What Is a DC Motor?
DC (Direct Current) motors are two-wire power and ground, continuous rotation motors. When these wires are connected to a power supply, a DC motor will start spinning until that power is removed. Most DC motors run at high rpm revolutions per minute.
DC motors can be used for computer cooling fans, radio-controlled cars, or other automotive applications. Mostly, the speed of DC motors is controlled using Pulse Width Modulation (PWM). Using this technology, we are rapidly turning the power on and off.
The percentage of time taken to cycle the on/off ratio determines the speed of the motors. For example, if the powers are cycled at 50%, half on & half off, the motor will spin at half your maximum speed fully on. Each pulse is so fast that motors appear to spin continuously without stuttering.
But how does a DC motor run? A DC motor has only one moving component – the rotor, which eliminates the complications caused by brushes in brushed motors.
And also, unlike brushed motors, the rotor has a ring of permanent magnets, while the coils are stationary. This setup eliminates the need for brushes.
The hard part comes in controlling the polarity of the current flowing through the coil and adjusting it to the speed of the rotor.
This can be achieved by measuring the emf back or by using a Hall effect sensor to measure the position of the magnets directly. Due to this, brushless DC motors are generally more expensive and complex, despite it having many advantages over brushed DC motors.
What Is a Servo Motor?
We can talk about servo motors as a combination of four things: a DC motor, a gearing set, a control circuit, & a position-sensor, usually a potentiometer.
Servo motors are known in the industry as motors that can be controlled most accurately. Compared to servos with standard DC motors, servos typically have three wires power, ground, and control.
Power is continuously applied to the servo motors. The servo controller controls the current draw to drive the motor.
This motor is designed for more specific tasks where the position needs to be defined. In addition, servo motors are used for precise tasks, for example, to move a robotic arm or robotic leg within a certain range, to move the camera to some object and etc.
Although we should mention that servo motors do not spin freely like a standard DC motor, unless it is modified for continuous rotation, there is no limit on the range of motion of these types of motors.
And instead of being the input signal that determines which position the servo should be in to rotate, continuous rotation relates the speed of the output and direction to the servo input.
However, for most servo motors, the angle of rotation is limited to 180 degrees forward and backward.
The servo motor receives a control signal that represents an output position and applies power to the DC motor until the shaft is turned to the precise position, determined by the position sensor. Unlike DC motors, a PWM signal is used to control the servo motor.
Durations of positive pulse determine the position rather than the speed of the servo shaft. A neutral pulse value usually around 1.5ms, dependent on the servo, keeps the servo shafts in the center position.
Increasing that pulses values will make servos rotate clockwise, and a shorter pulse will rotate the shaft counterclockwise.
The servo control pulse is typically repeated every 20 milliseconds depending on the servo motor. We must always tell the servo where to go, even if it means staying in that position.
When a servo receives a command to move, it will move into position and try to maintain that position even if an external force pushes against it.
A Brief Comparison Between Dc Motors and Servo Motors:
Servo motors are highly controllable. They were created with precision and accuracy in mind and provide users with essentially infinite resolution of what the output angle can be.
DC motors, while much less complex than servo motors, are also easier to control; Reverse the lead to change direction and change voltage to change speed.
These motors can be easily controlled, but their difference in complexity changes the resolution of the controls. Torques or speed range of DC motors is increased only by the number of types of DC motors present.
There are good chances that, given a certain speed & torque requirement, a DC motor has been developed to fit the bills.
However, the servo motor combines these dynamics speed/torque range with the ability to modify it at will. Their electronic feedback allows operators to set desired speed and power, and the servo motor will comply.
So, while there is a wide range of DC motors on the market with varying torque/speed, the average servo motor per unit can cover much of this range. The reliability of both these motors depends on the failure rate and maintenance.
When brushes on DC motors are consistently serviced and used responsibly, they can last for a long time without losing their functional characteristics. However, even if they do break down, their low cost allows buyers to replace only the broken motor.
Servo motors are not disposables but are also more reliables per unit. It was important to note that when the servo motor breaks down, the maintenance process is much more difficult.
Since error correction is paramount, the system must be adjusted for accuracy when new components are added, which is time-consuming & often requires experts technicians to reconfigure the system.
The efficiency of DC motors suffers from heats loss from brush contacts, but newer models are often rated with efficiencies of up to 85%, which is an impressive number.
Servo motors will naturally outperform DC motors in terms of efficiency, as precise electronics control the current & minimize losses. Even when servo motors are not in use, they still draw a current due to their feedback encoders, which means that servo motors must always be driven at some potential.
This is usually not the problem, but if regular power off is required, then servo motors will be harder to use. DC motors win when it comes to continuous output. They are largely unaffected by long runs with maintenance and can provide good torque during this period.
Servo motors are best used in intermittents applications where the motor does not spin continuously for very long, as operators want to reduce the chance of failure.
Servo motors can also only provide their peaks torque for a small percentage of the operating times, or they will be damaged, so they are best used as positioning motors & not for continuous rotation or spinning.
The price of the common dc motor has the best chance of competing against this highly accurate contender. Not only are they cheap per unit, but DC motors do not require additional controllers, amplifiers, & sensors to function well.
Servo motors & their accessories are more expensive to buy, install, maintain & replace, making them a risky option if long-term costs are a concern.
While they offer excellent torque, speed, and position control, servo motors and their price tags can scare off some buyers depending on overall project cost goals.
DC Motors VS Servo Motors:
DC Motor VS. Servo Motor is explained here, considering factors like the number of wires used in a two-wire or three-wire system. The assembly system is used, whether or not the motor’s rotation is continuous—examples of both motors.
The difference between DC Motor VS. Servo Motor is given below in a tabular form.
| Basic | DC motors | Servo Motors |
| Wire system | DC motor is a two-wire system known as power & ground | The Servo motor is the three-wire system known as power, ground & control. |
| Assembly | DC motors are individual machines with no assemblies. | It has assemblies of four things DC motors, gearing set, control circuit, and positions sensors. |
| Rotation | The movement of DC motors are continuous | Servo motor does not rotate freely & continuously like DC motors. Its rotation is limited to 180⁰ |
| Examples | DC motors are used in cars, wheels, fans, etc. | They are used in robotics arms, legs, or rudders controls. |
Direct Currents DC Motors, as already discussed in the articles DC Machine, is a device that converts electrical energy into mechanical.
It is based on the principles of Fleming’s left-hand rules that when a conductor is placed in a magnetic field that carries current, torque is produced that moves the motor.
According to Fleming’s left-hand rule, the forefinger, middle finger, and thumb, when extended vertically, represent the directions of the magnetic field, the direction of the current, & the direction of the force, respectively.
- Servo motor consists of the three-wire system known as power, ground, and control, whereas DC motor is the two-wire system known as power and ground.
- The servo motor consists of an assembly of four components: DC motor, gearing set, control circuit, and a position sensor. There is no assembly involved in a DC motor.
- Servo motor does not rotate freely & continuously like DC motor. Its rotation is limited to 180⁰ while the DC motor rotates continuously.
- Servo motors are used in robotic arms, foot or rudder control systems, and toy cars. DC motors are used in fans, cars, wheels, etc.
FAQ on DC Motors and Servo Motors
What is a DC motor?
A DC (Direct Current) motor is a type of electric motor that runs on direct current and has two wires, power and ground. It spins continuously when power is applied, and its speed is typically controlled using Pulse Width Modulation (PWM).
How does a DC motor work?
A DC motor has a rotor (the moving part) surrounded by permanent magnets, while the coils are stationary. When current flows through the coils, a magnetic field is created, causing the rotor to spin. The speed can be controlled by adjusting the voltage or using PWM.
What are common applications of DC motors?
DC motors are used in a variety of applications, including computer cooling fans, radio-controlled cars, and automotive applications.
What is Pulse Width Modulation (PWM) in DC motors?
PWM is a technique used to control the speed of DC motors by rapidly turning the power on and off. The ratio of on-to-off time determines the motor’s speed.
What is a servo motor?
A servo motor is a highly controllable motor that combines a DC motor, a gearing set, a control circuit, and a position sensor. It can precisely control the position of its output shaft.
How does a servo motor work?
A servo motor receives a control signal that specifies a desired position. It applies power to the DC motor until the shaft reaches that position. The position sensor provides feedback to ensure accuracy.
What are common applications of servo motors?
Servo motors are used in applications requiring precise control, such as robotic arms, camera positioning systems, and various types of precision machinery.
How does the control signal in a servo motor differ from that in a DC motor?
In a servo motor, the control signal is a PWM signal that determines the position of the shaft rather than its speed. A typical servo control pulse lasts around 1.5ms to keep the shaft in the center position.
Can servo motors rotate continuously?
Most servo motors cannot rotate freely and continuously like DC motors. They are typically limited to a rotation range of 180 degrees. However, some servo motors can be modified for continuous rotation.
What are the main differences between DC motors and servo motors?
- Wire System: DC motors use a two-wire system (power and ground), while servo motors use a three-wire system (power, ground, and control).
- Assembly: DC motors are standalone units, whereas servo motors include a DC motor, gearing set, control circuit, and position sensor.
- Rotation: DC motors rotate continuously, whereas servo motors are usually limited to 180-degree rotation unless modified.
- Applications: DC motors are used in cars, fans, and wheels, while servo motors are used in precise control applications like robotic arms and camera positioning.
Are servo motors more expensive than DC motors?
Yes, servo motors are generally more expensive due to their complexity and the additional components like control circuits and position sensors. They also require more expensive maintenance and installation.
Which motor is more efficient, DC or servo?
Servo motors are generally more efficient because they are precisely controlled by electronics, minimizing losses. DC motors can have high efficiency, but they suffer from heat loss due to brush contacts in brushed models.
Are servo motors suitable for continuous operation?
Servo motors are not ideal for continuous operation as they can only provide peak torque for short durations. They are best used in intermittent applications requiring precise positioning.
What is the maintenance requirement for DC motors and servo motors?
DC motors, especially brushed ones, require regular maintenance to replace brushes. Servo motors, while more reliable per unit, require more complex maintenance and recalibration by experts when they fail.
Which motor should I choose for my project?
The choice depends on your project’s requirements. If you need precise control and positioning, a servo motor is ideal. For applications requiring continuous rotation and simplicity, a DC motor is more suitable. Consider factors like cost, maintenance, and application needs when deciding.
