Old 02-24-2008, 01:03 PM
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AEAJR
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THE ROLE OF THE BEC IN YOUR ELECTRIC PLANE
by Ed Anderson
aeajr on the forums

In the world of electric motors the electronic speed control, ESC, takes the
place of the throttle used on fuel powered planes. It regulates the speed of the
motor by pulsing the power to the motor to achieve the desired motor speed.
However most ESCs also have two other functions, the LVC and the BEC.

The LVC, low-voltage-cutoff circuit, will cut power to the motor and preserve
power to the radio system so you can land your plane safely when the motor
battery is getting too low. In the case of lithium batteries, the LVC, can also
save your battery packs by preventing them from getting too low. If you started
with NiXX packs and have switched to lithium packs, be sure your LVC is set
properly or you could damage your lithium packs.

The BEC, the battery elimination circuit supplies power to the receiver and the
servos. It is the BEC that will be the main focus of this discussion.

The name, battery elimination circuit, comes from the fact that, in the "old
days" of electric planes, you had a battery pack to power the motor and another
one to power the receiver. In order to save weight, the BEC was introduced to
eliminate the need for that receiver battery pack.

In most of our radio systems, the receiver is designed to operate between 4 and
6 volts. To match this, the typical BEC supplies power to the receiver at about
5 volts by stepping down the motor battery voltage. However the higher the
voltage of the motor battery, the harder the BEC has to work to get the voltage
down to 5 volts. In doing this work the BEC generates heat. The greater the
voltage reduction, the more heat the BEC generates. As a result most BECs have
to be disabled if the motor battery pack is over a certain voltage. ESCs that
are designed specifically for high voltage use often do not have an integrated
BEC.

BECs are also rated by how many amps they can deliver to the receiver. The
greater the number of servos installed the greater the amperage the BEC must
deliver and the more heat it generates in the process. However, with most
integrated BECs, the higher the voltage of the motor battery pack the lower the
amperage the BEC can deliver. This is often where problems occur. It is this
heat load that leads to the need for a compromise as to how many servos an
integrated BEC can support.

For example, if the motor pack is 8.4V, then a given BEC might be able to
support 4 servos. If the motor battery voltage is higher, say 11 volts, then
the same BEC may need to be derated to handle only 3 servos. Since more heat
will be generated by the larger step down from 11 volts to 5 volts, the amp load
has to be reduced or the BEC will overheat.

Note that the voltage rating for the ESC may be different than the voltage
rating for the BEC. Your ESC may be rated for 14.8 volts but the BEC may have
to be disabled over 12 volts and you will have to power the receiver separately.
If you don't take note of this and pop in a four-cell lipo, your ESC may be fine
but your BEC may be heading for a failure, resulting in a crash.

According to Dimension Engineering, a maker of BECs, "Many people don't realize
that their ESC's BEC rating is misleading. With the linear BEC built into most
speed controls, the current rating decreases as pack voltage increases. For
example, several popular 25A ESCs with "3A" BECs are only capable of supplying
0.5A when running from a 3s pack".

If you are flying an RTF or "receiver ready" model, there may not be ESC/BEC
documentation included. As an example, the manufacturer of the plane may
designate that the plane takes an 8.4V pack. At that voltage the included BEC
may be fine. However, if you decide to pop in a three cell lipo, a problem may
only be a launch away. The BEC may do fine for a couple of flights, or maybe 5
minutes or may fail 100 feet out, and down you go.

We also have the variable of which servos are being used. Different servos draw
different amounts of current. If the current draw gets too high, the BEC will
get too hot causing a thermal shutdown of the BEC. This protects the BEC and
prevents a fire, but cuts the voltage to the receiver. The net effect is that
you lose all power to the radio system and you lose control of the plane.

In the case of an overheated BEC, if there is enough cooling air going through
the plane, the BEC may come back quickly as it cools. This could look like a
radio glitch, but it could be the BEC operating on the edge of total failure.
If your ESC is very hot when you land, the cause could be the BEC operating at
the edge of its capacity. When we see these glitches, we often think the
problem is the radio system, but in fact the cause could be the BEC.

A CASE STUDY

This pilot was flying a new Spektrum 2.4 GHz system. All was fine till the plane
suddenly went dead and crashed. All sorts of speculation were offered about what
the cause could be and much of it was focused on the Spektrum 2.4 GHz system.
After the plane was recovered, everything seemed to work OK so it must have been
a radio hit, right? However, due to the diligent work of the pilot, it was
determined that the BEC had failed due to overload. You can read the actual
account at this link in posts 2986 to 3006.
http://www.rcgroups.com/forums/showthread.php?t=604621&page=200

This is not the only account of this type that has been reported, but this was
one that was worked out over a short time with a very clear outcome. Note also
that the pilot had to run his test for several minutes before the failure
appeared. Thus, everything seemed fine at first; it seemed that the BEC was
handling the load. But over several minutes' heat built up in the BEC. Combine
this with the heat from the motor and the battery and, perhaps not enough
cooling airflow and the BEC shut down.

BE COOL FOOL!

With good airflow a BEC overload may be avoided. Regardless of what radio
system you are using, make sure you have enough cooling air going through your
electric plane. This is especially true of foam planes as the foam acts as an
insulator. You may have a cooling air vent in the front somewhere, but the heat
can't get out unless there is an exit air hole large enough to allow good
airflow. If you are pushing the limit on any part of your power or radio system,
not enough cooling air can cause damage or failure to your motor, ESC, BEC or
battery packs. The receiver could overheat or you could cook your servos.

How you fly your plane can also cause heat build-up. For example, an Easy
Glider that is flown for 1 minute to get to altitude might have enough airflow
to eliminate the built up heat. But if you fly it constantly for 10 minutes,
the heat build up could be enough to cook your BEC, your battery pack, or some
other part of the plane.

Be cool fool, and make sure you have enough airflow in your plane. If your
battery is very hot, or if your ESC is very hot, you may need more cooling.

OTHER CAUSES OF BEC PROBLEMS

You could be configured properly. Your BEC may be rated to handle your servo
count and you could have plenty of cooling air but still have problems. If you
have a servo push rod that is dragging or is otherwise placing a high load on
the servo, this can increase the amp draw of that servo. If that servo gets
stuck, the amp draw will go way up!

Servo loads are expected to be variable. A servo will move, put a load on the
BEC then come back to neutral and the current draw will drop. In between loads,
the BEC has a chance to cool. However a jammed servo will draw a lot of power
and that draw will be constant. You can see why it is very important that your
servos move freely, without binding. Check those control rods for kinks,
obstructions or things that could get in the way.

ENTER THE COMPUTER RADIO

In the past it was common to have 2 ailerons run off of one servo, so three
servos were typical of a 4-channel electric plane. With more and more people
using computer radios, there is a tendency to put 2 servos on the ailerons
meaning more load on the BEC.

Also, with a computer radio it is easy to add a little aileron to rudder mixing,
moving 3 servos at once. Now add a little up elevator in the turns and all four
servos are pulling power. Go to a full house electric sailplane, with flaps
following ailerons, rudder mixed in and a little up elevator in the turn and you
now have 6 servos, all moving at once. We begin to see how the BEC can become
challenged to keep up.

WHAT IF YOU NEED MORE?

If you need more power than the integrated BEC in your ESC can supply, or if
your motor battery voltage is higher than the BEC can handle, you will need to
disable the integrated BEC and put in a separate receiver pack or a separate
BEC. Many companies make after market BECs that can handle these higher
voltages or higher servo loads.

Note that there are different kinds of circuits that are used to create the BEC
function. There are linear BECs, which seem to be primarily what is found
integrated in with the ESC. These seem to be low cost but are more affected by
the voltage of the motor pack. Then there are switch mode BECs that seem to
tolerate these higher pack voltages better than the linear BECs. It appears
many of the after market BECs are of this type.

Regardless of what type you have, follow the instructions carefully or risk
losing your plane. And be sure to provide plenty of cooling air.

Listed below are some examples of after market BECs.

The Ultimate BEC
http://www.hobby-lobby.com/ubec.htm

Novak 3 amp BEC
http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXSGC1&P=7

For very large servo counts - 6 amps
http://www.horizonhobby.com/Products/Default.aspx?ProdID=SQBDBEC008V

Dimension Engineering has several BECs
http://www.dimensionengineering.com

The SMART BEC - Combines BEC and LVC that is Lithium aware
http://www.dimensionengineering.com/SmartBEC.htm

SUMMARY

The ESC is the heart of your electric power system. The BEC is the part of the
ESC that powers your radio system. Keep it cool and make sure you read the
instructions so you don't overload it. Forget these tips and you may be
picking up pieces of your plane, wondering what caused that crash.

Last edited by AEAJR; 02-24-2008 at 01:20 PM. Reason: Posted new ESC article at post 5 and move this one for more logical order.
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