CAMTECH/97/E/TM/3.0
TM Maintenance Handbook January’97
12
At light loads I
a
and hence
φ
will be
quite low and E
b
will be high, therefore
`N’ may become quite high. However
as the load on the motor increases,
speed will decrease rapidly & with
further increase in armature current,
the increase in flux is quite small
because of saturation effect & hence
the curve flattens out (as shown in
figure 1.11).
Similarly we see that at low speeds, high torque can be obtained, and with
increase in speed, less torque is developed.
This characteristics is particularly useful for
traction purposes, because requirement of
torque if high at starting, whereas once
the train is in motion the low friction
resistance reduces the torque requirement
for a given load.
This characteristics (as shown in figure 1.12) clearly indicated that DC series
motor should never be started without load, which is the case in traction.
Figure 1.11
Figure 1.12
