... SP 13560-971 Brazil. ** Federal University of Santa Catarina, Department of Electrical Engineering ... resistance between rotor cage and steel is very important to avoid parasitic .... multiple micro-welding points that cannot be eliminated by.
Rotor Cage of Single-Phase Induction Motors - Process Analysis Claudia A. da Silva*, Alvaro B. Dietrich*, Renato Lopes*, Renato Carlson** * Tecumseh do Brasil, R&D Electrical Motors São Carlos, SP 13560-971 Brazil ** Federal University of Santa Catarina, Department of Electrical Engineering Florianopolis, SC 88040-970 Brazil Abstract - Single-phase induction motor (SPIM) has been widely used in home appliances due to its simplicity, low cost, low noise and ruggedness. However, to produce a cost effective high efficiency SPIM is necessary to have a good knowledge and good control of process parameters. Dynamometer’s results of motors made with different process parameters show that rotor cages are more sensitive to process variations than stators and that they can to worsen the motor efficiency and torque characteristics. The aim of this work is to evaluate some steps of rotor cage process, namely rotor laminates coating, aluminum die-cast methods and rotor annealing and to analyze the effect of such parameters on motor performance. Results obtained from tests with a 2-pole, 100 W, high-efficiency SPIM are presented.
I.
INTRODUCTION
The popularity of cage induction motor is due to the simplicity of its rotor, which is made of laminated steel and cast aluminum. The basic steps for rotor cage production are laminates punching, bluing of laminates in furnace, stack assembling, casting, machining finish and finally the rotor annealing. However, despite of its simplicity, the rotor plays an important role in motor performance. Primary performance variations in motors usually come from the rotor and this is more visible in small motors. From the literature, we know that to assure the good resistance between rotor cage and steel is very important to avoid parasitic torques, which can considerably reduce the breakdown torques (BDT) and causes the presence of saddles and inflexion points on motor torque-speed characteristic. The main process variable that plays role in boundary resistance is the internal coating of rotor slots, which must show good resistivity and sufficient mechanical strength to withstand the aluminum cast process. Another important variable that affects the boundary resistance of rotor cage is the rotor heat treatment (annealing process) used to “detach” the rotor bars from steel and relief mechanical stress in cast aluminum. The rotor cage of small induction motors is traditionally made by high-pressure, where molten aluminum is pressed extremely fast by a piston inside a mould which enclosures the rotor stack and end-rings cavities. Although this casting process is widely used, it shows some intrinsic problems like adhesion of aluminum in laminate steel, which reduces the inter-bar resistance. Another problem is the dispersion of gases and aluminum oxide in the cast aluminum due its fast solidification, which reduces its conductivity. Lower inter-bar resistance and lower aluminum conductivity leads to reduction in maximum torque and reduces the motor efficiency.
The introduction of new worldwide legislations, demanding motors with higher efficiency, is a challenge for motor designers and companies, which look for new materials and production processes. Thus, rotor cage production is a topic of great interest. Indeed, in recent years some motor manufacturers have changed from high-pressure die cast to low-pressure or centrifugal cast and also from traditional annealing (in a furnace) to inductive annealing. Since any process needs to be well known and well evaluated, a study was made with small induction motors (2 pole, 50Hz and 100 W) in order to quantify the impact on its performance when different rotor laminates coatings and also different cast methods are used. The use of furnace and inductive high frequency annealing processes were also investigated in order to compare its effectiveness. Several cares were, taken to reduce experimental errors like the use of rotor laminates obtained from the same electrical steel coil and from the same punch tooling. Rotors, having the same process steps were always, processed in the same tools (e.g., all rotors made by high-pressure cast process were produced in the same cast machine and using the same molten aluminum). For each parameter analyzed, the number of rotors in the sample set was compatible to dynamometer accuracy and typical process variations. All rotors were, tested with the same stator. These fine cares were necessary to have a good evaluation of different process of study. The most important developments in motor process take place at industry and they are, most of the time, treated as secret information. Even though most of results presented in this paper are not new for experienced engineers that work for motor manufacturers, they are important for the fresh ones that look for information normally not available in the literature. II. ROTOR LAMINATION COATINGS The cores of electrical motors and transformers need to be, laminated to restrain the effects of eddy currents and to reduce the core losses. The steel laminates are insulated from each other by use of an insulating coating. Some motor manufacturers use grain non-oriented (GNO) fully processed electrical steel that can be supplied with standard organic or inorganic coatings. The semi-processed steel is supplied without coating, which is formed in the final stage at decarburizing furnace, in motor manufacturer facilities. In the literature is possible to find relevant papers that analyze the
importance of insulating coatings on non-oriented electrical steels even though some of these analyses did not evaluate directly the motor performance [1]-[3]. Small induction motor manufacturers due to cost restrictions largely consume the semi-processed electrical steel. In this case, laminates are usually heat-treated (decarburized) after the punching process, in order to reduce the carbon content and to increase the grain size as shown in Figure 1, both intended to reduce hysteretic losses. The elimination of burrs also takes place during the decarburizing process. Another important step at decarburizing furnace is the production of a thin, but usually good quality, Fe3O4 oxide layer to provide electrical insulation between laminates. The decarburizing process described above is worthwhile for stator laminates, which carry flux at line frequency, leading to significant iron losses in the stator cores. However, this process can be hardly justified for rotor laminates, due to cost-benefit reasons, since flux variations in most of rotor cores are of low frequency.
a) Not annealed laminate
and H2), humidity (dew point) and temperature (540 °C), since the quality and thickness of Fe3O4 film are strongly related to these parameters [5]. Nevertheless, bluing is enough to provide some rust protection, burr reduction and electrical insulation between laminates. Due to cast process of small and low cost rotors, the cage aluminum and rotor laminate steel are in direct electrical contact. Therefore, the bluing film, also found in the internal surface of rotor slots, contributes to reduce this electric contact or increase the inter-bar resistance. However, even with this coating, some currents flowing in rotor bars do not reach the end rings and flow amongst rotor bars, through rotor teeth steel. These inter-bar currents are, induced by stator fundamental m.m.f. wave and its harmonics, affecting negatively the motor torque-speed curve and efficiency (increasing the stray-load loss) [4][6]. For the torque-speed curve, the most noticeable influence is the reduction of maximum torque, which decreases as the cross-resistance decreases. To evaluate the influence of inter-bar resistance in the motor performance, tests were, carried out with rotor cages produced with semi-processed electrical steel having different coatings (zinc phosphate, manganese phosphate, thin and thick Fe3O4 films). All rotors used in this analysis were, fabricated by highpressure die-casting process and tested in dynamometer before and after the rotor annealing. The use of zinc and manganese as coating is not new. Zinc and manganese phosphates are, extensively used to improve corrosion resistance of finished products ranging from structural steelworks for buildings and bridges to tubes and screws [8]. However, its application as coating of electrical steels is not usual and was chosen due to its electrical insulation characteristics and availability for tests. TABLE I COATING FILMS (DATA MEASURED BEFORE ROTOR ANNEALING) Process
b) Annealed laminate Figure 1. Stator laminates (tooth section)
For standard motors, the most used heating treatment for rotor laminates is the so-called “bluing” process. This process takes place in a furnace with wet atmosphere, where a thin iron oxide layer is, precipitated on all rotor laminates surfaces, including the internal surfaces of slots. Since it is common and very convenient to use such furnace at the same time for rotor annealing, its temperature is limited at 450 °C to avoid geometrical distortions in the finished rotors. The quality of oxide layer in “blued” rotor laminates is poor when compared to that obtained in controlled atmosphere (N2
Thin Fe3O4 film (2 µm) Zinc phosphate film Manganese phosphate film
Breakdown Torque (N.m)
Efficiency (%)
1.41
82.1
1.44
82.6
1.17
81.3
1.41
81.8
Table I shows that zinc phosphate is not suitable to provide electrical insulation between aluminum cage and rotor steel. Although zinc phosphate shows good electrical insulation property (at least 10 times greater than thick Fe3O4 film, in Franklin tests) it shows noticeable low mechanical strength to friction. Therefore, it is supposed that this film cannot withstand the molten aluminum flow during the cast process and it is, plucked out or damaged. Manganese phosphate film has similar insulation property of zinc phosphate but with superior mechanical strength and
adhesion. Table I shows that the performance of manganese phosphate as slot coating is higher than zinc and close to the thin Fe3O4 film. The thick Fe3O4 film shows a good property to improve the motor efficiency when compared to bluing film and that can be associated to an increase in cross-resistance (better electrical insulation of rotor slot). III. ROTOR ANNEALING Rotor annealing is an important step of small rotor cage process if traditional high-pressure die-cast is used. The highpressure casting and fast aluminum solidification leads to very low rotor cage shrinkage, resulting in close mechanical and electrical contact of cage aluminum and rotor steel. If the coating of inner slot surfaces is poor, this contact produces multiple micro-welding points that cannot be eliminated by rotor annealing. Typically, the mechanical contact between rotor bars and steel can be relived, “detaching” the cage from steel. As a result, the electrical contact is decreased and the inter-bar resistance increased. Annealing cycle is very effective to reduce the inter-bar currents, leading to significant improvement in maximum torque and power factor. Additionally, annealing also relieves mechanical stress of cast aluminum, contributing to increase the aluminum conductivity. There are some standards types of rotor annealing. The thermal shock (also called hardening) consists in heating the rotor and next cooling it at cool water. The traditional heat treatment (also called tempering) involves in heating the rotor slowly to a desired temperature then holding at that temperature for long enough to enable the internal changes to take place and finally to cooling it slowly in air. The modern induction heating process relies on induced electrical currents within the material to produce heat. Advances in power electronics technology have made high-frequency induction heating a remarkably simple and cost-effective heating method for motor applications. To verify the effect of annealing in motor performance four ways of detaching rotor bars from steel were used: the thermal shock in cold water, inductive high frequency heating, heat treatment at 450°C (one cycle) and heat treatment at 450°C (several cycles). All assembled rotors have the laminates with thin Fe3O4 film and cage obtained by high-pressure die-casting process. Table II shows the obtained results. Table II shows that there is no significant difference among results obtained by annealing processes. Despite the minimal gain in results, the use of induction heating increases in motor industry because it is faster and cost effective.
TABLE II ANNEALING PROCESS Process
Breakdown Torque (N.m)
Efficiency (%)
Thermal shock in water
1.30
82.9
1.30
83.0
1.31
82.9
1.31
83.0
High frequency induction heating Heating treatment at 450°C (one cycle) Heating treatment at 450°C (extended cycle)
IV. CASTING PROCESS There are three basic methods for casting induction rotor cages. They are centrifugal, high-pressure and low-pressure die-casting [7]. The high-pressure cast is normally limited in application to low voltage motors and smaller HP medium voltage motors. This well-known and widely used process consists in injecting molten aluminum under high pressure through the mold. The molten aluminum must flow through bottom end-ring, rotor slots and must reach the upper end-ring, before solidifying. The normal pressure for this process ranges from 150-300 Kgf/cm2 and the rotor stack is not pre-heated. The aluminum must be injected rapidly so as to not solidify in the rotor bars slot in the lamination stack. As the aluminum is injected from one end of the rotor only, the resulting rotor casting may have metallurgical defects in the end ring at the end opposite of the metal injection. To avoid shrink holes or voids (porosity), as shown in Figure 2, it is necessary to apply some foundries principles. These principles are available in the literature and will not be presented here. The aluminum remains as the most used metal to produce rotor cages. However, important works presenting the use of copper are already available in the literature [10].
Figure 2. End ring of rotor cage with voids.
The low-pressure die casting process, a well-established process to produce engine components, chassis parts or wheels, recently started being used by motor manufacturers to produce rotor cage. The process consists of preheating the lamination stack to allow a much lower pressure injection of the aluminum into the rotor slots in the laminations. As a result, it is possible to have an extremely rugged cast rotor with improved performance and reliability but without the size limitations of conventional high-pressure die-casting
Even though the low-pressure casting also starts to be more known and used, the centrifugal one seems to be more promissory to replace the high-pressure casting and then the process starts to be deeply discussed at industry. The centrifugal cast machine occupies less space in production area and the process is cleaner as shown in Figure 3. Centrifugally casting of rotors employs dies, which allow the metal to be poured directly to both end rings. The molten aluminum is poured on the center axis of the rotor and then travels to the outside due to centrifugal force. As the molten aluminum passes through the rotor and dies cavities, it is impelled towards the outside diameter and becomes cooler than the metal that remains to flow. The metal at outside diameter is thereby at a lower temperature initially than the metal towards the center. This creates the desired temperature gradient and therefore the metal solidifies from outside diameter towards center and from bottom towards the top of the die, where the molten aluminum is poured. The resulting rotor is of high quality due to absence of shrinkage porosity leading to more efficient and cost-effective motors [9]. Table III shows results obtained with motors having rotor cage produced by highpressure and centrifugal casting method.
centrifugal cast as shown in Figure 4. An important result is the efficiency prior to rotor annealing. For the high-pressure casting, the annealing is a necessary step of the process because it reduces the inter-bar resistance. In the centrifugal casting, the annealing step is unnecessary, leading to a consistent reduction in manufacturing cost.
a) Centrifugal casting
TABLE III CASTING METHOD Process High pressure (not annealed) High pressure (annealed) Centrifugal (not annealed) Centrifugal (annealed)
Breakdown Torque (N.m)
Efficiency (%)
1.29
82.1
1.31
82.9
1.31
83.4
b) High-pressure casting
1.31
83.6
Figure 4. Rotor cages obtained by different cast processes
V. GROUND FINISHING FOR ROTOR OUTSIDE DIAMETER
Figure 3. Centrifugal cast machines
Results of Table III show the higher quality of centrifugal die-cast process when compared to high-pressure one. Even when the breakdown torque is almost the same, the gain in efficiency is significant. That happens due to the high density and low porosity of aluminum cage and rings obtained by the
It is known from the literature that rotor surface finishing is important to reduce the stray load losses and consequently to improve the motor efficiency. To evaluate these differences, two ways of machining the rotor external diameter were used. One standard rotor set was, machined on lathe and another rotor set was, machined and ground. By cost reasons, the machining on lathe is widely used in small induction motors process. The outer surfaces of tested rotors presented in Figure 5 are. As expected, the machined and ground rotor has a smoother surface than the standard one. That leads to a uniform air-gap that reduces the magnetizing current and improves the power factor. Results of tests showed little gain in efficiency and a small reduction in torques. The input current of motor having the machined and ground rotor is lower than the standard one, which can be associated to an improvement in power factor. Even though the outside diameter of ground rotors was identical to standard ones, the effective air-gap is slightly lower and, consequently, the magnetizing current decreases, improving power factor as shown in Table IV.
TABLE V PISTON WITH AND WITHOUT LUBRICANTS TABLE IV MACHINING PROCESS Process Machined on lathe Machined and ground
Input Current (A) 0.65 0.63
Process PF
Cast piston with lubricate Cast piston without lubricate
0.88 0.90
Breakdown Torque (N.m)
Efficiency (%)
15.5
82.9
15.5
82.7
ACKNOWLEDGMENT The authors would like to thank Mr. Sergio Bertachini and Henrique Queirós for their support during the planning and execution of the experimental work.
REFERENCES [1]
a) Machining on lathe
b) Machining and ground Figure 5 Rotors obtained by different machining processes
VI. CAST MACHINE PISTON LUBRICANTS The injection piston of tool used to cast the rotors by the high-pressure centrifugal casting method needs to be lubricated periodically. Typically, at each injection cycle a liquid lubricant is sprayed inside the piston chamber, before the molten aluminum is poured. Some amount of the lubricant is vaporized by the molten aluminum and can be diluted in the aluminum decreasing its conductivity due to the increase of porosity. In order to evaluate the effect of lubricant in the casting machine piston two sets of rotors were fabricated. The only difference between these two rotor sets is the absence of lubricant in the piston chamber before the molten aluminum pouring. Test results are presented in Table V, which shows that the use lubricant does not affect significantly the rotor performance.
M. Nakamura, T. Sugyama, M. Kitayama, “Magnetic and Insulating Properties of Electrical Steel with a New Insulating Coating”, IEEE Transactions on Magnetics, vol.17, no.3, pp. 1270-1274, May 1981. [2] A. Coombs, M. Lindenmo, D. Snell and D. Power, “ Review of the Types, Properties, Advantages and Latest Developments in Insulating Coatings on Nonoriented Electrical Steels”, IEEE Transactions on Magnetics, vol.37, no.1, pp. 544-557, January 2001. [3] P. Beckley, N. J. Layland, E. Hopper, D. Power, “Impact of surface coating insulation on small motor performance”, IEE Proc. Elect. Power Appl., vol.145, no. 5, pp. 409-413, September 1998. [4] J. Stepina, Single-Phase Induction Motors – Construction, Theory and Calculation, Magna Physics Publishing Division of Motorsoft, 2004. [5] J. A. de Castro, M. F. de Campos, N. A. de Castro, F. J. G. Landgraf, "Comparison of analytical and experimental results with finite volume models for the decarburizing phenomenon of electrical steels”, Proceedings of 17th CBECIMat, pp. 6527-6540, November 2006 [in Portuguese]. [6] R. Carlson, C. A. da Silva, N. Sadowski, M. Lajoie-Mazenc, Y. Lefevre, “Analysis of the effect of inter-bar currents on the performance of polyphase cage-induction motors”, IEEE Transactions on Industry Applications, vol. 39, no. 6, pp. 1674-1680, Nov-Dec 2003. [7] A. H. Bonnet, T. Albers, “Squirrel- rotor cage Options for AC Induction Motors”, IEEE Transactions on Industry Applications, vol. 37, no 4, pp. 1197-1209, July-Aug 2001. [8] United States Patent: US 4669904 - Manganese Surface Coated Steel Materials. Publication Date 1981. [9] “Centrifugal Casting of Large Integral Horsepower Electric Motor Rotors”, CCMCO (The Centrifugal Casting Machining CO, www.ccmcotulsa.com. [10] “Die cast copper rotors as strategy for improving induction motor efficiency”, Electrical Insulation Conference and Electrical Manufacturing Expo, pp.322-327, Oct. 2007.
