Regular transformer testing helps identify internal problems before they reduce system performance or create unexpected outages.

A digital low resistance ohmmeter, often called a DLRO, is used to measure very low resistance values in transformer windings, tap changers, bus connections, motor windings, and other low-resistance electrical paths. These measurements can reveal issues such as overheating, fatigue, corrosion, loose internal connections, damaged coils, tap changer wear, and poor joints.

For Canadian utilities, industrial facilities, electrical contractors, mining operations, pulp and paper plants, oil and gas sites, manufacturing facilities, and maintenance teams, low-resistance testing is useful because transformer failures are expensive. A transformer outage can affect production, safety systems, utility supply, process continuity, and repair schedules.

The goal of DLRO testing is simple: identify abnormal resistance before it becomes an operational failure.

Why Transformer Winding Resistance Testing Matters

The DC resistance of a transformer winding can help indicate the internal condition of that winding.

The source JM Test article explains that when the resistance of a transformer winding at ambient temperature is compared with hot resistance, it can indicate the internal temperature of the winding. It also notes that resistance readings can be taken at one-minute intervals while a hot winding cools, allowing the resistance at time zero to be estimated when the data is charted.

This matters because abnormal resistance can indicate:

• Overheating in the coils
• Fatigue in the winding
• Corrosion in the internal coil
• Loose or degraded internal connections
• Tap changer wear
• Poor contact resistance
• Winding damage

A transformer may still operate with these problems developing inside it. The risk is that the issue remains hidden until load, heat, vibration, or system stress turns it into a failure.

What a DLRO Measures

A DLRO measures very small resistance values, often in micro-ohms or milli-ohms.

In transformer testing, these values matter because winding and connection resistances are usually extremely low. A normal handheld multimeter is not the right tool for this type of work. A DLRO uses controlled test current and four-terminal Kelvin measurement methods to reduce the effect of test lead resistance and contact resistance.

In plain language, the instrument pushes current through the winding or connection and measures the voltage drop across it. From that, it calculates resistance.

That resistance value can then be compared with:

• Previous test results
• Similar phases
• Similar windings
• Factory data
• Acceptance limits
• Engineering expectations
• Temperature-corrected reference values

The number itself is only half the story. The real diagnostic value comes from comparison and trend.

Transformer Tests Performed with a DLRO

Low-resistance testing can be applied to several transformer winding configurations.

The source article states that low-resistance testing on transformers can address small, medium, large single-phase, large polyphase, and autotransformer windings. It also lists several test configurations, including dual windings, wye-to-wye, wye-to-delta, and delta-to-delta windings.

Dual Winding Transformers

For dual windings, the test current may be connected through the windings in opposed polarities.

This helps technicians evaluate winding resistance and identify abnormal differences that could indicate internal problems.

Wye-to-Wye Windings

For wye-to-wye windings, testing may be performed with or without a neutral connection.

The source article notes that the leg of the other winding can be connected to the potential lead to measure voltage at the internal connection.

This can help evaluate internal connections that would otherwise be difficult to assess directly.

Wye-to-Delta Windings

For wye-to-delta windings, a jumper may be used to connect current from the wye winding to the delta winding.

The JM Test article notes that this test mode can reduce test time.

Delta-to-Delta Windings

For delta-to-delta windings, test time may be improved by connecting the current jumper to the primary and secondary of the same phase in opposed polarities.

This helps reduce test time and improves test workflow when dealing with larger or more complex transformer winding arrangements.

Tap Changer Testing

Tap changers are used to improve voltage regulation. The source article notes that taps may be adjusted daily, and that excessive wear or loosening caused by vibration can be identified with low-resistance measurements.

This is important because tap changer problems are a common source of transformer reliability issues.

DLRO testing can help detect:

• Worn tap contacts
• Loose connections
• High contact resistance
• Vibration-related loosening
• Uneven resistance between tap positions
• Abnormal resistance jumps
• Contact degradation over time

Secondary Tap Changers

Consecutive tests can be performed on secondary tap changers where shorting-style taps are used.

The source article explains that large transformers may have many tap positions, and testing time can be reduced because the test current does not need to be shut off between tests.

Primary Tap Changers

Primary taps, or open taps, are different.

The article states that tests on primary open taps must be performed individually, with the test current shut off between tests.

That difference matters operationally. If the wrong test sequence is used, the technician may waste time, create poor data, or stress the equipment unnecessarily.

Current Capacity and Winding Saturation

The DLRO must have enough current capacity to saturate the transformer windings.

The source article explains that testing time depends on available test current and that large transformers require special attention before testing. It also warns that insulation between windings can store energy, similar to the dielectric in a cable, and must be discharged before testing.

This is not a small safety detail. Transformers are inductive devices. They can store energy, and test procedures must account for safe discharge before leads are removed or the next test step begins.

Test Current Limits

The source article also states that test current should be limited to the magnetizing current, about 1% of full-load current, because lower test currents reduce stress in the magnetic core and large test currents can create large forces that may damage the core.

For the Canadian version, present this as a technical rule from the source article, not as a universal substitute for manufacturer instructions or engineering procedure. Test current should always be selected based on the transformer, test method, instrument capability, and qualified electrical procedure.

Three-Phase Transformer Testing Considerations

When testing three-phase transformers, interaction can occur between the primary and secondary windings.

The source JM Test article states that this interaction is especially evident when testing transformers with wye and delta windings. It can be minimized by connecting the test current to flow through both primary and secondary windings, reducing mutual coupling between the windings and minimizing circulating current in the delta winding.

This is the kind of issue that can produce misleading readings if the technician does not understand the winding configuration.

Transformer resistance testing is not just “clip on the leads and press start.” The winding arrangement, tap position, test current, temperature, and discharge procedure all affect the quality and safety of the test.

Temperature and Resistance Readings

Resistance changes with temperature.

A heated motor or transformer winding may show higher resistance than the same winding at ambient temperature. As the winding cools, the resistance value may drop toward a previous ambient reference value. The source article makes this point in the DC motor testing section, and the same principle is important for transformer resistance interpretation.

For accurate transformer trend analysis, teams should record:

• Winding temperature or oil temperature where applicable
• Ambient temperature
• Test current
• Tap position
• Phase tested
• Winding configuration
• Instrument used
• Date and technician
• Previous comparable results

Without temperature context, resistance comparisons can be misleading.

DC Motor Bar-to-Bar Testing

The source article also covers DC motor bar-to-bar testing, which uses helical spring point probes to measure rotor bar-to-bar resistance. It notes that this test is commonly performed at a 10 A current level, with typical coil resistance measurements in the 6000 micro-ohm range.

The purpose is to identify broken or loose welds and solder connections between coils and commutator bars.

What the Test Can Reveal

Bar-to-bar testing can detect:

• Broken solder joints
• Loose welds
• Open coils
• Weak commutator connections
• Inconsistent coil resistance
• Heat-related resistance changes

The source article explains that resistance measurements should remain consistent and that higher readings may occur on a heated motor because of coil temperature.

Lap Winding Testing

In a lap winding, the windings are connected to bars lying next to each other.

The article explains that to test this arrangement, the current probe is placed at the end of the commutator bar and the potential probe is placed at the connection to the winding, also called the riser on the commutator bar. The operator then measures resistance between each set of bars, such as 1–2, 2–3, 3–4, and so on.

Abnormal readings can indicate a weak solder joint or a coil break.

Wave Winding Testing

In a wave winding, the coil path moves through multiple commutator bars before looping back around the armature.

The source article gives an example where a coil runs from commutator bar 1 to 6 to 11 to 16 and then loops back to bar 2. Measuring between bars 1 and 2 checks the full wave-wound coil loop. A break between internal bars may appear when measuring the beginning and end bars of that loop.

This section is useful because it shows a broader principle: low-resistance testing is not only about transformers. It is also valuable for rotating machines, bus connections, joints, bonds, and other low-resistance electrical paths.

Megger DLRO10HD Features

The source article specifically discusses the Megger DLRO10HD 10 Amp Digital Low Resistance Ohmmeter.

JM Test lists several key features, including:

• High or low output power selection for condition diagnosis
• Rechargeable battery or line power operation
• Continuous operation, even with a dead battery
• 10 A for 60 seconds
• High input protection to 600 V
• Heavy-duty case rated IP65 with lid closed and IP54 operational on battery operation
• Rotary switch with five test modes, including auto start on connection

For Canadian customers, availability, accessories, calibration options, and rental terms should be confirmed directly through JM Test Systems Canada.

Canadian Safety and Testing Considerations

Transformer testing with a DLRO should only be performed by qualified personnel using the proper electrical safety procedure.

Canadian teams should consider:

• Lockout/tagout requirements
• Verification of de-energized condition
• Stored energy discharge
• Transformer winding discharge
• Proper test lead connection and removal sequence
• Arc flash and shock hazard assessment where applicable
• Site-specific switching procedures
• Manufacturer instructions
• Instrument rating and calibration status
• CSA Z462 workplace electrical safety guidance where applicable
• Applicable provincial, territorial, federal, and authority-having-jurisdiction requirements

CSA Z462 is Canada’s workplace electrical safety standard and provides guidance on safe work procedures, PPE selection, qualified electrical worker criteria, safety devices, and work involving energized electrical equipment. It is also intended to be used with the Canadian Electrical Code and applicable Canadian safety regulations.

Do not treat this article as a complete transformer test procedure. It is educational content. Actual transformer testing should follow the equipment manufacturer’s instructions, site procedures, utility or facility standards, and qualified electrical engineering guidance.

Practical Takeaway

A digital low resistance ohmmeter is a valuable tool for transformer and motor diagnostics because it can detect abnormal resistance in windings, tap changers, internal connections, commutator bars, solder joints, and low-resistance electrical paths.

For transformers, DLRO testing can help detect overheating, corrosion, fatigue, loose internal connections, tap changer wear, and winding issues before they become outages. For DC motors, bar-to-bar testing can reveal weak solder joints, loose welds, open coils, and inconsistent winding resistance.

The test is simple in concept but not casual in execution. Transformer configuration, temperature, tap position, winding saturation, test current, stored energy, and discharge procedures all matter.

JM Test Systems Canada can support teams with DLRO rentals, Megger DLRO10HD equipment, transformer test equipment, electrical test equipment, and calibration services.