ESC

ELECTRONIC SPEED CONTROLLER

The purpose of a motor speed controller is to take a signal representing the demanded speed, and to drive a motor at that speed. With the purpose to vary an electric motor's speed and direction ESCs are often used on electrically-powered radio controlled models. An ESC can be a stand-alone unit which plugs into the receiver's throttle control channel or incorporated into the receiver itself, as is the case in most toy-grade R/C vehicles. Some R/C manufacturers that install proprietary hobby-grade electronics in their entry-level vehicles, vessels or aircraft use onboard electronics that combine the two on a single circuit board.

FUNCTION

Regardless of the type used, an ESC interprets control information not as mechanical motion as would be the case of a servo, but rather in a way that varies the switching rate of a network of field effect transistors, or FETs. The rapid switching of the transistors is what causes the motor itself to emit its characteristic high-pitched whine, especially noticeable at lower speeds. It also allows much smoother and more precise variation of motor speed in a far more efficient manner than the mechanical type with a resistive coil and moving arm once in common use.Most modern ESCs incorporate a battery eliminator circuit (or BEC) to regulate voltage for the receiver, removing the need for receiver batteries. BECs are usually either linear or switched mode voltage regulators.

DC ESCs in the broader sense are PWM controllers for electric motors. The ESC generally accepts a nominal 50 Hz PWM servo input signal whose pulse width varies from 1 ms to 2 ms. When supplied with a 1 ms width pulse at 50 Hz, the ESC responds by turning off the DC motor attached to its output. A 1.5 ms pulse-width input signal results in a 50% duty cycle output signal that drives the motor at approximately half-speed. When presented with 2.0 ms input signal, the motor runs at full speed due to the 100% duty cycle (on constantly) output.

HOW IS SPEED VARIED?

When we watch a film in the cinema, or the television, what we are actually seeing is a series of fixed pictures, which change rapidly enough that our eyes just see the average effect - movement. Now by switching the motor's supply on and off very quickly. If the switching is fast enough, the motor doesn't notice it, it only notices the average effect. As the amount of time that the voltage is on increases compared with the amount of time that it is off, the average speed of the motor increases. This on-off switching is performed by power MOSFETs. A MOSFET (Metal-Oxide-Semiconductor Field Effect Transistor). It is a device that can turn very large currents on and off under the control of a low signal level voltage.

If the supply voltage is switched fast enough, it won’t have time to change speed much, and the speed will be quite steady. This is the principle of switch mode speed control. Thus the speed is set by PWM – Pulse Width Modulation. The graph above shows the speed of a motor that is being turned on and off fairly slowly: 

Each switching on and off of the speed controller MOSFETs results in a little power loss. Therefore the greater the time spent switching compared with the static on and off times, the greater will be the resulting 'switching loss' in the MOSFETs. The higher the switching frequency, the more stable is the current waveform in the motors. The connection below shows the motor, esc and the battery

SWITCHING CIRCUIT FOR MOTORS

Figure shows how the stator coils are magnetized on supply of current and the next few steps indicate how the motor is moved by switching current on stator poles.

The steps above clearly explain the switching circuit of an electronic speed controller.