforms reach their peak.  If a resistor was connected to the A.C. source instead of an inductor, the voltage and current would be in phase.  In other words, they would both go through zero at the same time and would look like they were on top of each other, although they may have different amplitudes depending of the value of the resistor and the scaling of the graph.

Figure 4-2. Power Factor of an Inductor

Power Factor of an inductor

The power factor of a circuit is defined as the cosine of the angle, or phase shift,  between the voltage and current of the circuit.  The greater the angle between the voltage and current, the less efficient a circuit will be.  Power factor is usually expressed as a percent by multiplying the cosine of the angle by 100%.  Cosine of an angle can be found using most scientific calculators.  Figure 4-3 below shows the cosine of some angles. As the angle increases, the cosine decreases.

Here is an excerpt from Chapter 6 explaining how capacitors in parallel with an induction furnace can help the power factor problem...

Induction Furnace with Parallel Capacitors:
Figure 6-4 below shows a 200KW induction furnace with a power factor of 20% connected to an A.C. source and potentially a capacitor.  No capacitors have yet been connected to the circuit, as indicated by the 0 KVAR label beside the capacitor symbol. In the next figure, we are going to vary the furnace rated power, the furnace power factor, and the size of the capacitors in the circuit to see if the total power factor can be corrected by adding capacitors. Figure 6-4 is just to explain some of the aspects of the circuit. Looking at the notes in red on Figure 6-4, we can see that:

1)  The total power factor of the circuit is equal to the power factor of the furnace since no capacitors have yet been added.
2)  The total power factor is inductive since the total current lags the applied voltage by 78 degrees (the minus sign indicates the current lags the applied voltage).
3)  The total current of the circuit is equal to the current of the furnace since no capacitors have yet been added.
4)  The current of the furnace lags the applied voltage by 78 degrees (since the furnace power factor is 20% and cosine of 78 degrees is 0.20).
5)  Once capacitors are added, the total current will equal the capacitor current plus the furnace current, added vectorially to take into account their angles (we will do this for you).

Figure 6-4.  Induction Furnace with Parallel Capacitors Explained

Induction furnace with parallel

Figure 6-5 below shows an induction furnace with parallel capacitors in which we can vary the furnace rated power, furnace power factor, and size of the capacitors.  Perform the following steps to see the effects of capacitors on the circuit:

1)  Click on the drop-down list beside the capacitor symbol and choose 0 KVAR if it isn't already chosen (furnace rated power and furnace power factor should be 200 KW and 20% respectively) to confirm that the furnace has the same power factor and draws the same amount of current as shown in Figure 6-4 above.

2)  Click on the drop-down list beside the capacitor symbol and choose 1000 KVAR (furnace rated power and furnace power factor should still be 200 KW and 20% respectively).  Notice that the total power factor is now 99% capacitive.  This very nearly an ideal power factor of 100%.  The total power factor is capacitive because the total current leads the applied voltage by 6 degrees.  In other words, we have added slightly too much capacitance.  Notice also that the total current, or the current the power supply must provide, has been reduced from 2083 amps to 428 amps, lowering the power usage by almost 80%.  The current through the furnace coil is still 2083 amps, however.

3)  Click on the furnace rated power and choose 400 KW...


How Induction Furnaces Work is written in .html format so you can use your favorite internet browser to navigate through it using hyperlinks, bookmarks, and word searches.  How Induction Furnaces Work requires no software installation on your computer.  Just double-click on Table of Contents.html.

How Induction Furnaces Work is written by a Professional Engineer with 14 years industrial experience and 20 years experience teaching electrical engineering technology at the college level.

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