How Low-Voltage Motor Circuit Analysis Helps Predict Failures Early

By Howard Penrose

Low-Voltage Motor Circuit Analysis

We utilize virtually all testing methods for evaluating industrial and electrical systems for condition and root-cause analysis.  Depending on the investigation, we will select either high-voltage or low-voltage motor circuit analysis (MCA). 

For this article, we will discuss low-voltage MCA, which is not a reference to the voltage rating of the electric machine being tested. The output of these types of instruments is generally under 10 Vac with output frequencies under 2 kHz, but the applications span all machine sizes and voltages.

Using a hand-held ALL-TEST Pro 5 MCA device on a 13.2kV synchronous motor
Figure 1: Using a hand-held ALL-TEST Pro 5 MCA device on a 13.2kV synchronous motor
Data from the motor in Figure 1.
Figure 2: Data from the motor in Figure 1.

While there are high-voltage MCA devices, such as the Electrom in Figure 3, these may also have low-voltage tests built in, as shown in Figure 4.

Electrom D12 high output with low voltage and offline partial discharge and the ALL-TEST Pro 5
Figure 3: Electrom D12 high output with low voltage and offline partial discharge and the ALL-TEST Pro 5
Low voltage test results from the Electrom system.
Figure 4: Low voltage test results from the Electrom system.

The voltage output of the D12 in Figures 3 and 4 is higher than that of the ATPro system, a significant distinction.

The purpose of low-voltage MCA is to vary the applied frequency to seek imperfections in the insulation system due to material degradation, tracking, or damage, which all impact the inter-turn capacitance. 

Between conductors the values will be in inductance (milli-Henries, mH), impedance in ohms (Z), phase angle which identifies the purity of the capacitance (measure of the time relationship between voltage and current), I/F which is the impact on the circuit when the applied frequency is doubled, Q-factor which is the quality of the material, and then the insulation to ground and related capacitance and dissipation factor.

Understanding the measurements involves understanding that insulation materials are largely capacitive in nature.
Figure 5: Understanding the measurements involves understanding that insulation materials are largely capacitive in nature.
This includes the relationships on how different tests are affected by variations in capacitance.
Figure 6: This includes the relationships on how different tests are affected by variations in capacitance.

When dealing with a near-perfect sine wave produced by test instruments at different frequencies, the effects caused by variations in the capacitance of the insulation system can be determined.  This does not mean that the insulation system must be failed, destroyed, or have a direct short; however, as the insulation system degrades in small areas or overall, the results in each phase change.

Cutaway of a winding showing conductors in the slots, slot liners, and top sticks, and an encapsulation varnish.
Figure 7: Cutaway of a winding showing conductors in the slots, slot liners, and top sticks, and an encapsulation varnish.

Figure 7 is a cutaway of a winding showing a random-wound motor cutaway.  Each of the materials has a slightly different set of electrical properties.  Figure 8 shows the averaged and estimated interfaces between the materials in Figure 7 based on the supplied data.

The dielectric interfaces between the materials from Figure 7.
Figure 8: The dielectric interfaces between the materials from Figure 7.

If we assume that the particular machine is either operating at a voltage where partial discharge can occur over a period of time (i.e., >6kV) or for a short period of time at values under 6kV. 

Alternatively, for the cutaway, if the motor is on an inverter or variable frequency drive, then PD can occur even at lower voltages due to fast rise-time voltages.  The stages of failure in these instances, excluding high potential between odd turns, crossovers, and small gaps or voids, are illustrated in Figure 9.

Failure progression due to aging and contamination over time.
Figure 9: Failure progression due to aging and contamination over time.

Figure 10 represents data at 200 Hz and 400 Hz as the Figure 9 failure progresses, assuming multiple turns in Phase A are involved in the degradation. This assumes no interaction with a rotor.

Four stages of relationships between phases assuming a wye connection and he stator from Figure 7 with multiple turns involved.
Figure 10: Four stages of relationships between phases assuming a wye connection and he stator from Figure 7 with multiple turns involved.

The low voltage and high frequency are important for sensitivity, as shown in Figure 11, where lower voltages and higher frequencies result in greater sensitivity to small changes in capacitance within the winding circuit.  For the purpose of this article, we are using one pico-Farad for demonstration.

Relationship of frequency impacts on capacitance using 10Vac injection.
Figure 11: Relationship of frequency impacts on capacitance using 10Vac injection.

The same type of relationship also occurs in form wound machines and related insulation degradation by any number of causes.  One of the considerations in insulation system failure, however, is when the wire is scratched or damaged in a machine.

Damaged insulation materials, such as scratched or scraped conductors, before energizing using 10Vac at 400 Hz with a gap between conductors in varnish.
Figure 12: Damaged insulation materials, such as scratched or scraped conductors, before energizing using 10Vac at 400 Hz with a gap between conductors in varnish.

If we introduce a rotor and assume the winding is concentric, such that it represents a small motor with a resulting phase balance, in a healthy machine at 400 Hz, we may see something as shown in Figure 13.

Relationship between phases of a good winding in two positions as measured at 400 Hz and 10Vac.
Figure 13: Relationship between phases of a good winding in two positions as measured at 400 Hz and 10Vac.

When we introduce the different faults from Figure 10, this relationship, which includes the rotor (Figure 13), can be trended over time, including comparative testing, as shown in Figure 14.

Faults progressing using 10Vac at 400 Hz based upon a small motor with a rotor and concentric winding.
Figure 14: Faults progressing using 10Vac at 400 Hz based upon a small motor with a rotor and concentric winding.

Low-voltage frequency-based testing provides a lighter instrument (portable) with a high degree of accuracy.  Over the decades, multiple test devices have been developed that perform this style of testing, with several recognized names, including the ALL-TEST Pro 7. 

We have also noted that many traditional high-voltage test devices have been resisting the adoption of these disruptive technologies.  However, there are selective times for using either or both technologies, such as the adoption shown with the Electrom D12 and related series.