for Education and Industry
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:
This program explains:
1) a general
understanding of what an induction furnace does and how it does
2) a knowledge of the
terminology that accompanies the field of induction furnaces, and
3) enough knowledge to
pursue additional study if desired.
- how induction furnaces are built
- why the metal in the furnace melts
- what magnetic fields are
- what Oersted's Discovery was and why it is
- what electromagnets are and why they are
- why induction furnaces work on AC but not DC
- what Faraday's Law is and why it is important
- what power factor is
- how power factor can be corrected
- what effect installing capacitors in parallel with an
induction furnace has
- what effect installing capacitors in series with an
induction furnace has
- what effect power supply frequency has on an induction
How Induction Furnaces Work
Here is the Table of Contents of the
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?
4. The Power Factor Problem
The Power Factor Problem
Why Current Lags Voltage in an Inductor
5. How Capacitors Help the Power Factor Problem
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
Resistance vs Reactance
Connecting Capacitors in Parallel with an Inductor
Induction Furnace with Parallel Capacitors
7. Capacitors in Series with an Induction Furnace
Connecting Capacitors in Series with an Inductor
Induction furnace with Series Capacitors
8. How Power Supply Frequency Affects Furnaces with
Total Current vs Frequency
Furnace with Parallel Capacitors and Variable Frequency Power Supply
9. How Power Supply Frequency Affects Furnaces with Series
Total Current vs Frequency
Furnace with Series Capacitors and Variable Frequency Power Supply
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
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
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
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
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
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
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How Induction Furnaces Work requires no software
installation on your computer. Just double-click on Table
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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