DC Motors consist of two major parts, the part that rotates, or the “rotor”, and the part that is stationary, or the “stator”. A DC motor uses these parts to convert electrical energy into rotational mechanical energy using electricity and permanent magnets. Two types of DC motors are used in FIRST Robotics Competition: Brushed DC Motors and Brushless DC motors. Both types are useful in various robot applications, and both have their trade-offs.
Operating a brushed DC motor is simple; provide DC electrical power and the motor spins. In a brushed motor, the rotor consists of electrical winding wires and the stator consists of permanent magnets. Since the electrical part is spinning, there needs to be a way to connect the external power wires to the spinning rotor. This is accomplished through conductive “brushes” that make contact with the stator, automatically sequencing the power to make the rotor spin. Brushes make it easy on us, but they produce extra friction which reduces the efficiency of the motor.
Brushless DC motors don’t have brushes. They still have both electrical winding wires and permanent magnets, but the locations are flipped. The stator now consists of the electrical parts, and the spinning rotor consists of the magnets. This means there is no more brush friction within the motor, making a brushless motor more power-efficient. However, you can’t just give it DC power and expect it to spin. Without the brushes doing the sequencing for us, you must use a specialized motor controller that is designed for brushless motors to properly sequence the power and get the rotor spinning.
Stall Torque is measured when the motors RPM is zero and the motor is drawing its full Stall Current. This value is the maximum torque the motor is ever capable of outputting. Keep in mind the motor is not capable of outputting this torque for an indefinite period of time. Waste energy will be released into the motor as heat. When the motor is producing more waste heat than the motor body is capable of dissipating the motor will eventually overheat and fail.
Stall Current is the maximum amount of current the motor will draw. The stall current is measured at the point when the motor has torque that the RPM goes down to zero. This is also the point at which the most waste heat will be dissipated into the motor body.
Free Speed is the angular velocity that a motor will spin at when powered at the Operating Voltage with zero load on the motor’s output shaft. This RPM is the fastest angular velocity the motor will ever spin at. Once the motor is under load its angular velocity will decrease.
Operating Voltage is the expected voltage that the motor will experience during operation. If a robot is built using a 12 volt battery the Operating Voltage of the motor will be 12 volts. When controlling the RPM of the motor the DC speed controller will modulate the effective voltage seen by the motor. The lower the voltage seen by the motor the slower it will spin. DC motors have a maximum rated voltage if this voltage is exceeded the motor will fail prematurely.
The key metrics defined above are interrelated. Take some time to familiarize yourself with the definitions and how they connect together.
In order to ensure that an electric motor lasts as long as possible a few rules of thumb should be kept in mind:
- 1.Smooth loading - large torque spikes or sudden changes in direction can cause excess wear and premature failure of gear box components. This is only an issue when the torque spike exceeds the rated stall torque of the motor. When shock loading is necessary, it is best to utilize mechanical braking or a hard stop that absorbs the impact instead of the motor.
- 2.Overheating - when a motor is loaded at near its maximum operating torque it will produce more waste heat than when operating at a lower operating torque. If this heat this allowed to build up the motor can wear out prematurely or fail spontaneously.
- 3.Poorly supported output shaft, most motor output shafts are not designed to take large thrust forces or forces normal to the shaft. Bearings need to be used to support the axle when loads in these directions are expected.