Electric induction motors can power your business in many ways, from small fans to massive cranes. Regardless of size, most electric motors utilize the same essential components.
Housing – which protects the internal parts listed below
Stator – an electromagnetic cylinder wrapped with copper wire
Rotor – secondary electromagnetic cylinder that spins within the stator
Airgap – the predetermined space between the spinning rotor and stator
Shaft – allows the rotor to turn freely within the stator
Cooling system – common types include air passages, fans, or water jackets
Bearings – located at both ends of the shaft reduce friction and dissipate heat
Due to the rotational forces generated during operation, most electric induction motors produce some level of noise, usually described as a low-pitched hum. Any other noise or vibration could indicate an internal issue that needs correcting.
Electric Motor Noise Transfer Paths
To simplify the noise diagnosis process, it’s essential to understand which transfer path is being used during operation.
A Structure-borne noise path transmits sound from the stator mountings through the motor housing and frame as the stator spins. These vibration waves create noise and can eventually damage or weaken the motor housing.
An Airborne electric motor noise path occurs as vibrations travel through the motor frame or any other structural element. Once these waves reach the frame or resounding component, they are converted to the airborne sound waves you hear.
An Excitation motor noise path refers to the torque ripple created when couplings and gearboxes are energized or powered down. Torque ripple is a periodic increase or decrease in the output torque as the motor shaft rotates.
Determining the noise travel path can help diagnose the noise source and what corrective action to take.
Diagnosing Electric Motor Noise
To correct the noise problem, you must first determine whether the noise is mechanical, aerodynamic, or electromagnetic. Each type of noise and vibration can mean different things for your electric motor and your business.
Mechanical Motor Noise
Mechanical motor noise (and vibrations) are often associated with mechanical issues, including friction, misalignment, lack of lubrication, or loss of gear or bearing contact. The noise level typically depends on motor temperature, load, and rotational speeds.
The most common fixes for mechanical motor noise include:
- Repair, replace, or modify the ball bearings
- Install a seal or shield to protect bearings from contamination
- Adding chemical additives to reduce friction
- Installing a vibration absorption or isolation device
- Adjustments to the rotational speed of the rotor
- Dynamic rotor balancing
- Proper motor alignment
Aerodynamic Motor Noise
Unlike mechanical noise, most aerodynamic noise can be traced back to the venting, fans, or water-cooling system that dissipates heat build-up during operation. As the motor speed increases, so does the noise from the fan or cooling elements. This type of noise is unaffected by the motor load, as the noise is constant whether the motor is running under full, partial, or no load.
The most common solutions for aerodynamic noise include:
- Eliminating sharp edges to create smooth surfaces
- Minimize changes in the airflow direction
- Use gradual transitions in the cross-section of airflows
- Optimize the air gap between the stator and rotor
Electromagnetic Motor Noise
Electromagnetic noise and vibrations are the most prevalent disturbances in commonly used small and medium-sized electrical motors. This noise arises from motor vibrations within the audible range, caused by electromagnetic forces such as Maxwell stress.
Maxwell stress is the name of magnetic forces that arise from electromagnetic fields. It includes the repulsion and attraction forces between two magnets (stator and rotor), reluctant forces that can change magnetic field lines, and Laplace forces that impact electric current in external magnetic fields.
Here are the most common fixes to mitigate or manage electromagnetic noise:
- Avoid resonance between structural nodes and magnetic forces
- Reduce asymmetries so the tolerance is on
- Lamination roundness
- Magnet position/ magnetization
- Eccentricities
Electric Motor Noise and Vibration Analysis
Sound and vibration analysis should be essential to your preventative maintenance plan to maximize uptime while reducing unexpected repair costs and production shutdowns. Unfortunately, beyond a visual inspection, noise and vibration analysis requires specialized testing equipment and a certified technician to interpret the collected data.
As part of our commitment to keep you up and running, we’ve assembled a team of noise and vibration specialists (and the necessary testing equipment) to help troubleshoot, diagnose, and correct any electric motor noise issues you might have. You can call the Hi-Speed team with your electric motor noise questions at 800-713-0103 or send us an email.
Related articles
Here are some additional electric motor topics based on your interest in electric motor noise:
How to Protect Your Electric Motors with Vibration Analysis
Learn How to Reduce or Eliminate Electric Motor Downtime
Essential Guide to Getting Your Electric Motors Serviced and Repaired