Comparison and assessment of a different steel materials based on core losses reduction for three-phase induction motor

ABSTRACT


INTRODUCTION
During the recent years, the induction motors (IM) are utilized, instead of the DC motors, in many different applications including domestic and industrial uses such as heat pumps, wind generator, textile mills, machine tools, transposition, air conditioners, and robotics [1]- [4].In order to make the induction motors operate, there is no need for sliding contacts or permanent magnets, which make them too simple and have low manufacturing cost [5]- [8].With little exceptions, the electric motors are considered the major source of the mechanical energy supplies in industry.Along many years, induction motors were considered as the workhorse in industry.Most of these motors that used in industry are largely three-phase.However, in small power industries, single phase machines are so common [9].Many types of induction machines are presented in the markets with various powers ranging from several watts up to megawatts.
Despite induction motors have easy design as compare to a DC motors, but it has low efficiency because of high losses.The reducing of iron losses is considered one of the main points that related for improving the performance and developing the future of these machines [10].The determination of core losses and other losses in the electrical machines is one of the tasks that must be performed during the electromagnetic calculation.
Three phase induction motors are calculated by Ansoft RMxprt and tow dimension (transient mode) Application [11].The reduction of iron losses in a three-phase induction motor is depend on the quality of magnetic materials that used.Therefore, for optimizing the design each type of steel materials contains a digital core loss curves (magnetization curve and permeability curve) [12]- [14].The data of proposed steel material type that used in core can be added directly to the design of a three-phase induction motor program (ANSYS Maxwell).This program of simulation design will be able to speed up the time to see the effects of steel material type on the performance of the motor [15].This paper proposed a three type of steel materials in the core design for three-phase induction motor to present a comprehensive comparison and assessment in order to identify the core material that provides the most effective performance by iron losses reduction.

MOTOR CORE LOSSES CALCULATION
The model of three phase induction motor is designed by ANSYS Maxwell software.Also, from this software is calculated the following Losses: 1. Stranded Loss (R-loss) in the windings of motor is determined by (1): where  is the current and R  is the phase resistance.
During 100 years ago, fertile minds -Steinmetz (1900), Bertotti (1990) and Ionel (2010) struggling to develop the models that can calculated core loss at any time.By matching the physics that based on the interpolation function with core loss data, a common Bertotti model of core loss is the sum of eddy current losses, hysteresis losses and additional losses [12].

Core Loss (Iron Loss)
The core losses of motor are classified to eddy current losses, hysteresis losses and additional losses as follow: a. Eddy current losses (2)(3): where (  ) is coefficient of eddy current losses, () is the electrical conductivity and () is steel sheet thickness.b.Hysteresis losses (4): where: ( ℎ ) is coefficient of hysteresis losses, () is the frequency and (  ) is the maximum value of magnetic induction.c.Additional losses (5): where (  ) is the coefficient of additional losses.The iron losses will be ( 6): The Bertotti model expects all the coefficients of iron losses   ,  ℎ and   to be positive.But if are positive, then core losses curve will be increased monotonically then the flux density B increases.The Bertotti model did not foresee to the point of inflection as similar to the core loss models.The steel is a major material that used in the manufacture of electrical motors in home appliances, industrial machinery and transportation [16][17][18].Especially, it forms the basis of the motors and converts the magnetic energy with high efficient into electrical energy [10].
Both the magnetization curve ( − )and core Loss curve ( − )are require for looking them at the same time.In general, the higher efficiency is obtained when using types of steels with lower core losses [19].Therefore, in order to achieve the higher efficiency requires the reducing the magnetic current while reducing the core loss [20].The ( − ) curve is measured by the manufacturer as locus of tips of a series of hysteresis loops.The slope of permeability is increased with H increasing as shown in (7): It is known that the core losses vary according to the different of flow density and frequency [21].But it is also known to differ with the manufacturer's specifications.Each company has its own formula and recipe for manufacturing.By comparing the characteristics of curves for core loss, it can help to discover the manufacturer material that produces the lowest loss with flow density and frequency.
Three types of steel were used, after hardly obtaining the ( − ) curve and core losses curve for each of them separately and applied each of them to the three-phase induction motor in program Ansys Maxwell (transient mode) [22], [23].By designing the motor and entering the data of curves into the program after applying the type of steel and obtained the results of iron core losses with copper losses and assessment the performance of the motor in each type of steel.

DESIGN OF THREE PHASE INDUCTION MOTOR
The three-phase induction motor is designed in the Maxwell program using RMxprt that used a set of analytical and magnetic circuit equations to perform a more detailed analysis and more accurate performance forecasts.The general data of the motor characteristics are listed in Table 1 The shapes of slots for stator and rotor as shown in Figure 1 and Figure 2. The dimensions of slot for stator and rotor as shown in Table 2.The design of three phase induction motor is shown in the Figure 3.For the calculating purpose of core losses, the proposed material properties must be applied in the design of stator and rotor for motor core, while the aluminum material is used in the bars and the end ring of bars for motor rotor [24], [25].In the first case, Cobalt steel type (Hiperco 50) has been used as a core material.Figure 4 shows the magnetization curve ( − ) for Cobalt steel (Hiperco 50). Figure 5 shows the specific core losses curve that obtained by using Cobalt steel (Hiperco 50) at frequency ( = 60 ) and the thickness of steel sheet equals  = 0.635 .In the third case, Low Carbon Steel-SAE1020 has been used as a core material.Figure 8 shows the magnetization curve ( − ) for Low Carbon Steel-SAE1020. Figure 9 shows the specific core losses curve that obtained by using Low Carbon Steel-SAE1020 at frequency ( = 60 ) and the thickness of steel sheet equals  = 0.635 .

RESULTS OF CORE LOSSES AND PERFORMANCE CHARACTERISTICS
In this section, the ability of the proposed three types of steel materials for core design of a threephase induction motor in losses reduction and efficiency improvement are investigated.The motor is operated with each one of the proposed steel materials for no load condition and transient mode to obtain the results of core losses and check the effectiveness of these steel materials.Figure 10 shows the magnetic field distribution lines in the motor using Cobalt steel (Hiperco 50).

CONCLUSION
This paper proposed a three type of steel materials in the core design for three-phase induction motor by using "ANSYS Maxwell" program which supported the calculation of core loss and copper loss depend on coefficients that are calculated based on the data of loss curves.It is necessary to know the specific core losses for each type of steel material in order to identify the best type that provides the lowest core losses.From the results demonstrated that Cobalt steel (Hiperco 50) representing the better choice in comparison with other types of steel materials in terms of less core losses.Also, the results show that the copper losses not affecting by type of core material.

Figure 1 .
Figure 1.The shape of slot for stator Figure 2. The shape of slot for rotor

Figure 3 .
Figure 3.The design of three phase induction motor

Figure 4 .Figure 5 .
Figure 4.The magnetization curve ( − ) for cobalt steel (hiperco 50) Figure 7 shows the specific core losses curve that obtained by using Electrical Steel NGO-AK Steel's M-19 at frequency ( = 60 ) and the thickness of steel sheet equals  = 0.635 .

Figure 10 .
Figure 10.Magnetic field distribution lines in the motor using cobalt steel (hiperco 50)

Figure 11 .Figure 12 .
Figure 11.The input voltage of source

Table 1 .
: Parameters and specifications for three phase induction motor