28-07-2012, 03:54 PM
Solid State Devices Speed Control of DC Motors
DC Motors Speed Control_mod.pdf (Size: 242.22 KB / Downloads: 188)
Basics of DC Motors Speed Control
Up to the rated (base) speed (the speed corresponding to rated operation condition, rated armature
voltage, rated armature current, and rated field current), the armature and the field currents are kept
constant to maintain the torque at its rated value. The speed control within this range is carried out by
varying the armature voltage. Beyond the base speed, the speed control the achieved by varying the
field current. Within this range, the motor power is maintained constant and the torque is reduced with
the increase in the rotational speed as shown in Fig. 1. For series motor, the speed control beyond the
base speed is carried out using the armature current variation (since the armature and the field current
are the same in series motors) as shown in Fig. 2.
Speed Control using Power Semiconductor Switches
As discussed in previous lectures, there are several power semiconductor switches configuration that
can be used to provide variable output DC voltage. Single and three-phase AC-to-DC converters
(rectifiers) can be used to vary the input armature voltage and / or the field voltage of DC machines to
control it speed of rotation as shown in Fig. 3-a. Moreover, DC-to-DC converters can also be used for
speed control as shown in Fig. 3-b. the following subsections will briefly discuss some example of the
used power semiconductor converters in the speed control of DC motors.
Single-Phase Rectifiers
In these configurations, the armature and / or the field winding voltage can be varied using singlephase
full bridge AC-to-DC converters as shown in Fig. 4-a. This is a two quadrant converter, as
shown in Fig. 4-b, where the load current is always positive while the load voltage may be positive or
negative. The generated waveforms are shown in Fig. 4-c. Single-phase AC-to-DC semi-converters
can be also used for DC motors speed control as shown in Fig. 5-a. However, this is a first-quadrant
converter, as shown in Fig. 5-b, where the load current and voltage are always positive. The generated
waveforms are shown in Fig. 5-c. Usually the same AC supply is used as an input for the two
converters. The average value of the armature and / or field voltage (and hence the rotational speed)
can be varied and controlled by varying the firing angle of the thyristors used. The equation relating
the armature and the field voltage with the supply voltage and the firing angle can be expressed as
follows.