While BLDC motors can have good power density, their torque curve is not ideal for spinning a mass up to speed quickly because power output starts to drop rapidly when the motor is past the base speed. Field weakening can be used to extend this maximum power output region of a BLDC motor but performance gains are limited because BLDC motors are not designed for field weakening. Below is an idealized torque curve (to be tested later) of a Turnigy 2206 1400kV BLDC motor from my combat robotics collection.
Below base speed the motor is current limited. The speed controller is still capable of generating more torque by applying more current to the motor phases but should that happen the motor would get too hot and get damaged. Above base speed the motor is voltage limited - the back EMF from the rotating magnets in the rotor is high enough such that applying the full battery voltage to the phases cannot create the desired amount of current to flow through the winding. This forces the current in the motor to drop until it reaches 0. Base speed is the speed at which the EMF of the motor causes the current to drop below the current limit without any field weakening techniques.
This motor only reaches maximum power at one point, which is not ideal for a combat robot. Accelerating the weapon sweeps across the full motor speed range. We want a motor that has a consistent high power output to spin the weapon up as fast as possible. There are several types of motors that have a constant power speed range that are better suited for accelerating weapons for combat robots. Some motor types with constant power speed regions I looked at are: permanent magnet synchronous motors (PMSM), AC induction motors (ACIM), internal permanent magnet motors (IPM), synchronous reluctance motors, (SynRM), and switched reluctance motors (SRM - yes, very confusing!).
I chose to try designing an SRM because not having magnets in the rotor is going to make it significantly easier for me to manufacture. SynRM motors are appealing because they are slightly more power dense and would be manufactured in a similar way but I do not want to attempt to wind distributed windings for my first motor.
The construction of an SRM is relatively simple: the stator is similar to other DC motors and is wound with concentrated windings. What makes an SRM special is the rotor: there are no magnets, rather the rotor is shaped to 'conduct' magnetic flux from one stator tooth to the next. SRM type motors generate torque by magnetizing the ferromagnetic rotor and pulling it to the next set of stator teeth. The advantage that SRM motors have over BLDC motors is that the strength of the magnetic field in the rotor can be reduced at higher speeds which decreases the amount of back EMF. This creates a constant speed power region.
In the next part of this series I will explain how I built tools to simulate an SRM motor and the design process for the rotor and stator.