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ELECTRONIC SPEED CONTROLLER

By: Steve.D

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“We probably underestimate just how beneficial ESC technology… ESCs are at the core of the technology that makes hybrid cars a reality”. (Koral and Jonathan Feldkamp, 2009)
WHAT IT IS
An electronic speed controller or ESC is an electronic circuit which is used to take DC power from a battery and distribute to an electric motor. In order to do that, it is connected to a servo channel on the receiver and, rather than controlling the position of a servo, allows the signal from the receiver to control the speed and torque of the connected electric motor.
There are two types of electronic speed controller for the two different types of motor in common use:

• Brushed motor: the speed is controlled simply by restricting the amount of power delivered to the motor. The easiest way for the speed controller to do this is to supply the power is short pulses, many times a second. The longer the pulses, the more power, and the faster the motor turns. Brushed motors connect to the speed controller via two wires; to reverse the direction of the motor, the wires are swapped.

• Brushless motor: power is controlled in the same way, but the speed controller must electronically select between the multiple sets of windings in the motor to make it turn. Most modern brushless speed controllers are known as sensorless controllers and make use of the voltage generated (Back EMF) in the momentarily unpowered windings to determine the position of the motor. Virtually all brushless motors and controllers used in model helicopters have three wires; to reverse the direction of the motor, swap the connections of any two wires.
In brief, the changes are more than a lack of “brushes” to distribute power; the motors are built in a whole new way and with the ESC that would power them the power, efficiency and speed far surpassed previous brushed systems.
HOW IT WORKS
TRADITIONAL ESC

Early electric speed controls consisted of nothing more than a hefty variable resistor, the wiper of which was moved by a servo. This had the advantage of being simple, but was very inefficient at partial throttle settings. Such a control works by reducing the voltage to the motor, but this means that any voltage that does not appear across the motor terminals must appear across the speed control. For example, at half throttle, a resistor speed control that is controlling a motor drawing 10A from a 6-cell pack will have 3.6V across it, and 10A flowing through it. From our second law, that's 36W, which all becomes useless heat. This would be like running a 40W light bulb in the radio compartment of your plane. Furthermore, half the power being produced by the battery is being wasted. A resistor speed control is only efficient at zero throttle (when no current is flowing), and at full throttle (when there is no voltage drop across the speed control).

Some early electronic speed controls used ordinary (bipolar) transistors to switch the motor current. These generally have a 0.7V drop, regardless of the current flowing through them. This means a power loss. For example, at 20A (full throttle on a small 05 sized sport plane), this would result in a 14W loss (P = I × V = 20A × 0.7V = 14W).

MODERN ESC

Modern speed controls use MOSFETs (Metal Oxide Semiconductor Field Effect Transistors). Rather than having a fixed voltage drop like a bipolar transistor, a MOSFET has a fixed resistance when turned on. Therefore, the voltage drop depends on the current flow. A typical MOSFET used in inexpensive speed controls has 0.028 Ohms resistance. Using Ohm's law, we can determine the voltage loss. At 20A, this produces a 0.56V drop (V = I × R = 20A × 0.028 Ohms = 0.56V). We can use the second law to compute that the power loss would be 11.2W (P = I × V = 20A × 0.56V = 11.2W). The power loss can be reduced by using more MOSFETs in parallel, or using modern lower resistance MOSFETs. For instance, an Astro 211 speed control has a resistance of only 0.002 Ohms. At 20A, this would result in a 0.8W power loss. If it were being used with 10 cells at 20A, it would be less than a 0.4% loss (10 cells at 20A produces about 220W).

Theoretically, the speed control will be equally efficient at all throttle settings. (One could argue that it is more efficient at lower settings, because it spends more of its time in the 100% efficient off state.)

HOW TO CHOOSE IT

The general procedure of choosing the right ECS is to narrow the list down to ESCs that will get the job done, and then make your final selection based on price and preference. First, select ESCs based on their most fundamental features.

BRUSHED OR BRUSHLESS

R/C speed controllers are separated by the type of motor they work with, either brushed or brushless. If your motor has two wires, it is brushed, and you need a brushed speed control. If it has three wires, it is brushless motor, and you require a brushless speed control. An exception to these rules are ESCs that can work with both types of motor, however this feature is not commonly available. HeliDirect can be a good place to compare various brushless motors from different manufacturers.

CURRENT RATING

An ESC will have a power limit. To handle more power, the ESC needs to be larger, heavier, and is more expensive. It’s important to know the peak current your motor is going to pull at full throttle. This determines the current rating you should look for in an ESC. Always choose an ESC with a current rating that is higher than what you need. If the motor is going to pull 12A, a 25A-rated ESC is a much better choice than a 10A-rated one. The 10A ESC will probably overheat and cook, even if you only fly at half throttle. ESCs are relatively light and maintain great resale value, so this is one item in your power system where skimping isn’t worthwhile.

Choosing the correct type and identifying the minimum current rating are the two big steps. The next choices depend on your preferences. Here are some of the features and limits that can affect your selection.

VOLTAGE RATING

All ESCs have voltage limits. Some even have more than one! What is your battery voltage? Choose an ESC that is designed to work with an equal or higher voltage. Some ESCs are designed for low voltages (below 13V), some for medium voltages (below 25V), and some for high voltages (above 25V). You shouldn’t connect a high voltage battery to a low voltage ESC, but it is also wasteful to use a high voltage ESC with a low voltage battery. The second voltage rating that some ESCs have is based on their Battery Eliminator Circuit (BEC). For an ESC to provide power to your receiver and servos, it has to drop battery voltage down to 5V. This becomes difficult once battery voltage is above 13V, so usually a separate receiver battery or voltage regulator is required. Consider what is going to be powering your receiver and servos.

LOW VOLTAGE CUTOFF

To protect your lithium polymer battery pack from being discharged too much, most ESCs can shut down when they sense battery voltage has become too low. This is almost always a useful feature, as it can save your Li-poly battery from being permanently damaged.

PRICE

ESCs with the same current and voltage rating can vary in price. Investigate this large market, and put prices on the features that you want.

PROGRAMMABILITY

Some ESCs simply work out of the bag, like servos. Others can be fine-tuned and set up with exotic throttle profiles. The most advanced can be configured via a computer program and cable.

Article Source: http://depositarticles.com/

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