Electric Motors

To propel an electric vehicle, electricity must be converted to mechanical force. Electric motors use current flowing through wires to create magnetic fields that interact with other magnetic fields in the motor to create motion.

DC Motors

DC motors are electro-mechanical machines which take DC inputs to generate torque. These machines include Brushed DC, Brushless DC machines (BLDC), and homopolar machines among others. While some use mechanical commutators like brushes, some DC machines commutate electrically based on the rotor position.

Brushed DC Motor

Brushed DC Motors are the conventional hobbyist motor and come in two main varieties: Permanent Magnet Based (PMDC) and Field-Winding Based (FWDC). They use mechanical brushes to commutate the the flow of current through the rotor to generate a torque. Because of this, they are easy to control, and do not require a position sensor to track the rotation of the motor. For the PMDC machine, the flux generated by the rotor interacts with the permanent magnet flux to generate torque. For the FWDC machine, the permanent magnets are replaced by another set of coils which generate the stator flux. This allows the machines to operate at high speeds using flux weakening techniques.

For FSAE applications, Brushed DC machines can reduce system complexity; however, at higher power levels, these machines have low efficiencies and must have regular maintenance of brushes. In addition, these machines have relatively low torque and power density which can increase system mass and cause packaging issues. For these reasons, they are not commonly used in FSAE powertrains.

Brushless DC Motor

Brushless DC motors (BLDC) are another form of DC machines that commutate electrically (i.e. without brushes). BLDC machines typically consist of a rotor with permanent magnets and a stator which is wound such that the EMF has a trapezoidal shape. While the structure shares many similarities with the AC Synchronous machines, the distinction is in the control methodology and corresponding winding structure.

Rather than using a high resolution control algorithm like Field Oriented Control, BLDC motors use a heavily discretized, six-step control method. This is implemented by commanding a DC voltage (averaged through PWM) through two of the motor phases based on the rotor position. This is implemented by Hall Effect sensors embedded in the machine which determine which of the six possible phases to regulate current in.

In addition to the Hall Effect sensor used for commutation, BLDC motors often use an encoder to track the rotational position and velocity of the motor for control purposes.

BLDC motors are advantageous for their high torque density (higher than SMPM machines); however, the six-step method generates a large amount of torque ripple which reduces efficiency. BLDC machines also cannot flux weakening with conventional techniques, so problems can arise at higher speeds if voltage limited.

AC Synchronous Motors

Known for their sine waves. Typically uses a resolver to track the rotation of the motor. Can also use sensorless feedback based on the back EMF of the motor.