Motor windings are an integral part of electrical motors used in most mechanical or electrical devices. These are copper or aluminum wires wrapped around the cylindrical shaft that create a magnetic field to rotate the motor shaft. 

The resistance of motor windings is crucial for efficient operation, proper current flow, and overall motor reliability. Its measurement helps detect faults such as shorted turns, loose connections, or insulation issues. 

Accurate testing of winding resistance requires the right tools and expertise, as even small deviations can signal potential problems. So only a skilled team can identify hidden faults early, ensuring your motors run safely and reliably.

Technomax, with its skilled professionals and industry-standard testing practices, provides exactly this assurance. Their systematic approach to motor winding resistance testing helps industries detect potential faults early, prevent downtime, and extend the lifespan of critical equipment.

This article will guide you through the basics of motor winding resistance testing, explain why it’s important, and offer professional insights to help you make the right choices for maintaining your electrical motors.

What is Motor Winding Resistance?

Motor winding resistance is the resistance of copper wires or bars from one end to the other that wind a motor.  

If ‘I’ is the current passing through the coil, and ‘V’ is the applied voltage, the winding resistance ‘R’ (Ω, ohms) of a motor winding can be determined using Ohm’s Law:

R = V/I

For an electric motor to run efficiently, it must maintain a specific and predetermined winding resistance value. This value depends on the motor’s size, voltage rating, type of winding, temperature, and manufacturer's design.

With the advancement in motor technologies, the winding resistance value is decreasing. So the check for these low resistance values can be determined using a high-precision resistance meter. 

Motor winding resistance varies with the temperature, conductor material (copper or aluminum), wire size, winding length, the quality of connections, motor age and condition, and environmental factors such as moisture and dust.

The graph below, winding resistance increases with temperature, along with the other factors that influence it.

Fig.Variation of Winding Resistance with Temperature

Because wire resistance varies with temperature, performing temperature correction is essential to ensure that measurements accurately reflect the true resistance under standard or reference conditions.

Why Winding Resistance Is an Important Measurement?

Among all the tests conducted on motor windings, the winding resistance test is an important measurement as it detects problems that other tests cannot. These include:

1. Resistive connections, internally or externally 
2. Disconnected strands in multi-strand coils
3. Partial blowouts in random-wound multi-strand coils
4. Use of the wrong wire gauge in coils or feeder cables
5. Open or shorted coils
6. Hard shorts between turns or coils
7. Incorrect turn counts
8. Wrong coil or group connections

Signs Your Motor Windings May Need Testing 

The common signs that show your motor windings may need attention include:

1. Visual Signs: Smoke, burning smells, or visible damage are clear warning signals.
2. Overheating: A motor that becomes very hot may have winding or internal faults.
3. Unusual Noises: Grinding, buzzing, or humming can indicate mechanical or electrical problems.
4. Reduced Performance: Slower operation or lower output shows the motor isn’t running efficiently.
5. Frequent Tripping: Breakers or fuses that keep blowing suggest electrical issues.

Why Testing Motor Windings is Crucial?

Motor failure brings the industrial operations to a halt, time loss, missed deadlines, and unnecessary stress. A winding resistance test is like a health check for your motor. It helps to:

1. Spot early damage to the motor
2. Ensure safety from electrical hazards
3. Improve the performance and efficiency of the motor
4. Extend the motor life

How to Check Motor Winding Resistance?

Motor winding resistance is checked using a DC resistance (DCR) test. A precise ohmmeter measures the resistance between winding terminals to detect open or shorted coils, poor connections, and incorrect turn counts. 

Motor Winding Resistance Test

A winding resistance test measures the electrical resistance of a motor’s windings (coils) to check their condition. The test is usually done with a digital multimeter or a specialized winding resistance meter, and temperature correction is often applied because resistance changes with temperature.

The step-by-step guide on how to test motor windings demonstrates how our experts inspect and measure each motor to ensure safe, efficient, and reliable operation.

1. Visual Inspection

• Exterior Check: Look for cracks, dents, or damage.
• Wiring & Connections: Inspect for frayed wires, loose terminals, or corrosion.
• Discoloration: Burn marks may indicate overheating.
• Environment: Ensure the motor is clean, dry, and free of contaminants. 

2. Continuity Test

• Purpose: Confirms windings are intact.
• Method: Set the multimeter to continuity mode and test each winding end-to-end.
• Result: Beep or continuity reading = good; no signal = open circuit. 

3. Insulation Resistance Test

• Purpose: Checks insulation to prevent electrical leakage.
• Method: Use a megger, connecting one lead to the winding and the other to ground.
• Result: Typically >1 MΩ; lower values indicate moisture or insulation damage.
• Tip: Test each winding individually. 

4. Winding Resistance Test

• Purpose: Ensures all windings are balanced and functional.
• Method: Use a multimeter on low-ohm setting; measure U-V, V-W, W-U for 3-phase motors.
• Result: Values should be within 2-3% of each other; large differences indicate internal faults.

5. High-Pot Test (Dielectric Strength)

• Purpose: Checks if windings can withstand high voltage.
• Method: Connect the high-pot tester between winding and ground; gradually increase voltage.
• Result: Excessive leakage current = weak insulation.
• Safety: Use caution; follow manufacturer guidelines.

Maintaining Motor Windings

Motor windings can be maintained efficiently for better life and performance by:

• Testing Windings: Check resistance and detect faults early.
• Cleaning Motor: Remove dust and debris to prevent overheating.
• Monitoring Load: Operate within rated limits to avoid stress.
• Lubricating Parts: Follow guidelines to reduce wear.
• Keeping  Records: Track all inspections and maintenance activities.

3-phase Induction Motor Winding Resistance

3-phase motors are widely used in industries to drive machines like pumps, fans, conveyors, and compressors because they are efficient and reliable. Inside a 3-phase motor, there are three sets of windings (coils). These three windings work together to make the motor run smoothly. Checking the winding resistance of each phase is important because:

• It ensures that all three windings are balanced and working correctly.
• Detects problems like open coils, shorted turns, or poor connections that could cause motor overheating or failure.
• Helps maintain the motor’s efficiency, safety, and lifespan.

Since these three windings operate together, each one must be checked separately to confirm that the motor is in good condition and ready for reliable operation.

Tools Used for Motor Winding Resistance Test

There are specialised tools that provide direct measurement of winding resistance, helping detect open circuits, shorted coils, poor connections, and incorrect turn counts.

• Multimeter / Ohmmeter: Measures winding resistance directly by connecting to motor terminals; suitable for small motors or quick checks.
• Motor Winding Ohmmeter: High-precision instrument designed specifically for accurate measurement of motor and generator windings.
• Winding Resistance Test Meters / Sets (e.g., WRM-40, Megger MTO106, Raytech WR-50): Used for field or lab testing; often includes automatic temperature compensation for precise readings.

When to Consult a Professional?

Even with the right knowledge and tools, there are times when expert help is the smartest choice. 

Don’t hesitate to reach out if:

• Severe Electrical Faults Appear: Complex problems need advanced diagnostics that only trained technicians can provide.
• Rewinding Is Required:  Motor rewinding is precise work that should be done by experienced specialists.
• Safety Is at Risk: If you feel unsure or uncomfortable with electrical testing, stop immediately and call a professional.
• Your Motor Is Under Warranty:  Avoid DIY fixes that could void your warranty coverage.

Conduct Motor Winding Resistance Test with Technomax

Being the leading engineering company in UAE, Technomax perform motor winding resistance tests with precision and care. Our experts measure each winding, check for deviations, and identify early signs of potential issues.

We compare readings across all phases and account for temperature effects, giving you a clear picture of your motor’s health. This helps prevent faults, plan maintenance, and extend the life of your motors.

At Technomax, we go beyond motor winding resistance testing by providing comprehensive electrical inspections and advanced condition monitoring services. Our expertise ensures reliable performance, reduced downtime, and extended equipment life. Partner with Technomax today to safeguard your assets with trusted, industry-leading solutions.

Whether it’s a small, medium, or large motor, our testing keeps your equipment safe, efficient, and compliant.

Book your Motor Winding Resistance Test with Technomax today for a reliable, professional assessment!

FAQs

1. How much resistance should a motor winding have?

The resistance of a motor winding should always be a small, finite value — never zero. For small motors, it is usually in the range of a few ohms. For medium motors, it falls in the fraction of an ohm range. For large motors, it is typically in the milliohm range. Regardless of size, the winding resistances must be balanced across all phases within about 2–3%.

2. What is the winding resistance test?

A winding resistance test measures the electrical resistance of a motor’s windings (coils) to check their condition. 

3. What is the winding resistance of a 7.5 kW motor?

In general, 7.5 kW motors have low winding resistance, typically ranging from a few milliohms to a few ohms. For three-phase motors, all windings should be nearly equal, with deviations kept within 2–3%, ensuring balanced performance and safe operation.

4. How many ohms is a 3-phase motor?

A three-phase motor winding normally has 5 to 50 ohms, depending on the power and design of the motor.