Electronic Speed Controllers (ESC’s)
The purpose of an electronic speed controller is to control the speed of the power. There are many ways of doing this such as the old style variable resistors which allowed different amounts of current to reach the brushed motor. Today there are much more effective modern ways such as Pulse Width Modulation (PWM).
Pulse Width Modulation
PWM basically means varying the length of electrical pulses. These electrical pulses sent to the motor can be longer or shower, as shown in this diagram.
This is a wave diagram. As you can see the wider, longer pulses have a higher average voltage, and the narrower a lower average voltage. The reason for using pulses is because transistors can either handle off or on, then can’t be half on or half off. The Electronic Speed controller times the pluses so that longer pulses sends more power to the motor and shorter less.
The benefit of using PWM is that the voltage going to the motor stays the same, so this means the motor can be turned a lot slower and more accurately.
The frequency of these pulses is very very high, so the motor should not jerk as it is running.
With brushless systems the ESC performs another function which is to create a rotating magnetic field. As explained in the motor post the coils just ahead must turned on and the coils behind the must be turned on with the current in the opposite direction. This causes the rotor to turn. The ESC must control which coils are turned on, and vary the amount of electricity sent using PWM.
In our brushless system there is 3 wires coming out one end and 2 the other. There is also a 3 pin plug as I will explain later. The first two wires go to the battery, one is positive the other negative, they’re just normal DC power wires used for powering the ESC. The electricity is then manipulated and sent as pulses down the other three wires, which connect to the motor to power the coils. The reason there is 3 wires is because of the brushless system, IE they allows the creation of the rotating magnetic field.
3 pin plug
The three pin plug are the industry standard. They are used for powering and controlling small peripherals such as servos. The first pin is a ground pin, the second being power, usually at 5 volts the 3rd pin is the data pin. The idea is commands are sent from the central control unit, which the ESC interprets and changes the motor speed correspondingly. Other components include servos, lights, radios which are controlled using the low to high values. The data pin again uses Pulse Width Modulation, a small width means a small value and a high width means a high value from 0% – 100%. This system is used because a lot of servos simply smooth the pluses to convert them into an analog voltage (such as for controlling analog servos).
Most ESC’s have another function called a BEC (battery eliminator circuit). A BEC is used to create a stable voltage of 5V from whatever the battery pack might be. It then sends this voltage down the 3 pin plug’s power leads. These 5 volts are used to power the rest of the system these items would be:
- Radios, RC and Video.
- Control unit and sensors.
- Small lights.
- Anything else that requires 5 volts. The rule of thumb is electronic systems are the 5 volts and electrical systems the main battery.
BEC’s are an excellent power source for most models. However in extremely sensitive applications they have some drawbacks, this is because a lot of BEC’s suffer from voltage sag under heavy load and ripple voltages from the PWM of the motor. This means that for powering something like the Control unit, sensors and radios can be quite risky. Instead I would power things which require heavy loads and aren’t sensitive such as servos with the BEC. For my drone I will be using a separate power source for those electronics such as a standalone battery or a much higher quality standalone BEC with built in redundancy.
This diagram explains how an ESC is wired. The Radio receiver is interchangeable with the controller.
Electronic Speed Controller Ratings
Now I’ll explain the different types of ratings for ESC’s.
Max continuous current
The maximum continuous current is the max amount of current the ESC can sustain for continued periods of time. Much like the motors continuous current rating. Exceeding this for long periods of time will cause the ESC to burn out. This is due to too much heat being generated thus destroying the ESC. It is important to get an ESC with a higher Max continuous current then the current the motor will draw continuously. IE hovering.
Max burst current
Again this is a current rating, it is the maxium burst current that the ESC can handle. Note the ESC will not be able to sustain this, and a burst should only be used in small couple of second bursts. However due to the nature of quadcopters, the current does not really jump in burst. Therefore you should look at the maximum continuous current to be high. The higher the current ratings the better. ESC’s that don’t need to work hard won’t have as much risk of failing, and they won’t need as much cooling either because they won’t get that hot.
This is the voltage rating range. It is the max range in which the ESC can operate. A voltage higher than this could cause the ESC to pop as the internal parts will be subject to high electrical pressure. A too low voltage could mean it won’t turn on or function properly.
I shall explain some additional features you might find in an ESC
This causes the motor to act as a break slowing it’s movement. It is useful for things like cars but in quad copters it can cause the propellers to stop, crashing the drone. It should be turned off.
As explained in the battery section it is very damaging to lipos if their voltage drops too low. To remedy this some ESC’s have cutoff features which kill power to the motor in the event of low battery voltage. This again is useful for things like cars where the car will just stop. However if power is cut to a quad copter it will just fall out of the sky. This feature should be switched off.
This feature electrically isolates the data pin from the ESC. Doing this can dramatically reduce the level of radio interference which can be created especially with very high currents. You cannot have both opto-isolation and BEC working at once in an ESC though quite a few allow you to select at installation which of the 2 features you want to use.
Timing mode is similar to PWM and controls how the on/off switching in the motor. There are two type:
– Soft timing; for 2,4,6 pole motors.
– Hard timing; 6 or more pole motors.
Hard timing increases both the motor revolutions and the current (up to 20%) with the same propeller and battery pack when compared to soft timing. Basically this means how fast the rotating magnetic field should rotate.
There are many many different features to an ESC, but the most important one is to pick the right one for you motor. The voltage you’ll be running it at and to make sure the ESC it up to supplying the amount of current the motor will draw.