Electric Motor Mechanics and Manufacturing Methods--Varnish Impregnation

New varnish resins are having an increasingly larger impact on the final designs of new varnish impregnation machines and systems. Not only are process times and temperatures changing, but likewise the size of the systems is also changing com pared with many older conventional systems now using heat-cured varnishes. This does not mean that a new trickle-varnishing system is necessarily better than a conventional conveyorized system. The following basic capital equipment goals still need to be met and justified when evaluating a new system:

_ Can the new system meet the production level required?

_ Can the new system be integrated into an existing line?

_ Can the cost of the new system be justified? Does the return on investment meet the company's requirements?

_ Are there any environmental issues that may determine the type of equipment that is purchased?

_ Will the machine produce a good product? Following are descriptions of a number of different varnish impregnation methods and machines, as well as a description of the key areas in specifying, operating, and maintaining these systems.

Applying varnish to motors, transformers, and armatures is not a new process; consequently, most of the various methods of applying varnish have been around for a long time. The most common methods include dip-and-bake, dip-and-spin, and trickle systems. Some of the parameters that determine what type of system should be used are as follows:

_ Type of varnish to be used-solvent based, water based, or low volatile organic compound (VOC)

_ Part size and weight

_ Quantity

_ Conveying and handling preferences

_ Available floor space

Smaller parts which are more likely to be processed in larger quantities may tend to be processed automatically on a continuously moving system--for example, armatures for hand power tools or vacuum cleaners.

Larger parts which are more difficult to handle and are typically manufactured in small lots or by the piece lend themselves to a more manual system--for example, large-frame industrial-duty motors.

Following are descriptions and examples of the various systems.

Dip-and-Bake Systems

The dip-and-bake system is probably one of the most common conventional methods of applying varnish. In this process, the complete stator and coil assembly is sub merged in a tank of varnish, either under normal atmosphere or under vacuum or pressure.

Various parts may be either preheated or at ambient temperature depending on the type of varnish being used. The size of the part and configuration of the windings will determine the amount of time it takes for the varnish to fill the voids in the slots and windings. Submerging the part for various time intervals, weighing the part, and electrically testing the part after baking will determine the proper dip time. One disadvantage of dipping parts in varnish is that removal of the varnish in unwanted areas is required after baking, thus adding to the cost of the finished assembly.

Conventional dip-and-bake systems using convection heat typically have dip times in the range of 5 to 30 min for larger parts and bake times of 2 to 4 h at 350°F (205°C). Times range from 30s to 5 min for smaller parts. Parts coated with water based varnishes will operate at lower temperatures.

Production lines processing over 200 parts per hour most often can justify and utilize a dip-and-bake system.

Dipping systems primarily use three different processing methods.

1. Indexing rack system. In the first method, a part is set on a rack or lowered by a hoist into the varnish. It’s then removed from the varnish, allowed to drain, and set into a chain-type conveyor system. Once the rack is loaded onto the chains, it’s conveyed through the oven and allowed to cure for the recommended time.

Transformers are often manufactured by this method.

2. Batch oven system. The second method of curing uses the same dipping system, but the coated rack of parts is placed into a tray oven or batch oven. Midsized stators are often processed by this method. Curing is normally completed all at one level. Stacking the racks with separators is required if multiple levels of parts are to be cured in the oven.

3. Continuous system. In the third method, the part is hung on an overhead conveyor which either is indexed on timed intervals or is continuously moving. The conveyor travels through a preheat zone, goes on through the dip tank into the bake oven, and returns back to the loading/unloading area. Quite often, cooling by forced ambient air or chilled air is incorporated into the system as the final step in the process. Parts are hung on the conveyor either by single wire hooks or on a multipart fixture. Multipart fixtures and drag-through dip systems are very common and can process the highest volume of parts per hour.

Dip-and-Spin Systems

The dip-and-spin method is typically used to apply varnish to motor stators. The stator is hung from a hook or is set on a pallet with the bore up, and is then submerged in varnish. After dipping or submerging in varnish, the part is allowed to drain. After a short drain time, the stator is spun about its bore axis to remove excess varnish. The type of varnish used is normally mixed at a low viscosity, allowing only a thin film of varnish to be left on the part after spinning. Corrosion resistance and bonding of the lamination are reasons to use this type of varnishing system.

Dip-and-spin systems are often utilized in the processing of hermetic motors or motors that see a wet environment, such as pool pumps. In such cases a varnish coating over the entire core and coil is crucial.

A palletized conveying system can also be used. The parts are loaded on a pallet that is pushed or conveyed through a continuous oven. The pallets serve a dual purpose of supporting the parts and containing spilled varnish.

The heating and conveying process for the dip-and-spin method may be the same as for the conventional dip system.

Trickle Systems

Trickle systems take their name from the method by which the varnish is applied.

Varnish is dispensed in controlled volumes and locations over an armature or stator.

The benefits of trickling varnish are as follows:

_ The ability to apply varnish where desired

_ Reduction of post-curing cleanup operations

_ The ability to fill the slots and coil with varnish

_ Reduction of the volume of varnish used

_ Reduction of the loss of varnish

_ Minimization of processing times

These goals are met by choosing the right type of heating method, handling method, and varnish for the part being processed.

Trickle systems typically include a load station for placing parts on a moving conveyor; a preheating zone, which ensures better absorption of varnish; a trickle station or stations; a holding setup zone; a postheating zone; and, in some cases, a cooling zone after the varnish has catalyzed. Finally, the parts are taken off the line and sent to the next process.

Each individual function of a trickle system can make use of different methods of completing each of these tasks.

Heating Methods:

_ Convection-moving high-velocity heated air over the part

_ Infrared-radiant heating by controlling the intensity of light supplied to the part

_ Resistance-attaching a resistance heater to the leads of the part and energizing the coil

_ Ultraviolet-curing a particular type of coating by UV energy Conveying Methods

_ Continuous belt or slats-used for large motors

_ Continuous chain with fixture-used for armatures and stators

_ Chain on edge with spindle-used for armatures

_ Turret type-used for armatures and small parts

_ Single-station or multistation spindle

Fixturing and Rotation. An important issue that has not been described earlier is the requirement that parts be held correctly and rotated.

Stators are most often held and conveyed by securing a fixture in the inner diameter or bore of the part. It’s important that minimal contact be made so that bonding of the fixture to the part does not occur. An expanding mandrel of some sort is usually developed.

Armatures are normally held up to the commutator by the shaft, assuming that the shaft is long enough. Common methods of securing the armature are by a drill chuck, a collet, or a clamping device.

Most conventional trickle systems incorporate part rotation in the preheating, varnish application, and postheating stages of the process. Rotation is considered important for uniform part heating and uniform distribution of varnish over the circumference of the part.

Some manufacturers have been successful in applying varnish to stators without rotating the part. In this approach, the part is placed in a fixture with the bore facing up. Varnish is applied from the top only and is controlled so that minimal or no dripping occurs out of the bottom.

Varnish Application Methods

Meter-Mix Pumping Systems. These are pumping systems that have two cylinders pumping a fixed volume of varnish with each stroke. The two separate volumes are forced and mixed into a single tube and out a single dispensing head. This dispensing head then directs the varnish to the correct location.

Provisions need to be made to prevent the contents of the two separate cylinders from mixing inside the pumping equipment. Curing of the varnish in the tubing, mix head, and occasionally the cylinder has most likely happened at least once to anyone who has used this type of pumping system. Most pumps are designed so that they can be broken down and cleaned if this happens.

Meter-mix pumping systems can be used with most varnish formulations that need controlled and accurate mixing. These systems are by far the most reliable method of dispensing accurate volumes of varnish. Consequently, the cost of such systems is higher than those used with other methods. A manufacturer is most likely to use this type of pumping system when mixing has to occur just prior to dispensing the varnish, such as with a quick-setting catalyzed varnish or when the process requires strict control.

Peristaltic Pumps. Many older systems that used solvent-based varnishes on armatures utilized this type of pumping system. Often there are multiple feeder lines that continuously pump the varnish as the armatures travel past the tubing. Occasionally, a system indexes or cycles a number of parts past a fixed number of trickle heads. By doing this, the size of the equipment and system is often minimized.

Since the solvent-based varnishes cure by heat, and heat is generally kept away from the trickle area, the resin that drips off is often recycled back to the main holding tank, remixed, and reused.

With the new resins which catalyze after dispensing and heating, intermittent dispensing of a controlled volume of varnish on an individual part is becoming more common. Dispensing times and volumes based on the part size are normally determined by testing. The actual varnish dispensing time is based on the volume of varnish needed to properly coat the part and the rate at which the part can accept the varnish with minimal dripping or runoff. Part temperature is critical with these new resins. If a part is too hot, the varnish often thins too much when it touches the part.

On longer core stacks, the varnish sometimes sets up in the slots and prevents a complete slot fill. If the temperature is too low, proper filling of the slots does not occur and the final varnish film will appear uneven.

Maintenance of peristaltic pumping systems requires periodic replacement of tubing. Since varnish is not exposed directly to the pumping components, the pumps remain relatively maintenance-free. Newer systems that dispense varnish to a single part at a number of precise locations require fewer pumps and pumping lines.

TBL. 18 Energy Costs of Using Three Varnish Resins

TBL. 19 Annual Energy Operating Costs with Three Varnish Resins City Dip and bake Trickle, 100:3 Trickle, 100:1

Process Times and Temperatures

Newer varnish formulations cure in minutes rather than hours. In the past, conventional varnishes required preheat temperatures of about 300°F (149°C) and bake temperatures of 350°F (177°C), whereas newer catalyzed varnishes have preheat temperatures in the range of 150 to 200°F (65 to 93°C) and bake temperatures just slightly higher. As a result of these lower cure temperatures, utility costs are also considerably reduced.

A generalized comparison of the energy cost for processing parts using a dip-and-bake resin and two available chemically cured trickle resins is shown in Tbl. 18 and 3.19. Identical 48-frame stator-coil assemblies were used in the comparison and some data were estimated. The data are not exact and are for comparison purposes only.

Control Systems

With the shorter process times seen with new low-VOC varnishes, process control information has become a more dominant issue. Most systems can make use of programmable logic controller (PLC) systems. PLCs can simplify the design by reducing the number of electrical components in the control cabinet. They are also now more often utilized to collect data and system operating information.

PLCs can be used to monitor the following components on a system.

_ Conveyor travel-speed and index times

_ Part detection through photo eyes or proximity switches

_ Auto/manual run modes

_ Varnish dispensing-start and stop

_ System operating conditions such as heating-system status or motor operation

_ Safety alarm circuits for operator intervention

Recipes can also be developed to incorporate many of the listed items for individual parts, so an operator can simply pick a part number to run a certain recipe.

Trickle heads can be integrated within a recipe to deposit varnish in correct locations. Linear actuators can be added for continuous varnish application from the trickle heads.

Strategically locating the control panel may enable all of these controls to be located in the same panel so that an operator can perform any necessary loading and unloading operations and still monitor the system.

Operational Maintenance Items

The operational maintenance of the system pertains to those items which need to be inspected by an operator or checked electrically to ensure that the system is operating within the control parameters.

These checks are not common to all of the previously mentioned systems.

Varnish Holding Tanks

_ Periodic cleaning for buildup of varnish

_ Heating and cooling systems

_ Circulation systems

_ Holding and fill tanks

_ Automatic or manual liquid levels Trickle Equipment

_ Tubing line replacement

_ Pump-speed settings

_ Trickle head location and setup

_ Trickle head volume and flow control (closeups)

_ Photo eye settings for automatic detection of parts

Preventive Maintenance

_ Care of conveyor systems

_ Oven systems maintenance: Fan bearings; Motors; Sheaves, belts, and drive units; Burner flame safety and spark ignitors

_ Maintenance of pumps varnish-dispensing

Conclusion

It’s evident that it takes years to learn all the tricks of the trade in operating and maintaining the various varnish impregnation systems.

Fortunately, the systems are evolving into systems that are smaller and less complicated. A thorough evaluation of the size of a part, the quantity per hour being produced, and the desired final quality.