EMF Equation
description
Transcript of EMF Equation
• EMF induced in DC generator:
Directly proportional to flux per pole (Ф). Directly proportional to speed (N).
• Polarity of the induced emf:
Depends on the direction of magnetic field.Depends on the direction of rotation.
• In the first half of the revolution , current flows always along ABLMCD i.e. brush no 1 in contact with segment a.
• In the next half revolution, the direction of the induced current in the coil is reversed.
• But at the same time the position of the segments a and b are also reversed which results that brush no 1 comes in touch with that segment b.
• Hence, the current in the load resistance again flows from L to M.
EMF Equation
Let,
Ø= flux per pole in weber
Z = Total number of conductor
P = Number of poles
A = Number of parallel paths
N =armature speed in rpm
Eg = emf generated in any on of the parallel path
Flux cut by 1 conductor in 1 revolution = P * φ
Flux cut by 1 conductor in 60 sec = P φ N /60
Avg emf generated in 1 conductor = PφN/60
Number of conductors in each parallel path = Z /A
Eg = PφNZ/60A
• Methods of excitation ( Supply to field winding)
• In a separately excited DC generator, the field winding is excited by an external independent source.
• IF - Field Current
• Eg – Generated EMF
• Ia - Armature current
• IL - Load current
• V – Terminal voltage across the Load.
Self-excited DC Generators
• These are the generators whose field magnets are energized by the current supplied by themselves.
• In these type of machines field coils are internally connected with the armature.
• Due to residual magnetism some flux is always present in the poles.
• Field coils are connected across the output voltage of the armature.
• In a shunt-wound generator, the field coils consist of many turns of small wire as less amount of current flow through it.
• If Ia=100 A, then If = 2 to 10 A (approx.)
• In these type of generators, the field windings are connected in series with armature conductors.
• So, whole armature current flows through the field coils as well as the load.
• As series field winding carries full load current, it is designed with relatively few turns of thick wire.
• The resistance of series field winding is therefore
very low (nearly 0.5Ω ).
• Compound wound generators have both series field winding and shunt field winding.
• One winding is placed in series with the armature and the other is placed in parallel with the armature
The generators in which shunt field winding is in parallel with both series field and armature winding