DESIGN OF AN ENERGY EFFICIENT AUTOTRANSFORMER FOR STARTING AN INDUCTION MOTOR

Induction motors are widely used in most of the Industries. Induction motors draw very high current in the order of 5 to 8 times the rated current during starting. Starting of a higher rating Induction motor using Autotransformer has been found great importance in limiting the starting current and improving the starting torque compared to star-delta starter. This paper discusses the design and analysis of an energy efficient Autotransformer starter of rating 6.6 kV, 110 kVA to start an induction motor of rating 6.6 kV, 250 KW. The Autotransformer was designed with on-load tap changing facility of eight tapping’s ranging from 50 % to 85% with each step tapping of 5%. Laser Scribed 23ZDMH85 material and conductor of current density 3.07A/mm are proposed in the present work to reduce the total losses. The feasibility of achieving a reduction of about 22% in total losses compared to conventional transformer is discussed in detail. At 50% tapping, unity power factor the estimated efficiency of the autotransformer is 99.21%.


INTRODUCTION
The increasing demand for electricity in India, all electric utility companies are going for energy efficient electrical equipment.Ninety percent of the motors used in industries are induction motors.When a 3-phase induction motor of higher rating is switched on directly from the mains, it draws a starting current of about 5 -8 times the full load (depending upon on the design) current.Also, this will cause a drop in the voltage affecting the performance of other loads connected to the mains.Therefore, starters are used to limit the starting current drawn by the 3-phase induction motors [1].
Common methods employed for starting an induction motor are Direct-on-line starter (DOL), Star-Delta starter, Autotransformer Starter and Soft starters.DOL starters are used to start induction motors of rating less than 5 HP and for starting medium rating induction motors, Star-Delta starters are used.For starting higher rating induction motors, Autotransformer starters are used and for very high rating induction motors, Soft starters are used.
The work carried out by the earlier researchers in the area of autotransformer starter design and development is briefly presented in this section.
Daniel J Rogers investigated characteristics of contact operated under new hybrid diverter design for on-load tap changer [2].
Wenzhou et al. investigated the effects of laser scribing on surface coating and magnetic properties of silicon steel [3].
Ajay et al. presented an overview of the transformer design, optimization and evaluation of energy efficient transformers from the literature related to this field for the past forty years [4].
Harsha et al. discussed and analysed different motor starters (conventional electromechanical and electronic starters), disadvantages and energy saving features of each type of starter [5].
Larry B Farr has discussed high voltage stress failures of auto transformer starter ratings ranging from 2400V to 11kV and the cause for failure [6].
Sewan et al. proposed several auto transformer arrangements to enhance the power quality of high current DC power supply [7].
In the present work, design and analysis of Autotransformer starter of rating 6.6 kV, 110 kVA to start an induction motor of rating 6.6 kV, 250 KW is discussed in detail.

Significance of Autotransformer
In Star-Delta starter the starting current is limited by a factor of (1/3).This limitation is overcome in Autotransformer starter.The starting current is given by K * I , where K is the transformation ratio and I sc is the short circuit current.
An auto transformer starter is suitable for both star and delta connected motors.The starting current and torque can be adjusted to any desired value by selecting the suitable tapping position in an auto transformer.The schematic diagram of auto transformer is shown in Fig. 1  [1].Initially the starter is connected to start position where reduced voltage gets applied to the stator of the induction motor, once the motor reaches 80% of its rated speed the starter switch is positioned to RUN and full voltage gets applied to the stator.

On-Load tap changer for an Autotransformer
Interruption of huge starting current results in arcing and damages the contacts.By using on-load tap changing mechanism, arcing at the contacts is avoided and the life of the contacts is increased.Fig. 2 to Fig. 4 shows the tap changing mechanism from 50% tapping to 55% taping position.The tapping has to be changed from 50% to 55%.First toggle switch T 2 is opened, the total load current gets diverted and flows through S 1 to the load.Now S 2 is moved to 55% as shown in Fig. 3(a).Now T 2 is closed as shown in Fig. 3(b) and there will be circulating current.In practice this current is limited by connecting reactors in the circuit and losses due to this current are minimized.In the next step T 1 is opened and the total load current flows through S 2 .Selector switch S 1 is moved to 55% tapping as shown in Fig. 4 and T 1 is closed.The total load current gets divided equally and flows through the switches S 1 and S 2 .

DESIGN OF AUTOTRANSFORMER
The flow chart for designing autotransformer is shown in Fig. 5.The design of 6.6 kV, 110 kVA auto transformer is carried out in accordance with Indian Standards.The materials used for designing the autotransformer are ferromagnetic core, conductors, insulating materials, sealing materials etc.For energy efficient application, to minimize the core loss, laser scribed 0.23 mm thickness 23ZDMH85 CRGO silicon steel which has specific loss of 0.56 watts/kg and conductor of current density 3.07 A/mm2 is considered.The tapings are provided from 50% to 85% with a step voltage of 5%.

Design of stepped core
In a Non-step Lap joint the flux which crosses the air gap contributes to "leakage of flux" and therefore requires more no-load current to achieve the required flux density.Further the over-saturation of flux at the corner joints [8] also leads to higher magnetostriction of the core, which is the main cause of noise level in a transformer.
In stepped Lap core, there will be more layers of laminations available for distributing the flux resulting in lower losses at the corner joints.Therefore, in conventional transformers, non-step lap core is used and in energy efficient transformers step lap core is used.In the present work, to minimize the core losses, Laser scribed 0.3 mm thickness 23ZDMH85 CRGO silicon steel which has a specific loss of 0.56 watts/kg was used.

Parameters of a stepped core
Volts per turn is given by k √kVA (1) For designing a 6.6 kV, 110 kVA energy efficient transformer, k value was chosen as 0.5.Therefore, from equation ( 1 Substituting the values of (V/T), frequency=50Hz and B=1.5T in equation (3), Net area, A i is obtained and is equal to 15893 mm 2 .Gross area is given by .17465 From above, D = 150 mm and 8 steps are considered in the present case.Between any two consecutive steps from step 1 to step 6, difference of 10 mm is maintained and to balance in last two steps 30 mm difference is maintained.Gross area = stack width * stack height Net area = Gross area * packing factor Here, packing factor is taken as 0.97.The stepped core parameters are shown in Table 1.

Design of common winding
An autotransformer has a single winding, the primary and secondary circuits therefore have a number of winding turns in common.The schematic of autotransformer is shown in Fig. 6.
The maximum current in the common winding is given by * √ * .* ≅ 9.63A.The cross section of conductor is given by . .

2.95mm
(4) Axial forces are more than radial forces, hence we consider turns axially.

2.58
The window height for an autotransformer starter can be 2 to 4 times the diameter of the core.In the present work, the window height is considered as 2.5 times the diameter of the core and is equal to 375 mm.For 6600 V application, as per IS 2026, the clearance required between core and inner winding is As per IS2026, hot spot temperature is 98 o C and yearly weighted average ambient temperature in India is 32 o C. Therefore, the maximum winding temperature is 66 o C (98-32).Maximum permissible temperature gradient is .≅ 14.5 ° C.

Design of series winding
When an autotransformer operates at 50% of the rated voltage, the maximum current that flows in the series winding of 360 turns is 9.63 A. In the present work, conductor of diameter 2 mm with current density 3.07 A/mm 2 was considered.The series winding parameters were estimated using the equations given in section 2. The schematic of core is shown in Fig. 7.

Centre limb
The schematic of centre limb is shown in Fig. 8.

Top yoke
The schematic of top yoke is shown in Fig. 9.For stack = 38 mm, stacking = 0.97 and density = 7.65g/cc, the weight of top yoke is calculated using equation ( 22) and is equal to 18.55 kg.The weight of bottom yoke = weight of top yoke = 18.55 kg.

Design of tank
The top view of transformer tank with three phase windings is shown in Fig. 10.The bottom plate does not contribute to any heat dissipation and on top surface the LV and HV bushings are present, do not allow any heat dissipation by tank wall surface.Therefore, only four sides of the tank were considered.As per standards [10] 500 W/m 2 is dissipated.Surface Area is given by 2(l + b) * h = 2(0.775+0.32) *0.78 =1.708 m 2  (33) Heat dissipated by tank = 1.708 * 500 = 850W Heat dissipated in Radiators is given by Heat to be dissipated -Heat dissipated by Tank = 1450 -850 = 600 W.
Considering the temperature of oil as 45 o C, as per standards [10], the heat dissipated by the radiator is 400W/m 2 .Therefore, the estimated surface area is given by (600/400) = 1.5m 2 .

Design of Radiator
It is a standard practice [12] to mount the radiator top pipe at a distance of 90mm from the lid and 10mm clearance at bottom of tank.From Fig. 11, height of the radiator is given by height of the tank -clearances -pipe diameter = 780 -100 -145-10 = 520 mm (34) Surface area/fin=0.226m 2 , no of fins is given by surface area / surface area per fin and is approximately equal to 12 fins.

EFFICIENCY OF AUTOTRANSFORMER
The efficiency of an autotransformer is given by Where, x = fraction of load or percentage of load

Efficiency of conventional Autotransformer
As per IS standards 2026, the maximum permissible current density is 3.63 A/ mm 2 and this value is considered for computing load losses for a conventional autotransformer.All the parameters defined in section 2 are estimated and summarised below: The cross section of conductor = 2.65 mm 2 , diameter of bare wire = 1.85 mm, length of common winding = 315 mm, turns per layer = 139 turns for first two layers & 82 turns for the last layer, No of layers for common winding = 3, length of mean turn = 0.5683 mm, length of wire = 615 mm, resistance per phase @ 75 °C = 1.63Ω, weight of the conductor = 14.66 kg, stray loss of the conductor = 0.051%, specific loss = 1.02, build factor = 1.3, core losses = 390W, load losses of common winding = 465W, load losses of series winding losses = 512W, tank losses = 110W, total losses in conventional auto transformer is given by core losses + series winding loss + common winding loss + tank loss = 390 + 465 + 512 + 110 = 1477W (36) Total load losses is given by Total losses-core losses = 1477-390 = 1087W (37) Efficiency of a conventional auto transformer η c for 50% taping at unity power factor is given by %η From above total reduction in losses in energy efficient autotransformer compared to conventional autotransformer is given by 1477 -1165 = 312W (40) In a year total number of units saved is given by 0.312*24*365 = 2733 kWh (41) The life of the transformer is expected to be 30 years.Therefore, total number of units saved in 30 years = 2733*30 = 81993 kWh.At present, cost of one unit = Rs.5.25 Total savings in Rs = 81993*5.25= Rs.4,30,469

CONCLUSIONS
This paper provides a detailed analysis of a design of a 6.6 kV, 110 kVA energy efficient Autotransformer for starting a three phase induction motor of rating 6.6 kV, 250 kW.In the present work, core losses are reduced by selecting superior material in place of conventional material i.e. laser scribed 23ZDMH85.The core losses were brought down from 390 W to 190 W by using this superior material.To reduce the load losses, lower current density conductor 3.07 A/mm 2 was preferred in lieu of standard value 3.68 A/mm 2 (IS -2026).The total estimated losses for an energy efficient autotransformer and conventional autotransformer are 1165W and 1477W respectively and there is a significant reduction in losses ISSN 1335-8243 (print) © 2016 FEI TUKE ISSN 1338-3957 (online), www.aei.tuke.sk of about 22% when compared to a conventional transformer.
In the present work, the total losses of the energy efficient autotransformer are 1165 W which are well within the permissible limits [13] and thus validates the method of estimation of total losses with IS standards.The life of the autotransformer was assumed to be about 30 years and the total net savings will be about Rs 4,30,469.The estimated efficiency of the autotransformer operating at 50% tapping, unity power factor with energy efficient autotransformer and conventional autotransformer was 99.21% and 98.8% respectively.This detailed information helps an electrical engineer to design an energy efficient autotransformer of any rating.

Fig. 1
Fig. 1 Schematic diagram of auto transformer starter for starting an induction motor S 1 & S 2 represent selector switches, T 1 & T 2 represents toggle switches and I L represents the load current.During starting, tap changer is set to 50% tapping to provide required starting torque.As shown in Fig. 2, S 1 & S 2, T 1 & T 2 are in closed position, current flowing through the selector switch is half the total load current (I L /2).

Fig. 3 (
Fig. 3(a) On-Load tap changer with S 1 at 50% and S 2 at 55% tapping and T 2 open

Fig. 6
Fig. 6 Schematic diagram of an autotransformer From equation (4), the diameter of the bare conductor, b is 2mm.The total insulation between two bare conductors is 0.25 mm.Therefore, insulated conductor diameter, b i is 2.25 mm.For 6.6 kV application, as per IS standard 2026, yoke to common winding clearance is 30 mm.Common winding length is given by window height -(2*end clearance) = 375 -(2*30) = 315 mm (5) 9 mm.Inner diameter, ID of common winding is given by = D + (2*clearance) = 150 + (2*9) = 168 mm (7) Radial height of common winding is given by (b *oil duct between two layers) + ISSN 1335-8243 (print) © 2016 FEI TUKE ISSN 1338-3957 (online), www.aei.tuke.sk(no of ducts*oil duct between two layers) = (2.25*3)+ (2*3) = 13 mm (8) Outer diameter, OD of common winding is given by Inner diameter + radial height = 168 + (2*13) 10), lmt = 568 mm = 0.568 m.Length of conductor is given by lmt * no of turns * no of limbs = 0.568m*360*3 = 615 m (11) Volume of conductor is given by area of conductor * length of conductor (12) Bare weight, w of common winding conductor is given by Volume of conductor * density of copper * 200 loss, W s is given by * * * Stray loss factor * Current density, J is 3.07 A/mm 2 and load loss is always defined at 75 o C (20 + 55), since average ambient temperature is 20 o C and as per IS2026 winding temperature in India is considered as 55 o C. Load loss, W L is given by Load loss factor * bare weight * (current density) 2 * stray losses (16) As per Indian standards, Load loss factor is 2.4 and load loss at 75 o C is 2.4 * 17.2 * 3.07 .1 ≅ 390WThe temperature gradient is given by LoadLoss No of limbs * no of surfaces * heat dissipation factor * winding length * length of mean turn * .

2 .
Inner diameter of series winding = 206 mm Outer diameter of series winding = 232 mm Length of mean turn = 0.688 m Length of conductor = 744 m Bare weight of conductor = 20.8kg Insulated weight of conductor = 21.5 kg Stray loss = 0.051% Load loss = 475W Temperature gradient = 2.6 As per IS2026, tank loss is given as 1W/kVA.For 110 kVA transformer, total tank losses are 110W.Total load loss is given by Load losses in common winding + Load losses in series winding + Tank losses = 475 + 390 + 110 = 975W (18) 2.4.Estimation of core dimensions 2.4.1.Estimation of Length and center distance of core [11]

Table 1
Stepped core parameters