D ESIGN OF A B RUSHLESS D OUBLY - FED I NDUCTION M OTOR F OR A DJUSTABLE S PEED D RIVE A PPLICATIONS By SINDHUJA SAYEEBABA Edited By Sarath S Nair m

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

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

Schematic of BDFM

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

B RUSHLESS D OUBLY F ED M OTOR

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.

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.

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.

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.

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'

 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.

D OUBLY F ED W OUND F IELD IM D RIVE

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.

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.

A set of symmetrical sinusoidal currents of frequency f 1 are flowing in the primary windings of the first machine, a counter-clock wise rotating field of w 1 =2пf 1 /p is set up in the airgap The second machine is of 2q poles, the rotor currents will set up a rotating field of w 2r =2пf 2 /q which will rotate in clock-wise with respect to the rotor. f l is fixed and f 2 is varied properly, the rotor speed can be controlled to be w r = 2п (f 1 +f 2 )/(p+q) Hence, the combination acts like a 2(p+q) pole machine.

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 T e =pM p (i qs1 i dr1 -i ds1 i qr1 ) +qM q (i qs2 i dr2 -i dr2 i qr2 )

Comparison of BDFIM &IM 2/6 P BDFIM 4 P IM NEMA FRAME SIZE SLOT/PHASE/POLE6/2 3 HORSEPOWER 10 EFFICIENCY (%) POWER FACTOR National Electrical Manufacturers Association (NEMA)

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

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 used to supply the primary windings. Hence a brushless synchronous machine can be realized

MERITS T he 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

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

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 [3] [4]

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