Introduction to Electrical Systems Course Code: EE 111 Course Code: - - PowerPoint PPT Presentation

Introduction to Electrical Systems Course Code: EE 111 Course Code: EE 111 Department: Electrical Engineering Department: Electrical Engineering Instructor Name: B G Fernandes Instructor Name: B.G. Fernandes E‐mail id: bgf@ee iitb ac in E‐mail id: bgf@ee.iitb.ac.in

Sat Nov 7, 2009 EE 111: Introduction to Electrical Systems

• Prof. B.G.Fernandes

1/8 Lecture 38

Sub‐Topics:

• Classification of dc machines
• Performance characteristics

Sat Nov 7, 2009 EE 111: Introduction to Electrical Systems

• Prof. B.G.Fernandes

2/8 Lecture 38

Review ⇒ speed of F = 0 & there is saliency ⇒ speed of Fs= 0 & there is saliency ⇒ ‘V’ induced in armature conductor is ac t t b h t t d ⇒ commutator + brushes convert ac to dc → mechanical rectifier b h fitt d t th t t t d th ⇒ brushes are fitted to the stator, mounted on the commutator ⇒ direction of current in all the armature ⇒ direction of current in all the armature conductors on one side of pole is the same & armature mmf axis is fixed → it is always at 900 w.r.t. field axis.

r

F

• F

90 or δ 90 ∠ = =

Sat Nov 7, 2009 EE 111: Introduction to Electrical Systems

• Prof. B.G.Fernandes

s

F

90 or δ 90 ∠

3/8 Lecture 38

⇒ unlike other machines(IM & syn.), in dc machine, ‘δ’ does not change with load δ does not change with load ⇒ if ‘F’s is held constant, ‘F’r(ia) will change ∵ δ 900 F (I ) & F (I )can be changed independently ∵ δ = 900, FS(IF) & Fr(Ia)can be changed independently

(more later)

∵ brushes are in contact with commutator ∵ brushes are in contact with commutator wear/tear, regular maintenance ⇒ may not be suitable for high speed application y g p pp (arcing may take place) ⇒ Eo → open circuit voltage w.r.t. IF for

• p

g

F

constant ‘ω’ ∴ E α I

Sat Nov 7, 2009 EE 111: Introduction to Electrical Systems

• Prof. B.G.Fernandes

∴ Eo α IF

4/8 Lecture 38

⇒ due to armature reaction air gap flux reduces. stator eq. ckt: ⇒ at steady state, VF = RFIF rotor eq. ckt: ⇒ Induced emf in armature, E Kφ rotor eq. ckt:

Sat Nov 7, 2009 EE 111: Introduction to Electrical Systems

• Prof. B.G.Fernandes

E = Kφω

5/8 Lecture 38

classification of dc machines: separately excited dc generator(S E) ⇒ field is connected to a separate dc source separately excited dc generator(S.E): source

2 F F F

v P R =

F

⇒ for given VF, PF ↓ as RF ↑ → use thin conductor & there would be large no of turns → use thin conductor & there would be large no. of turns external characteristics: → variation of V with I at constant ω → variation of Vt with IL at constant ω

t a a

V E I R = −

a L

I I = E Kφ

Sat Nov 7, 2009 EE 111: Introduction to Electrical Systems

• Prof. B.G.Fernandes

E Kφω =

6/8 Lecture 38

• as Ia ↑, ‘φ’ due to Ia also ↑ &

due to armature reaction, → E α (air gap flux) ω due to armature reaction, air gap flux ↓ → E α (air gap flux) ω ⇒ power is developed in the armature, Ra comes in the main path of power flow, so it should be small. ⇒ needs a separate dc source ⇒ instead if there is residual magnetism main path of power flow, so it should be small. ⇒ instead if there is residual magnetism → connect the field across the arm. t i l terminals → as the rotor starts rotating ll ‘V’ i i d d i h

Sat Nov 7, 2009 EE 111: Introduction to Electrical Systems

• Prof. B.G.Fernandes

→ small ‘V’ is induced in the armature

7/8 Lecture 38

→ this ‘V’ drive a small ‘I’ in the field vr

F a F

v i R R Δ ≅ + → this Δi flows in N (field winding) thi φ ith id th id l fl → generates its own flux → this ΔiF flows in NF (field winding) → this φ can either aid or oppose the residual flux → if it aids, then there is ↑ in air gap flux hi ↑ h ‘V’ i d d i h & Δi → this ↑ the ‘V’ induced in the armature & ∴ ΔiF → ‘V’ builds up & attain a steady value → m/c develops ‘V’ at its own & it is due to residual magnetism h h lf d d

Sat Nov 7, 2009 EE 111: Introduction to Electrical Systems

• Prof. B.G.Fernandes

→ hence the name self excited dc generator

8/8 Lecture 38