Types of Motors

DC Motors - The most common are brushed or brushless. There are also stepper motors and servo motors.

Brushed DC Motors are one of the simple and are found in many appliances, toys, and automobiles. They use contact brushes that connect with a commutator to alter current direction. Excellent torque at low speeds (measured in revolutions per minute or RPM). One drawback is that brushes and commutators tend to wear relatively quickly as a result of their continued contact, requiring frequent replacement and periodic maintenance.

Brushless DC Motors do not use brushes. They 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, generate less noise, and have higher power density than brushed DC motors. They can also be mass-produced and resemble an AC motor with a constant RPM, except powered by DC current. Drawbacks include that they are difficult to control without a specialized regulator and they require low starting loads and specialized gearboxes in drive applications causing them to have a higher capital cost, complexity, and environmental limitations.

A Stepper Motor is driven by pulses; it rotates through a specific angle (step) with each pulse. They're found in printers, machine tools, and process control systems and are built for high-holding torque that gives the user the ability to move from one step to the next. They have a controller system that designates the position through signal pulses sent to a driver, which interprets them and sends proportional voltage to the motor. They are relatively simple to make and control, but they draw maximum current constantly. Small step distance limits top speed and steps can be skipped at high loads.

Servo motorsare another popular hobby market motor and are used for position control without precision. Their popular applications include remote control applications like RC toy vehicles and robotics. They consist of a motor, potentiometer, and a control circuit and are mostly controlled through pulse width modulation (PWM), through the sending of electrical pulses to the control wire. Servos can be either AC or DC. AC servos can handle higher current surges and are used for industrial machinery, whereas DC servos are for smaller hobbyist applications.

AC Motors - AC motors include synchronous and induction (asynchronous).

	Synchronous With synchronous motors, rotation is synchronous with the frequency of the supply current. These motors are often used to drive the rotating trays in microwave ovens; reduction gears in the motor unit can be used to obtain the appropriate rotational speeds to heat food.
	Induction (Asynchronous) With induction motors, the rotation speed varies with frequency; but the movement is not synchronous. In the past, these motors were often used in electric fans and washing machines.

What to consider when purchasing a motor?

There are several characteristics that you need pay attention to when selecting a motor but voltage, current, torque, and velocity (RPM) are most important.

Current is what powers the motor and too much current will damage the motor. For DC motors, operating and stall current are important. Operating current is the average amount of current the motor is expected to draw under typical torque. Stall current applies enough torque for the motor to run at stall speed, or RPM. This is the maximum amount of current the motor should be able to draw, as well as the maximum power when multiplied by the rated voltage. Heat sinks are important are constantly running the motor or are running it at higher than the rated voltage in order to keep the coils from melting.

Voltage is used to keep net current flowing in one direction and to overcome back current. The higher the voltage, the higher the torque. The voltage rating of a DC motor indicates the most efficient voltage while running. Be sure to apply the recommended voltage. If you apply too few volts, the motor will not work, whereas too many volts can short windings resulting in power loss or complete destruction.

Operating and stall values also need to be considered with torque. Operating torque is the amount of torque the motor was designed to give and stall torque is the amount of torque produced when power is applied from stall speed. You should always look at the required operating torque, 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 must make sure the stall torque is strong enough to lift the weight of the robot. In this instance, torque is more important than speed.

Velocity, or speed (RPM), can be complex regarding motors. The general rule is that motors run most efficiently at the highest speeds but it is not always possible if gearing is required. Adding gears will reduce the efficiency of the motor, so take into account speed and torque reduction as well.

These are the basics to consider while selecting a motor. Consider an application’s purpose and which current it uses to select the appropriate type of motor. An application’s specifications such as voltage, current, torque, and velocity will determine which motor is most appropriate so be sure to pay attention to its requirements.
Do you have any additional tips regarding selecting motors? Let us know at [email protected].