Understanding Electric Motor Rewind

Electric motor rewind, also known as motor winding, is a specialized process performed in motor repair shops. This procedure involves replacing a motor stator’s copper windings to restore performance.

Motor rewind is not just about wrapping wires. It's a meticulous process involving the removal and replacement of the existing copper windings, which are integral to motor efficiency.

By conducting an electric motor rewind, you rebuild the motor to its optimal condition, ensuring longevity and consistent performance. Motor rewind is a cost-effective way to extend the life of the machine at a fraction of the price of a new electric motor.

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Now, we'll delve into the types of failures that three-phase stators can face and rewind procedures.

Types of Failures in Three-Phase Stators

Three-phase stators play a pivotal role in distributing the energy that the motor consumes. The symmetrical power distribution in stators means that they are often used in high-power applications.

However, with the rigorous demands placed on them, stator failure can occasionally occur due to various reasons, leading to a range of issues that affect the motor's performance. Here are some common types of failures in three-phase stators: 

1. Winding Insulation Failure:

• Ground Insulation Failure: When the insulation between the copper motor windings and the stator core breaks down, it can result in a ground fault. This can lead to short circuits and motor overheating. Grounding can occur either in the slot or at the edge of the slot.
• Phase-to-Phase Insulation Failure: Breakdown of insulation between different phases of the stator windings can cause phase-to-phase faults, resulting in unbalanced currents and motor instability. This type of insulation failure is typically caused by contaminants, abrasion, vibration, or voltage surge.

2. Shorted Turns:

• Turn-to-Turn Short: Short-circuits between adjacent turns of the same winding can occur due to insulation degradation or mechanical damage. This can lead to current imbalances and overheating. This type of insulation failure is typically caused by contaminants, abrasion, vibration, or voltage surge.
• Inter-Turn Short: Short-circuits between turns of different windings in the same phase can disrupt the motor's magnetic field and cause performance issues.  

3. Phase Imbalance:

• Imbalanced Winding Resistance: If there is a significant difference in resistance between the windings of the three phases, it can lead to uneven phase currents and reduced motor efficiency.
• Unbalanced Supply Voltage: Variations in the supply voltage between phases can also result in phase imbalance, affecting motor performance. Unequal voltages usually are caused by unbalanced loads on the power source, a poor connection at the motor terminal, or a high resistance contact (weak spring). Note: A one-percent voltage unbalance may result in a six- to ten-percent current unbalance.

4. Rotor Misalignment or Eccentricity:

• Rotor Eccentricity: If the rotor is not perfectly centered within the stator, it can cause air gap variations, leading to increased vibration, reduced efficiency, and potential mechanical failures. 

• Shaft Misalignment: Misalignment between the rotor shaft and the stator can cause excessive mechanical stress and increased wear on motor bearings.

5. Overheating:

• Overloading: Operating the motor at higher loads than its rated capacity can lead to overheating, insulation degradation, and premature failure. Note: Under-voltage and over-voltage (exceeding NEMA standards) will result in the same type of insulation deterioration. 

• Inadequate Cooling: Insufficient cooling due to blocked ventilation or cooling system malfunctions can also result in motor overheating. 

6. Vibration and Mechanical Stress:

• Rotor Imbalance: An imbalanced rotor can lead to increased vibrations, which can cause mechanical stress on the stator windings and bearings. 

• Mechanical Damage: Physical impacts or stresses on the stator, such as from shock or excessive external loads, can lead to winding damage and failures. 

7. Corrosion and Contamination:

• Moisture Ingress: Moisture entering the motor can corrode copper windings and insulation, leading to electrical breakdown. 

• Dirt and Contaminants: Dust, dirt, and contaminants can accumulate inside the motor, reducing insulation resistance and potentially causing short circuits. 

8. Aging and Wear:

• Insulation Aging: Over time, the insulation materials used in the stator windings can degrade, reducing their effectiveness and increasing the risk of failures. 

• Bearing Wear: Wear and tear on motor bearings can lead to increased axial and radial movement, potentially causing misalignment and damage to the stator. 

• Locked Rotor: Severe thermal deterioration of the insulation in all phases of the motor normally is caused by very high currents in the stator winding due to a locked rotor condition. It may also occur as a result of excessive starts or reversals.

Why Choose Electric Motor Rewind?  

The benefits of electric motor rewind include not just restoring motor efficiency but also cost savings. Instead of replacing the entire motor, copper winding replacement techniques can be employed to breathe new life into the machine. Motor restoration also promotes sustainability by reusing existing materials and reducing waste. 

It's essential to identify and rectify the causes of three-phase stator failures to prevent recurrent problems. 

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Here's an overview of the electric motor rewind process: 

1. Assessment: The first step in motor rewind is to assess the condition of the electric motor. This involves disassembling the motor and inspecting its various components, including the windings, stator, rotor, and bearings. The technician will also check for any signs of overheating, contamination, or damage. 
2. Disassembly: Once the assessment is complete, the motor is carefully disassembled. This involves removing the rotor, stator, and other components, taking care not to damage any parts during the process. 
3. Winding Removal: The old windings, which may have deteriorated or suffered damage, are removed from the stator. This is a delicate process to ensure that the stator's core remains intact and undamaged. 
4. Winding Replacement: After the old windings are removed, new copper or aluminum wire windings are carefully installed in the stator slots. The number of turns and wire gauge must match the motor's original specifications to ensure proper performance. 
5. Insulation: Insulation is a critical component in electric motor windings. Insulating materials such as varnish, tape, or epoxy are applied to insulate the new windings and prevent electrical shorts. The stator is dipped in a varnish and baked in an oven to cure, which provides a protective coating. Vacuum Pressure Impregnation (VPI) is a specialized process used to enhance the insulation and protection of the windings in electric motors, transformers, and other electrical equipment. 
6. Reassembly: Once the new copper windings are in place and properly insulated, the motor is reassembled, and all components (i.e. bearings) are carefully installed. 
7. Balancing and Testing: The motor is balanced to ensure that it runs smoothly and does not produce excessive vibrations. Electrical tests, including resistance, impedance, and insulation resistance tests, are performed to ensure the motor's electrical integrity. 
8. Painting and Finishing: After successful testing and balancing, the motor may be painted and finished to protect it from environmental factors and improve its appearance. 
9. Reinstallation: The motor is then ready to be reinstalled in its original application, and it should perform like a new motor if the rewind was done correctly. 

Motor rewind can be a cost-effective alternative to purchasing a new motor, especially for larger and custom units that cost more. Benefits of electric motor rewind include improved motor efficiency, equipment failure prevention, and reduced production downtime.  

Whether it's the deterioration of copper windings or other components, addressing the root causes of three-phase stator failures ensures a longer lifespan for the motor. Through motor restoration, businesses can also enjoy cost savings, better performance, and a more sustainable approach to machinery maintenance.  

Understanding the benefits of electric motor rewind and recognizing the common signs of stator malfunction can help in timely intervention and optimal motor performance. 

Contact Square One Electric Motors & Pumps to have rewind work performed by qualified professionals.