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Background Information on Electrics

This article will focus on the basics of electricity. This information will help you to understand what is happening in your electric flight system.

As with any technical area, there are some terms that need to be understood before proceeding any further.  These are the volt, ampere and ohm.  They all have some bearing on how the electrons move in a wire, this being the basis of electricity.

• Volt (V) - The volt is a measure of electrical potential.  It is like the force pushing the electrons through the wire.  The larger the electrical potential (the voltage), the larger the push.
• Amp (I) - The amp (ampere) is a measure of the number of electrons moving past a single point in a wire or other conductor.  The more electrons moving past a point in the wire, the larger the current (as measured in amps).
• Ohm (R) - The ohm is a measure of the resistance of the wire or other conductor to the passage of the electrons through it.  The higher the resistance, the fewer electrons can pass through the point in the circuit.

Now that we have these terms, look at how they are related.  The fundamental principle for electricity is something called Ohm's law.  It states that the voltage in an electrical circuit is equal to the amount of current in amps multiplied by the resistance of the circuit in ohms.

V = I x R

Another similar equation states that the power (watts) of a system is equal to the volts times the current

P = I x V

Substituting the value of the voltage with the first equation and we can re-write this as follows:

P = I x I x R

where power is equal to the current squared and multiplied by the resistance.   These are very important equations that have many applications in electric flight.   For instance, we can immediately see that using low quality wire will increase the resistance in the circuit which then will reduce the volts reaching the motor.  This translates directly to reduced thrust and efficiency.  Another example is the use of a shunt, as described in the last newsletter.  The shunt introduces a very small resistance into the system and this is translated into a very small voltage drop across the shunt which is proportional to the current flowing through the shunt.

Some other basic principles to remember are as follows:

• electrons flow from the negative terminal to the positive terminal.
• current is measured by inserting an ammeter or shunt into the circuit. Never put an ammeter directly across a battery or other voltage source, even for an instant!
• voltage is measured across a conductor by use of a voltmeter. A voltmeter on the terminals of a battery will show the voltage potential of a battery. A voltmeter on the terminals of the motor shows the voltage reaching the motor.
• the current in an electrical system is always the same, no matter where in the circuit the ammeter is placed. The current depends on the total resistance of the system.
• the voltage in an electrical system depends on where it is being measured. Increased resistance from wiring or connectors will decrease the voltage as measured at the motor.

There are several practical implications from this information.  For instance, you can introduce a shunt or ammeter anywhere into your circuit and know that you are measuring the correct current in the system.  Measuring the voltage at the motor terminals and comparing that to the battery pack voltage will give you information on the total resistance of your system.  If the voltage is too low, look for poor connectors or other factors which introduce resistance into the system.  Knowing the voltage at the motor terminals and the current in the circuit will let you evaluate the power of the system and determine if you are exceeding the capabilities of your motor.  A ferrite motor should not be run at more than about 100 watts. A small cobalt motor can easily handle 150 watts.

Resistance is Futile! (and a waste of power)

In the previous newsletter, the basics of electrical theory were presented with some suggestions as to how this information can be used.  One of the items discussed was ohms as a measure of resistance.  This article will continue to explore how resistance affects an electric system.

From the previous article, recall that according to Ohm's Law, volts = amps x ohms.   We can rearrange this equation to read amps = volts/ohms.  Therefore, as the resistance in a circuit is increased, the amps that the circuit can manage is reduced, if the voltage stays the same.  For example, if the electrical system we have has 1 ohm of resistance and 15 volts measured, then the current flowing through the system is 15/1 = 15 amps. If the resistance is increased to 2 ohms and the voltage is maintained at 15 volts, the current is decreased to 7.5 amps.  Therefore, it is clear that the more resistance in a system, the less current will pass through it.

If two resistors are placed in series, one after the other, the total resistance in the circuit is then the sum of the two resistors.  In other words, Total Resistance = Resistance 1 + Resistance 2. If more resistors are added in series, then the resistance continues to increase by the amount of resistance added.  If two 1 ohm resistors are placed in series, the total resistance is the sum of the two resistors, or 2 ohms.

If two resistors (R1 and R2) are placed in parallel (side by side), the situation is completely different.  In this case a completely different equation is used and the total resistance is the product of the resistance values over the sum of the resistance values.  This is clearly shown in the following equation:

Total R = (R1 x R2)/(R1+ R2)

Therefore, putting two 1 ohm resistors side by side does not lead to a total resistance of 2 ohms as it would if they were placed in series.  Instead, using the equation above, it can be seen that the total resistance is actually 0.5 ohms or one-half the value of each resistor.  This is why stranded wire is just fine for electric flight applications.  Although each strand may have a higher resistance than we would like, by putting so many of them together into one wire it reduces the overall resistance to a desired value.  The stranded wire also has the benefit of being more flexible than solid wire.

In electric flight applications, we must ensure that all possible sources of resistance are minimized as much as is practicable to ensure the maximum amount of current through a system.  Recall also that the current measured anywhere in a circuit will be the same since the current relies on the total resistance of the circuit.  This is why we must look for all possible sources of resistance.

When examining your electric flight power system, first look for under-sized wires.   The amount of resistance of a wire is determined by the gauge of a wire. Also note that the resistance of a specific gauge of wire is not affected whether it is stranded or solid.  Twelve gauge solid wire has the same resistance per foot as 12 gauge stranded wire. Since the resistance is determined by the length of wire used, another thing to look for is excess wiring.  Keep all wires as short as possible.

Next, look at the connectors.  Are they really good ones or are they the same ones that originally came with the system?  Keith Shaw determined that the Tamiya style connectors (white plastic) that are standard on most pre-made battery packs have a resistance of 1.4 milliohms per connection while sermos-style connectors have a resistance of only 0.27 milliohms per connection.  While this may not seem like a lot of difference, keep in mind that all resistances add up and will reduce the performance of your electric system.

Keith Shaw further explored the effects of wiring and connectors on a system when he looked at the difference between a best and worst case situation.  The best case situation used 12 gauge wiring with a resistance of 1.8 milliohms per foot and sermos-style connectors.  The worst case situation used 16 gauge lamp cord with a resistance of 5.94 milliohms per foot and less effective connectors with a resistance of 4 milliohms per connection.  It was calculated for a small system using only four cells, four connections, 20 inches of wire and a 20 amp current flow.  Since power to the motor is proportional to the square of the voltage, the losses from resistance would also follow similarly.  The best case scenario had a calculated power loss of 4 percent while the worst case scenario had a power loss of 24 percent! Keith Shaw also found that for the same system, the power loss increases as the number of cells is reduced.  Therefore, a 24 cell system will be less affected by poor connectors than a 7 cell system.

So, in summary, resistance in a power circuit is very important and will affect how much power the system can deliver.  Since electric flight has a low power to weight ratio as compared to wet fliers, it makes sense to ensure that your system is capable of delivering all the power possible.  Therefore, check your power system and make sure that you have the proper gauge of wire and good connectors.  Also, reduce any unnecessary wiring and connectors to further improve the power in your system.