Understanding the Electronic Speed Control
By Ed Anderson
When we look at model airplanes that have electric motors as opposed to liquid fuels, the things we notice first are the quiet electric motor and the battery. However there is a component that sits between them called the electronic speed control that is really the master control point for all power in the plane. We are going to look at its make-up and how it does its job.
On the surface we can see that the electronic speed control, the ESC, takes over the function of the throttle servo that would operate the carburetor in a glow or gas powerd plane. Just as the throttle servo controls the speed of these wet fuel motors, the ESC controls the speed of the electric motor. But there is more to it than that.
The first thing that we want to recognize is that there are two different kinds of ESCs that are specific to the type of motor they control. There are brushed motors, such as the speed series or the Mabuchi motors, and then there are the brushless motors. Each type of motor needs a different electronic speed control.
Understanding the Wires
When you look at an electronic speed control, you notice that you have three sets of wires. Typically two sets of thick wires and one set that looks like a servo wire.
Two of the thick wires, typically black and red, connect to the battery. The ESC will usually be marked to tell you which are the battery wires. They would connect to the battery as red to red and black to black.
A second set of wires, typically thinner than the battery connection wires, has a plug on the end that looks like a servo plug. This will be connected to the receiver and will serve two purposes as it sends power to the receiver and gets signals from the receiver.
If we look at the wires on this plug they usually run from a dark or black wire on one side to a light or white wire on the other side. I am going to use black, red and white for this discussion. Yours may be dark brown, orange, yellow or something similar.
The black and red wires feed power to the receiver which in turn distributes power out to the servos and other accessories that are plugged into the receiver. Note that the red wire is in the center. This is the power wire. Since it is in the center you can insert the plug into the receiver either way and nothing bad will happen. You won’t get any response from the servos if you put it in wrong, but you won’t damage anything. Note that, on some older systems, particularly Airtronics radio systems, the red wire was on the end. If you plugged it in the wrong way it could damage the receiver and possibly the servos. However the center red design has been fairly universal for many years.
The third wire, the white wire is the signal wire that sends commands from the receiver to the ESC to tell it how to control the motor. As you move the throttle control on your transmitter, the receiver gets the command and passes it up the white wire to the ESC so it knows how much speed you want from the motor.
There is a third set of wires that go to the motor. The ESC is usually marked to show which wires are the motor wires. If this is a brushed motor ESC then there will be two wires, typically red and black.
On a brushed motor ESC, if we connect red to red on the motor, and black to black, the motor will turn in the expected direction. If we reverse them the motor will spin in the opposite direction.
On a brushless ESC, you will have three wires going to the motor. If there are colors, you can match color to color as well. However if the colors don’t match don't worry. Connect them up and observe the direction of the motor. If it is spinning in the wrong direction, reversing any two wires will correct this.
Note that on some older brushless motors there were additional wires that attached to a sensor in the motor. However, unless you have an old motor and ESC combination you won’t see that on any of the current designs.
Some ESCs have an integrated switch. In most cases this will allow or prevent the motor from running and pass or block power to the receiver. However it typically does not stop the flow of current from the battery to the ESC. In fact, even if there is no switch there is always current flowing to the ESC which will drain the battery.
It is for this reason that you should never leave your battery connected when you store your plane. This small current drain will take your battery to zero charge over time. If you are using NiCd or NiMh, the damage may be minor. If you are using Lithium batteries, you lithium battery pack will likely be ruined. So, don’t leave your battery connected unless you are preparing to fly.
The connector/plug that goes to the receiver is standardized. It is the same wire scheme and plug type as is used for the servos. Today all makers, except Futaba, use the universal plug.
On the Futaba J plug you have the same wiring scheme but there is an extra tab on the plug that insures the connector is inserted properly into the receiver. If you have a receiver that accepts this slotted plug it will also accept universal plugs. However if you have a receiver that expects the universal plug, then you will need to trim off this tab with a hobby knife or you can sand it off. Once trimmed, the plug will work fine.
Battery and motor connectors are not as simple.
There is an emerging standard for motor/ESC connection on brushless motors. The connectors are round and are called bullet connectors. Most brushless motor/ESC makers seem to be using these now, so on brushless motors this connector standard seems to be established. However, for brushed motor connections there is no standard.
On the motor side we have the option of not using a connector as we can solder the motor and ESC wires together. This works fine if you don’t plan to remove the motor or the ESC and it gives the best connection. However if you do have to remove one of them for service, you will need the soldering iron in order to take the connection apart.
On the battery side we always use a connector so that we can remove the battery for charging and storage. When flying electric planes it is common to have several battery packs so the connector allows us to remove one pack and insert a fresh one while the first is charging.
Whatever batter or motor connector you use, make sure that is has a current, amp, rating that is larger than what the motor is likely to pull. The reason the wires for these links are thicker is that the battery has to deliver high current to the motor as opposed to the relatively small current that goes to the receiver. If the connector can’t handle the flow, it will heat up and potentially be damaged. Likewise, if the connector can’t handle the current the motor will never develop full power. Too light a connector can also cause a serious voltage drop.
This lack of standards leads to situations where you buy a motor that has one connector, your battery has a different connector and your ESC has a third type. Or, as seems to becoming more common, none of them have connectors and you have to add your own.
My suggestion is to standardize connectors. Once standardized, any motor or battery connection that doesn’t have your standard connector gets a connector replacement. It takes time and soldering but with one standard, all of your batteries will work in any plane for which they are appropriate and you can move motors and ESC around as you desire.
This will also simplify your battery to charger connections. One or two adapters for your charger will handle all of your batteries. Just make sure the connector you use can handle the current.
I have three standards. For brushlesss motors, I use the bullet connectors. For small brushed motors and batteries in very small light planes where the current will typically be under 5 amps, I use the red BEC connectors. These are sometimes called GWS connectors as they are common on GWS motors, batteries and ESC. They are small and light and are well suited for small light planes.
For my high current applications I use the Deans Ultra connectors. They can handle high currents, are easy to solder and can be easily removed and reused. However there are many other high current connector that are equally as good. As long as it can handle the current flow, it will be fine.
Sizing an ESC
Electronic Speed Controls are sized according to how many amps they can control and the voltage that they can handle. So you may see an ESC marked as 20 amps and 7-10 NiXXcells or 2-3 cell Lipo. That says it can handle a 20 amp flow using a battery pack that ranges between 7.4V and 12 volts. If you use it with a motor/battery system that is outside this range it will likely fail. When it fails it may simply not run the motor or it may also cut power to the receiver, which will lead to a crash.
You size your ESC according to the motor and the battery you are using. I won’t go into how we determine what the motor and battery will need. That is covered in another article. It is enough to say that, if your motor is going to draw 20 amps you will need an ESC that is rated for at least 20 amps. There is no problem having an ESC that is rated for more amps than you need, but and ESC that is rated below the expected current load will likely lead to a failed ESC.
The same goes for the voltage. Use your ESC outside the voltage it is designed for and you can expect it to fail.
Your ESC will likely have an integrated battery elimination circuit, a BEC. This is the part that delivers the power to the receiver. Always check the specs for the BEC. While the ESC might be able to handle 14.4 volts, the instructions may say that for uses above 11.1V you may have to disable the BEC. There is a complete article on the BEC, so I won’t go into it here. Let’s just say you need to check this.
I recommend that you always have at least a 20% margin between the amp requirements of your motor and the rating of your ESC. This way you will know you will not be overloading the ESC. A bigger margin is also fine.
How the ESC controls the Motor
Motors are rated by Kv, which means the number of revelations the motor will turn when you apply 1 volt of electricity. So a 1200 Kv motor will spin at 12,000 rpm if you apply 10 volts.
From this you might imply that the ESC changes the voltage to the motor in order to change the speed of the motor, but that is not the case. If you look at the specifications for your ESC you will probably see a frequency number. This might range from 2 KHz to 12 KHz or higher. This is related to how fast the ESC can pulse power to the motor. You see your ESC is not a variable resistor that adjusts the voltage to the motor, it is a fast switch that pulses power to the motor.
You can think of this as a duty cycle control. How long will the ESC leave the power on till it turns it off? Then, how long will it be off before it turns it back on? There is no need for you to know this cycle time, only that on every on cycle your motor is getting the full voltage of your battery.
I take the time to explain this because people mistakenly believe that if they run their motor at partial throttle they are sending reduced voltage to the motor. If the motor is not supposed to get more than 7.4 volts and you put in an 11.1V battery, running the motor at ˝ throttle does not reduce the voltage to the motor. It is getting 11.1V hits every time the ESC switches on. On a brushed motor that is receiving too much voltage, this will typically produce arcing which will burn up the brushes on the motor. In addition to this arcing on brushed motors, this higher electric pressure may push too much current that will overheat the motor.
If you have had a motor “burn up” even thought you usually ran it at a partial throttle setting, this may be the reason. Understanding how the ESC controls your motor will help you diagnose problems.
Note also that, since the ESC is switching power on and off it is also producing electromagnetic pulses, or radio waves. The electronics in the ESC will typically be designed to reduce or shield some of this radio wave noise, but it can’t block it all. This is why we recommend keeping the ESC and the receiver as far apart as possible as this ESC noise can interfere with the receiver. If you are getting “glitching” or odd pulses to your servos, these may be coming from ESC noise bothering the receiver. Try moving things around.
Other Components in the ESC
I am going to address these in later articles, but there are typically two other components that are integrated into your ESC. We already mentioned the BEC. The other is the LVC, the low voltage cutoff. These are not directly involved in controlling the speed of your motor, but as you will see in the articles that are focused on these that they are very valuable parts of your ESC that you will want to understand.
The electronic speed control is the power system controller for your airplane. Its various components distribute power to the receiver and control the speed of the motor. Understanding how it works will give you the ability to properly size and install the ESC and to diagnose problems in the system.
What's Inside an Electronic Speed control?
MY MOTOR WON'T RUN - WHAT'S WRONG?
A tip for new electric pilots - Setting the throttle to zero
Before most Electronic Speed Controls, ESC, will allow the motor to run they require that you move the stick to zero throttle. This is a safety feature that prevents the motor from coming on the moment you connect the battery.
But, is your throttle stick it really at zero?
There is a trim on the throttle channel, just like the other channels. On glow planes they use this to set the idle, so the motor won't shut off when they go to zero throttle position. In other words the throttle isn't really at zero.
But we don't have to worry about idle on electric models. So we want the throttle to be able to go to zero.
If your throttle trim is set to the center, then your throttle channel may not really be going to zero. This can result in your ESC not arming and not allowing your motor to run. If this happens to you, move that trim on the throttle channel till it is all the way down, to zero. Now connect the battery and see if the ESC will arm and the motor will run.
This came up because a friend with a new electric plane had this problem. When he called for support, they thought it was a defective ESC and sent him a new one. But that one did not work either. So he called me. Well, I have been down this path before, so after trying a few other things, we moved the trim all the way down.
Bingo! His motor now works and all is right with the world.
Just a tip from someone who has seen this a few times before.