Chapter 6

Chapter 6 - 6-1 Chapter 6 Basic Principles of...

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Unformatted text preview: 6-1 Chapter 6 Basic Principles of Electromechanical Energy Conversion Exit 2001 by N. Mohan Print TOC " ! 6-2 Electric Drive Electrical System Motoring mode Generating mode Exit 2001 by N. Mohan Electrical Machine Pelec Mechanical System Pmech Pelec Pmech TOC " ! 6-3 Basic Structure ❏ Construction Stator Air gap Rotor ❏ Multi-pole structure S N S N N N S S N S 2 pole 4 pole salient pole Sufficient to consider a 2-pole machine Exit 2001 by N. Mohan TOC " ! 6-4 Production of Magnetic Field ❏ Radial field (H,F,B) in the air gap is (t ) H positive if away from the center magnetic axis ❏ Using Ampere’s law, field r is (t ) produced by the stator is, Hs = Fs = H s ! g = N sis 2 & Flux density in air gap Bs = µo H s 2001 by N. Mohan lg Ns N sis 2!g mmf acting on air gap Exit µ =∞ θ Bs (t ) + stator surface π 0 rotor surface − π 2 − N s is µo 2l g π 2 N s is µo 2l g 2π TOC θ " ! 6-5 Basic Principles of Operation ❏ Force on a current carrying conductor subjected to an externally-established magnetic field f em = B ! i ❏ emf induced in a conductor moving in a magnetic field e= B!u Exit 2001 by N. Mohan TOC " ! 6-6 Electromagnetic Force external B field B f em l f em fem i " $# # % subtract fem = B ! i & "$% & & [ Nm] Exit 2001 by N. Mohan [Wb / m2 ] [ m] [ A] " $# # % add resultant ❑ Force Direction Higher concentration to lower concentration TOC " ! 6-7 Induced EMF Force on positive charges fq = q ( u × B ) + fq + B (into paper) In this example a net positive charge accumulates at the top and a net negative charge accumulates at the bottom u fq− − Magnitude of induced emf e B l u & = "$% & & [V ] [Wb / m2 ][ m][ m / s ] fq+ B (into paper) Polarity of induced emf is given by f q and is independent of current flowing u through the conductor Exit 2001 by N. Mohan TOC " ! 6-8 Application of Basic Principles δ ❏ Assumptions x Uniform Bs , radial in direction x Rotor current of constant magnitude but polarity changes with position x counter-clockwise torque is positive ❏ Force acting on the conductor ir ωm + er − stator magnetic axis Tem Electrical system f em = Bs ( N r I ) l ir and torque on coil Tem = 2 f em r = 2Bs ( N r I ) lr torque remains constant as rotor turns ❏ emf induced in coil er = 2econd = 2N r Bs lrω m r −I 0 er 0 I 360 o 180 o E δ −E Tem δ 0 Exit 2001 by N. Mohan δ TOC " ! 6-9 Energy conversion ir δ −I Motoring Mode + er − ir Tem stator magnetic axis 360 o 180 o 0 er ωm I E 0 δ −E Tem Electrical system δ 0 ir δ Regenerative braking Mode −I ir + er − Electrical system Exit 2001 by N. Mohan δ 0 er ωm Tem stator magnetic axis 0 I 360 o 180 o δ E δ −E Tem δ 0 TOC " ! 6-10 Power losses and Energy efficiency ❏ Motor losses ! Conduction losses, PR ! Iron losses, Phe ! Friction and windage losses, Pfw ! Switching losses, Psw Pin ! Stray losses, Ps ! Conduction losses ! Switching losses ηdrive = η PPU × ηmotor 2001 by N. Mohan Electrical Machine Motor losses Po Po Po = Pin Po + Ploss ❏ PPU losses Exit P P U PPU losses Ploss = PR + Phe + Psw + Pfw + Ps ηmach = Electric Drive '#### (##### # ) (80 − 90%) TOC " ! 6-11 Motor Ratings ❏ Safe Operating Area Speed Continuous Safe Operating Area Re duced torque due to lack of cooling 100% 0 100% Torque ❏ Rated speed, torque and power Prated = ω rated Trated ❏ Motor temperature rise due to losses ❏ Expanded safe operating area during transients and for intermittent operation Exit 2001 by N. Mohan TOC " ! 6-12 Summary ❏ What is the role of electric machines? What do the motoring-mode and the generating-mode of operations mean? ❏ What are the definitions of stator and rotor? ❏ Why do we use high permeability ferromagnetic materials for stators and rotors in electric machines? Why are these constructed by stacking laminations together, rather than as a solid structure? ❏ What is the approximate air gap length in machines with less than 10 kW ratings? ❏ What are multi-pole machines? Why can such machines be analyzed by considering only one pair of poles? Exit 2001 by N. Mohan TOC " ! 6-13 Summary ❏ Assuming the permeability of iron to be infinite, where is the mmf produced by machine coils "consumed"? What law is used to calculate the field quantities, such as flux density, for a given current through a coil? Why is it important to have a small air gap length? ❏ What are the two basic principles of operation for electric machines? ❏ What is the expression for force acting on a current-carrying conductor in an externally established B-field? What is its direction? ❏ What is slot shielding and why can we choose to ignore it? ❏ How do we express the induced emf in a conductor "cutting" an externally established B-field? How do we determine the polarity of the induced emf? Exit 2001 by N. Mohan TOC " ! 6-14 Summary ❏ How do electrical machines convert energy from one form to another? ❏ What are various loss mechanisms in electric machines? ❏ How is electrical efficiency defined and what are typical values of efficiencies for the machines, the power-processing units, and the overall drives? ❏ What is the end-result of power losses in electric machines? ❏ What is meant by the various ratings on the name-plates of machines? Exit 2001 by N. Mohan TOC " ! ...
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This note was uploaded on 02/06/2012 for the course EE 4002 taught by Professor Scalzo during the Fall '06 term at LSU.

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