Basic Electrics for R/C models

Basic Electrics for R/C models

 

BASIC ELECTRICS FOR R/C MODELS

BY JASON SAGAIDAK (MMAC education officer)

Ok so you want to try electric r/c model aircraft but don’t know where or how to start?

Been disappointed with the performance of that ARF electric model you purchased?

Got an old IC model at home that you thought might make a good electric one?

THEN READ ON

Right –o-then ,I don’t claim to know everything about electric r/c modelling however I can put you in touch with some good basic knowlage to get you confidently on your way, to power and purchase parts that will work for your application.

There are a couple of items you will need to purchase to put into your tool kit.

  1. Rev / min counter (optical tacho) you may already have one
  2. Watt-Voltage-current meter
  3. Calculator (there is a bit of maths need to work it out)

What does KV on a motor mean?

Ok in electrical terms KV = kilo volts however in motor terms K = RPM thus meaning RPMV = RPM per VOLT

So this means that if you have a 1400kv motor and you intend to use a 3cell  li-poly battery you then take the source power voltage and multiply it by the motor KV.

  • 2 cell li-poly = 7.4volts
  • 3 cell li-poly = 11.1volts
  • 4 cell li-poly = 14.8volts
  • 5 cell li-poly = 18.5 volts
  • 6 cell li-poly = 22.2 volts

Example

Formula is RPMV where RPM=1400 and V=11.1

Therefore  RPM x V = 1400 x 11.1

                                    = 15,540 rpm

 

Lets say you have a 3600 kv motor and use a 3 cell li-poly battery what would be the rpm?

RPM x VOLTS = 3600×11.1

= 39,600rpm

 

What if I don’t know the KV of the motor?

Ok using your optical tacho and the volt-watt-amp meter connect a li-poly battery run the motor up to full power and get the tacho and voltage reading.

The test motor I used gave a reading of 26,400 rpm and I use a 3cell li-poly.

Example

Using RPM / V=KV  , rpm = 13,400 and  v=11.1

Therefore  RPM / V = 13400 / 11.1

=12,070 KV thus meaning the motor is probably around the 1200kv area.

Why is KV important?

The KV is important when choosing the right motor for the airframe you wish to power. For example a large model such as a 80in scale warbird your going to want a low KV motor so it will swing a large propeller (22”x10”). If you where to choose a high KV motor chances are the propeller would not protrude out past the cowel.

  • Low KV = high torque & lower rpm
  • High KV = low torque & high rpm

Low KV motors are commonly used in larger slower aircraft and high KV motors are used in smaller quicker aircraft.

So to recap high KV are a generally small motors and low KV are generally large motors.

WHAT IS POWER

Ok this is where OHM’s law comes into play. OHM’s law states that V=I x R , where

  • V = volts ,
  •  I = current (amps) &
  •  R = resistance (ohm)

Therefore using ohm’s law we also know that power is measured in watts thus giving W = V x A where

  • W = Watts
  • V = Volts
  • I = Current (amps)

Now why do we need to know the power rating of the motor? This tells us how much power the motor is consuming to run flat out. The spec’s given on the motor spec sheet are usually the max rates. If the motor is pushed to use more power then burning out of the motor is going to result.

Now this is going to be important when selecting prop and speed controller sizes with the battery combination you wish to use, and of coarse the model your going to put it into.

We will come back to this bit later

HOW TO UNDERSTAND MOTOR SPEC’S INFORMATION

Here is an example of a brushless motor spec information.

D2830-11 1000kv Brushless Motor

 

 

Suggested Prop: 8×4 (4S) ~ 10×7 (2S)

So from the information  on the table above we can gather the following important information.

  • The KV , (1000 rpm/v)
  • Max current in amps , ( 21amps)
  • Max voltage , ( 15volts)
  • Power in watts , ( 210watts)

We can also get information on the shaft size and physical size of the motor as well. This info is equally important when matching motor to aircraft, ie will it fit in the cowel…etc

The motor manufacture usually provides the prop sizes at the nominal voltage used this is very important to take note of too. In this case on 4 cells they used a 8×4 while on 2 cells a 10×7 was used.

Lets look at the rpm difference between the to prop sizes

  • 14.8 x 1000 = 14,800 rpm on the 8”x4” prop
  • 7.4 x 1000 = 7,400 rpm on the 10”x7” prop

It’s most likely that each prop combo produced the same thrust , however the same plane with the smaller prop and higher rpm would be quicker than the larger slower prop as there is considerably more drag from the prop diameter.

Ok lets’ look at the next example

 

Required; 

6S~9S Li-Po

Test Data:
22v – 21×13 Prop – 40A – 5100g Thrust
22v – 19×12 Prop – 29A – 3500g Thrust

Suitable for sport and scale airplanes weighing 9 to 15 pounds (4–6.8 kg).

So from the information  on the table above we can gather the following important information.

  • The KV , (210 rpm/v)
  • Max current in amps , ( 60amps)
  • Max voltage , ( 33volts)
  • Power in watts , ( 2000watts)

So lets have a look at the tested rpm

  • 22.2 x 210 = 4,662rpm now look at the current diff using the same voltage 40amps compared to 29amps this was achieved only by changing prop size. This is always good to remember when trying to work within the current rating spec’s.

LI-POLY BATTERIES

This seems to be the most confusing for people to come to gips with so here goes I’ll try to explain it as best I can.

Lets look at a commonly used li-poly battery used.

2200 25c 3S1P what does this info mean.

  • 2200 = 2.2amps this is the C rating of the battery
  • 25C = is the discharge rate the battery can supply without melt down
  • 3S1P = 3cell series and the 1P = 1pack parallel

This means that a 2200  25C pack can deliver 2.2 x 25 = 55amps continuous

 

2200 30-35C 4S1P

  • 2200 = 2.2amps
  • 30-35C = discharge rate with it’s maximuim short burst
  • 4S1P= 4 cell series 1pak

This means that a 2200 30C pack can deliver 2.2 x 30 = 66amps with short burst 77amps.

How long can I fly for on a charge

Right-O this seems to be the first thing that people ask when discussing batteries is ,” how long will I get out of this battery.”

Think of the battery as a fuel tank and we all know that we can buy different size fuel tanks for IC. It is all dependant on room in the model, well the same goes for li-poly batteries.

Think of the battery this way for a sec,

  • 6oz tank = 850mh li-po
  • 8oz tank = 1000mh li-po
  • 10oz tank = 1250mh li-po
  • 12oz tank = 1500mh li-po & so on

They are a store energy and in this case size does matter there is a trade off though with more capacity comes disadvantages such as

  • Physical size (larger)
  • Weight
  • Longer to charge between flights

To work out how long a battery will last use the following formula

Ib / I x 60 where

  • Ib = battery amps (ie 2.2A)
  • I   = current draw (ie 12A)
  • 60= seconds to put into min

Example

Lets look at a model  that uses a 2200mh 25C battery. The first thing you need to do is hook up your watt-volt-amp meter and run the motor flat out and get the reading, repeat again at ½ throttle and get the reading.

The plane I used is the axn floater (night flier) 2200mh battery at full throttle pulled 16.81Amps and at ½ throttle pulled 7.27amps therefore flight time are

  • 2.2 / 16.81 x 60 =7.89 min at full throttle
  • 2.2 / 7.27 x 60 =18.15 min at ½ throttle

So here we can see that the same size battery can give us a 8min flight and with a bit of throttle management can also give 18min flight.

Lets use the same plane but change the size of the battery to 1000mh 25C one.

  • 1 / 16.81 x 60 = 3.56 min at full throttle
  • 1 / 7.27 x 60 = 8.25 min at ½ throttle

So the flight time was reduced in proportion to the capacity size of the battery.

Lets look back at the motor data to see what size battery one would need to get a 8min flight on our aircraft we are going to bring back to life.

First thing we need to transpose the formula to = Ib

Therefore from Ib / A x 60 = T

We get Ib = T x A / 60 

T=8 , A=21  =>Ib = 8 x 21 / 60

Ib=2.8amp or 2800mh pack

From the hobbyking website this was the closest in mh

 

Take a look at the physical size of the battery to see if it will fit. If it will then lets work out what flight time one should expect.

3 / 21 x 60 = 8.5 min at flat out.

Now one can see that this battery is a 20C so 3 x 20 =60amps the motor is only 21amps so we know this will work well and not get too hot delivering its power.

 

Next time i’ll talk about prop’s sizes with motor KV

Hope this has been a help.