As we know any electric motor can be used as a servo motor if it is controlled by a servomechanism. Similarly, if we control a DC motor using a servomechanism, it will be called a DC servo motor. There are different types of DC motor, such as shunt wound DC motor, series DC motor, separately excited DC motor, permanent magnet DC motor, brush less DC motor etc. They mainly include separately excited DC motor, permanent magnet DC motor and brush less DC motor. Used as a server.
Separately Excited DC Servo Motor:
DC Servo Motor Theory:
Motors that are used as DC servo motors usually have separate DC sources for the field winding and the armature winding. Control can be archived by controlling either field current or armature current. Field control has certain advantages over armature control and on the other hand armature control also has certain advantages over field control. The type of control applied to a DC servo motor is decided depending on the specific application.
Let us discuss the working principle of DC servo motor for field control and armature control one by one.
Field Controlled DC Servo Motor Theory:
The figure below illustrates the schematic diagram of a field controlled DC servo motor. In this arrangement the field of the DC motor is excited by the amplified error signal and the armature winding is energized by a constant.
The field is controlled below the knee of the magnetizing saturation curve. On this part of the curve the mmf varies linearly with the excitation current. This means that the torque developed in a DC motor is directly proportional to the field current below the knee of the magnetic saturation curve.
From the general torque equation of a DC motor, it follows that, torque T ∝ φIa. Where, φ is the field flux and IA is the armature current. But in a field-controlled DC servo motor, the armature is excited by a constant current source, so here Ia is constant. Therefore, T ∝ φ
Since the field of this DC servo motor is excited by the amplified error signal, the torque of the motor i.e. the rotation of the motor can be controlled by the amplified error signal. If the constant armature current is large enough, every small change in field current causes a corresponding change in torque on the motor shaft.
The direction of rotation can be changed by changing the polarity of the field. The direction of rotation can also be reversed using a split-field DC motor, where the field winding is split into two halves, one half of the winding wound clockwise and the other half wound counterclockwise. I happen. The amplified error signal is given at the junction point of these two parts of the field as shown below. The magnetic field of both parts of the field winding is opposite to each other. During motor operation, the magnetic field strength of one half of the section dominates the other depending on the value of the amplified error signal fed between the sections. Because of this, the DC servo motor rotates in a certain direction according to the amplified error signal voltage. The major disadvantage of field-controlled DC servo motors is that the dynamic response to a fault is slow due to the long time constant of the inductive field circuit. The field is an electromagnet so it is basically a highly inductive circuit so due to a sudden change in the error signal voltage, the current through the field will reach its steady state after a certain period of time which is the time constant of the field circuit. depending on. This is why the field control DC servo motor arrangement is mainly used in small servo motor applications.
The major advantage of using a field control scheme is that, as the motor is controlled by the field – the controlling power requirement is much less than the rated power of the motor.
Armature Controlled DC Servo Motor Theory:
The figure below shows the schematic diagram for an armature-controlled DC servo motor. Here the armature is energized by the amplified error signal and the field is excited by a constant current source.
The field is driven well past the knee point of the magnetizing saturation curve. In this part of the curve, for a large change in magnetizing current, there is a very small change in mmf in the motor field. This makes the servo motor less sensitive to changes in field current. Actually for an armature controlled DC servo motor, we don’t want this, the motor must respond to any change in field current.
Again, the field flux is maximum at saturation. As we said earlier, the general torque equation of a DC motor is, torque T ∝ φIa. Now if φ is large enough, there will be a significant change in motor torque for every small change in armature current Ia. This means that the servo motor becomes very sensitive to the armature current.
Since the armature of a DC motor is less inductive and more resistive, the armature winding time is considerably less. This causes a sudden change in armature current due to a sudden change in armature voltage. This is why the dynamic response of an armature controlled DC servo motor is much faster than that of a field controlled DC servo motor. The direction of rotation of the motor can be easily changed by reversing the polarity of the error signal.
Permanent Magnet DC Servo Motor:
Field control is not possible in case of permanent magnet DC motor because the field here is a permanent magnet. In this case the working principle of DC servo motor is similar to armature controlled motor.