Can Light Bulbs Be Used to Test Electric Machine Health?

​
As electrified powertrains become more common, accurate testing of electric machines (motors and generators) is becoming a critical part of the diagnostic process. With the high cost of electric drive units and transmissions, determining the condition—or state of health (SOH)—of these components requires reliable and repeatable data.
Despite this, simplified methods continue to circulate in the aftermarket, including the use of light bulbs to evaluate electric machine condition. While this approach may appear practical, it does not provide the level of insight required for accurate diagnostics.

Myth vs. Fact

MYTH
Light bulbs can effectively be used to determine the conditions of EM SOH instead of buying “all of that expensive equipment” by merely observing the brightness and pulsing of light bulbs on each Phase while the EM is being rotated.

FACT
The SOH or condition of an EM cannot be determined merely by using light bulbs. The light bulb brightness and strength of pulse (while rotating the Rotor) is an extremely subjective test measurement that does not provide measurable test data that is based in a Recommended Practice or Standards. No such test can be found in any of the organizations that are responsible for developing contemporary Best Practices or Standards, and no Standards organization would ever consider it a viable test to determine SOH.

Why Light Bulb Testing Falls Short
Light bulb testing relies on visual interpretation—specifically brightness and pulsing—to draw conclusions about machine condition. The issue is not that the method produces no response, but that the response cannot be quantified or validated.
Without measurable data, there is no way to:

• Compare results across tests
• Identify trends or degradation
• Confirm accuracy against known standards

This turns what appears to be a diagnostic process into a subjective observation.

What Light Bulb Testing Can Actually Do
In very limited scenarios, light bulb testing may indicate the presence of a basic electrical condition. For example, when testing a permanent magnet machine with all three phases connected in series, a non-pulsing bulb may indicate an open circuit.
However, even this use case is highly dependent on configuration. If phase coils are connected in parallel, the system may still produce a pulsing light even when a fault exists. Additionally, this method provides no useful information for induction machines or separately excited rotor systems.
This reinforces an important distinction: detecting a simple fault is not the same as determining state of health.

What Proper Electric Machine Testing Requires
Accurate electric machine testing is based on measurable, multi-variable data. Determining SOH requires evaluating how the machine performs across multiple electrical and magnetic characteristics.
Key measurements typically include:

• DC resistance
• Inductance
• Impedance
• Capacitance
• Phase angle
• Current-to-frequency relationship
• Dissipation factor
• Insulation resistance

These metrics must be analyzed together to provide a complete picture of machine condition. No single measurement—and certainly no visual indicator—can provide this level of insight on its own.

Standards and Best Practices
Electric machine testing is guided by established standards and recommended practices developed by industry organizations. These include:

• IEEE 1415
• IEEE 43
• EASA 100

These standards define testing methodologies that are validated, repeatable, and grounded in engineering principles. They are developed and reviewed by subject matter experts and are designed to ensure consistent and accurate results across applications.
The absence of light bulb testing from these standards is not an oversight—it reflects the method’s lack of diagnostic validity.

Why This Myth Persists
The appeal of light bulb testing is easy to understand. It is inexpensive, simple to perform, and does not require specialized equipment.
However, this simplicity comes at the cost of accuracy.
In environments where technical training or access to proper tools is limited, these types of methods can gain traction. Over time, they become accepted as “good enough,” even when they fail to provide reliable results.
In professional settings, however, decisions must be based on data—not assumptions.

The Cost of Inaccurate Testing
Using incomplete or unreliable methods to evaluate electric machines can lead to a range of issues, including:

• Misdiagnosis
• Unnecessary component replacement
• Increased labor time
• Reduced profitability
• Customer dissatisfaction

Given the cost of electric drive systems, even a single incorrect diagnosis can have significant financial and reputational impact.

Final Takeaway
Electric machine state of health cannot be determined using subjective methods such as light bulb testing.
Accurate diagnostics require measurable, multi-variable data obtained through validated testing methods aligned with established standards.
Reliable data leads to informed decisions, confident repairs, and better outcomes for both technicians and customers.

Contact Us
If you would like to review Electric Machine testing or test equipment, contact us at:

📩 [email protected]

We welcome professional Electric Machine collaboration.

References

• IEEE 1415
• IEEE 43
• EASA – 100

Disclaimer
This content is provided for general informational purposes only. It is based on publicly available data, standards, and published sources available at the time of release. It does not constitute advice of any kind. Information is provided as-is, without warranties, and no liability is assumed for actions taken based on this content.

---