Page 73 - Servo Motors and Industrial Control Theory -

66 4 Electrical DC Servo Motors

DC Servo Motors

Separately Excited Permanent Magnet Brushless
DC Motors

Ceramic Magnet Rare Earth magnet

Fig. 4.1 Types of DC servo motors

4.2 Types of Power Unit

The recent development in power electronics is now possible to control large cur-
rent with low power control signal. One of these devices is thyristor. The operation
of a thyristor is similar to a transistor with an extra connection of a gate. When a
low power level control signal is connected to the gate, large current flows through
it and when the control signal is removed the conduction stops when the current be-
comes zero. With this method, large DC servo motors can be controlled. For smaller
motors, transistors are used to control the armature current. A simple diagram of
thyristor controlled separately excited DC servo motor is shown in Fig. 4.2.
The power supply is connected to the input of the thyristor. The period of con-
duction depends on when the control signal Vc with respect to the position of input
sine wave is excited. In this way, a portion of the sign wave will pass through the
thyristor. The motor, because of rotor inertia, only responds to the average of output
voltage. In Fig. 4.2, the armature resistance and inductance are shown by R and L.
The output voltage is connected to the armature through brushes. The output volt-
age although is rectified is a series of pulses. For half wave rectification, the output
frequency is 50 Hz. When a full wave rectification is used, the output voltage fre-
quency is 100 Hz. Single phase, two phase, and three phase half or full wave recti-
fication produces output voltage 100, 200, and 300 Hz respectively. Of course, the
higher the frequency of the output voltage, the smoother operation of the motors is
achievable. The rotor fluctuation of speed will generate noise in sensors. Carefully
designed velocity, position, and current transducers must be used various types of
power supply for DC servo motors is shown in Fig. 4.3.
In the pulse width modulated (PWM) power unit, the power supply is first recti-
fied with full wave rectification method. The DC conduction of the output voltage
to the motor is controlled by transistors for smaller motors and with thyristor for
larger motors. The output voltage will be a series of pulses with controlled mark
to space ratio. This is shown in Fig. 4.4. With this method output, voltage of high