AC induction motors are used everywhere in Canadian industry.

They run pumps, compressors, fans, conveyors, crushers, mills, process equipment, HVAC systems, water treatment equipment and production lines.

When one of these motors starts losing torque, vibrating, overheating, making noise or tripping protection, the problem is not always easy to find.

The fault could be in the stator winding, insulation system, bearings, rotor, power supply, driven load, coupling, alignment, controls or the process itself.

Rotor bar problems are often overlooked because they are less common than many other motor failures and may not show up during a basic inspection. The source JM Test article notes that rotor bar problems are infrequent in many applications, but when they occur, they are easy to miss and can waste a lot of diagnostic time.

A rotor bar test can help answer a practical maintenance question:

Is the AC rotor likely part of the problem, or should the team keep looking elsewhere?

What Are Rotor Bars?

In a squirrel-cage AC induction motor, rotor bars are conductive bars embedded in the rotor.

They are connected at each end by end rings. During motor operation, current is induced into the rotor by the rotating magnetic field from the stator.

That induced current helps create torque.

The source JM Test article describes rotor bars as bars in an AC rotor that carry current induced in the rotor by the stator.

When rotor bars or end-ring joints become cracked, fractured or highly resistive, the rotor may no longer produce torque evenly. That can create current imbalance inside the rotor, local heating, vibration, speed fluctuation and mechanical stress.

What Causes Rotor Bar Failures?

Rotor bar failures are often linked to thermal and mechanical stress.

Common contributors include:

• Frequent motor starts
• Starting under heavy load
• Excessive load conditions
• High inrush current
• Repeated acceleration cycles
• Heat cycling
• Rotor casting defects
• Porosity in die-cast rotors
• End-ring connection problems
• Rotor eccentricity
• Mechanical stress
• Vibration
• Overheating
• Poor operating conditions
• Process overloads

The original JM Test source says frequent starts, especially under full or excessive load, can draw very high current. That heat can expand the bars, fracture the joint between a rotor bar and end ring, worsen resistance at the fracture, and eventually lead to cracking.

In simple terms, every severe start stresses the rotor.

One hard start may not cause a failure. Repeated hard starts over time can contribute to fatigue, heating and cracking.

What Happens When a Rotor Bar Breaks?

A broken or cracked rotor bar changes how current flows through the rotor.

Instead of current flowing evenly through all bars, current may be pushed into neighbouring bars. That can create local hot spots and make the fault worse over time.

JM Test’s source article explains that with a resistive fracture, current is diverted to adjacent bars, increasing the current in those bars and causing localized hot spots. In severe cases, this can contribute to rotor warping or additional cracked bars.

A rotor bar fault can lead to:

• Reduced torque
• Uneven torque
• Torque pulsation
• Speed fluctuation
• Higher vibration
• Higher motor noise
• Rotor heating
• Local hot spots
• Rotor imbalance
• Bearing stress
• Poor acceleration
• Higher current draw
• More difficult starts
• Process instability
• Possible rotor-to-stator contact in severe cases

PdMA describes rotor health as the integrity of rotor bars, rotor laminations and end rings in squirrel-cage induction motors, and notes that rotor defects can influence other motor fault zones.

Why Broken Rotor Bars Are Easy to Miss

Rotor bar problems can be difficult to diagnose because the motor may still run.

A motor with a rotor fault may:

• Start normally under light load
• Run acceptably at some speeds
• Fail only under load
• Vibrate only at certain operating points
• Show symptoms similar to bearing or alignment problems
• Show symptoms similar to power quality or process issues
• Pass some basic electrical checks
• Fail intermittently

This is why teams can spend time replacing bearings, checking couplings, changing overload settings or inspecting the driven machine before the rotor is considered.

A rotor bar test helps narrow the search.

It does not replace the full motor investigation, but it can help decide whether the rotor deserves closer attention.

Secondary Effects of Broken Rotor Bars

Broken rotor bars can create more than one problem.

The source JM Test article lists several secondary effects, including sparking concerns in hazardous areas, higher current in healthy bars, rotor core damage from temperature, torque and speed oscillations, premature bearing wear, rotor bending and imbalance, and possible contact between lifted rotor bars and the stator winding.

In practical maintenance terms, a rotor bar issue can create symptoms across the whole machine:

Electrical Effects

• Uneven rotor current
• Distorted magnetic field
• Increased heating
• Changed current signature
• Possible arcing in severe cases

Mechanical Effects

• Torque pulsation
• Speed oscillation
• Increased vibration
• Bearing stress
• Rotor imbalance
• Possible rubbing

Operational Effects

• Poor starting
• Reduced torque
• Unstable process control
• Increased downtime risk
• More frequent trips
• Higher maintenance cost

A rotor fault may begin as an electrical problem and become a mechanical reliability problem.

How an AC Rotor Bar Test Works with the iTIG

The Electrom iTIG is a motor tester and winding analyzer used for high-voltage and low-voltage motor tests. Electrom describes the iTIG as a tester with more than 20 high-voltage and low-voltage tests, including high-frequency surge and partial discharge testing.

The source JM Test article focuses on the iTIG used with the RTR-03 Rotor Bar Clamp.

According to the JM Test article, the Rotor Bar Clamp is connected to one phase lead of a motor and to the front panel of the iTIG. With the motor running, the iTIG displays the sinusoidal power wave. A stable wave suggests the rotor is likely good, while peaks moving back and forth indicate a possible rotor bar issue.

Electrom’s test feature summary also lists a Rotor Influence Check, or RIC test, for broken rotor bars and eccentricity.

The value of this test is speed.

If the motor can run safely, the technician can collect useful rotor condition information without immediately dismantling the motor.

What the Waveform Tells You

The waveform is important because the rotor fault changes the current pattern as the damaged bar moves through the motor’s magnetic field.

In the source article, JM Test explains that if the waveform peaks move back and forth, a rotor bar issue has been detected. That peak movement is caused by current fluctuations as the damaged bar passes by the poles.

A stable waveform generally supports the conclusion that the rotor is not showing the tested rotor bar fault pattern.

A moving or unstable peak can indicate a rotor bar problem.

The waveform can also be captured at different moments in time for reporting, giving the maintenance team documented evidence rather than only a verbal opinion. The JM Test article notes that wave captures can be stored and included in reports.

Why This Is Different From a Basic Motor Check

A basic motor check may include:

• Visual inspection
• Nameplate review
• Winding resistance
• Insulation resistance
• Polarization index
• Current measurement
• Voltage measurement
• Bearing inspection
• Vibration measurement
• Thermal inspection

These tests are useful, but they may not directly confirm a cracked or broken rotor bar.

Rotor bar testing focuses on a specific fault area: the rotor.

The iTIG and related Electrom test platform can support multiple motor tests, including surge tests, insulation resistance, DC hipot, PI, DAR, winding resistance and rotor-related checks depending on model and configuration. Electrom’s test summary lists phase-to-phase surge comparison, partial discharge, DC hipot, insulation resistance, PI, DAR, winding resistance, impedance, inductance and RIC testing among available iTIG measurements.

This makes the tool useful when a shop or maintenance team wants one platform for several motor diagnostic checks.

AC Rotor Bar Testing vs Surge Testing

The original URL uses “AC rotor surge tester,” but the article is really about rotor bar testing using an iTIG motor tester and rotor bar clamp.

It is important to avoid confusing every motor surge test with rotor bar testing.

Surge Testing

Surge testing is generally used to evaluate winding insulation weaknesses and turn-to-turn insulation issues.

Electrom describes iTIG surge testing as using high-frequency pulse repetition rates to find insulation weaknesses.

Rotor Bar Testing

Rotor bar testing looks for rotor-related issues such as open, cracked or broken rotor bars.

The source JM Test article says the iTIG Rotor Bar Clamp setup monitors the motor’s sinusoidal power wave while the motor is running, and waveform peak movement can indicate a rotor bar issue.

These are related because they can be part of the same motor test platform, but they are not the same diagnostic question.

When to Suspect an AC Rotor Problem

Consider rotor bar testing when a motor shows symptoms such as:

• Loss of torque
• Hard starting
• Slow acceleration
• Unusual vibration
• Speed fluctuation
• Higher noise
• Overheating
• Repeated trips
• Increased current under load
• Poor performance under heavy load
• Frequent starts under load
• History of severe duty
• Unexplained bearing wear
• No clear stator or insulation fault
• Process instability linked to the motor

The source JM Test article summarizes the end result of rotor bar problems as possible torque loss, torque and speed fluctuation, vibration, higher noise, hot spots, overheating, arcing and rotor lamination damage.

A rotor test is especially useful when other checks are not giving a clear answer.

Industries Where Rotor Bar Testing Matters

AC induction motors are common across Canadian heavy industry.

Rotor bar testing can be useful in:

• Mining
• Oil and gas
• Pulp and paper
• Water and wastewater
• Manufacturing
• Food processing
• Utilities
• Power generation
• Cement and aggregates
• Marine facilities
• Grain handling
• HVAC plants
• Chemical processing
• Sawmills
• Material handling
• Compressor stations

The motors most worth testing are usually those where failure would create high downtime cost, safety risk, production loss or difficult replacement work.

Why Frequent Starts Matter

Motor starting is one of the highest-stress moments for an induction motor.

During start-up, current can be much higher than running current. If the motor starts under heavy load, starts repeatedly, or starts in harsh conditions, rotor heating and mechanical stress can increase.

The source article identifies frequent starts under full or excessive load as a common reason rotor bars fail because those starts draw very high current and generate heat.

Applications with frequent starts may include:

• Crushers
• Conveyors
• Pumps
• Compressors
• Fans with high inertia
• Mixers
• Mills
• Process equipment with jam conditions
• Equipment with poor control tuning
• Motors starting under excessive mechanical load

If a motor has a history of difficult starts, the rotor should not be ignored during troubleshooting.

Rotor Bar Faults and Vibration

A broken rotor bar can create torque pulsation.

Torque pulsation can show up as vibration, speed variation or mechanical stress on the driven system.

The source JM Test article says broken bars can cause torque and speed oscillations in the rotor, which can provoke premature bearing wear.

This is why rotor faults can sometimes be mistaken for:

• Misalignment
• Imbalance
• Bearing failure
• Soft foot
• Coupling problems
• Mechanical looseness
• Driven-load issues

Vibration analysis may help, but rotor electrical testing can give another angle on the same problem.

Rotor Bar Faults and Hazardous Areas

The source article notes that broken bars can cause sparking, which is a serious concern in hazardous areas.

That matters for facilities where flammable gases, vapours, dusts or combustible materials may be present.

Examples include:

• Refineries
• Chemical plants
• Oil and gas facilities
• Grain handling sites
• Wastewater facilities
• Paint or coating areas
• Certain mining operations
• Fuel handling areas

Any testing on a running motor in or near a hazardous location must follow the site’s hazardous-area classification, permit system, equipment limitations and safe work procedure.

A rotor bar test should never be treated as permission to keep a questionable motor running in a hazardous environment.

Running-Motor Test Considerations

The rotor bar clamp method described in the source article is used while the motor is running.

That means the test plan should consider:

• Whether the motor can run safely
• Whether operations has approved the test
• Whether the driven equipment is safe to operate
• Whether guards and covers remain in place
• Whether the test point is accessible
• Whether the area is classified or hazardous
• Whether the tester and accessories are suitable for the environment
• Whether the worker is qualified
• Whether arc-flash and shock hazards have been assessed
• Whether PPE and safe approach boundaries are defined
• Whether lockout is needed for any setup or connection work
• Whether the motor is on a VFD or soft starter
• Whether connected electronics must be protected for other test types

CCOHS notes that electrical safety rules in Canada are enforced by provincial, territorial and federal jurisdictions and that the Canadian Electrical Code is referenced across Canadian jurisdictions, with local amendments. CCOHS also notes that occupational health and safety legislation may include requirements for hazardous energy control and PPE.

Canadian Electrical Safety Context

Motor testing can involve energized equipment, rotating equipment and stored energy.

Canadian maintenance teams should treat this as qualified electrical work.

CSA Z462:24 reorganized requirements around establishing an electrically safe work condition and added that absence of voltage must be verified at each point of work. It also includes updates related to shock and arc-flash safety, qualified workers and electrical safety procedures.

Before testing, confirm:

• Site electrical safety procedure
• Motor control centre access rules
• Energized work limits
• Shock and arc-flash assessment
• Required PPE
• Qualified worker requirements
• Drive or starter isolation requirements
• Stored-energy discharge requirements
• Lockout requirements
• Area classification requirements
• Test equipment ratings
• Manufacturer instructions

The test instrument can help diagnose the motor, but it does not replace electrical safe work planning.

What Other Tests May Be Needed?

A rotor bar test is useful, but it should not be the only motor test.

A complete motor assessment may include:

• Visual inspection
• Nameplate review
• Operating history review
• Load history review
• Start count review
• Insulation resistance testing
• Polarization index testing
• Winding resistance testing
• Surge comparison testing
• Partial discharge testing where applicable
• Current signature analysis
• Vibration analysis
• Thermal imaging
• Bearing inspection
• Alignment check
• Air gap review
• Power quality review
• VFD or starter review
• Process load review

PdMA notes that rotor testing technologies such as inductance measurements and current analysis can help identify early changes in the rotor’s magnetic signature.

A single test can point the team in the right direction. Multiple tests give better confidence.

What the iTIG Can Test

The source JM Test article lists several tests associated with the Electrom iTIG, including:

• Megohm resistance tests
• Hipot tests
• Surge tests
• Rotor bar tests for open or cracked AC rotors
• Step voltage tests
• Polarization index tests
• Dielectric absorption tests
• Armature tests for DC rotors
• Form coil tests
• Transformer tests

Electrom’s current iTIG page describes the iTIG as a field or benchtop motor tester and winding analyzer with high-voltage and low-voltage test functions, reporting options and configurations for different budgets.

That makes the platform useful for motor shops, reliability teams and industrial maintenance groups that need more than a basic megohmmeter.

Reporting and Maintenance Records

Rotor bar testing is more useful when results are documented.

A useful report may include:

• Customer name
• Site or plant
• Motor asset number
• Motor horsepower or kW
• Voltage rating
• Motor speed
• Manufacturer
• Serial number
• Application
• Date of test
• Tester model
• Accessory used
• Test conditions
• Waveform capture
• Technician notes
• Observed symptoms
• Recommended next action

JM Test’s source article says the iTIG waveform can be captured at different rotor positions and stored for inclusion in reports.

Good records help compare motor condition over time and support maintenance decisions during shutdown planning.

When Renting a Rotor Test Instrument Makes Sense

A motor analyzer or surge tester may not be needed every week.

Rental may make sense when:

• The test is needed for one shutdown
• A critical motor is showing symptoms
• A maintenance team needs temporary diagnostic capability
• A motor shop needs extra capacity
• A reliability team wants to confirm a suspected rotor fault
• A project requires documented testing
• Buying a full motor analyzer is not justified
• Existing equipment is out for calibration or repair

JM Test Canada’s rental page states that the rental division includes instrument and controls, electrical, communications, gas detection, utility products and mechanical equipment, and that JM Test provides precision test, measurement and calibration solutions across Canada.

For the Canadian article, the availability claim should stay conditional. Confirm the exact Electrom iTIG model, Rotor Bar Clamp availability, rental terms, accessories, calibration documentation and shipping timeline before quoting.

What to Confirm Before Testing an AC Rotor

Before planning an AC rotor bar test, confirm:

• Motor type
• Motor voltage
• Motor horsepower or kW
• Motor speed
• Rotor type
• Whether the motor can run safely
• Whether the motor is loaded or unloaded
• Whether the motor is on a VFD
• Whether the motor is in a hazardous area
• Whether operations has approved the test
• Whether the technician is qualified
• Whether the correct iTIG model is available
• Whether the RTR-03 Rotor Bar Clamp is included
• Whether waveform reporting is required
• Whether other motor tests are also needed
• Whether the site requires permits or specific PPE
• Whether the test result will be used for repair planning, shutdown planning or failure analysis

Rotor testing should be planned around the motor, the process and the safety requirements.

Common Mistakes to Avoid

Assuming Rotor Bars Are Fine Because the Motor Still Runs

A motor can run with a rotor fault, especially at lighter load.

Replacing Bearings Before Checking Rotor Behaviour

Broken rotor bars can cause torque and speed oscillations that contribute to vibration and bearing wear.

Treating a Rotor Bar Test as the Whole Diagnosis

Rotor bar testing is one part of motor diagnostics. Stator, insulation, bearing, power supply and load conditions may still need review.

Ignoring Frequent Starts

Frequent starts under heavy load are a major rotor stress factor.

Testing Without Operations Approval

A running motor test can affect process equipment and may introduce safety risk if not controlled.

Ignoring Hazardous-Area Risk

Broken rotor bars can create sparking concerns, and test activity in hazardous areas must follow the site’s approved procedure.

Confusing Surge Testing with Rotor Bar Testing

Surge testing and rotor bar testing can be available on the same platform, but they answer different diagnostic questions.

Not Capturing a Report

Waveform captures and notes make it easier to support maintenance decisions later.

Practical Takeaway

AC rotor bar problems are not the most common motor fault, but they can be costly when missed.

A broken or cracked rotor bar can lead to torque loss, speed fluctuation, vibration, overheating, hot spots, noise, bearing stress and possible damage to other parts of the motor. The original JM Test article explains that damaged rotor bars can divert current to adjacent bars, create local hot spots, cause torque and speed oscillations, and contribute to rotor core or stator damage in severe cases.

The Electrom iTIG with the Rotor Bar Clamp provides a practical way to look for rotor bar problems on a motor that can run. The test monitors the sinusoidal power wave, and moving waveform peaks may indicate a rotor bar issue.

JM Test Systems Canada can support industrial maintenance and reliability teams with electrical test equipment, motor test equipment, rentals and calibration support where available. Confirm current Canadian availability, iTIG model, Rotor Bar Clamp accessory, reporting capability, rental terms and service details before booking.