Electric Flight Specialist since

Electric Flight Specialist since 1999

FAQs

Welcome to our FAQ's page, please scroll down the page to the answer you require! Hopefully we have info relating to your query on :-

Brushed motors, Brushless Motors, What is Kv, A more technical explanation of KV and Torque, How do I decide which motor/battery to use in a model, How do I calculate the power being produced by a motor?, How much Power is required to fly the way you want?, Li-Po and battery charging, What is BEC, What is U BEC,

Motor Types

There are two main types of electric motor available for model use, brushed and brushless motors.

"Brushed Motors".

Most are generically named after the Graupner "Speed" range. (Speed 280, 300, 360, 400, 480, 500, 600, 650, 700, 820, 900) although not all are designed for electric flight. Others supply similar sized motors named accordingly.

Generally speaking the number refers to the length of the motor casing in mm multiplied by 10, i.e. the case of a 600 motor is approximately 60.0mm long not including the drive shaft but including the length of any protruding bearings. Some motors have external brush gear and the length of this is not included in the number. Each size has variations of voltage and current rating. Some have plain bearings better quality motors have ball races. (BB suffix) Generally speaking if particular model specification calls for “600 motor” you have a good chance of selecting a suitable motor and controller package if the normal and max current ratings are followed when selecting controller and battery packs.

Ferrite magnets are common in the cheaper motors and rare earth magnets such as cobalt or neodym are used in some of the more expensive. The stronger the magnetic field the better it will perform. An external flux ring can be wrapped around the outside of some motors to improve the magnetic circuit / efficiency of the motor.

A permanent magnet is fixed around the inside of the external casing and encloses the rotor that has the windings and commutator fixed to it. The DC current to the motor windings passes through carbon brushes that bear on the rotating commutator by spring pressure. We use high currents so the wear on the brushes and the commutator can be high generating significant heat, this wastes battery power. The motor is controlled by an Electronic Speed Controller (ESC)

"Brushless Motors"

This type of motor must not be connected directly to a DC supply. They require a special brushless ESC that electronically performs the job that the commutator does in a brushed motor. As with brushed motors there are many sizes of motors available that have been developed for model use. Unfortunately different manufacturers and suppliers do not follow the same naming process! Many motors being referred to by physical dimensions (sometimes internal and sometimes external) This gives little indication of the performance and leads to confusion when selecting motors. To make things worse the Kv rating (revolutions in 1000's / per volt applied) is usually incorporated into the jumble of numbers. Confused! don't worry.

When we test the motors we supply they are given a power rating and equivalent IC comparison along with recommendations for prop size, this removes some of the confusion.

Advantages of brushless motors:- Better power to weight ratio and ability to operate at high RPM. Higher efficiency so run cooler. Little or no maintenance. Greatly reduced radio interference. Wide operating voltage.

The ESC is more complex than brushed type and therefore can be costly. However economy “Basic” controllers with limited programming are available if ultimate performance is not required. A good example of "you get what you pay for"!

Standard (In-runner) brushless motors are generally the reverse of a brushed motor. The windings are fixed around the inside of the external casing and enclose the rotor that has permanent magnets fixed to it. The rotor turns the central drive shaft that is supported in bearings. Because the windings are fixed the electrical connections can be brought out directly. This simple configuration reduces losses, as the only mechanical friction is from the shaft bearings.

Out-runner brushless motors use a different configuration. The magnets are fitted to the outer case that revolves (hence the name) around a central stator that has the windings on it. The stator is fixed to the front plate of the motor which is the only part of the outer case that does not revolve so the connections to the windings come out through the front. It is hard to visualize (and explain) but think of the outer casing being like the drum in a washing machine only the drive shaft comes through the centre of the drum, i.e. through the centre of the fixed front plate. The motors windings are inside the drum! Out-runners produce higher torque and can drive larger props without a gear-box but because most of the motor revolves, care is need to ensure nothing can get close to the motor when it is in use.

"What is Kv?"

Kv refers to the number of revolutions (in 1000's) a motor turns per volt at no load or with no prop. If you have a 1000kV motor, using a 3 cell battery will make it spin at 11.100rpm (3 cells at 3.7v each) x 1000 (with no prop on it). A 2 cell will only make it spin at 7,400rpm(7.4v x 1000kv). You need to select the Kv required for your needs i.e.; type of plane and performance required.

For a given power input, a lower Kv allows the use of a larger diameter prop, giving higher thrust at the expense of top speed, a higher Kv turns a smaller prop, at higher RPM resulting in a higher top speed but in lower thrust.
If you intend to hover, have fast climb, good acceleration and are able to use a larger diameter prop but do not want top speed, then low Kv is preferable. High KV will turn a small prop at high revs but without the torque – better suited to high speed flight.
Here's a fact - a 2s pack will turn a larger prop than a 3s pack! On 2s you can turn a larger prop before you reach a particular motor's amp limit. If you tried to swing that same prop on 3s you will exceed the max current rating of the motor.

"A more technical explanation of KV and Torque"!
Torque is a motor constant (kt) and inversely proportional to the voltage constant (kv). As kv increases, kt decreases. This means a higher kv motor has a lower kt and draws more current on less volts to make its power. The lower kv motor will get you to the same point with more cells but less current. (I x V=Watts)
A high Kv motor is good for running a small prop (6"-7"-8"), very fast, for speed. A 1800Kv motor can potentially turn at 18000rpm on 10v (not with a prop on, but never mind that). A 850kv motor can only turn at 8500rpm max on 10v, but it has more torque - so it can turn a bigger prop (say 10"-11"), admittedly more slowly, but generate a lot of thrust at the expense of speed.
Use a high Kv in-runner motors (say 4400Kv) you want to use a low gear (say 6.6:1)gearbox to turn a big prop very efficiently - you can get useful power out of one of these on lower voltages. You need to be careful ! Do not exceed the max rpm (typically 50,000rpm) of the motor. High Kv in-runners are usually not useful for direct drive because they draw too many amps, even with very small props.

"How do I decide which motor/battery to use in a model?"
The simple answer is, calculate the power needed (watts), to fly the model (weight in pounds/kg) in the way you would like it to fly. If this sounds complicated, trust us, it isn't! Click here for our simple guide. There are many other factors which could be taken into account but we don't want to do that,

"How do I calculate the power being produced by a motor?"

By multiplying the voltage (V) of the battery pack by the current drawn ( I ) from the pack you have the power (Watts) going into the motor. Of course this is input power and the actual power out is dependant on the efficiency of the motor, but for simplicity most people use the input power when calculating power required for a given model. The simplest method of measuring the voltage and current is to use a "Watt meter" this invaluable device calculates the watts and allows you select the correct components for your power train. Without this ability you do not know if the battery / motor / controller are operating within their respective limits.

"How much Power is required to fly the way you want?"

It takes about 30 watts per pound of aircraft to stay in the air. 40 to 50 watts per pound are needed to take off from a grass field or (ROG) rise off ground. With more than 60 watts per pound, you will be aerobatic if the airframe is capable. At 100 watts per pound and above, full aerobatic performance is possible. Motor efficiency is getting better with advances in technology so less power is required if using brushless motors.

"Li-Po and battery charging FAQs"

Please also see our guide to using Lithium batteries

A battery is made up of individual cells. Each cell of a Lithium Polymer battery has a nominal voltage of 3.7v, and they are connected in series to obtain the required voltage. This results in a 2-cell Li-Po being referred to as a 7.4v (2s) Li-Po, and a 3-cell Li-Po being referred to as an 11.1v (3s) LiPo.

Packs or cells connected in parallel are given a "p" designation. this can be confusing as sometimes even a single pack of say 3 cells is called 3s1p! Best to ignore this parrallel description initially as it is usually only used with more advanced motor battery installations. For clarification however, two 2200 mAh packs of 3 cells connected in parallel would be designated 3s2p this would have a nominal voltage of 11.1v but combined capacity would be 4400 mAh i.e. 2 x 2200 mAh

LiPo cells must maintain a voltage between about 3.0 volts and 4.2 volts, and allowing a cell to go much above or below this voltage range can result in damage to the cell.

The only way to assure that each cell stays within this voltage range is to charge them individually to a specified voltage (balance charging), or to bring each cell to the same voltage as the other cell(s) in the pack (balancing).

A special connector on LiPo battery packs, called a 'balance connector', makes this possible. The specification / type of connector varies according to manufacturer. (JST-XH is probably the most common style of connector currently in use).

Some balancing devices simply balance the cell voltages - they do not actually charge the cells. We recommend the use a Balance Charger that monitors and charges the individual cells of a LiPo. You do not need to balance packs every time they are charged.

We strongly recommend using cell voltage checker before and after the pack is used. This enables you to “monitor and understand” the condition of your battery packs. The use of this cheap simple device coupled with a little understanding relating to battery packs will pay dividends in battery life and peace of mind on safety issues.

Li-fe and Li-ion battery packs also require the voltages to be monitored although actual cell voltages differ due to the chemistry of the cells, they are more tolerant to max / min cell voltages, this fact should be considered when selecting speed controllers and charging equipment.

What is a BEC?

BEC ( Battery Elimination Circuit) is an electronic feature of some speed controllers which provides a power supply for receiver and servos. Power is taken from the main battery pack and regulated to 4.8-6v which feeds to the receiver via the 3wire speed controller / receiver lead. It is worth noting that the +ve and -ve pins of each receiver channel output are common therefore the voltage /current supplied by the BEC in the speed controller lead when plugged into the relevant channel assigned to motor control will power the receiver and the dedicated battery input pins will not be used. For this reason if the BEC function of a controller is not required, simply remove the +ve wire/contact from the plug and isolate with heat shrink tube. Reinstating the wire at any time will restore the BEC function. The BEC function has limitations and care should be taken to ensure the current limit of the device is not exceeded. Servos draw varying amounts of current depending on specification load and quality. Cheaper micro servos can often be found to draw high current.

Some speed controllers have a linear BEC which is simply a resistive voltage dropping device, this has limitations especially when working on higher cell count battery packs (the higher the cell count the less the number of servos you can drive!) Better quality controllers utilise a “Switch BEC” which electronically regulates the voltage / current and is better able to supply a regulated voltage and higher current.

What is a U BEC?

U-BEC (Universal BEC) is a separate Battery Elimination Circuit which can be connected to the main flight pack to supply a regulated voltage to the Rx and servos, as it is a separate unit it is possible engineer the installation to have the current capacity suitable for the number of servos required. The limitation of the “onboard” BEC in many ESC’s is it’s ability to supply a suitable current and voltage especially when high cell counts are used. This is one of the reasons why many high amp controllers are have no BEC facility these are usually referred to as OPTO although the OPTO function means the ESC is electrically de-coupled from the RX (signals to drive the motor controller are passed between Rx and controller but current to drive servos can not pass because of the optical de coupling. Hence the need to use a separate Rx battery or BEC (usually referred to as a u-BEC, which is actually a voltage regulator that reduces the main flight pack voltage to 4.8 or 6v to power the Rx and servos. The optical isolation reduces the chance of interference passing from controller to Rx via the wiring. Price of U-BEC varies, possibly a little more than a good quality Rx battery but there is usually a weight saving and of course it’s one less battery to charge and probably more reliable, (how many crashes are the result of discharged or defective Rx packs!)

More FAQ's being compiled! Please ring or mail if you require help.