- DC Servo Motor Definition: A DC servo motor is a DC motor configured to respond to control signals from a servomechanism for precise motion control.
- Control Methods: DC servo motors can be controlled either by adjusting the field current (field control) or the armature current (armature control), each with distinct advantages.
- Field Controlled Motor: This type of motor adjusts torque through field current changes, suited for applications needing stable but less dynamic response.
- Armature Controlled Motor: In this setup, motor response is quicker due to direct armature current adjustments, ideal for applications requiring fast response times.
- Permanent Magnet Motor: Permanent magnet motors use a constant magnetic field, and control is achieved by varying armature current, simplifying the control mechanism but restricting field adjustability.
Any electrical motor can function as a servo motor when controlled by a servomechanism. Specifically, a DC motor controlled this way is called a DC servo motor. Common types of DC motor include shunt wound DC motor, series DC motor, separately excited, permanent magnet DC motor, and Brushless DC motor. Typically, separately excited, permanent magnet DC motor, and brush less DC motor are preferred for servo applications.
Separately Excited DC Servo Motor
DC Servo Motor Theory
DC servo motors typically use separate DC sources for field winding and armature winding. They can be controlled by adjusting either the field or armature current. Each method has its benefits: field control is stable while armature control responds more quickly. The choice of control method depends on the motor’s specific application.
Let’s explore the working principles of field control and armature control in DC servo motors.
Field Controlled DC Servo Motor Theory
The figure below illustrates the schematic diagram for a field controlled DC servo motor. In this arrangement the field of DC motor is excited be the amplified error signal and armature winding is energized by a constant current source.
The field is controlled below the knee point of magnetizing saturation curve. At that portion of the curve the mmf linearly varies with excitation current. That means torque developed in the DC motor is directly proportional to the field current below the knee point of magnetizing saturation curve.
From general torque equation of DC motor it is found that, torque T ∝ φIa. Where, φ is field flux and Ia is armature current. But in field controlled DC servo motor, the armature is excited by constant current source, hence Ia is constant here. Hence, T ∝ φ
As field of this DC servo motor is excited by amplified error signal, the torque of the motor i.e. rotation of the motor can be controlled by amplified error signal. If the constant armature current is large enough then, every little change in field current causes corresponding change in torque on the motor shaft.
The direction of rotation in a DC servo motor can be reversed by changing the field’s polarity or using a split field motor. In a split field motor, the field winding is split into two halves wound in opposite directions. An amplified error signal controls the dominance of magnetic field from each half, determining the motor’s direction based on the signal’s voltage. The main disadvantage of field control DC servo motors, is that the dynamic response to the error is slower because of longer time constant of inductive field circuit. The field is an electromagnet so it is basically a highly inductive circuit hence due to sudden change in error signal voltage, the current through the field will reach to its steady state value after certain period depending upon the time constant of the field circuit. That is why field control DC servo motor arrangement is mainly used in small servo motor applications.
A major advantage of using a field control scheme is that it requires much less power than the motor’s rated power, making it energy-efficient.
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 amplified error signal and field is excited by a constant current source.
The field is operated at well beyond the knee point of magnetizing saturation curve. In this portion of the curve, for huge change in magnetizing current, there is very small change in mmf in the motor field. This makes the servo motor is less sensitive to change in field current. Actually for armature controlled DC servo motor, we do not want that, the motor should response to any change of field current.
Again, at saturation the field flux is maximum. As we said earlier, the general torque equation of DC motor is, torque T ∝ φIa. Now if φ is large enough, for every little change in armature current Ia there will be a prominent changer in motor torque. That means servo motor becomes much sensitive to the armature current.
As the armature of DC motor is less inductive and more resistive, time constant of armature winding is small enough. This causes quick change of armature current due to sudden change in armature voltage. That is why dynamic response of armature controlled DC servo motor is much faster than that of field controlled DC servo motor. The direction of rotation of the motor can easily be changed by reversing the polarity of the error signal.
Permanent Magnet DC Servo Motor
Field control is not possible in the case of permanent magnet DC motor as the field is a permanent magnet here. DC servo motor working principle in that case is similar to that of armature controlled motor.
