How Motors Work and How to Choose the Right Motor

Motors are found almost everywhere. This guide will help you understand the basics of electric motors, the available types, and how to choose the correct one. The basic question is which type and which specifications should I choose, and these are important.

How does the motor work?

Electric motors work by converting electrical energy into mechanical energy to create motion. Forces are generated within the motor through the interaction of a magnetic field with alternating (AC) or direct current (DC) current in the windings. As the strength of the current increases, the strength of the magnetic field also increases. Keep in mind Ohm's law (V = I*R); the voltage must increase to maintain the same current as the resistance increases.

Electric motors have a range of applications. Traditional industrial uses include blowers, machines and power tools, fans and pumps. Hobbyists typically use motors in smaller applications that require motion, such as robots or modules with wheels.

Motor type:

There are many types of DC motors, but the most common are brushed or brushless motors. There are also vibration motors, stepper motors, servo motors, etc.

DC brushed motors are one of the simplest motors and are found in many appliances, toys and cars. They use contact brushes connected to a commutator to change the direction of the current flow. They are cheap to produce, easy to control, and have excellent torque (measured in revolutions per minute, or RPM) at low speeds. Some disadvantages are that they require frequent maintenance to replace worn brushes, limit speed due to brush heating, and generate electromagnetic noise from brush arcing.

Brushless DC motors use permanent magnets in their rotor assembly. They are popular in the hobby market for aircraft and ground vehicle applications. They are more efficient, require less maintenance, produce less noise, and have higher power density than brushed DC motors. They can also be mass-produced and are similar to AC motors with a constant RPM, except powered by DC current. However, there are disadvantages, including that they are difficult to control without specialized regulators, and that they require low startup loads and dedicated gearboxes in drive applications, resulting in their higher capital cost, complexity, and environmental constraints.

Vibration motors are used in applications that require vibration, such as cell phones or game controllers. They are produced by the electric motor, and an imbalance of mass on the drive shaft causes the vibrations. They can also be used in non-electronic buzzers that vibrate in order to sound or for alarm or doorbells.

Stepper motors are your friend whenever precise positioning is involved. Found in printers, machine tools, and process control systems, they are designed for high holding torque, enabling the user to move from one step to the next. They have a controller system that specifies positions via signal pulses sent to the driver, which interprets them and sends a proportional voltage to the motor. They are relatively simple to make and control, but they constantly draw maximum current. The small step size limits the top speed and the number of steps can be skipped under high load.

Servo motors are another popular hobby market motor used for imprecise position control. Their popular applications include remote control applications such as RC toy cars and robots. They consist of a motor, potentiometer, and control circuitry, and are controlled primarily through pulse width modulation (PWM), which sends electrical pulses to a control line. Servos can be AC or DC. AC servos can handle higher current surges and are used in industrial machinery, while DC servos are suitable for smaller hobbyist applications. To learn more about servos, check out our How Servo Motors Work article.

AC motors are divided into three basic types: induction motors, synchronous motors, and industrial motors.

Induction motors are called asynchronous motors because they do not move at the same constant rate or turn less than the frequency provided. In an induction motor, slip (the difference between actual speed and synchronous speed) is required to generate torque, the twisting force that causes rotation. The magnetic fields around the rotors of these motors are caused by induced currents.

The rotor of a synchronous motor rotates at a constant rate when supplied with alternating current. Their magnetic fields are generated by permanent magnets. Industrial motors are designed for three-phase high power applications such as conveyor belts or blowers. AC motors can also be found in home appliances and other applications such as clocks, fans and disk drives.

Things to consider when buying a motor:

There are several characteristics to look for when selecting a motor, but voltage, current, torque and speed (RPM) are the most important.

Electric current powers the motor, too much current can damage the motor. For DC motors, operating current and stall current are important. Operating current is the average amount of current the motor is expected to draw at typical torque. The stall current provides enough torque for the motor to run at stall speed or 0RPM. This is the maximum amount of current the motor should be able to draw, and the maximum power multiplied by the rated voltage. A heat sink is important to run the motor continuously or at higher than rated voltage to keep the coils from melting.

Voltage is used to keep the net current flowing in one direction and overcome the reverse current. The higher the voltage, the greater the torque. The rated voltage of a DC motor indicates the most efficient voltage for operation. Be sure to apply the recommended voltage. If the applied voltage is too low, the motor will not work, while too much voltage will short out the windings, resulting in loss of power or complete damage.

Handling and stall values also need to be considered along with torque. Operating torque is the amount of torque the motor is designed to provide, and stall torque is the amount of torque produced when power is applied from stall speed. You should always look at the operating torque you need, but some applications will require you to know how far you can push the motor. For example, with a wheeled robot, good torque equals good acceleration, but you have to make sure the stall torque is high enough to lift the weight of the robot. In this case, torque is more important than speed.

Speed or velocity (RPM) can be complicated for motors. The general rule is that motors run most efficiently at their highest speed, but if gearing is required, this is not always possible. Adding gears reduces the efficiency of the motor, so there is also a reduction in speed and torque to consider.

These are the basic elements to consider when selecting a motor. Select the appropriate motor type by considering the purpose of the application and the current it uses. Application specifications such as voltage, current, torque and speed will determine which motor is best, so it is important to pay attention to its requirements.