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What are the Cuts on the Rotor of Motor’s Armature?

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What is Purpose of the Notches or Grooves in the Armature of a Motor?

The grooves in the armature slots are used to balance the weight of the core, ensuring smoother rotation of the motor and avoiding sparking of the carbon brushes during motor operation.

The notches in the rotor don’t serve any electrical purpose; they are provided after a repair or previous rewind for dynamic balancing, ensuring perfect and balanced rotation of the motor armature. Some manufacturers use epoxy weights, while others remove material (e.g., cuts on the armature core) during assembly to balance the rotor.

Rotor balancing, also known as weight balancing, should be done prior to installation. Running an unbalanced motor with electrical noise will damage bearings and wear out the shaft due to vibration. This practice is more common in motors used for applications like CNCs, which require precision.

To address these issues, carvings are made during the factory process using special cutters to balance the motor. Another approach involves adding weight in the slots above the top sticks, which is more effective than removing core iron.

What is Rotor Balancing?

Rotor balancing is the process of adjusting the mass distribution of a motor’s rotor to ensure even weight distribution along its axis and across multiple planes. This process minimizes vibration and noise during operation, enhances the rotor’s stability, and prolongs the lifespan of bearings and other components.

Balancing is achieved by adding or removing material from specific points on the rotor to achieve both static and dynamic balance, resulting in smooth and efficient motor performance.

Types of Unbalance in the Motor’s Rotor

Static Unbalance:

Static unbalance happens when the rotor’s mass is uneven along its axis.
It causes the rotor to wobble when stationary.
Corrected by adjusting weight on the axis.

Couple Unbalance:

Couple unbalance occurs when there is a mass difference at both ends of the rotor.
It causes the rotor to tilt and produce a rocking motion.
Corrected by balancing weights at opposite ends.

Dynamic Unbalance:

Dynamic unbalance involves uneven mass distribution in multiple planes.
It leads to vibration during rotation.
Requires balancing weights in both position and magnitude.

Related Post: Difference Between Static Balancing and Dynamic Balancing

Reasons of Cuts in the Motor’s Armature

Rotor cuts in the armature, specifically for dynamic balancing after rewinding the motor’s armature slots, serve to ensure that the rotor is evenly balanced during rotation. The primary functions of these cuts include:

Reducing Imbalance: Rotor cuts help to correct any imbalances in the rotor assembly. Even slight imbalances can cause vibrations, noise, and wear on the bearings, leading to reduced efficiency and potential mechanical failure. By strategically removing material, the rotor can be brought into balance.
Reduce Vibration and Heat: Properly balanced rotors rotate more smoothly and with less vibration, which improves the overall stability and performance of the motor. This is especially important in high-speed applications where even minor imbalances can have significant effects because when a metal part vibrates, its temperature increases and generate excessive heat.
Reducing Noise: Imbalances in the rotor can cause noise during operation. Dynamic balancing helps to minimize this noise, leading to quieter motor operation.
Extending Bearing Life: Vibration caused by an unbalanced rotor can lead to heat and excessive wear on the motor bearings. By dynamically balancing the rotor, the stress on the bearings is reduced, thereby extending their lifespan and reducing maintenance costs.
Enhancing Motor Efficiency: A well-balanced rotor operates more efficiently, as less energy is lost to vibration and noise. This leads to improved performance and energy savings over the life of the motor.

The grooves in the iron core are sections of the rotor machined away to balance the armature using the single-plane method. Once the wire is wound around the armature, it will be out of balance, causing vibration when the armature spins. This produces excessive heat, causing wear and tear on the bearings and other parts.

Dynamic balancing is typically achieved by placing the rotor on a balancing machine, which measures the imbalance and indicates where material should be removed using a special cutting tool (or sometimes added) to achieve balance. Balancing cuts are made in precise locations based on these measurements to ensure the rotor is balanced both statically and dynamically.

Removing portions of the iron core slots is the most cost-effective method to achieve static and dynamic balance of the wound rotor. However, this method has a significant drawback: the loss of contact surface between the rotor core and the stator. Consequently, the magnetic flux flow weakens. To mitigate this disadvantage, more precise fabrication is needed to reduce invasiveness.

Motors found in grinders don’t have many places where material can be removed, so the laminations are what is worked with. Other motor designs, like squirrel-cage motors, have places on the end faces of the armature where holes can be drilled, and some are made with cast bumps that can be removed for balancing.

Placing cuts in the rotor should be the last resort for achieving rotor balancing. Precise fabrication prior to manufacturing and assembly should be done to achieve rotor balance. This includes the laminated core fabrication, rotor shaft manufacturing, winding assembly and termination, varnish, and electrical insulation film, etc. Unexpected noise and vibration may be generated due to internal air turbulence in the rotor plugs and cuts.