By Chase Fell
Vice President of Engineering
Advances in ultrasonic technology have brought new opportunities for efficiency and productivity in repair and service for industrial electric motor and generators. Ultrasonic cleaning can make for a faster and better repair job with potential improvements in product quality and safety for shop employees. Proper application of ultrasonic cleaning can save costs for industrial users of electric machines with a faster turnaround and a potentially longer lasting repair when compared to jobs using conventional cleaning techniques. Applications for the repair service center include DC armatures, wound rotors, synchronous rotors, motor parts and heat exchangers.
ULTRASONIC CLEANING: HOW IT WORKS
The term ultrasonic is defined as “having a frequency above the audibility limit of the human ear.” In general, the range of frequency in healthy human hearing is 20hz – to 20khz. Energy for ultrasonic cleaning is the result of rapid changes of pressure in the cleaning liquid. These changes lead to the formation and collapse of bubbles in the liquid or cavitation. When the bubbles collapse a shock wave is created that effects a tiny explosion. These waves accelerate the action of the cleaning detergent and serve to blast away contaminants attached to the object being cleaned. (See Figure 1.)
The wave energy in the liquid is created by an ultrasonic emitter or transducer that transforms AC energy into ultrasound. Similar technology is used in medical imaging and parking sensors in modern trucks and automobiles. Alternating electrical current is converted into sound waves at the resonant frequency of transducers affixed to the bottom or sides of the cleaning tank.
Piezoelectric transducers are the most common energy device used in ultrasonic cleaning systems. These transducers convert mechanical pressure to electric waves. Piezoelectric crystals change size and shape when pressure is applied; AC voltage makes them oscillate at the same frequency and produce ultrasonic waves. When a voltage is applied across the ceramic through the electrodes, the ceramic expands or contracts (depending on polarity) due to changes in its lattice structure. This physical displacement causes a sound wave to propagate into the cleaning solution.
Magnetostrictive transducers are also used as energy emitters in ultrasonic cleaning equipment. These devices consist of laminations of a ferromagnetic material which are bonded to the cleaning tank containing liquid to be ultrasonically activated. An electric coil produces an oscillating magnetic field which causes the ferromagnetic laminates to vibrate at their resonant frequency. These transducers look very similar to an iron core transformer and perform in much the same way. The length of the lamination stack is chosen so that the core is resonant at the desired ultrasonic frequency. This type of emitter is used in industrial applications and generally has a lower operating frequency when compared to the piezoelectric device.
The amount of energy (watts per gallon) required for ultrasonic cleaning depends on the frequency of the waves, the size of the tank and the type of parts to be cleaned. The level of energy required diminishes as the size of the tank is increased. Larger tanks generally require less watts/gallon because there is less reflection of the ultrasonic waves and the energy flows more efficiently. The range of energy required is from 5-200 watts per gallon.
Very tiny bubbles form and grow due to alternating positive and negative pressure waves created by the transducers against the tank. Bubbles grow until they reach resonant size and then they explode. The temperature inside the bubble is very high and when the bubble bursts near a hard surface, the bubble creates a fast-moving jet action. With the combination of pressure, temperature, and velocity, the jet action breaks up adhesion of contaminates stuck to the part. Because of the tiny size of the jet and the relatively large energy, ultrasonic cleaning can reach into small crevices and remove entrapped soils very effectively. The lower the frequency, longer the time to rupture, the larger the bubbles and the greater the intensity of the explosion. (See Figure 2.)
Higher frequency ultrasonic gives shorter rupture times, smaller bubbles and implosion energy is lower. The smaller the bubble, the less chance of damage to the substrate and the greater the ability of the system to successfully clean hard-to-reach areas of the part. Air trapped in the system compromises the cleaning action. Frequency choice depends on the nature of the parts and the contaminants.
Remove heavy debris from the part with steam cleaning, brushing or other approved method. Verify that the cleaning tank is clean and free of debris and contaminants. Detergents used in ultrasonic cleaning for motor repair shops should be non-ionic and slightly alkaline with a ph slightly greater than 7. Follow the manufacturer’s recommendations for mixing of the cleaning agent. Fill the ultrasonic tank with the cleaning solution to the level recommended by the manufacturer, preferably to a level sufficient for covering the part completely with the detergent liquid. Heat ultrasonic bath to the specified temperature, usually in the range of 120º-180ºF. Degas the solution for approximately 10 minutes or longer until bubbling has stopped. Immerse the part into the ultrasonic bath. Close the tank lid if possible. Cleaning time will vary based on geometry of part, the contamination and the adhesion. Oil skimming may be required. After the cleaning cycle, remove part for ultrasonic bath and rinse. Bake to dry as required.
SYSTEM DESIGN CONSIDERATIONS
Small tanks utilize stainless steel baskets for immersion of the parts for cleaning. For cleaning armatures, rotors and other large parts, tank design should include a stainless work rest at the bottom of the tank. With this feature part damage is mitigated and the bottom of the tank is protected from puncture and abrasion. (See Figure 3.)
Due to the complexity of the parts to cleaned in the service center consider adjustable power features, pulse mode power and a degassing feature. Tanks with shaft wells can maximize cleaning efficiency for rotors and armatures. Automatic oil skimmer features can save time and can help keep the system functioning properly.
The service life of the detergent is a major consideration when evaluating the cost of maintenance in an ultrasonic system. Detergent should be on a regular maintenance schedule and for best results some applications may require fresh detergent for every application. Detergent costs are in the range of $1 per gallon of mixed solution. So changing detergent in a 500-gallon tank would cost about $500 plus the cost of the water and labor. The surfaces of the tank should be cleaned, and the filters serviced. The generator should be cleaned and checked as well as the pump and associated lines and electrical connections.
Armatures with low megger readings may be contaminated with carbon beneath the risers in an area where steam cleaning is ineffective. The same is true for synchronous rotors with low megger readings. Brush holders and other intricate parts can be cleaned completely and quickly. A heat exchanger with clogged passages can be processed in the ultrasonic system and returned to service quickly when conventional cleaning methods are not effective.