STUDY OF OPTIMIZATION OF SQUIRREL CAGE INDUCTION MOTOR USING DCR TECHNIQUE

ABSTRACT
Induction Motors are most widely used Electric Motors best suitable for industrial applications as well as lowpower applications in daily used domestic equipment’s because of their unique advantage compared to other types of motors like DC and Synchronous Motors. Electric Motors do not transfer 100% of the input Electrical power into kinetic Mechanical Power. Some percentage of electrical power is lost during the conversion into Mechanical Energy. These losses are occurred in motor windings, core and rotating mechanical parts decreases the energy efficiency of Motors. Now a days power crisis condition is creating, it is very important to manufacture the motors which are more efficient than the standard Motors which are already available.

The AC Induction Motor is continuously improving by optimizing stator and rotor design with winding and best material used for electric current properties. With given exterior and interior motor dimensions used, the potential efficiency gain is limited and costly. DCR (Direct Current Resistance) technology is a new invention to decrease losses by using copper winding instead of aluminum for the conductor bars and end rings in rotors to improve its efficiency and performance. This paper reviews the past development in squirrel cage rotor designs, importance of Energy efficient induction motors, efficiency enhancement, implementation of DCR technology and the optimization of 3- Phase Squirrel Cage Induction Motor.

DCR 기법을 이용한 농형 유도 전동기의 최적화 연구

유도 전동기는 DC 및 동기 전동기와 같은 다른 유형의 모터에 비해 고유한 장점으로 인해 산업용 응용 분야는 물론 일상적으로 사용되는 가정용 장비의 저전력 응용 분야에 가장 적합한 가장 널리 사용되는 전기 모터입니다. 전기 모터는 입력 전력의 100%를 운동 기계적 동력으로 전환하지 않습니다.

기계 에너지로 변환되는 동안 전력의 일부가 손실됩니다. 이러한 손실은 모터 권선, 코어 및 회전하는 기계 부품에서 발생하여 모터의 에너지 효율을 감소시킵니다.

오늘날 전력 위기 상황이 발생하고 있으므로 이미 사용 가능한 표준 모터보다 더 효율적인 모터를 제조하는 것이 매우 중요합니다.

AC 인덕션 모터는 전류 특성에 가장 적합한 권선과 재료를 사용하여 고정자와 회전자 설계를 최적화하여 지속적으로 개선되고 있습니다. 주어진 외부 및 내부 모터 치수를 사용하면 잠재적인 효율성 향상이 제한되고 비용이 많이 듭니다.

DCR(직류 저항) 기술은 회전자의 도체 바와 엔드 링에 알루미늄 대신 구리 권선을 사용하여 효율과 성능을 향상시켜 손실을 줄이는 새로운 발명입니다.

본 논문에서는 농형 회전자 설계의 과거 개발, 에너지 효율적인 유도 전동기의 중요성, 효율성 향상, DCR 기술 구현 및 3상 농형 유도 전동기의 최적화를 검토합니다.

Keywords

Induction Motor, Squirrel Cage Rotor, Losses, Energy Efficiency, Optimization and DCR Technology.

I. INTRODUCTION

In a Three phase induction motor, three phase ac supply is given to the Stator winding through which current starts to flow and it produce the flux. The flux from the stator winding, flowing through the air gap uniformly links the rotor winding. It induces EMF in rotor winding and current in rotor winding produces torque to rotate the rotor of induction .
This motor is also called Asynchronous motor because their operating speed is slightly less than synchronous speed because of slip in the rotor during the running condition.
Similar to the other Rotating Electrical Machines, a three-phase Induction Motor also consists of Stator, Rotor, Shaft, Bearings, Cooling fans and End plates, it completely enclosed in machine frame for protection from atmospheric conditions.
Electrical conductivity is a key operating parameter in determining which type of material to be used in rotor conductors. Conductors of better conductivity with low resistance which result in more Efficient Transfer of Electrical Energy.
The Materials have good conductivity are Silver, Copper, Gold & Aluminum. Since silver and copper are costly and expensive materials and their property for carrying heavy current and high voltage never permits as a choice of conductor in electrical machines. So the recognizable choice is either aluminum or copper for making winding for stator and rotor.
We know that the Efficiency is the ratio of the amount of work produced to the amount of energy consumed.
The power losses are the difference between input and output powers and they can be categorized into five categories:

Iron losses / constant losses: magnetic losses occur in core laminations.
Stator copper losses / variable losses due to current in stator winding.
Rotor copper losses due to current in rotorwindings with high and low rotor resistance.
Wind age and friction losses due to mechanical parts like bearing and fans used for motor cooling.
Stray load losses which are negligible but also affect the motor efficiency.

II. OPTIMIZATION

The design of electrical machine is both science and an art of electrical and mechanical engineer. A science it follows establishment and universally accepted physical and mathematical principles which have been verified by experimental methods and an art is that knowledge of these principles is often inefficient to produce correct and economic design of the motor. This can only be achieved by correct conclusions based upon judgment and through understanding of the subject.
The design of all electrical machines consist of essential solution of many difficult and different engineering difficulties and normally these difficulties are closely interconnected to a bigger or a lesser degree.
The aim of optimization in the design of electrical machines is to select the best solution for a given problem from the assembly of possible results. The optimization procedure which involves the choice of various variables in such a manner so that the design in respect of a particular feature is the best result and at the same time fulfilled all limitations imposed on its performance as compared to other machines.
Therefore optimization is the effective process of finding a set of conditions required to achieve the best result from a given condition.
A characteristic feature of optimization in designing of electrical machine parts is the presence of conflicting or opposing effects on the efficiency of induction motor in all conditions.
In recent day the major challenge in designing the rotating machine are:

It would be Energy Efficient
It must be Economical to design

It is clear that the new challenge in machine design is to optimize the efficiency and manufacturing price both also. The best design will be acquired by the compromise of two main factors i.e. cost and performance, the two exerting opposing influences.
Performance optimization means motor is high efficient, has very low losses and low temperature rise during the working operation. Where, the cost optimization means the overall manufacturing cost of the motor should be reduced.

III. ENERGY EFFICIENCY OF THE INDUCTION MOTORS

An “Energy Efficient” Motor produces the same shaft output (H.P.), but absorbs less input power (KW) from the supply than a standard motor of same rating.
Efficiency = Output power / Input power
= (Input power – losses) / Input power
= 1 – (losses / Input power)
Energy – efficient motors, also called premium or high efficiency motors which reduces the energy losses and save the money. Motors qualify as „Energy Efficient‟ if they achieve or surpass the efficiency level. In
conventional design, the cost of motor increases while attempting to reduce the losses because the high-quality material used.

IV. SQUIRREL CAGE ROTOR DESIGNS

The Squirrel cage rotors are manufactured by two methods:

Fabrication methods
Die Cast method

A fabricated Rotor is one in which the Rotor bars are individually inserted and then shorted together on each
end rings made of same material as used for making Rotor bars generally copper material is used. The Rotor
bars may be aluminum, copper or the alloys of aluminum or copper. The end rings are usually the same
material as the bars.
But in energy efficient induction motor copper material is mostly used.
Die casting is a method which involve the injecting molten metal at a high pressure into a mold or cavity (called
a “Die”) in order to manufacture a part rapidly and frequently. Die casting is usually used in high production
volume applications to manufacture small or medium size parts of the Induction machines.
Hence, Die Casting is a inexpensive manufacturing method comparing to fabrication technique because
fabrication is a time consuming, increase high labor cost and doesn’t appropriate for industrial volume
production requirements now a days because demand is increasing rapidly due to development of country.
Based on the selection of conducting material for rotor winding and the after development of manufacturing techniques, following types of rotors design exist for a squirrel cage induction motor for being more energy efficient:

Fabricated Copper Bar Rotor (CuBar)
Fabricated Aluminum Bar Rotor (AlBar)
Aluminum Die Cast Rotor (DAR)
Copper Die Cast Rotor (DCR)

V. COPPER DIE CAST ROTOR (DCR) TECHNOLOGY

For the three phase low voltage Induction motor, the used material for the cage rotors construction is
aluminum because of lower value when compared to copper material which was convenient for the existing technological results.
As we know that the copper’s resistivity is lower than that of aluminum and therefore the copper cage
rotor losses reduce with the ratio of resistivity of copper to resistivity of aluminum. As a result energy
efficiency increased.

Hence for the same current requirement, the substitution of copper for aluminum results in 36.08 % reduction in copper losses. It helps in important improvement in energy efficiency and reduction in temperature rise during the working operation of induction motor which is loaded continuously for many hours during production in industries.
Die casting is a procedure involving injecting molten metal at a high pressure (1500 – 25000 psi) into a mold or cavity in order to manufacture a part speedily and repeatedly. Naturally, Die casting is done with low melting temperature metals available in the market given their typically lower price of processing. Sometimes, higher melting temperature metals such as ferrous alloys are also used in die casting, but this is rare given the higher processing charges.
DCR method or DCR rotors are those in which conducting material is copper in place of aluminum and rotor is manufactured by Die casting method which make it more energy efficient. The molten material is the copper and its alloys in place of traditional light weighted aluminum rotor.
The former challenges of Die casting copper, which are higher temperatures and pressures compared with die casting aluminum material used , have been resolved with the development of a Die casting process using Nickel base alloy Die inserts operated at high temperature. Due to which it become more suitable at high working temperature.

VI. BENEFITS OF DCR ROTOR

Die cast Copper Rotors can offer advantages in three ways:

Enhancement in motor energy efficiency in induction motor operation
Reduction in overall manufacturing cost
Reduction in motor weight

If motor re-design efforts are merely to improving energy efficiency, it is projected that the new design of
induction motor with DCR could achieve 92.5% Efficiency. This DCR motor creates a super-premium efficient motor with an efficiency level higher than currently available induction motors available.
The higher conductivity of copper compare to aluminum, guarantees reduction of motor energy losses by 15 –24 % and rotor I²R losses by 30 – 42%, leading to upgrading of the overall efficiency of the motor by at least 2.5% - 5.0 %, for the same slot design of the rotor of induction motor.
Copper Die cast rotors operate at 11 – 130 C less than aluminum Die cast rotors, subsequent in doubling the life of insulation used in motor, thus increasing the service life of the motor during the operation. Copper rotors reduce operating temperatures due to their lower I²R losses. Raised temperatures accelerate degradation of the insulation of motor’s winding, in the long run leading to failure.
Even though the cost of Die-casting a copper rotor is higher than that of die-casting aluminum rotor, the overall price of the motor utilizing the copper rotor can be lower. Due to the higher efficiency of the copper rotor, the overall length of the rotor (and motor) can be reduced, while still matching the performance of a motor utilizing an aluminum rotor bar.

VII. CONCLUSION

From the above conversation about induction motor it can be concluded that:
By keeping the main dimension and other design parameters constant, replacing copper in place of aluminum in Die-cast rotors, the rotor winding resistance losses get reduced because of the higher conductivity of the copper material and its alloy. It assured helps in improving the energy efficiency level and decreases the temperature rise of the motor during its working operation. It is surely that the performance optimization of a standard efficient Induction Motor.
On the other hand when the performance parameters (as losses, efficiency, temperature rise, output power etc.) remains constant and aluminum is replaced by copper in Die cast rotor, the main dimensions can be reduced. It decreases the overall manufacturing price and over all weight of the motor. It is certainly the cost optimization of the induction motor.
By following the above methods we can make energy efficient induction motor according working operation.

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