Figure 6-15.--A two-speed de controller with shunt field rheostat.
Table 6-5.--Operation of slow-speed circuit
The motor will run at low speed until coil LM is
de-energized. The LM and HM contractors are
mechanically interlocked to prevent both from closing
at the same time.
DC Speed Selection
The speed of dc motors is determined by the amount
of current flowing through both the field winding and
the armature winding. If resistance is added in series
with the shunt field (fig. 6-15), the current through the
shunt field winding will be decreased. The decreased
field strength will momentarily decrease the amount of
cemf produced, and the motor will sped up. Once the
motor speeds up the amount of cemf will rise and again
limit the armature current.
In a similar manner, a decrease in resistance
increases the current flow through the field windings,
momentarily increases the production of cemf, and
slows the motor down.
The operation of the slow-speed circuit is shown in
table 6-5.
6-10