Design of a Brushless Doubly-fed Induction Motor For Adjustable Speed Drive Applications

1. Design of a Brushless Doubly-fed Induction Motor For Adjustable Speed Drive Applications www.technologyfuturae.com By SINDHUJA SAYEEBABA Edited By Sarath S Nair www.technologyfuturae.com

2. OVERVIEW • Introduction • Adjustable Speed Drives (ASD) and its need • Evolution of the BDFM • Principles of Operation • Design of a Brushless Doubly-fed Induction Motor • Comparison of BDFIM and Induction motor • Potential Applications • Merits and demerits • Summary www.technologyfuturae.com

3. INTRODUCTION • BDFIM is an AC machine which can operate as both a motor and a generator • The stator has two sets of winding one is power winding which is connected to the grid and the control winding which is supplied from a converter • A specially designed rotor assembly tries to focus most of the mutual magnetic field to follow an indirect path across the air-gap.Hence brushes are absent. • Power is supplied to the rotor by means of electromagnetic induction. Winding sets are excited independently and actively participate in the electro-mechanical energy conversion process, which is a criterion of doubly-fed electric machines www.technologyfuturae.com

4. www.technologyfuturae.com Schematic of BDFM

5. The shaft speed is adjusted by varying the frequency of the control winding. As a doubly-fed electric machine, the rating of the frequency converter need only be fraction of the machine rating. • A variety of rotor structures can be fitted into the stator namely BDFIM and BDFRM. • In BDFIM, the rotor is made of laminated steel and a nested cage of aluminum or copper rods and the operation is pertained to the squirrel-cage induction motor • In the BDFRM, the rotor is made of salient pole of laminated steel, similar to the operation of synchronous reluctance motor www.technologyfuturae.com

6. Brushless Doubly Fed Motor www.technologyfuturae.com

7. Adjustable Speed Drives (ASD) • The equipment to control the speed of machinery. Also known as Variable speed Drive(VSD) • Primary function of adjustable speed drive is to control the speed, torque, acceleration, deceleration and direction of rotation of a machine. • The speed control of either an induction motor or a synchronous motor is done by adjusting the frequency of the power supplied to the motor. www.technologyfuturae.com

8. Speeds must change to adapt to various tasks for improved product quality, production speed or safety. • There are two electric adjustable speed drives namely DC drives and AC drives. • Electrical drive is preferred due to its low initial cost, high efficiency, low maintenance requirements, excellent performance, saves energy and versatility. www.technologyfuturae.com

9. Need For The Electrical Drive • In IM at start high inrush current cause high power loss which leads to insulation failure and affect the performance of the utility. • When the motor is operated at a minimum load magnetizing current is drawn and the PF is very low due to the inductive load. • When the motor operates at a PF less than unity, the current drawn by the motor is not sinusoidal in nature. www.technologyfuturae.com

10. Power factor is the ratio of real power to reactive power. PF=P/ (Irms*Vrms) • Poor power factor is due to the phase displacement or due to the high harmonic current. • The PF is very important and the customer is forced to maintain the full-load condition for the entire operating time or else pay penalties for the light load condition. • In applications like cranes or hoists, stop and reverse of the motor is needed often • So for all the above problems there is a need for an electric drive which runs at various speed ranges. www.technologyfuturae.com

11. Evolution Of The BDFM • The adoption of AC brought the induction machine into industrial service. However industry required variable speed operation • Fundamental operating speed for an induction machine is fixed by the mains frequency • The common method of controlling the speed of induction machines was the introduction of series resistance to the rotor • Later on connecting two induction machines together allowed to achieve three different speeds of efficient operation. Such an arrangement was known as `cascaded induction machines' www.technologyfuturae.com

12. cascaded connection consists of two wound-field induction machines coupled mechanically to the same shaft and load The slip power of the first machine is therefore used to excite the second machine . The stator windings of the second machine can be short-circuited or terminated in external resistances Thus the doubly-fed motor control can be realized without the use of brush gears. Three different efficient speeds of operation could be achieved. www.technologyfuturae.com

13. Doubly Fed Wound Field IM Drive www.technologyfuturae.com

14. Two multiphase winding sets with similar pole-pairs are placed on the rotor and stator • A multiphase slip ring assembly is used to transfer power to the rotating winding set and to allow independent control of the rotor winding set. • Rotor winding set actively participates in the energy conversion process with the stator winding set. www.technologyfuturae.com

15. Principles of Operation • The operating principles of the BDFIM are illustrated with the help of cascade connection of induction motor consisting of two wound field induction machines having different pole numbers of 2p and 2q which are coupled mechanically to the same shaft and load. www.technologyfuturae.com

16. A set of symmetrical sinusoidal currents of frequency f1 are flowing in the primary windings of the first machine, a counter-clock wise rotating field of w1=2пf1/p is set up in the airgap • The second machine is of 2q poles, the rotor currents will set up a rotating field of w2r=2пf2/q which will rotate in clock-wise with respect to the rotor. • fl is fixed and f2 is varied properly, the rotor speed can be controlled to be wr= 2п (f1+f2)/(p+q) Hence, the combination acts like a 2(p+q) pole machine. www.technologyfuturae.com

17. The first machine, usually connected to the 60 Hz utility, acts like a high-slip induction machine while the second machine can be in synchronous machine action or induction machine action • The electromagnetic torque found in the d-q reference frame is Te=pMp (iqs1idr1-ids1iqr1) +qMq (iqs2idr2-idr2iqr2) www.technologyfuturae.com

18. Comparison of BDFIM &IM www.technologyfuturae.com National Electrical Manufacturers Association (NEMA)

19. BDFIM • Ratings: Motor: 10 HP Drive: 5 HP • Prices: • Motor: $811 Drive: $1450 Total: $ 2261 • SQUIRREL CAGE IM DRIVE • Ratings: Motor: 10 HP Drive: 10HP • Prices: Motor: $614 Drive: $2375 • Total: $2989 (32% higher than BDFlM) www.technologyfuturae.com

20. POTENTIAL APPLICATIONS • Turbo-machinery • hydro power generation • commercial and industrial heating • ventilation and air conditioning • Wind power generation • If the BDFM is maintained at its synchronous speed, the secondary windings need to supply DC excitation to the motor and a fully rated variable frequency power converter can be usedto supply the primary windings. Hence a brushless synchronous machine can be realized www.technologyfuturae.com

21. MERITS The merits of this emerging technology can be summarized as • Brushless • High power factor operation when operating as VSD • Minimum system cost • Energy saving DEMERITS • poor electromagnetic design • Avoids slip ring assembly www.technologyfuturae.com

22. SUMMARY In this paper the basics of BDFIM, evolution of BDFM, design of the brushless doubly-fed induction motor, comparison of BDFM with IM based on design and cost, potential applications, merits and demerits of the BDFIM are considered www.technologyfuturae.com

23. REFERENCES [1] Yuefeng LIAO,’ Design of a Brushless Doubly-Fed Induction Motor for Adjustable Speed Drive Applications’, GE-Corporate Research and Development Center, NY 12301, USA. [2] Paul C. Roberts,’ A Study of Brushless Doubly-Fed (Induction) Machines’, University of Cambridge, September 2004. [3] www.microchiptechnologies.com [4] www.google.com www.technologyfuturae.com

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