The term "inductance" can refer to "mutual induction", that is when an electric circuit generates voltage as a result of the current variation in another circuit, or to "self-induction", that is when the electric circuit generates voltage as a result of the variation of current flowing in it. In both cases, the inductance is given by the ratio between the voltage and the current, and the relative unit of measurement is the henry (H), defined as 1 volt per second divided by amperes. Since henry is a fairly large unit of measurement, inductance is generally expressed in millihenry (mH), one thousandth of a henry, or in microhenry (uH), one millionth of henry. Several methods for measuring the inductance of an inductor coil are illustrated below.
Steps
Method 1 of 3: Measure the Inductance from a Voltage-Current Ratio
Step 1. Connect the inductor coil to a waveform generator
Keep the wave cycle below 50%.
Step 2. Organize the power detectors
You will need to connect a current sense resistor, or a current sensor, into the circuit. Both solutions will need to be connected to an oscilloscope.
Step 3. Detect current peaks and the time interval between each voltage pulse
The current peaks will be expressed in amperes, while the time intervals between the pulses in microseconds.
Step 4. Multiply the voltage delivered to each pulse by the pulse duration
For example, in the case of a voltage of 50 volts delivered every 5 microseconds, it would be 50 times 5, or 250 volts * microseconds.
Step 5. Divide the product between voltage and pulse duration by the peak current
Continuing with the previous example, in the case of a current peak of 5 amperes, we would have 250 volts * microseconds divided by 5 amperes, or an inductance of 50 microhenry.
Although the mathematical formulas are simple, the preparation of this test method is more complex than the other methods
Method 2 of 3: Measure the Inductance Using a Resistor
Step 1. Connect the inductor coil in series with a resistor whose resistance value is known
The resistor should have an accuracy of 1% or less. The series connection forces the current to cross the resistor, as well as the inductor to be tested; the resistor and the inductor must therefore have a common terminal.
Step 2. Apply a sinusoidal voltage to the circuit, at a fixed peak voltage
This is achieved through a waveform generator, which simulates the currents that the inductor and resistor would receive in the real case.
Step 3. Check both the input voltage and the voltage on the common terminal between the inductor and resistor
Adjust the frequency of the sinusoid until obtaining, at the connection point between the inductor and the resistor, a maximum voltage value equal to half the input voltage.
Step 4. Find the frequency of the current
This is measured in kiloHertz.
Step 5. Calculate the inductance
Unlike the calculation of the inductance from the current-voltage ratio, setting up the test in this case is very simple, but the necessary mathematical calculation is much more complex. Proceed as follows:
- Multiply the resistance of the resistor by the square root of 3. Assuming you have a 100 ohm resistance, and multiplying this by 1.73 (which is the square root of 3 rounded to the second decimal place), you get 173.
- Divide this result by the product of 2 times pi and the frequency. Considering a frequency of 20 kiloHertz, we get 125, 6 (2 * π * 20); dividing 173 by 125.6 and rounding to the second decimal place yields 1.38 millihenry.
- mH = (R x 1.73) / (6.28 x (Hz / 1000))
- Example: considering R = 100 and Hz = 20,000
- mH = (100 X 1.73) / (6, 28 x (20.000 / 1000)
- mH = 173 / (6, 28 x 20)
- mH = 173/125, 6
- mH = 1.38
Method 3 of 3: Measure the Inductance using a Capacitor and a Resistor
Step 1. Connect the inductor coil in parallel to a capacitor whose capacitance value is known
By connecting a capacitor in parallel with an inductor coil, a reservoir circuit is obtained. Use a capacitor with a tolerance of 10% or less.
Step 2. Connect the tank circuit in series with a resistor
Step 3. Apply a sinusoidal voltage to the circuit, with a fixed maximum peak
As before, this is achieved through the waveform generator.
Step 4. Place the oscilloscope probes on the circuit terminals
Once this is done, switch from low frequency values to high ones.
Step 5. Find the resonance point
This is the highest value recorded by the oscilloscope.
Step 6. Divide 1 by the product between the square of the energy and the capacity
Considering an output energy of 2 joules and a capacity of 1 farad, we would obtain: 1 divided by 2 squared multiplied by 1 (which gives 4); that is, an inductance of 0, 25 henry, or 250 millihenry would be obtained.
Advice
- In the case of inductors connected in series, the total inductance is given by the sum of the values of the single inductances. In the case of inductances in parallel, however, the total inductance is given by the reciprocal of the sum of the reciprocals of the values of the individual inductors.
- Inductors can be built underneath as a cylindrical, toroidal core, or thin film coil. The more the windings of an inductor, or the larger its section, the greater the inductance. Longer inductors have a lower inductance than shorter ones.
