How to Measure the Inductance of a Transformer and Calculate Theoretical Vout

The purpose of this Instructable article is to demonstrate how to measure the inductance of a transformer. This demonstration is prepared especially for PHYSIOTHERAPY students for a better understanding of terms related to transformers in ELECTROTHERAPY lectures.

Instrumentation refers to the devices and techniques used for measurement, control, and data collection. Electrotherapy, on the other hand, involves the use of electrical energy as a medical treatment to alleviate pain, improve circulation, repair tissues, and stimulate muscles.

In this context, understanding how to properly use instrumentation to measure inductance is crucial for accurate applications in electrotherapy devices, where transformers play a significant role in managing electrical currents safely and effectively.

Today, more advanced power supplies are being used, which not only include transformers but also possess advanced features such as patient isolation and short circuit protection. However, during the period when basic electrotherapy techniques, such as galvanic current and faradic current, were commonly applied, power supplies with transformers were quite widespread. These advancements in modern power supplies enhance patient safety by preventing unintended electrical exposure while still allowing effective electrotherapy treatments

Equipment Needed:

1. LCR meter (Inductance, Capacitance, Resistance meter)
2. Transformer to be tested
3. Connecting wires

On the input side of the transformer, you will identify two pins designated for the connection of 220 V AC power supply. Meanwhile, on the output side, you will find three pins associated with the 9 V output, which allows for multiple configurations depending on the specific application requirements. Understanding the function of each pin is crucial for ensuring proper connections and optimal performance of the transformer.

The three pins on the output side of the transformer serve specific purposes, allowing for various configurations in different applications. Typically, one pin acts as a common or reference point, while the other two pins provide positive and negative outputs.

This dual voltage output capability is essential for applications requiring a dual polarity power supply. For instance, in electromyography (EMG) systems, which measure the electrical activity of muscles, having access to both positive and negative voltages is crucial for obtaining accurate measurements.

The flexibility provided by dual voltage output significantly enhances the functionality of EMG systems, enabling more effective examination and analysis of various muscle activities

Preparing the LCR Meter

Power on the UT603 LCR meter.

Select the Mode: The display will show measurement modes like "L," "C," or "R." Choose the "L" (inductance) mode.

Select the Range: Turn the setting knob on the LCR meter to 20H.

Making Connections

Use the connection cables to connect the input pins of the transformer to the LCR meter.

Performing the Measurement

Once the connections are made, the device will calculate and display the inductance value on the screen.

Note down the measurement values displayed.

On the transformer’s output side, there are typically three pins

Center pin (common or ground): This is often the reference point for measuring.

Two outer pins: These provide the positive and negative voltages.

Making Connections:

To measure the inductance of one side of the winding, connect the LCR meter to the center pin and one of the outer pins.

To measure the inductance across the full winding, connect the LCR meter to the two outer pins, bypassing the center pin.

Since the output side of the transformer has a lower inductance value, we adjusted the LCR meter to a lower range compared to the input side. While we measured the input windings using the 20H setting, for the output side, we started at 20H and gradually decreased the range, obtaining the most accurate reading at 200mH

Taking the Measurement of one side of the winding:

Attach the meter’s probes to one outer pin and the center pin,

Press the measurement button on the LCR meter in "L" mode to begin the inductance measurement.

The inductance value will be displayed on the LCR meter’s screen, typically measured in Henrys (H).

Taking the Measurement of other side of the winding:

Attach the meter’s probes to other outer pin and the center pin,

Press the measurement button on the LCR meter in "L" mode to begin the inductance measurement.

The inductance value will be displayed on the LCR meter’s screen, typically measured in Henrys (H).

Taking the Measurement of full winding:

Attach the meter’s probes between the two outer pins ,

Press the measurement button on the LCR meter in "L" mode to begin the inductance measurement.

The inductance value will be displayed on the LCR meter’s screen, typically measured in Henrys (H).

Interpreting the Results

Measuring from Center to Outer Pin: This gives the inductance of half of the transformer winding, commonly used for single voltage output.

Measuring Between the Outer Pins: This gives the inductance of the entire winding and is important for understanding the full behavior of the transformer’s dual voltage output.

By following these steps, you can accurately measure the inductance of a transformer’s output with three pins, taking into account the center tap and dual voltage configuration. This method helps in assessing both individual winding inductance and the overall inductance across the transformer’s full output.

The ratio of the input voltage Vin to the output voltage Vout is equal to the square root of the ratio of the input inductance Lin to the output inductance Lout.

Lin=17.2H=17200mH

Lout1=37.6mH

SquareRoot(Lin/Lout)=Vin/Vout

SquareRoot(17200mH/37.6mH)=(220V/Vout)

SquareRoot(457.446)=220/Vout

21.387=220/Vout

Vout=220/21,387

Vout1=10.286V

Lin=17.2H=17200mH

Lout2=38.1mH

SquareRoot(Lin/Lout)=Vin/Vout

SquareRoot(17200mH/38.1mH)=(220V/Vout)

SquareRoot(451,443)=220/Vout

21.247=220/Vout

Vout=220/21,247

Vout2=10.354V

Lin=17.2H=17200mH

Lout=154.7mH

SquareRoot(Lin/Lout)=Vin/Vout

SquareRoot(17200mH/154.7mH)=(220V/Vout)

SquareRoot(111.182)=220/Vout

10.544=220/Vout

Vout=220/10.544

VoutFull=20.959V