Production Testing for Electric Motors in Household Appliances

Selling to more than 100 countries worldwide, the Turkish company Arçelik is a manufacturer of household equipment and entertainment electronics with an impressive research and development department. Acoustic tests are carried out and vibration measurements taken during the production of electric motors to ensure that given tolerances are met for quality control purposes. Such measurements are made using laser vibrometers. The non-contact features of this technology enable measurements to be taken at a high production rate.

Maintenance-Free Sensor System

The Polytec IVS Industrial Vibrometer shown in Figure 1 uses a laser beam as a measurement probe and is particularly useful for taking measurements in production plants. It cannot be controlled externally - it is configured via the internal serial interface. This means that it is impossible to accidentally change settings during service or maintenance work.

Figure 1: Example of a measurement using a laser vibrometer to characterize an electric motor.

This optical measurement technology has additional advantages, including:

• Installation is simplified.  Mechanical fixturing and direct contact with the tested object are eliminated.
• The small probe size allows ordinarily inaccessible regions to be measured.
• Probe mass, eg of an accelerometer, doesn’t affect the vibration characteristics of the test specimen.
• The technology can be quickly adapted regardless of the type or version of the test specimen and test environment.

Customized Test Procedures

The two most frequently used methods for testing the motors are to take measurements while the motor is warming up and/or to take measurements at a constant RPM (Figure 2). For this reason, a test bench must have flexible control of the motor speed. This includes both acquisition and monitoring of the actual RPM on the tested motor. Records are taken of the structure-borne sound vibrations at the individual measurement points. Software is then used to evaluate these.

Figure 2: Time-speed diagram for the two different measurement procedures.

The RPM is specified by setting a control voltage which is dependent on the test stand. For example, a function generator without feedback could be used, or the motor speed could be set directly and actively regulated. With a function generator, an encoder or an analog voltage proportional to the motor speed must be used to measure the actual motor speed.

The Polytec QuickCheck test software is used to evaluate and control the test procedure. QuickCheck has an integrated function generator which enables the setting of a trapeze-shaped speed profile. This profile has an adjustable increase, an adjustable maximum voltage and a variable duration of the constant voltage. The overall duration of the measurement cycle corresponds with the measurement time. An alternative approach is to specify the measurement time according to the production cycle and adjust the motor speed profile accordingly.

Vibration signals are measured simultaneously with the setting of the motor speed. Either the motor speed specifications or the actual measured speed can be used as a basis with which to divide the measurement data into sections.  A section can incorporate traversing the RPM range from 500 to 2,500 RPM or from 8,000 to 15,000 RPM as shown in Figure 2. The software searches the time domain relevant to the velocity signal and analyzes the vibration signals obtained within this range. Every motor can have a different time signature as a result of its dynamic behavior.

Efficient Evaluation

An evaluation model for the test bench is available with QuickCheck, making such tests straightforward to carry out (Fig. 3). An RPM range is initially given for this analysis. The QuickCheck software will only analyze measurement data that corresponds with the individual measurement channels in the relevant time section. It will then use an adjustable block size to calculate the spectrogram for this data. This data is used to calculate various characteristics which can then be compared to specified limits. For example, the band energy or the peak value in the frequency range from 100 to 300 Hz. Within an RPM range, any number of characteristics can be calculated with the software. Likewise, any number of RPM ranges can be defined and it is even possible for these ranges to overlap.

Figure 3: Screen shot of the evaluation module in the QuickCheck test software.

A specific time domain can be selected when measuring at a constant speed. Using the options above, it is possible to set and calculate characteristics. The measured time signals can be saved for subsequent order analysis with off-line analyses.

A table in the QuickCheck test software details all characteristics calculated in this way. All characteristics must be within their specified limits for the motor to pass the test and progress to the next manufacturing step. The measurement values along with their timestamp can be saved in a database. The serial number of every motor can be read by bar code or data-matrix code with QuickCheck.  The value can then be saved alongside the corresponding measurement values. This makes it possible to trace individual production and testing results at any given time for every product.

QuickCheck Software

QuickCheck is a multichannel, PC-based software for carrying out semi or fully automatic process monitoring and quality control based on the vibration behavior of the products.

QuickCheck is able to acquire and evaluate the measurement signals, make the pass/fail decision, control the test routine and communicate with the customer’s process control system. www.polytec.com/software

This information has been sourced, reviewed and adapted from materials provided by Polytec.

For more information on this source, please visit Polytec.