Page 75 - Servo Motors and Industrial Control Theory -

68 4 Electrical DC Servo Motors

Fig. 4.5 Ideal speed torque speed
characteristic of DC servo
motors Tarque

4.3 Speed Torque Characteristic of DC Servo Motors

One of the important properties of DC motor is its speed torque characteristic. Ide-
ally when the voltage is connected to the motor, it must provide a constant torque to
accelerate the rotor and when the required speed is reached the torque must become
zero. This is shown in Fig. 4.5
The important property of DC servo motor is its speed torque characteristic.
The voltage equation ignoring the inductance may be written as,
v: = RI+C ω
m m
And the torque is proportional to the current,

T: = KI
t
The values of K is the motor torque constant and is defined by manufacturers. When
t
a voltage is applied initially, the speed is zero and a large current flows through the
armature and it produces a large torque to accelerate the rotor. As speed is gradually
increased, the current/torque reduces until the steady speed is reached. The initial
large current is almost ten times of its rated power. This is shown for various speeds
in Fig. 4.6.

4.4 DC Servo Motors in Open and Closed Loop Velocity
Control

First, the velocity control will be studied. The voltage equation of the DC motor
ignoring the Inductance can be written as

V: = RI + C ω m
m
The torque produced by the motor is proportional to the current as

T: = KI
t