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DC Motor vs Servo Motor

Almost every electromechanical device/equipment that we see around us is driven by an electric motor. Electric motors are used to power hundreds of devices in our everyday life from air conditioning systems, and security cameras to DVD players. They do so by converting electrical energy into mechanical energy. With so many design variables to consider including wire system, construction, acceleration, torque, and drive circuitry, selecting the right electric motor for a given application can be quite a task. As such, this article intends to help you choose between a DC Motor and a Servo Motor.

DC Motors Overview 

Essentially, electric motors rely on elementary electromagnetism for their operation. Such that when a conductor carrying an electric current is placed in a magnetic field, the resultant magnetic field exerts a force on the conductor. The force exerted is directly proportional to the strength of the external magnetic field and the amount of current in the conductor.

A DC motor receives its input power from a direct current source. It then converts the received electrical energy from the direct current source into mechanical energy (e.g., shaft rotation). Its working mechanism is based on the fact that a conductor carrying an electric current and placed in a magnetic field will experience a magnetic force that will cause it to spin (rotate) with regard to its original position. The armature is the conducting part of a DC motor, and the field windings provide the magnetic flux. A typical DC motor comprises two primary parts – the stator and the rotor. The stator forms the magnetic field system (it houses the field windings and receives the supplied power), while the rotor forms the armature system which brings about mechanical rotations.

DC motors have a wide range of applications including but not limited to elevators, rolling mills, steel mills, locomotives, and excavators. They power numerous mechanical systems that require rotational output motion, such as lathe machines, milling machines, and grinders. The choice of the size and type of a DC motor depends on the power requirements for the specific application. For instance, a DC motor used to operate security cameras or DVD players cannot be used to drive an excavator.

DC Motor Construction  

Generally, the DC motor assembly consists of a Stator, Rotor, Yoke, Poles, Field and Armature Windings, Brushes, and Commutator.

The functions of the aforementioned parts of a DC motor assembly include:

The Yoke/Frame: This is a magnetic frame made from cast iron or steel. The yoke is an essential component of the DC motor’s stator. Its primary role is to support the armature and provide a protective casing for the motor’s intricate inner workings. It protects the DC motor’s magnetic poles and field winding. It also provides structural support for the field system.

Poles: The magnetic poles of a DC motor are screwed into the yoke’s inner wall. Poles are made up of the pole core and the pole shoe; the two parts are stacked together using hydraulic pressure. The pole shoe has a large cross-sectional area that enables it to spread the produced magnetic flux throughout the air gap between the stator and rotor, thereby reducing resistance losses. In contrast, the pole core is characterized by a smaller cross-sectional area for holding the pole shoe over the yoke. Slots for the field windings, which generate the magnetic field flux, are also included on the pole shoe.

Field Windings: Copper wire coils are used to create the field windings of a DC motor, and they are coiled across the slots in the pole shoes in such a way that when a field current flows through them, they create poles with opposite polarities. This generates a magnetic field. And in a DC motor, the rotor armature rotates within the magnetic field created by the field winding, effectively cutting the magnetic flux.

Armature Winding: It’s coupled to the motor shaft, which provides output power to the motor drive equipment. Due to its location on the rotor, the armature winding experiences magnetic losses as a direct result of the varying magnetic field along its path of rotation. Because of this, magnetic losses like hysteresis and eddy current losses are minimized using an armature core constructed from several layers of low-hysteresis silicon steel lamination for the rotor.

Note: The bearing provides the rotating armature/motor shaft with free and smooth rotation to reduce friction.

Slots made from the armature core material are used to secure the armature winding, which consists of multiple rounds of copper wire wound around the whole circumference of the armature core. Due to the tremendous centrifugal force created by the spinning of the armature in the presence of supply current and magnetic field, the slot apertures are closed with fibrous wedges to prevent the conductor from flowing out.

Commutator: This is a cylindrical device with copper segments that are stacked and insulated from one another by the mica. It is responsible for commuting or relaying the supply current from the mains to the armature winding. In some DC motors, brushes made with a graphite structure are used to relay the current from the external circuit to the commutator.

Working Principle of a DC Motor

When the DC motor is connected to an external direct current supply, its field windings get excited, thereby developing alternate north and south magnetic poles. The supplied DC current then flows through the armature windings. The armature conductors under the south pole will carry the current in one direction while those in the north pole in the opposite direction. The direction of the armature movement is determined using Fleming’s left-hand rule.

The current carrying armature conductors are within the magnetic field; so a magnetic force acts on them. All the forces in the armature conductors sum up, exerting a torque that causes the armature to rotate. As each armature conductor moves from one side of the brush to another, its current gets reversed. But since it moves under the influence of opposite polarity, the force direction in the conductor does not change. This implies that the motor will continue to rotate in the same direction.

Servo Motors Overview

A servo motor can be described as a rotary (or linear) actuator that facilitates accurate and precise control of angular (or linear) position, velocity, and acceleration. It can be seen as an assembly of a DC motor, a gearing set, a potentiometer, and a control circuit. The DC motor is coupled to a feedback sensor for position feedback. The feedback sensor detects the difference between the preset speed or position and the actual speed or position and provides a corrective mechanism (adjustment) to achieve the targeted position or speed.

Servo motors are suitable for use in a closed-loop control system. They are commonly applied in Computer Numerical Control (CNC) machines, automated manufacturing, hydraulics, and robotics. For example, servo motors are widely used to control the positioning and movement of elevators in radio-controlled airplanes. This is due to their high accuracy and precision. To achieve such high precision, a servo motor requires a sophisticated controller–usually a dedicated control module designed to explicitly provide servo control. Note: The rotation of servo motors is limited to 180 degrees.

In terms of rotation control, servo motors are classified into three primary categories: positional rotation, continuous rotation, and linear rotation servo motors. Positional rotation servo motors are the most common type. They are used for small-scale applications where only a moderate precise positioning is required. These do not provide speed control or continuous rotation. Some have physical stops built into the gear mechanism to prevent the motors from turning beyond their limits. Compared with positional rotation servo motors, continuous rotation servo motors can turn clockwise and counter-clockwise at varying speeds.

Servo Motor Construction

As mentioned before, a servo motor is an assembly of four components: a DC motor, a gearing set, a potentiometer, and a control circuit (encoder circuit), as shown below. Since the DC motor rotates at a very high speed and thus at low torque. The gear arrangement inside the servo motor lowers the DC motor’s rotational speed. This causes the torque to increase. Thus, the gear system of the servo motor is designed to lower rotational speed while increasing output torque.

A DC servo motor comprises of five parts: the stator and rotor windings, bearing, shaft, and encoder. The stator winding, also known as field winding, wounds around the stationary part of the servo motor. The rotor winding, also called the armature winding, wounds around the servo motor’s rotating part. Bearings are used to support the movement of the motor shaft in order to reduce friction. They are of two types: front bearing and back bearing. The shaft provides the output power of the motor to the connected equipment or machine. The encoder is made up of a feedback sensor that detects the actual rotational speed of the motor shaft.

Working Principle of a Servo Motor

Servo motors have a closed-loop control system which comprises a comparator and a feedback path. The comparator is used to compare the actual position of the rotor to the desired position and generates an error signal. The diagram below shows a block diagram of a typical servo motor control system:

A Servo Motor Control System

Servo motors are generally controlled using Pulse Width Modulation (PWM) method. An electrical signal of varying lengths is sent to the motor. The width pulse is varied in that range of 1 to 2 milliseconds and then transferred to the servo motor by repeating 50 times per second. The pulse width thus controls the position of the rotating shaft.

Differences between Servo Motors and DC Motors 

There are numerous differences between servo motors and DC motors. For instance, servo motors can be precisely controlled. They also offer a practically infinite resolution for the output angle and are designed with extremely high accuracy and precision. On the other hand despite their simplicity compared to servo motors, DC motors can still be easily controlled by reversing the leads to change the motor’s direction or adjusting the voltage to change its speed. Hence, both servo and DC motors can be controlled easily, but the difference in their complexity affects the precision and accuracy with which such control can be done.

DC motors are available in extensive varieties which expands their possible torque/speed range. For example, if you have a specific requirement for speed or torque, you can easily find a DC motor that will do the job. In contrast, servo motors feature a wide speed and torque range as well as the ability to precisely control the two parameters. Also, the servo motor can be controlled electronically to achieve the desired speed and power output levels. As a result, while DC motors come in a wider variety of torques/speeds, the typical servo motor can cover a broader range of these two parameters just as a single device.

When it comes to constant or sustained power output, DC motors are the best. DC motors can provide adequate torque for an extended period of usage. This is possible if regular (periodic) maintenance of the DC motor is performed. Conversely, due to their susceptibility to failure, servo motors are best suited for intermittent applications in which the motor is not required to spin continuously for extended periods. Even so, servo motors can only deliver their maximum torque for a small fraction of their operating time (1% of their duty cycle) before they are damaged, making them more suitable for positioning applications than continuous rotation or spinning.

In terms of acquisition costs, low cost is the common DC motor’s best chance at competing with this highly accurate rival–the servo motor. DC motors are advantageous because they can operate effectively without the costly addition of a controller, amplifier, and feedback sensors. If you’re worried about your budget, servo motors and their peripherals can be a risky investment in the long run. Thus, although servo motors are highly desirable due to the precision with which they can be controlled in terms of torque, speed, and position output, compared to DC motors their high investment costs may deter some buyers.

Comparison in terms of Reliability and Efficiency 

Newer models of DC motors are often rated with efficiencies of up to 85%, which is impressive given the heat losses from the brush contacts that usually reduce the efficiency of older models. However, since the servo motor’s precision electronics control the supply current and reduce losses, it will inherently be more efficient than the DC motor. It’s worth noting that servo motors always require some power level due to the current they draw even when they’re not in use, thanks to their feedback encoders. While this is usually not an issue, it can make working with servo motors more challenging.

Maintenance requirements and failure rates are also key factors in determining the dependability and reliability of these motors. DC motor brushes can last a very long time without losing their functional characteristics if properly maintained and used. Nonetheless, if the DC motor fails, it can be easily replaced due to its low purchase price. In contrast, servo motors cost more than conventional DC motors, but they also last longer. However, fixing a servo motor is a tedious and stressful task. This is due to the critical nature of error correction associated with faulty servo motors–in such a case the servo motor system must be fine-tuned whenever a new part is added; this can be a time-consuming process and often necessitates the assistance of highly trained technicians.

Below is a comparison summary table for DC Motors and Servo Motors:

Comparison Parameter DC Motors Servo Motors 
Torque and Speed Range Moderate High (up to 6000 RPM) 
Controllability  Easy (simple) Complex 
Efficiency Moderate (average) High 
Reliability Moderate High 
Ideal Application  Continuous Applications Intermittent Applications  

This entry was posted on October 26th, 2022 and is filed under Automation, Hardware Comparison, Mitsubishi, Uncategorized. Both comments and pings are currently closed.

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