Dc machines 2

motors

1. 
There are four different methods for supplying the dc current to the motor There are four different methods for supplying the dc current to the motor
or generator poles:or generator poles:

– – Separately excitation;Separately excitation;

– – Shunt connectionShunt connection

– – Series connectionSeries connection

– – CompoundCompound

2. Field Coil
ArmatureV
f
+
-

3. Series Field Coil
Armature
R
A

4. 
Shunt Field CoilShunt Field Coil
Armature
R
A

5. Torque – Power ConversionTorque – Power Conversion

.If Ta is the torque exerted by the prime mover .If Ta is the torque exerted by the prime mover
running at n rps, then the power developed is:running at n rps, then the power developed is:
Ta. Ta. 2 π 2 π n n watt watt

.and the mechanical power converted into .and the mechanical power converted into
electrical power in the armature is:electrical power in the armature is:
E E
⋅

⋅
Ia Ia wattwatt
Then Ta Then Ta 2
⋅
2
⋅
ππn n = = E E
⋅

⋅
IaIa

6. Total Loss in a DC machineTotal Loss in a DC machine

7. Loss in a DC machineLoss in a DC machine

1. Copper losses1. Copper losses
These losses occur due to currents in the various These losses occur due to currents in the various
windings of the machinewindings of the machine

8. Loss in a DC machineLoss in a DC machine

2. Iron or Core losses2. Iron or Core losses
These losses occur in the armature of a d.c. machine and are due to the rotation of These losses occur in the armature of a d.c. machine and are due to the rotation of
armature in the magnetic field of the poles. They are of two types viz., (i) hysteresis armature in the magnetic field of the poles. They are of two types viz., (i) hysteresis
loss (ii) eddy current loss. loss (ii) eddy current loss.

(i) Hysteresis loss(i) Hysteresis loss

Hysteresis loss occurs in the armature of the d.c. machine since any given part Hysteresis loss occurs in the armature of the d.c. machine since any given part
of the armature is subjected to magnetic field reversals as it passes under of the armature is subjected to magnetic field reversals as it passes under
successive poles. Fig. (1.36) shows an armature rotating in two-pole machine. successive poles. Fig. (1.36) shows an armature rotating in two-pole machine.
Consider a small piece ab of the armature. When the piece ab is under N-pole, Consider a small piece ab of the armature. When the piece ab is under N-pole,
the magnetic lines pass from a to b. Half a revolution later, the same piece of the magnetic lines pass from a to b. Half a revolution later, the same piece of
iron is under S-pole and magnetic lines pass from b to a so that magnetism in iron is under S-pole and magnetic lines pass from b to a so that magnetism in
the iron is reversed. In order to reverse continuously the molecular magnets in the iron is reversed. In order to reverse continuously the molecular magnets in
the armature core, some amount of power has to be spent which is called the armature core, some amount of power has to be spent which is called
hysteresis loss. It is given by Steinmetz formula. hysteresis loss. It is given by Steinmetz formula.

9. Loss in a DC machineLoss in a DC machine
a low value of Steinmetz hysteresis co-efficient e.g., silicon
steel.

10. Loss in a DC machineLoss in a DC machine
a low value of Steinmetz hysteresis co-efficient e.g., silicon
steel.