Principles of Electric Machines and Power Electronics Chapter 4 DC Machines Third Edition P. C. Sen
Chapter 4 DC machine Electric machine Type: rotating machine Applications: generator (electric source) motor (electric load) Main contents Introduction of electric machines DC machine basics Armature winding Armature voltage Electromagnetic torque Magnetization (saturation) curve DC machine classifications Fig_1-1
Electric machine Convert electrical energy to mechanical energy and vice versa Generator: prime mover rotate rotor shaft to get electric output (voltage and current) Motor: apply electric voltage (current) to machine to get mechanical output at rotor shaft (torque and speed) One machine can be used as generator or motor Electric system: voltage and current electric power Mechanical system: torque and speed --- mechanical power
Machine types DC machine Use or generate DC power Applications: Automobile(Air condition, oil pump), robots, power tool AC synchronous machine Use or generate AC power Rotational speed is the same with the electrical speed Applications: power generator, house hold appliances, Electrical vehicle Induction machine (Asynchronous machine) Use or generate AC power Rotational speed is slower than electrical speed Applications: house hold appliances, Electrical vehicle
Structure of DC machine Structure Stator (field) winding: static Rotor (armature) winding: rotating DC current through field winding Commutator: rotating Brush: stationary Features Precise torque and speed control Adjusted wide speed range Constant mechanical torque Rapid acceleration and deceleration Application Mainly for DC motor application DC generator is limited due to the wide use of AC power Large dc motor: conveyors, cranes, paper mills Traction motors: transit cars and locomotives Control device: tachogenerator for speed sensing; servomotor
Physics behind motor Current-carrying conductor in a magnetic field is subject to electromagnetic force Electromagnetic force (Lorentz force): Ԧf = Ԧi B l Direction: right-hand (screw) rule
Operating principle of Motor
Operating principle of Motor- commuter and brush
Physics behind generator Conductor moving in magnetic field generates motional Electromotive Force (EMF) ε (volt) ε = v B l The direction of the EMF: right-hand (screw) rule
Operating principle of generator- commuter and brush Current reversal by commutator and brush
Operating principle of Generator Too large ripple! Multi-turns Series connection Shifted physically Sum voltage: DC voltage with ripple
Four-pole DC machine--mechanical angle vs electrical angle One mechanical cycle, two electrical cycles are encountered ed p 2 md Pole pitch: distance between centers of two adjacent poles One pole pitch 180 o ed 360 p o md
Armature windings Turn---Coil---Winding
Lap winding of a dc armature high current low voltage Current paths: #1: 5-10-7-12; #2: 8-3 - 6-1 ; #3 13-2-15-4 ; #4:16-11-14-9 number of parallel paths(a)= number of machine poles (p)= number of brushes
Wave winding of a dc armature Current paths: #1:[8]10-47-36-23-12-49-38-25-14-1-40-27-16-3-42-29-18-5-44-31 [13] #2:[8] 21-34 [20] #3:[7] 19-32 [19] #4:[7] 8-45 [20] #5:[19] 47-6-17-30-14-4-15-28-39-2-13-26-37-50-11-24-35-48 [2] #6:[1] 7-20 [13] #7[1]46-33[14] #8[2] 9-22 [14]
Wave winding of a dc armature number of parallel paths (a)= 2 There may be two or more brushes Used for high voltage low current
Armature voltage in dc generator e Blv e B( ) 2 l r, e B 2l r t m t m B p p, et m A 2 rl E N N e p a a a t m Np Ea Ka m, Ka, a p or a 2 a Ka machine or armature constant
Torque production in dc motor f Bli Ia fc B( ) l ic B( ) l a I I pi Tc fcr B( ) l r, Tc Bl r a a 2 a a a a T 2NTc T K I a a Right hand: thumb current; fingertip Lorenz force
Armature output power and DC machine loss DC Generator outputs electric power Pa EaIa Ka Ia m DC motor outputs mechanical power P T K I a m a a m
Example 4.1 (practice)
DC machine representation- two circuits Field flux is in d-axis direction Armature current flux is in q-axis direction
Magnetic circuit of two-pole dc machine Magnetic path: pole, air gap, rotor, yoke of the stator (laminated Ferrite) Field MMF: F p = N I Magnetic reluctance R Magnetic flux: φ = F p /R
Flux-mmf (field excitation) relation in dc machine At low values of flux, flux is determined by reluctance of air gap; At high values of flux, saturation occurs.
Magnetization curve: open-circuit armature voltage vs field excitation mmf Induced voltage is proportional to flux times speed Np Ea Ka m, Ka, a p or a 2 a