TRICKLE IMPREGNATION OF ELECTRIC MOTORS & GENERATORS This paper is intended to give a practical overview of trickle impregnation, with particular reference to the stators and rotors of medium and large frame motors and generators (typically up to NEMA 680­ frame). It is aimed specifically at those who have little or no experience of the process. Following a brief description of a typical impregnation cycle using a Newtech machine, the benefits of the process, compared with traditional impregnation techniques, will be discussed. The paper is written from the perspective of an equipment supplier, with examples and observations based on the experiences of our customers. What is trickle impregnation? Trickle impregnation is a process for impregnating the windings of stators and rotors with insulating resin. It is characterised by the fact that the resin is trickled in a continuous stream and at a controlled rate directly onto the windings, usually without it being applied to the iron core. Why impregnate electrical windings at all? Copper winding wire is usually coated with a tough, electrically­insulating enamel during its manufacture, so that coils wound onto soft iron cores can be made up from many turns of wire in order to maximise the magnetic flux generated when a current flows in the coil. In operation, the coils of motors and generators are subjected to high transient voltages, high mechanical forces due to torque reaction and also to vibration, due to the alternating current, which increases in force as load increases. Coils are impregnated for the following reasons:
a) To give to the windings a degree of mechanical strength, particularly in the unsupported end windings and where the coils enter the slots of the stator, or rotor, in order to resist the forces due to torque reaction; b) To prevent wires from rubbing against each other due to vibration caused by the changing magnetic fields, which would eventually wear the insulating coating, causing a short circuit; c) To displace air (which is a thermal barrier) from inside the windings and replace it with resin, in order to improve conduction of heat from the centre of the windings to the iron core, or to the outside air around the end windings; d) To provide a degree of environmental protection against the ingress of moisture, impact by dust and particles of debris, etc.; e) To improve the electrical insulation of the windings, between coils at different potentials and between the windings and earth (i.e. the motor frame). How widely applicable is the trickle impregnation process? Trickle impregnation is applicable to most stators and wound rotors, from micromotors and F.H.P motors to large frame motors and generators, with the possible exception of high voltage motors (e.g., traction motors) which have end windings so heavily wrapped in insulating tape / glass banding that it is difficult for the resin to penetrate and enter the windings. Trickle resins The most widely used trickle impregnation resins are heat­curing, two­part unsaturated polyester or polyesterimide resins. The resin is dissolved in a monomer such as styrene or vinyl toluene, which gives flowability and which becomes chemically combined in the resin by the process of polymerisation. Epoxy resins are also suitable for trickle impregnation.
On Newtech Trickle Impregnation Machines, if the two parts are mixed in a narrow ratio, typically from 1:1 to 5:1, the two components are pumped separately until immediately prior to the point of application, where they are mixed using a static mixing nozzle. Mixing immediately prior to application generally increases the likelihood of the resin behaving in a more predictable manner. It also reduces the ‘house­keeping’ procedures which must be applied when using a pre­mixed resin. If the resin is mixed in a wide ratio, such as 100 or 50 parts resin to 1 part catalyst, the resin is usually pre­mixed prior to dispensing and treated as a single part resin. Unmixed resin can typically be stored for 3 to 6 months under the correct conditions; mixed resin typically has a pot life of 3 to 30 days at 20°C, less at higher temperatures (dependent on the formulation).
Single­part resins are available which behave in a similar way to two­part resins but are more stable at lower temperatures with a more precisely defined 'trigger' temperature. The stability of these resins at lower temperatures, with pot lives of typically 6 months, offers benefits in terms of simplicity of the resin dispensing system, as well as minimising the ‘house­keeping’ procedures required.
A number of low temperature curing resins are also available. Generally these are triggered by mixing a catalyst with the resin during dispensing. The types of low temperature curing resins of which we are currently aware use a mix ratio of, typically, 1 part catalyst to 100 parts resin; once mixed, the resin cures within a relatively short time, typically 5 to 30 minutes. Low temperature curing resins may be suitable for small products, but, in our opinion, are less suitable for large products which take a long time to trickle and must be processed individually for the best results. The mix ratio is too critical to allow for inaccuracies and there is a risk of the resin gelling prematurely before the windings are fully impregnated. 3. Typical machine cycle for the trickle impregnation of a medium or large frame stator or wound rotor on a Newtech machine Newtech has developed a range of machines for the trickle impregnation of stators and wound rotors for motors and generators up to NEMA 680­frame; 8­, 6­ and 4­ spindle machines, for processing smaller stators in high volume, a twin­spindle machine, capable of processing two stators up to NEMA 400­ frame simultaneously, a range of single­spindle machines for processing stators up to NEMA 590­ frame, and a large single­spindle machine for processing stators and shaft­mounted wound rotors up to NEMA 680­frame. Load Stator A stator, or rotor, is held on the machine with its axis horizontal; stators are held in the bore, either on a hydraulically actuated expanding, three­jaw mandrel, or a manually actuated three­jaw chuck; shaft­ mounted rotors are held between centres.
The machine can be loaded using an overhead crane, by slinging stators in the bore using a C­hook. Alternatively, a range of manually operated or powered transfer units are available, which are particularly suitable for flowlines in which stators are handled on pallets. Newtech are able to provide a range of loading systems, which can be tailored to suit a particular customer’s requirements. Connect coils to heating current The windings are connected to an intelligent, fully controlled power supply through a terminal block on the spindle and heating current is fed via sliprings mounted behind the faceplate. The number of terminals (and sliprings) depend on the different types of windings to be processed. Winding temperature is measured continuously throughout the cycle by a non­contacting infra­red pyrometer which controls heating current during the various stages of the process. The windings are heated in process by passing a controlled direct current through them, relying on the resistance to provide self­heating of the wire according to the I²R relationship. Current level is calculated by the machine at the start of the cycle and is determined uniquely for each product based on its winding specification. Direct current (DC) is used to avoid the problems which the reactance of the windings would cause if alternating current (AC) was used ­ namely very poor power factors, very
high voltages, and hardware that would need to be vastly over­rated for the real heating power supplied to the windings. Furthermore, because the power system settings are determined uniquely for each winding, heating is done at the optimum power factor. The method of heating the windings has a major effect on impregnation quality. Resistance heating is quickest and most efficient and results in reasonably even temperature distribution throughout the winding. Oven curing is very inefficient for many reasons (outlined in section 4). Direct radiant, or convection heating (e.g. hot air blowers) is also inefficient for large products, resulting in uneven temperature distribution with most of the heat energy absorbed by the iron. Hot air blowers also drive off the more volatile constituents from the surface of the resin resulting in reduced bond strength without properly curing the resin in the centre of the winding. Windings of some three­phase stators and most wound rotors are connected internally in either a star, or a delta configuration. If the star point were accessible, windings in star could be treated as three separate phase windings connected in parallel; however, the star point is usually not accessible. Consequently, it is not possible to achieve a balanced heating effect purely by using DC current. Furthermore, it is usually not feasible to use AC current for the reasons given earlier in this section. Newtech has developed a method of achieving a balanced heating effect in internally connected three­ phase windings by switching DC current around the three phases on a timed basis using a thyristor switching circuit. Set process parameters The operator enters a small amount of product information via a keypad, or recalls the information from a database (i.e. weight of copper, resistance, resin volume and winding type). The Newtech approach is to use computer control to as great an extent as possible, to make as many process variables as possible adjustable through software (within limits, where appropriate) and to interlock all machine functions through the controller. This has the benefits of simplifying the operator's console whilst maintaining total flexibility. Also, by providing a display which prompts the operator to enter product data, and continuously informs about machine status with messages in English (or the local language), machine operation has been made simple and fool­proof. As well as having an internal database, which can store the processing information for many different products, and which can be recalled by entering the product number via a keypad, the machine can also be connected to an external database, with virtually unlimited storage capacity, and to which many machines can be connected. Such systems must be developed in conjunction with a customers particular information system, to ensure compatibility. Resistance test Newtech has developed a system for measuring winding resistance with the windings connected for processing. Resistance is measured at the start of the cycle, prior to heating, to check that the correct winding data has been entered into the controller and that the windings have been connected correctly and are not damaged. If the resistance differs from the expected value by more than the allowable tolerance, a warning message is displayed to inform the operator and the cycle aborted.
Rotate component Spindle speed is programmable between approx. 5 and 30 rpm. There is a reversing facility which allows the spindle to oscillate during the trickle phase, on a timed basis (pre­set, typically between 2 and 5 minutes between reversals), in order to achieve an even distribution of resin; the spindle runs in one direction at all other times during the cycle to help retain the resin. Incline component
The product is inclined at a slight angle (programmable typically up to 14°) to aid penetration of the resin into the windings. Raise to trickle temperature The winding temperature may be raised above ambient to, say, 50 ­ 70°C (120 ­ 160°F) during trickling to reduce viscosity of the resin to help it flow into the windings, and to ensure it is applied under consistent, repeatable conditions. Trickle resin The resin is pumped by peristaltic pumps, driven by a geared DC servo motor, which allows precise control of resin rate and total volume. Resin is delivered to the spindle via semi­rigid P.T.F.E pipe where, in the case of a 50:50 ratio resin, it is passed through a proprietary, static mixer nozzle immediately prior to application. Resin flows into the windings due to the effects of gravity and capillary action. Usually, there are four nozzles to deliver resin onto the non­lead end winding (the uppermost winding when the product is inclined), two onto the outside and two inside. There is also an auxiliary nozzle, which can be sited to deliver resin to the outside of the lead end overhang, to wet out the tapes securing the cables to the end winding. The auxiliary nozzle is opened automatically at a specific point in an automatic cycle (typically after 80 ­ 90% of the resin has been dispensed).
Return to horizontal Once the windings are full of resin, the stator or rotor is returned to the horizontal and rotation continues.
Raise to gel temperature The mixed resin reacts very slowly at room temperature; polymerisation is triggered by raising the temperature. Care must be taken not to promote too fast a reaction, as it is exothermic (i.e. releases heat) in which case the temperature may rise uncontrollably; if this happens, large amounts of vapour may be given off due to boiling, and the resultant resin matrix may be seriously weakened by porosity and consequent cracking. Polymerisation is usually done in two controlled steps. The first stage of polymerisation is 'gelation', when the resin starts to cross­link at many sites throughout the windings and is marked by a rapid increase in viscosity. Gelation temperature is typically between 85° and 120° C (185° and 250°F) and is held for between one and fifteen minutes, dependent on the resin and size and type of winding.
Some single­part resins do not need to be held at an intermediate gel temperature, and can be heated straight up to cure temperature.
Raise to cure temperature The final stage is 'curing', when polymerisation is completed. Curing temperature is typically between 120° and 150°C (250° and 300°F) and is held for between one and fifteen minutes. Curing ensures development of a fully cross­linked resin matrix, giving maximum bond strength particularly at elevated temperatures. Generally, once polymerisation of heat­curing resins is initiated, it must be taken to completion to achieve maximum physical properties. It is far better with large, high value products to gel and cure the resin gently at lower temperatures and for longer times, rather than risk damaging, or weakening it by processing it more quickly at higher temperatures, which would also result in greater emissions of V.O.C’s. Diagram (1) shows typical process temperatures throughout the impregnation cycle of a large stator using a heat­curing UP resin.
Diagram 1 ­ typical heating cycle for a heat­curing U.P. resin Resistance test Resistance is measured at the end of cycle (corrected for temperature) and compared with the original value, to check whether the windings have sustained any damage during the cycle; if so, a warning message is displayed.
Disconnect and remove product The spindle stops in a controlled orientation, with one of the jaws in the 12 o'clock position and the electrical terminals adjacent to the operator's console, to facilitate loading and unloading. The stator or rotor is available for assembly very soon after impregnation. 4. The benefits of trickle impregnation compared with traditional impregnation techniques Excellent product quality The trickle impregnation process results in very high slot fill (typically greater than 95% of the air in the windings is displaced by resin) resulting in improved thermal conductivity and mechanical strength. Consequently, the size of the motor may be reduced for a given output and the life of the motor may be extended. Once determined, the optimum process conditions can be repeated for consistently high product quality. The dip & bake and VPI processes result in variable, often poor impregnation quality (i.e. low slot fill) because a significant proportion of the varnish is lost due to drainage, both before the component is put into the oven and, as the viscosity of the resin initially reduces when the product is placed in the oven (prior to gelation occurring), it has a tendency to drain from the windings (once this resin has flowed into the bottom of the oven, it can no longer be used). Also, in the case of a solvented varnish, a significant proportion of its content (i.e. 50 ­ 60%) is lost as the solvent evaporates. Environmentally friendly If the optimum process conditions are used, the amount of volatile organic compounds (V.O.C’s) released into the atmosphere during trickle impregnation can be very low. Some of our customers have recorded emissions of between 1.9 and 3.3% of total resin amount (by weight) for IEC 200­frame to 225­frame stators. Some resin manufacturers claim emissions of V.O.C’s. as low as 1% for their resins. There are a number of reasons for the lower emission of V.O.C’s.: During the gel and cure stages of the trickle process, the monomer forms part of the cured resin structure. On the other hand, solvented varnishes, which are widely used for dip & bake impregnation, represent a serious
environmental problem because of the large amounts of solvents (predominantly aliphatic and aromatic hydrocarbons) which are discharged into the atmosphere, typically 50 ­ 60% of the total resin volume (solvented varnishes rely on the evaporation of solvent in order to cure). With the trickle impregnation process, the resin is applied directly onto the windings, minimising the surface area from which fumes can be emitted, whereas with the dip & bake and VPI processes, the iron is also coated in resin, creating a much greater surface area from which the emission of V.O.C’s can occur. Short cycle times Typically 10 minutes to 2 hours floor­to­floor time (dependent on product size, etc.) On­line process A stator or rotor is completely impregnated and cured in a single machine cycle, minimising the amount of handling required, and reducing the risk of damage to products. The trickle process is well suited to flowline manufacture on a ‘Just­In­Time’ basis, which reduces the amount of work­in­ progress (W.I.P.), as well as reducing the lead time of motors that are required urgently. Our customers have also reduced the number of stators held in storage after impregnation by about 50%. With the dip & bake and VPI processes, products must be taken out of the assembly flowline (typically for between 8 and 48 hours) and processed in batches, which increases W.I.P, handling, risk of damage, storage space, etc. It is also inefficient to process small numbers (i.e. less than a full batch). Products processed individually As Newtech trickle impregnation machines allow many of the process parameters to be adjusted individually, the optimum process conditions can be selected for each product and for different resins. Economical resin usage During the trickle impregnation process, resin is applied only where it is required ­ in the windings; the dip & bake and VPI processes also coat the iron with resin which, apart from a small number of applications, is not desirable, and increases resin usage (this resin must also be removed prior to assembly ­ see below). Furthermore, as the product is continuously rotated throughout the process and the resin is cured by resistance heating during the trickle impregnation machine cycle, there is negligible resin spillage. During the curing stage of the dip & bake and VPI processes, a great deal of resin can be lost from the windings due to secondary drainage and cannot be re­used. We are aware of one motor manufacturer who removes over 30 tonnes of waste resin from the bottom of their dip and bake oven annually. Not only is this exceedingly wasteful of resin, the cost of disposal is significant. One of our customers calculated that, when they were vacuum impregnating stators, between 60 to 65% of the resin applied to IEC 225­frame to 250­frame stators was lost due to secondary drainage during oven curing. With trickle impregnation, the same manufacturer is using approximately 40% less resin (by mass) than they previously used with vacuum impregnation, but a much greater quantity of resin remains in the windings (see diagram 2, below). The savings on resin cost alone for this customer justify the trickle impregnation process, before all of the other financial, quality and environmental benefits are considered.
Diagram 2 ­comparison of resin loss during direct current and oven curing of IEC 250­frame stator Little or no cleaning of the iron, fewer secondary operations required The fact that the iron becomes coated during the dip & bake and VPI processes usually means that a secondary cleaning operation must be performed, often requiring special machinery, brushes or abrasive wheels, dust extraction equipment, handling equipment and an operator to run the machine. Powdered resin removed from the iron is hazardous, flammable and expensive to dispose of. During trickle impregnation, minimal resin is spilled onto the iron. Energy efficient Oven curing is very wasteful of energy because the iron is heated to the same temperature as the windings (typically, it takes between 3 and 5 times more energy to heat the iron than is required to heat the copper). Direct electrical heating of the windings uses 10 ­ 20% of the energy required for oven curing. The ovens must also be pre­heated, or maintained at temperature. Minimal heating of the iron core also means that it remains relatively cool, allowing the stator or rotor to be handled sooner after impregnation. Space efficient The dip & bake and VPI processes usually require a large, centralised plant, which is usually built­in (i.e. it cannot easily be moved and sited elsewhere). Newtech’s trickle impregnation machines are compact, self­contained, requiring no special foundations and can be transported, installed and moved easily. Less space is also required for product storage and handling. Labour efficient One operator can typically look after four or five trickle impregnation machines. Stable resins requiring very little 'house­keeping' A dip or VPI tank usually contains a large amount of varnish or resin (i.e. tonnes). Maintaining this material in good condition requires very carefully planned and rigorously followed ‘house­keeping’ procedures. If these procedures are not followed, the material may not be at the correct viscosity, it may become contaminated by material from the windings (e.g., waxes) and, if the varnish does deteriorate such that it cannot be used, the value of the lost varnish may be very high and the disruption to production in emptying and refilling the tank very severe. Also, such a large volume of resin or varnish represents a serious fire hazard. On the Newtech trickle impregnation machines, there is only a very small amount of unreacted liquid resin present, minimising the fire hazard and simplifying ‘house­keeping’ procedures. In the case of an even ratio resin, the volume of mixed resin present on the machine is negligible.
5. Summary In the majority of cases, the trickle impregnation process offers significant financial, quality, production and environmental benefits when compared to the dip & bake and VPI methods for the impregnation of medium and large frame motors and generators.