Skip navigation
All Places > Keysight Blogs > General Electronics Measurement > Blog > Author: bill griffith

Measuring inrush current is always interesting. In some devices, inrush current can be surprisingly high (10x or higher than their steady current). Excessive inrush current can damage components and pc boards designed for lower steady-state currents. To avoid damage, many devices include a protection circuit to limit the inrush current. Typically, large inrush current lasts for a few cycles before returning to a steady-state current. Inrush current is measured as a peak current and is useful for sizing fuses or designing additional protection circuitry.

 

An inrush current of an inductive load.

Figure 1. An inrush current of an inductive load.

 

It is also important to consider what phase the AC voltage is at when it is applied to the DUT. The turn-on phase can significantly affect the inrush current. A couple of voltage waveforms with different turn-on phases are provided below.

 

Various turn-on phases of an AC voltage. Left 0 degrees, middle 30 degrees, and right 180 degrees.

Figure 2. Various turn-on phases of an AC voltage. Left 0 degrees, middle 30 degrees, and right 180 degrees.

 

When using a mechanical switch, you have no control over the turn-on phase. The device’s inrush current will be completely unpredictable since it is dependent on the phase of the AC voltage. To control the voltage turn-on phase and thus predict the inrush current, you can use Keysight’s AC6801B AC source. A peak hold measurement determines the inrush current while a peak measurement is used for steady-state current.

 

Current waveform with peak hold and steady state peak current.

Figure 3. Current waveform with peak hold and steady state peak current.

 

AC6801B AC source measurement panel displaying AC current peak, peak hold, and rms.

Figure 4. AC6801B AC source measurement panel displaying AC current peak, peak hold, and rms.

 

Determining the maximum inrush current

 

Most electronics contain switching power supplies as they are incredibly efficient. The test setup below tests an external switching power supply with an AC source and a 30 W load.

 

Test setup to measure the inrush current of the 12 VDC supply.

Figure 5. Test setup to measure the inrush current of the 12 VDC supply.

 

The steady state 2.26 A peak current drawn from the AC source.

Figure 6. The steady state 2.26 A peak current drawn from the AC source.

 

A series of measurements are made to determine the maximum inrush current. The first measurement uses a 0-degrees turn-on phase, and the second uses 10 degrees. Each subsequent measurement uses a 10-degree higher turn-on phase. Configuring the AC source for inrush current measurements is a two-step process.

 

1) Setting the turn-on phase from the front panel of the AC source.  

Setting the turn-on phase from the front panel of the AC source.

2) Clear the peak hold measurement from the front panel.

Clear the peak hold measurement from the front panel.

 

The 12 VDC power supply output will turn on 1.3 seconds after the AC power is applied. Only after turning on its output will it be drawing steady state current. In Figure 7, two different time scales are used to display the inrush current. The screen capture on the left shows the voltage phase and the inrush current. The screen capture on the right shows the inrush current and the 1.3 second delay before the steady-state current.

 

The voltage applied to a 12VDC power supply and the current it draws with two different time scales is shown. You can see the details of the inrush current spike on the left. On the right, after 1.3 seconds, the power supply turns on and pulls steady-state current.

Figure 7. The voltage applied to a 12VDC power supply and the current it draws with two different time scales is shown. You can see the details of the inrush current spike on the left. On the right, after 1.3 seconds, the power supply turns on and pulls steady-state current.

 

Graphing the inrush measurements versus phase for the 12 VDC power supply reveals a trend. The inrush currents are lower when the voltage is turned on with a phase of 0 and 180 degrees. This is because at a phase of zero and 180 degrees, the voltage is turning on at zero volts.

 

Inrush current for a capacitive device, 12 VDC power supply vs. phase.

Figure 8. Inrush current for a capacitive device, 12 VDC power supply vs. phase.

 

Devices with capacitive input will have low inrush currents when the voltage is turned on at zero volts. At zero volts a sinewave has its maximum rate of change, this change causes an inductive load to create their largest inrush current. An inductive load will have its maximum inrush current at zero and 180-degrees.

 

A simulated graph of inrush current vs. phase for an inductive load.

Figure 9. A simulated graph of inrush current vs. phase for an inductive load.

 

Limiting inrush current

 

Inrush current can be limited by designing a device with lower reactance. An example is lower capacitance or lower inductance. Another possibility is to turn on a small part of the device and synchronize the rest of the turn-on to the AC line, taking advantage of the phase with the lowest inrush. The 12 VDC power supply tested delayed the turn on of its output. A third possibility is adding a negative temperature coefficient (NTC) current limiting device to your design. The NTC device initially has a high impedance, which reduces the inrush current. As the NTC device warms up, its impedance is reduced. The steady-state current is not affected by the NTC. Knowing the steady-state current and maximum inrush current helps in selecting the right NTC.

 

Using an NTC to reduce the inrush current into capacitive device.

Figure 10. Using an NTC to reduce the inrush current into capacitive device.

 

Summary

 

To accurately measure the maximum inrush current, it is essential to consider the turn on phase. The design of the device will affect the phase at which the maximum inrush current occurs. Some devices will have to be designed to limit the inrush current. Adding an NTC current limiting device to your design will limit inrush current. Several measurements need to be made to select the right NTC. An AC6801B AC source can be used to characterize the inrush current of a device quickly, helping you design a device with inrush current that you and your customers can trust.

Electronics are often designed to work anywhere in the world with local power. An AC source can create the various voltage-frequency combinations used in different countries and even measure the current and power consumption. 

Figure 1 Power supply that accepts 100 Vac to 240 Vac

 

 

 

 

 

 

 

 

 

 

Figure 1. A power supply that accepts 100 Vac to 240 Vac.


You can automate the process using Visual Basic for Excel to program an AC6801B AC source. Excel lets you capture and share results in a neatly organized table. In this example, we chose eleven countries to demonstrate the wide variety of voltages and frequencies used around the world. Some countries use multiple voltages and plug types, and those displayed are selected to illustrate a broad range. The table is easily modifiable, and you can add more countries by increasing the rows as well as the country count in cell D6.

 

Figure 2 table with various voltages and frequencies

Figure 2. An example table of various voltages and frequencies. Row number and column letter shown in red.

 

The Visual Basic program reads the voltages and frequencies from the table and sets up the AC source to output them. The program pauses for the number of seconds provided in column F, allowing the test device to settle. The AC source then makes several measurements and adds them to each row of the table.

 

Figure 3 AC source adds measurements to each row in table

Figure 3. The AC source output is set to 100 Vac at 50 Hz. Three measurements are added to the table after the delay. The process is repeated for each country.

 

Getting started with Visual Basic for Excel

If you are not currently using Visual Basic for Excel, you need to display the developer tab. It is included in Excel but is not shown by default. You can find instructions online for adding the tab to the version of Excel you use. The second step is adding Keysight IO Libraries Suite. In this example, all the commands are sent to the AC source using the Keysight IO library. With the software installed, open the developer tab and create the global variables.

 

Figure 4 Opening VBA project

Figure 4. Opening the VBA project and creating the global variables.

 

Opening a connection to the AC6801B

The following subroutine creates a connection to communicate with an instrument based on its VISA address. The VISA address is read from the table, making it easy to update. You can use the Keysight Connection Expert, which is installed with the IO Suite to read the instrument address. Watch this YouTube video to see the steps to locate the instrument VISA address and verify the connection.

 

Figure 5 Subroutine to open a connection to instrument

Figure 5. Subroutine to open a connection to the instrument.

 

You can insert a button onto the spreadsheet (Developer > Insert > Form Control > Button). You are then prompted to add the name of the subroutine that will run when the button is clicked.

 

Figure 6 Excel with controls and LXI address

Figure 6. The spreadsheet with the first two controls and the LXI address.

 

Resetting and configuring the instrument

The subroutine initConfig_Click() has three primary roles: to clear the AC source, to configure DUT protection, and to select the upper range and turn on the AC source with a known voltage. Once the instrument is reset, it is set to output only AC voltage, and the voltage is limited by a min and max value. Obviously, overvoltage can cause damage to a device, but too low a voltage is just as harmful. In the case of a power supply, too low a voltage leads to higher current to meet the power needs of the load. In addition, a current limit is set to protect the DUT from damage due to excessive current. The time required to run the test is reduced by using a single range. The AC6801B is a dual range source and provides additional current in its low range. For our device, the upper range has enough current to test the device. Lastly, a voltage-frequency combination is selected, and the output is turned on. The primary goal of this program is to demonstrate some of the AC source’s capability. With the output turned on, the AC source displays measurements on the front panel.

 

Figure 7 Subroutine to put AC source in known state
Figure 7. Subroutine to put the AC source in a known state.

 

Step through the list of countries

The createSequence_Click() subroutine populates the table with the power measurements for each country. It uses the number specified in cell D6 to determine the number of loops necessary to complete the table. In each loop, the voltage-frequency is pulled from the table, and the source is set to output the combination. The power measurements are made after the delay in column F, which allows the device to adjust to the new voltage and frequency. Three power measurements were selected from a choice of 17 different measurements. As each measurement is made, the results are added to the table.

 

Figure 8 Subroutine to apply voltage-frequencies from table
Figure 8. Subroutine to apply the voltage-frequencies from the table and populate the measurements.

 

Closing the instrument connection

Closing the instrument connection releases the resources. Once the connection closes, an error is generated if you attempt to send commands to the instrument.

 

Figure 9 Closing instrument connection
Figure 9. Closing the instrument connection.

 

Reading instrument errors

The read error subroutine is useful while creating or modifying a program. It pulls the error strings from an instrument one string at a time. You may need to run it multiple times to clear all the errors. It is designed to run independently of the rest of the program. It assigns resources, opens the IO, and then closes the IO.

 

Figure 10 Subroutine to query error strings
Figure 10. A subroutine to query the error strings.


Summary

The universal adapter test program is designed to demonstrate some of the AC6800 Series capabilities and a method to document the measurements using Excel with Visual Basic. Adding a main program to call each of the subroutines simplifies the program because it’s not necessary to connect each routine to a separate control button. The program ran many times over a two-week period using the AC6801B, and the results are repeatable. Often the results are identical when formatted to show a single decimal place. The program is easily modifiable to make additional measurements. A couple of real-world applications are to characterize a group of power supplies and statically determine the max power usage or verify results from a remote facility. Using a second AC source, it is easy to reproduce results with a production facility located on the other side of the globe.

A search of the internet provides articles and videos showing the general operation of a synchronous motor. I thought it would be interesting to make some actual measurements and display some real-world voltage and current waveforms. I was fortunate to find a synchronous motor that had external wiring to its two coils, allowing independent characterization of each coil. The motor is small, but the characterization would scale to a larger motor.

 

Synchronous Motor Basics

An AC signal powers a synchronous motor. It uses the positive/negative power cycle to create a changing magnetic field. Our motor uses two coils that are 90 degrees out of phase. The fields change such that the magnetic rotor turns to keep aligned with the fields. Synchronous 3-phase motors are popular due to the simplicity of using each phase to create a rotating field. You can view many good animations of how a synchronous motor works on the internet.

 

Drawing of synchronous motor with two coils

Figure 1. A drawing of a synchronous motor with two coils. The colors represent the colors of the motor leads.


The AC source causes the motor to rotate at a rate proportional to its frequency (typically measured in Hz). Our example motor is powered by a 60 Hz AC signal and rotates at 3600 RPMs. The synchronous motor can turn in either direction. It starts turning toward the attracting magnetic force, which could be either of the coils depending on its configuration. A minimum voltage is required to create a magnetic field strong enough to cause the rotor to spin. Once spinning, the AC source voltage does not affect the speed of rotation.

 

Changing location of AC source
Figure 2. Changing the location of the AC source to cause a different direction of rotation.

 

Electrical Connections to the Motor

I wanted to characterize the voltage and current through each coil. I used our PA2201A power analyzer to characterize each coil. Each channel of the power analyzer has an input for voltage and an input for current. It can make  measurements of each input or measurements that combine two inputs. While not shown in Figure 2, I added a switch to connect the AC source to coil A or coil B.

 

Drawing of motor with power analyzer connected
Figure 3. An electrical drawing of the motor with a power analyzer connected. The power analyzer connections are color-coded. Channel 1 is yellow and Channel 2 is green.

 

The AC6801B AC source supplies the 110 VAC input power. The AC source will characterize all the power parameters for the motor. We can also vary the frequency of the input voltage.


Real-World Measurements

We will start with the AC source connected to the coil A as well as channel 1 of the power analyzer, shown in Figure 3. We expect to see the 110 V on channel 1 (top yellow waveform) of the power analyzer. Notice the coil B voltage shown on channel 2 (top green waveform) lags channel 1 almost perfectly by 90 degrees.

 

Screen capture of PA2201A power analyzer
Figure 4. Screen capture from the PA2201A. The display is split into three parts: the top is voltage, the middle is current, and the bottom is power. Yellow waveforms represent channel 1, coil A, and green waveforms represent channel 2, coil B.


I do not work with inductors and capacitors every day and was surprised that coil B receives much more current and power. If you look at the equations for reactance, it makes sense. The capacitor reduces the reactance X in the branch with coil B, X = XL - XC. More current flows through this branch, which creates more voltage across coil B.


Reactance through motor
Figure 5. The diagram shows the reactance through each branch of the motor.

 

PA2201A power analyzer display
Figure 6. Measuring the power of channel 1, coil A, and channel 2, coil B, with PA2201A power panels.

 

The AC source is measuring the input to the motor, which is consuming 0.05 ARMS, 5.1 W, and 5.4 VA. Coil B uses most of the energy. I was also surprised to learn the coil's reactance is nearly 50% resistive, 50% inductance. A phase angle ɸ of 45% or power factor PF .707 would be precisely a 50% mix, and the measurements were only slightly more inductive. You can see the phase angle (the delay between voltage and current) in the Figure 6. Lastly, I had expected the current waveforms to be more sinusoidal. In reality, the motor acts as a transformer and sends energy between the coils. If I remove the capacitor, you can see 50 V across coil B; No current flows, as I left the branch open.

 

 PA2201A power analyzer display

Figure 7. With the capacitor removed, the motor can turn in either direction. Energy in coil A creates a voltage on coil B. On the left, the motor is turning in a counterclockwise direction (normal), and on the right it is turning clockwise. Notice in the first case, the coil B voltage (shown in green) lags, and in the second it leads.

 

Reversing the Motor Direction

Applying the source voltage to coil B instead of coil A causes the motor to run in the opposite direction.

 

Motor drawing
Figure 8. Applying the source voltage to coil B, which causes the motor to turn clockwise.

 

PA2201A power analyzer display
Figure 9. Measurements of the two coils with the source connected to coil B.

 

The coils are not perfectly symmetrical, as the power through each branch is a bit lower than in the counterclockwise setup. Using the AC source, the overall power measurements are also lower, reduced to 4.8 and 5.1 VA. You will notice the voltage in coil B, green waveform, leads coil A. You can compare Figure 9 and Figure 6 to see the differences.

 

Changing the Source Frequency

The AC source can vary its frequency from 40 Hz to 500 Hz. Changing the frequency of the source affects the speed of the motor. It also changes the impedance of the circuit, as the reactance of the capacitor and inductors are a function of frequency.

 PA2201A power analyzer display
Figure 10. The waveforms on the left were captured with a source frequency of 40 Hz, and on the right the source frequency was increased to 80 Hz.


Summary

I was impressed with the motor’s overall power factor. The AC source measured the PF to be 0.94. Earlier, we measured the current through each branch of the circuit. The total current consumed by the motor is a point-by-point summation of the two current waveforms (i.e., the sum of the current through each of the coils). A PF of 0.94 indicates that the summation has nearly identical phase to the input voltage. The power analyzer can sum the two current waveforms and display them overtop the input voltage.

 

PA2201A power analyzer display
Figure 11. The pink waveform represents the total current used by the motor. It is a mathematical summation of the current waveforms shown at the bottom of the screen.


As you can see, the system has a PF near 1.0, as the input current and voltage have nearly identical phase.


The capacitor used with our motor is considered a run or permanent split capacitor, as it maintains the 90-degree shift in the coil voltages. People sometimes generically refer to a capacitor used with a motor as a start capacitor. A start capacitor is disconnected by a switch once the motor starts spinning.


To overcome the difficulties in changing the speed of our single-phase motor, the manufacturer added a gearbox so it spins at 33.3 RPMs.


The AC6801B AC source made it easy to supply the required AC power (110 VAC at 60 Hz) while making power measurements. The measurements include current, real power, apparent power, reactive power, and power factor. The PA2001A was used to characterize the voltage and current through each coil.