Unsourced material may be challenged and removed. Electronic load controller pdf-size electric vehicles also have systems to control the speed of their drive motors. By adjusting the duty cycle or switching frequency of the transistors, the speed of the motor is changed.
The rapid switching of the transistors is what causes the motor itself to emit its characteristic high-pitched whine, especially noticeable at lower speeds. A brushed motor can have its speed controlled by varying the voltage on its armature. Industrially, motors with electromagnet field windings instead of permanent magnets can also have their speed controlled by adjusting the strength of the motor field current. A brushless motor requires a different operating prnciple.
The speed of the motor is varied by adjusting the timing of pulses of current delivered to the several windings of the motor. Brushless AC motor controllers are much more complicated than brushed motor controllers. Computer-programmable speed controls generally have user-specified options which allow setting low voltage cut-off limits, timing, acceleration, braking and direction of rotation. Reversing the motor’s direction may also be accomplished by switching any two of the three leads from the ESC to the motor. Generally the higher the rating, the larger and heavier the ESC tends to be which is a factor when calculating mass and balance in airplanes. A higher number of cells connected will result in a reduced power rating and therefore a lower number of servos supported by an integrated BEC, if it uses a linear voltage regulator. A well designed BEC using a switching regulator should not have a similar limitation.
Most mass-produced electric cars use AC motors, which allow the ESC to capture energy when the car coasts, using the motor as a generator and slowing the car down. In others, such as the Nissan Leaf, there is only a slight “drag” effect when coasting, and the ESC modulates the energy capture in tandem with the conventional brakes to bring the car to a stop. DC motors instead of AC, because of their lower cost and simpler wiring. However, DC motors cannot be used for regenerative braking, so these vehicles cannot travel as far on the same battery, all other factors being equal. DC motors also cannot safely run at RPMs as high as AC motors can, so many custom-built electric cars retain a multi-speed transmission of some kind. Since electric motors have full torque from zero RPM, the vehicle can still start off in a high gear, but starting in a lower gear allows for quicker acceleration, lower current draw and less wear and tear on the motor. ESCs used in mass-produced electric cars with AC motors usually have reversing capability, allowing the motor to run in both directions.