Electromechanical Systems & Actuators
Dr. Adel Gastli Electromechanical Systems & Actuators DC MACHINES These slides are the contributions of: Dr. A. Gastli, Dr. A. Al-Badi, and Dr. Amer Al-Hinai DC Machines LEARNING GOALS Introduction Application of DC Machine Advantages & Disadvantages of DC Machine Construction of DC Machine Field System Armature Commentator Brush Principle of Operation Faraday’s Law Armature Voltage & Developed Torque Classification of DC Machine Permanent Magnet Self-Excited Separately-Excited DC Machine Representation Magnetization Curve (Saturation) DC Motor & Generator Equations Power Flow & Efficiency Torque-Speed Characteristics Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators Starting of DC Machine 2
Introduction Most of the electrical machine in service are AC type. DC machine are of considerable industrial importance. DC machine mainly used as DC motors and the DC generators are rarely used. DC motors provides a fine control of the speed which can not be attained by AC motors. DC motors can developed rated torque at all speeds from standstill to rated speed. Developed torque at standstill is many times greater than the torque developed by an AC motor of equal power and speed rating. MCTE3210: Electromechanical Systems & Actuators Dr. Adel Gastli 3 Application of DC Machines The d.c. machine can operate as either a motor or a generator, at present its use as a generator is limited because of the widespread use of ac power. Large d.c. motors are used in machine tools, printing presses, fans, pumps, cranes, paper mill, traction, textile mills and so forth. Small d.c. machines (fractional horsepower rating) are used primarily as control device-such as tachogenerators for speed sensing and servomotors for position and tracking. Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 4
Application of DC Machines DC Motor Paper Mills Oil Rigs Steel Mills Dr. Adel GastliMining Robots MCTE3210: Machine Electromechanical ToolsSystems & Actuators Petrochemical 5 Advantages & Disadvantages Of D.C. Motors Advantages High starting torque Rapid acceleration and deceleration. Speed can be easily controlled over wide speed range. Used in tough gobs (traction motors, electric trains, electric cars, .) Built in wide range of sizes. Disadvantages Needs regular maintenance Cannot be used in explosive area High cost Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 6
Introduction Electric Machine Mechanical Input Electrical Output Generator Electrical Input Motor Mechanical Output Electromechanical Energy Conversion Electrical system v i Ideal Electric Machine ω T Mechanical system Motor Energy Flow v i T ω Generator MCTE3210: Electromechanical Systems & Actuators Dr. Adel Gastli 7 Construction of DC Machine Parts of a DC Machine Armature core Leading pole tip Armature winding Field coil Rotation Pole axis Shaft Pole core Trailing pole tip Pole face Dr. Adel Gastli Field yoke MCTE3210: Electromechanical Systems & Actuators 8
Construction of DC Machine Shaft Armature Commutator Stator Field coil Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 2 Pole DC Machine 9 Construction of DC Machine: Field System The field system is to produce uniform magnetic field within which the armature rotates. This consists of Yoke or frame: Acts as a mechanical support of the machine 2000HP DC Motor field System Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 10
Construction of DC Machine: Armature The rotor or the armature core, which carries the rotor or armature winding, is made of sheet-steel laminations. The laminations are stacked together to form a cylindrical structure Teeth Slots The armature coils that make the armature winding are located in the slots Slots for wedges Non-conducting slot liners are wedged in between the coil and the slot walls for protection from abrasion, electrical insulation and mechanical support Cooling ducts for air circulation Portion of an armature lamination of a dc machine showing slots and teeth Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 11 Construction of DC Machine: Armature Armature of a DC Machine Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 12
Construction of DC Machine: Commutator Commutator: is a mechanical rectifier, which converts the alternating voltage generated in the armature winding into direct voltage across the brush. It is made of copper segments insulated from each other by mica and mounted on the shaft of the machine. The armature windings are connected to the commutator segments. Commutator Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 13 Construction of DC Machine: Brush The purpose of the brush is to ensure electrical connections between the rotating commutator and stationary external load circuit. It is made of carbon and rest on the commutator. Commutator and Brushes Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 14
Construction of DC Machine: Armature Winding Top coil sides Top coil sides Bottom coil sides Commutator 1 2 3 Brush Elements of Lap Winding 1 2 Brush Elements of Wave Winding MCTE3210: Electromechanical Systems & Actuators Dr. Adel Gastli 15 Construction of DC Machine: Armature Winding End connection Conductors Turn Dr. Adel Gastli Coil Winding MCTE3210: Electromechanical Systems & Actuators 16
Construction of DC Machine: Armature Winding Lap Winding a b c d e a b c d 1 2 3 4 5 6 7 g 9 10 11 12 13 14 15 16 17 18 19 20 S N S N f 8 e h g f 19 20 21 1 2 3 4 5 6 8 9 10 11 12 13 14 15 16 17 18 19 - - - 7 - h - Ia - a b p Icoil // paths brushes poles - - Systems - & Actuators MCTE3210: Electromechanical Dr. Adel Gastli 17 Construction of DC Machine: Armature Winding a d c b 1 Wave Winding e 2 3 4 5 6 7 8 j b c S N k g f h g f 17 18 19 20 21 1 2 3 4 - 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 - - - j k Ia i h - Icoil Dr. Adel Gastli e d 11 12 13 14 15 16 17 18 19 20 21 S N i 9 10 a MCTE3210: Electromechanical Systems & Actuators - a 2 Nb. of // paths 18
Principle of Operation The Faraday Disk and Faraday’s Law An emf is induced in a circuit placed in a magnetic field if either: the magnetic flux linking the circuit is time varying or there is a relative motion between the circuit and the magnetic field such that the conductors comprising the circuit cut a cross the magnetic flux lines. φ Magnet N ω S V Brush 1st form of the law is the basis of transformers. 2nd form is the basic principle of operation of electric generators. Copper disk Conducting shaft MCTE3210: Electromechanical Systems & Actuators Dr. Adel Gastli 19 Principle of Operation The right-hand rule and generator action V Velocity, u Voltmeter Flux density, B Conductor rails Φ B.A Φ B.l.s d Φ dB .l.s dt dt ds ds e B.l. ,u dt dt e Faraday’s law or flux cutting rule Dr. Adel Gastli u emf, e Moving conductor B e l e Blu MCTE3210: Electromechanical Systems & Actuators 20
Principle of Operation ω Without Commutator φ N S l1 Field pole N-turn coil Slip rings v brushes v External circuit t MCTE3210: Electromechanical Systems & Actuators Dr. Adel Gastli 21 Principle of Operation With Commutator ω S N v coil Commutatorb segments a brushes t v Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 22
Single-Phase Full wave Rectifier MCTE3210: Electromechanical Systems & Actuators Dr. Adel Gastli 23 Multi-Pole Machines If p is the number of poles, then p/2 cycles of variation of the flux are encountered every complete mechanical rotation. 360 o md One pole pitch 180 ed p o θ B(θ) N Pole pitch θ ed p θ md 2 N π N S 2π 3π θ S 4π θed θmd π S S N 2π θed : electrical degrees or angular measure in cycles θmd : mechanical degrees or angular measure in space Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 24
Principle of Operation: Armature Voltage Emf conductor Emf Total p.Φ . N m Flux / Re v. p.Φ 60 time / Re v. (60 / N m ) Emf conductor Number of conductor / path p.Φ . N m Z p.Φ .Z . N m Emf Total / 60 60 a a where p number of poles Z total number of armature conductors a number of parallel paths, 2 for wave and p for lab. Φ flux per pole (Weber) Nm speed of the motor in the revolutions per minute (rpm) time of 1 revolution 60/Nm (sec) MCTE3210: Electromechanical Systems & Actuators Dr. Adel Gastli 25 Principle of Operation: Armature Voltage Let ωm 2 .π . N m ω .60 Nm m 60 2 .π ωm speed of the motor in radians per second Emf Total p.Φ .Z .ω m p.Φ .Z ω m .60 . 60 a 2 .π 2 .π .a Emf Total K a .Φ .ω m Ka: armature constant Ka p .Z 2 .π .a Generated voltage : generator operation Back emf : motor operation Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 26
Developed (or Electromagnetic) Torque Consider the turn shown in the following Figure. 2πrl p Area per pole A pΦ Φ A 2π r l B Flux density Ic Current / conductor is Ia a fc B l The force on a conductor is Ia a Tc f c r B l The torque developed by a conductor is Te The total torque developed is Φ p Ia Ia r a 2π a Zp Φ I a E I K aΦ I a a a ωm 2π a MCTE3210: Electromechanical Systems & Actuators Dr. Adel Gastli 27 Production of Unidirectional Torque and Operation of an Elementary ω ω N F a b b a F S I 1 2 Position of conductor a under N-pole N F b F a I 1 S b a 2 Position of conductor a under S-pole B I F With this configuration the torque is unidirectional and independent of conductor position Left-hand rule Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 28
Classification of DC Machine Separately excited DC Machine Self-excited Short Shunt Long Shunt Shunt Series Cumulative Permanent magnet Differential Compound MCTE3210: Electromechanical Systems & Actuators Dr. Adel Gastli 29 Classification of DC Machine Field Field Armature Field Armature Shunt Separately excited A1 φf F1 Series A1 φf φs S1 F2 Armature S2 F1 S1 F2 S2 A2 A2 Short-shunt Dr. Adel Gastli φs Motor operation Generator operation Long-shunt MCTE3210: Electromechanical Systems & Actuators 30
Classification of DC Machine A1 φf F1 φs S1 F2 A1 φf S2 S1 F2 F1 φs S2 A2 A2 Cumulative compound Differential compound Motor operation Generator operation MCTE3210: Electromechanical Systems & Actuators Dr. Adel Gastli 31 DC Machine Representation The mmf’s produced by the field circuit and the armature circuit are in quadrature. q-axis d-axis q-axis Field Armature d-axis Armature mmf Field mmf φa Armature mmf Φ Field mmf φf Dr. Adel Gastli Saturation Linear Flux-mmf relation in a dc machine Fp MCTE3210: Electromechanical Systems & Actuators 32
Magnetization (or Saturation) Curve of a DC Machine Ea Φ Speed ωm Saturation Linear 0.5 ωm If Nf Flux-mmf relation in a dc machine If Magnetization curve The magnetizing curve is obtained experimentally by rotating the the dc machine at a given speed and measuring the open-circuit armature terminal voltage as the current in the field winding is changed. Magnetization Curve Represents the saturation level in the magnetic system of the dc machine for various values of excitation mmf . MCTE3210: Electromechanical Systems & Actuators Dr. Adel Gastli 33 Dc Motors Equations Separately Excited DC Motor It Ia Ra Rfw ωm E a Vt I a R a Vt Rfc If Vf Vf Rf I f E a K aΦωm Te K a Φ I a ¾ Rfw: resistance of field winding. ¾ Rfc: resistance of control rheostat used in field circuit. ¾ Rf Rfw Rfc: total field resistance ¾ Ra: resistance of armature circuit, including the effect of brushes. Sometimes Ra is shown as the resistance of armature winding alone; the brush-contact voltage drop is considered separately and is usually assumed to be about 2V. Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 34
Dc Motors Equations Shunt or Self-Excited DC Motor If It Ia Ra Rfc Vt ωm E a Vt I a R a E a K aΦω m , Vt I t R L , Rfw V f R f I f Vt Te K a Φ I a Ia It I f MCTE3210: Electromechanical Systems & Actuators Dr. Adel Gastli 35 Dc Generator Equations Separately Excited DC Generator Ia Vf (Rfw Rfc)I f Rf I f IL Ea Vt Iara ra ωm Rfw Rfc If Dr. Adel Gastli Vt Ea RL Ea KaΦ ωm Vt ILRL Ia IL Vf MCTE3210: Electromechanical Systems & Actuators 36
Dc Generator Equations Self-Excited DC Generators 1. Shunt generator If Vf RfI IL Rfc ra Rfw ωm RL Vt E a K aΦ ω m Vt I L RL Ea Ia IL I MCTE3210: Electromechanical Systems & Actuators Dr. Adel Gastli Vt E a V t I a ra Ia f f 37 Dc Generator Equations 2. Series Generator Ia IL Vt Ea Ia (ra Rs ) ra Ea Rs Vt RL I L Ia I f Ea KaΦsωm Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 38
Dc Generator Equations 3. Compound DC Generator If If IL Rfc Rs Vt Rfw Short Shunt Vt Ea E a K a (Φ Vt Ea I a Ra I L Rs Rs Ea Rfw Ia Ra Ia Ra Rfc IL sh Φ s )ω Vt Ea I a (Ra Rs ) m IL Ia I f IL Ia I f If Long Shunt Ea I a Ra R fw R fc If Differential Cumulative Vt R fw R fc Ea K a (Φ sh Φ s )ωm E K (Φ sh Φ s )ωm a a Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 39 Power Flow and Efficiency IL If DC Generators Ia Rfc Ra Rs Vt Ea Rfw Ea I a Pinput Pmech Pshaft Rotational losses η η Dr. Adel Gastli η Va I a Pinput Vt I L Vt I L Poutput Pelectrical I 2f R f I L2 Rs I a2 Ra Poutput Va I L Poutput Poutput Losses Vt I L I 2 R Rotational Losses Vt I L Electromechanical Systems & Actuators EMCTE3210: I Rotational Losses a a 40
Power Flow and Efficiency IL If DC Motors Ia Rfc Ra Rs Vt Ea Rfw Pinput Pelectrical Vt IL Va IL I 2f Rf IL2 Rs η η η Dr. Adel Gastli Poutput Pinput Vt I L Va Ia Ea Ia Poutput Pmech Pshaft Ia2 Ra Rotational losses Pinput Losses Pinput I 2 R Rotational Vt I L Losses E a I a Rotational Losses MCTE3210: Electromechanical Systems & Actuators Vt I L 41 Torque-Speed Characteristics Separately excited & Shunt motors (φ is independent of the load torque ) V t E a I a ra E a K aΦ ω m ωm Ia Vt I a ra K aΦ T K aΦ I a ωm Vt K aΦ Therefore , V ra ωm t T K aΦ (K aΦ)2 ra Slope ( K Φ ) 2 a T Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 42
Torque-Speed Characteristics Series motors E a Vt I a ( R a R s ) E a K aφ ω m φ K1I f K1I a Neglecting saturation E a K a K 1 I aω m K s I aω m Vt R Rs a KsIa Ks ωm But T K aφ I a K a K 1 I a2 K s I a2 ωm Vt Ks T Dr. Adel Gastli Ra Rs Ks MCTE3210: Electromechanical Systems & Actuators 43 Torque-Speed Characteristics Compound motors Cumulative Compound AT t AT shunt AT series Differential Compound φ t φ shunt φ series ωm Dr. Adel Gastli Shunt motor Vt ra T K aφ t ( K aφ t ) 2 MCTE3210: Electromechanical Systems & Actuators 44
Starting of DC Machine If a d.c. motor is directly connected to a d.c. power supply, the starting current will be dangerously high. Ia Vt E a ra Ia ω 0 Ea 0 at starting Starting Vt ra Since ra is small, the starting current is very large. The starting current can be limited by the following methods: 1- Use a variable-voltage supply. 2- Insert an external resistance at start, as MCTE3210: Electromechanical Systems & Actuators Dr. Adel Gastli shown in the Figure. 45
Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 3 Introduction Most of the electrical machine in service are AC type. DC machine are of considerable industrial importance. DC machine mainly used as DC motors and the DC generators are rarely used. DC motors provides a fine control of the speed which can not be attained by AC motors.
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