Improve Thermal Performance for Ironless Brush DC and Brushless DC Motors

Electric motor manufacturers must ensure that a motor’s instant internal temperature never exceeds its various components’ maximum allowable temperature. Depending on the design of the motor and the materials used, thermal phenomena will dictate the motor’s performance. Motor designers typically have two options to enhance motor performance without overheating and damaging its components: increase power conversion efficiency by generating less heat for a given mechanical power output or improve the motor's cooling capability through heat dissipation.

Heat dissipation is proportional to the delta between the inside temperature of the motor and the outside, or ambient, temperature. It also depends on the thermal resistance: The lower the thermal resistance, the easier and faster the heat will be led outside the motor. Keep in mind that the coil is the most critical component because this is where joule heating occurs. Here are some common heat transfer and mitigation concepts and challenges for DC motors:

Ironless Brush DC Motors. When a coreless brush DC motor’s coil temperature rises, heat will transfer in two steps with different thermal resistances: from the coil to the tube, and from the tube to the ambient environment. Heat generation and dissipation at steady-state is a matter of balance. Assuming the electrical current flowing through the coil is not excessive, the coil temperature will rise and the heat dissipation will also increase to a point where the thermal energy in the motor is constant over time. The components’ temperatures will no longer vary, and the coil will stabilize at a certain temperature.

Should the coil temperature be slightly above this stabilized value, the slightly increased dissipation power will allow the temperature to lower to the stabilized value and reach steady-state. Remember to account for the coil’s electric resistance at a given temperature.

Brushless DC Motors. With brushless DC motors, the rotating permanent magnet creates iron losses: a source of heat. Iron losses are proportional to the motor speed, so they can be neglected at low speed. However, they tend to become greater than Joule losses at high speed. Therefore, torque must be kept lower at high speed. Brushless motors can reach high speeds because they are not limited by a mechanical brush-collector commutation system.

Brushless DC motors’ two heating sources can be seen as a trade-off between Joule losses at high torque and low speed, and iron losses at high speed and low torque. However, the thermal challenge remains: keep the coil temperature below its maximum allowable temperature.

Torque Performance During Transient Operation. If an application requires high torque for a short duration, a motor can be supplied with an electric current that exceeds the motor’s maximum continuous current as long as the coil temperature does not exceed its maximum allowable temperature. Slotless brushless motors are particularly well-suited for short peak torques.

When it comes to periodic duty cycle, the highest torque during a cycle can, to some extent, exceed the motor’s maximum, continuous torque, depending on the torque profile and the duration of each step in the cycle. If the duration of one cycle, or repeating period, is significantly shorter than the thermal time constant, an equivalent continuous torque value, or current value, may be calculated as a quadratic mean (RMS). This current can be considered as a continuous value over time as long as it is not greater than the motor’s maximum continuous torque.

Look For a Motor Supplier That Understands Your Challenges

Motors can be improved to address many challenges, according to each application’s duty cycle, environment and critical success factors such as delivering the highest torque, highest speed, best energy efficiency to prolong battery life, or operating at the coolest possible temperature. Be sure to seek a motor supplier that understands customer challenges and assists with choosing the best fitting DC motor for each application.

For more information, read the complete white paper or contact an engineer.