2. DC Machines
The armature of a 6-pole dc machine is wound with a wave
winding containing 664 conductors.
(a) Calculate the generated emf when the flux per pole is 0.06
Wb and speed is 250 r/min.
(b) At what speed must the armature be driven in order to
generate 550 V, If
(i) the flux is reduced to 0.058 Wb
(ii) the flux is kept constant at 0.06 Wb
3. Types of dc generators
• Dc machines are classified according to the type of connection
between the armature and field windings
Separately Excited dc Machines
Self excited Shunt Connected dc generator
Self Excited Series Connected dc generator
Compound Excited dc Machine
4. Separately Excited dc Machines
• The field winding is separately excited from an independent dc
source such as battery or a small dc generator known as an
exciter
5. Shunt Connected dc generator
• The field winding is connected in parallel with the armature
winding
6. Series Connected dc generator
• The field winding is connected in series with the armature
winding.
• The armature current is also the field current
7. Compound Excited dc Machine
• It employs the combination of the two previous methods.
8. Armature Reaction
• When the field is excited and the machine is on no-load,, the
flux is uniformly distributed and the magnetic neutral axis
coincides with the geometrical neutral axis as shown
10. Armature Reaction
• The resultant flux is the sum of the two fluxes and is distribute
as shown in Fig
11. Armature Reaction
• In the leading pole tip, flux is strengthened, while in the trailing
pole tips the flux is weakened.
• The increase in flux in the leading pole tips is less than the
decrease in flux in the trailing pole tips
• Because in the former the core becomes more saturated while
int he latter the core becomes less saturated
• Armature reaction results in distortion of the field distribution
and weakens the field
12. Armature Reaction
• The distortion shifts the magnetic neutral axis by an angle
• The coil being short-circuited by the brush has emf induced in it
because it no longer lies in the magnetic neutral axis as was the
case at no-load.
• The brush must be advanced so that they coincide with the
MNA, so that commutation occurs on the MNA axis
• In order to produce a given flux per pole at full load, the field
mmf calculated on no-load must be increased to allow for the
demagnetizing effect of the armature reaction
14. Separately Excited Generator
• Decrease due to voltage drop across the armature circuit
resistance
• Decrease due to a fall in generated emf caused by
The demagnetizing effect of the armature current
Decrease due to a fall in speed of the prime mover
17. Example
A separately excited dc generator is rated 100 kW, 500 V 2400
r/min and has an armature circuit resistance of 0.1 Ω, field circuit
resistance of 25 Ω, . The machine delivers rated load at rated
voltage and speed. Calculate
No load voltage
Voltage regulation
25. Compound Generator
• The demagnetizing effect of armature reaction is proportional to
the armature current.
• It can therefore be neutralized by using a series winding wound
on the main field poles but connected in series with the
armature winding
• The compound dc generator combines the characteristics of the
shunt and series generators
• When series winding is so connected that its flux aids the main
flux the drop due to the demagnetizing effect of armature
reaction can be eliminated by proper choice of the number of
series turns.
