Brushed vs. Brushless Motors
Brushless motors are a new and exciting technology for the FIRST Robotics Competition. While the fundamentals of DC Motors apply to both brushed and brushless motors, brushless motors have some unique advantages over brushed motors.
What is a Brushed Motor? (top)
A brushed DC motor uses an internal commutator (called brushes) that reverses the polarity of the coils as they pass permanent magnets fixed to the outside housing of the motor. The brushes have to be in contact with the motor’s rotor in order to conduct current to the coils.
To control a brushed motor, a voltage is applied to the motor’s leads. The more voltage is applied, the faster the motor will spin. The direction of the motor is controlled by reversing the polarity of the input voltage. For example, a brushed motor will spin clockwise when a positive voltage is applied to the (+) lead and a negative voltage is applied to the (-) lead. That same motor will spin counterclockwise when a negative voltage is applied to the (+) lead and a positive voltage is applied to the (-) lead.
What is a Brushless Motor? (top)
As the name suggests, brushless motors don’t have brushes. This means that there isn’t a commutator built into the motor, so commutation must be done by an external controller. Brushless motors are typically 3-phase motors, where each phase can be energized using a voltage. When a voltage is applied to a phase, it produces a magnetic field. This magnetic field attracts the phase to a permanent magnet located on the rotor (inrunner motor) or to the outer housing (outrunner).
To control a brushless motor, an external controller energizes (or commutes) each of the motor’s phases in a sequence. The higher the applied voltage is, the more attracted a phase is to a permanent magnet and therefore the faster the motor will move. The direction of the motor is determined by the order the phases are energized.
In order to know which phases should be energized, brushless motors have two options:
1.) Measure the motor’s back-electromotive force (back-emf). By measuring the back-emf voltage of a phase, the controller can measure detect which commutation step the motor is currently at.
2.) The more common method is to use a rotation sensor. Typically this is done with an array of (3x) hall effect sensors spaced 120° apart.
Advantages of Brushless Motors (top)
There are many advantages brushless motors have over their brushed counterparts. First, understand that brushed motors have some problems related to their construction:
- Brush Lifespan - Brushes will wear out over time leading to degraded performance and eventual failure. This cannot be avoided, and can put FIRST teams at a significant disadvantage, as their motors may not perform the same as the season progresses. This is why many teams use new brushed motors every year and some have even started replacing them mid-season to ensure peak performance
- Heat - The friction of the brushes rubbing on the rotor produces heat. Another source of heat is electrical arcing that occurs during normal operation
- Efficiency - The brushes rubbing on the rotor also creates a torque load on the motor. This reduces a brushed motor’s efficiency. Brushed motors are typically less than 75% efficient
- Electrical Noise - Brushed motors produce a lot of electrical noise which affects the analog signals on sensor cable that run close to the motor
The construction of brushless motors have several advantages:
- Longer Lifespan - Since there are no brushes to wear out, a brushless motor lasts much longer. In theory, the bearings in the motor will be the first thing to fail
- Cooler - Since there’s less friction in a brushless motor, brushless motors run much cooler than brushed motors
- Higher Efficiency - Brushless motors can reach a peak efficiency of around 90%
- Less Electrical Noise - Since there’s no arcing in a brushless motor, they produce much less electrical noise
- Smaller Size - Since brushless motors are more efficient they tend to be smaller than brushed motors with similar power output
- Inrunner & Outrunner Motors - The construction of brushless motors allows for two different types of motors. An “Inrunner” motor where the center shaft spins like it would on a brushed motor. These are good for high speed / low torque applications. There’s also “Outrunner” motors where the outside of the motor spins. These are good for low speed / high torque applications
In the world of FRC, there’s a huge advantage to using brushless motor since they’re smaller, lighter, and more efficient. FRC teams building robots using only brushless motors will have a big advantage on weight over teams using no brushless motor or a mix of brushed and brushless motors.
Additionally, an FRC robot has a fixed amount of power since teams are limited to a single battery that has a limit on how much current it can produce at any given moment. In order for teams to get more power out of their robots, the only option is to increase the efficiency of the motors on their robot.
Brushless Motor Control (top)
The topic of brushless motor control can get pretty deep. For the sake of this guide, we’re going to stick to a few topics that are important to FRC.
Sensored vs. Sensorless
Brushless controllers need to know, or at least make an educated guess about the location of the shaft. This way, the controller know what phases need to be energized or set neutral. There are two ways this can be done:
- Sensored Commutation - This is where a sensor, like a hall effect or magnetic encoder, is used to track the location of the shaft. Sensored commutation is great at producing torque at extremely low speed and handling high dynamic loads
- Sensorless Commutation - This is where the controller is reading the motor’s back-EMF (Electromagnetic Frequency) to know the position of the shaft. Sensorless commutation is great at high speed, but performs poorly at low speed
Sensorless performance is so much better at high speed applications, that many controllers will use sensored commutation at low speed and then transition to sensorless as the motor accelerates.
Trapezoidal Commutation is the most basic form of brushless commutation. With trapezoidal commutation, rotation is split into six steps of 60 degrees. With each step, the controller energizes two of the three phases of the motor. An example of a full revolution would look like:
The name trapezoidal commutation comes from the shape of the graph when you plot the motor’s back-EMF.
One drawback of trapezoidal commutation is that there is a torque ripple every time a step changes. This is caused by the phases not being able to energize instantaneously during the transition. The result of the torque ripple is lower average torque and an audible noise from the motor.
Field Oriented Control
Brushless motors produce the most torque when the motor’s phases are orthogonal (90°) from the permanent magnets.
Field Oriented Control (FOC) is a modern form of brushless motor commutation that uses sinusoidal commutation. Instead of applying voltage to two of the three phases like trapezoidal commutation does, FOC applies voltage to all three phases all the time. It can do this by varying the amount of voltage applied as the motor spins.
The advantages of FOC are:
- No torque ripple, the motor transitions between steps since the phases always have a voltage applied to it
- Better control of the motor’s phases. This allows the controller to make sure that the motor’s phases are always orthogonal from the permanent magnets
- More torque at lower speed
- Since more torque is being produced, there’s an increase in the amount of mechanical power the motor is producing
- More mechanical power means an increase in the motor’s efficiency
- Less vibration and noise during operation. This also extends the life of the motor