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Measure Vibration Signals from an AC Motor in Time and Frequency Domains

2020-05-03  |  9 min read 

Why Is It Necessary to Measure or Monitor Vibration Signals from an Electric Motor?

Measuring vibration signals from an electric motor is critical to understanding its condition. Many types of products in various industries use motors, from home appliances such as fans and washing machines to wind turbines, industrial water pumps, compressors, large rotary equipment, and conveyor systems. In many industries, especially heavy industries and manufacturing, processes run continuously 24 hours a day, seven days a week. Such industries cannot afford any unplanned equipment breakdowns that will cost them millions of dollars.

So, what does a vibration signal from an electric motor look like? In this blog post, we use an AC motor as an example of measuring vibration signals. It might seem that the vibration is a pure sinusoidal signal, depending only on the power source’s line frequency. That is not entirely correct, but the AC motor vibrations do heavily link to the line frequency. The synchronous speed of an AC motor depends on the number of poles it has and the power source’s line frequency. The speed dependency relationship is based on this formula:

AC-motor-speed-formula
The vibrations of an electric motor can come from various parts and sections of its construction. An AC motor will experience vibrations from the shaft alignment, ball bearing defects, broken or cracked rotor bars, loose rotor bars, loose winding slots, winding insulation integrity, loose connections, and other moving parts. See Figure 1.

AC-motor-parts
Figure 1. Internal parts of an AC motor

When you combine all the small- and large-magnitude vibrations from the internal parts of an AC motor, the resultant vibration measured will look very complex, as in Figure 2.

Figure 2. Vibration signal captured from an AC motor in the time domain

It is difficult to decipher this complex vibration waveform only in the time domain. After converting this information via fast Fourier transform (FFT), you can view it in the frequency domain, as in Figure 3.

AC motor frequency domain vibration analysis
Figure 3. An AC motor frequency domain vibration analysis

How to Make Vibration Signal Measurements Using a Data Acquisition System (DAQ)

Here is a basic setup, as shown in Figure 4.

Vibration-measurement-setup
Figure 4. Overall AC motor vibration setup

Device under test

The device under test is an AC inductive motor with a variable frequency drive to control the speed of the motor. Depending on the application, you can test the vibration characteristics of the AC motor while at a particular constant speed or various constant speeds or during a dynamic change of speeds.

Sensor selection

Accelerometers are the most suitable sensors for vibration measurements. Before selecting an accelerometer sensor, take note of a few key considerations, such as the dynamic range or magnitude of the vibration, the frequency range, the temperature range, form and fitting to match the structure of the device under test, environmental noise, and elements. This information will help you narrow down the model you need for your application.

Before selecting the model, it is essential to know the major types or classes of accelerometers.

Type

Accelerometer sensors can be piezoelectric, capacitive, or piezoresistive. Industrial applications often use the piezoelectric type because of its ruggedness and rigidity. The most common capacitive type is micro-fabricated using MEMS fabrication technology. Electronic devices such as mobile phones and the Internet of Things devices frequently use the capacitive sensor. The piezoresistive type is best-suited for shock testing because it can handle > 200 g.

Impedance

There are two types, high-impedance, and low-impedance sensors. High-impedance sensors can withstand temperatures > 120 °C. Low-impedance sensors are more common as they can interface with test instruments using long cables without loss of signal quality. This type commonly interfaces with Integrated Electronics Piezoelectric (IEPE) standard-compliant test instruments.

Sensitivity

There are high- and low-sensitivity sensors. Use high-sensitivity sensors for low-amplitude vibrations and low-sensitivity sensors for high-amplitude vibrations. The sensitivity of the sensor should match what your test hardware input can measure.

IEPE-accelerometer-sensor
Figure 5. An IEPE accelerometer sensor connects to the input of the DAQM909A digitizer module

Since the device under test is an AC motor, the choice of sensor for this experiment is an accelerometer with medium to high sensitivity that is IEPE compliant, is a piezoelectric type, and has a low impedance. The model selected also fits the dynamic range, frequency range, and temperature range of the application.

Test hardware selection

The test hardware used is a modern data acquisition system, the Keysight DAQ970A mainframe, which can accommodate up to three module slots. The module used is the Keysight DAQM909A four-channel simultaneous sampling digitizer, with each channel capable of sample rates up to 800 kSa/s. If your application must monitor more multiple dynamic signals simultaneously, you can configure three digitizer modules together in this mainframe to get up to 12 channels of simultaneous sampling inputs.

In this experiment, we are using one single-axial accelerometer to illustrate the vibration condition of the AC motor. It is an IEPE sensor, so it requires a constant current to excite the sensor. The DAQM909A delivers the constant current source to the sensor and also reads the dynamic vibration (voltage) signal from the sensor. See Figure 6.

digitizer-iepe-setup
Figure 6. Simplified circuit diagram of the DAQM909A digitizer module connecting to an IEPE accelerometer sensor

Remote DAQ software

The Keysight PathWave BenchVue DAQ application software connects to the DAQ970A via a USB interface. You can make the same connection via a standard LAN interface. This software increases your overall test and measurement productivity and provides the following features:

  • controls and sets up your DAQ hardware with an intuitive point-and-click user interface
  • automates your test sequences without the usual time-consuming traditional programming overheads
  • shows results graphically, whether it is a time domain chart, a frequency domain chart, or statistical data outputs
frequency domain vibration analysis
Figure 7. Keysight’s PathWave BenchVue DAQ software showing an FFT frequency domain chart of a zoomed-in vibration measurement of an AC motor

How to perform the setup

Here is a three-minute video clip of my colleague demonstrating how to set up the hardware and DAQ software to make the vibration measurements of the AC motor.

Video: Measuring Vibration of a Motor Using DAQ973A and DAQM909A

Summary

This blog post provides insights into sources of vibrations inside an electric motor and how complex the vibration signals from an operating AC motor are. It also demonstrates how easy it is to set up Keysight’s DAQ970A mainframe, DAQM909A digitizer module, and an industry-standard accelerometer to measure these vibration signals.

For more information on Keysight’s data acquisition system, please visit www.keysight.com/find/DAQ.

The remote PathWave BenchVue DAQ software comes with the purchase of the DAQ hardware. You can check out the software here: Data Acquisition Control & Analysis.

To learn more about dynamic data acquisition, please click and download the Dynamic Data Acquisition System white paper.