A BASIC GUIDE TO SUCCESSFUL |
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| chapter four | |
| Switch or Speed Controller ? | |
| A motor control
switch is simply that, on or off. In its simplest form it
can be a micro switch glued to the side of a normal servo
such that the servo arm depresses the switch as it
rotates in response to a transmitter stick movement,
switching on the motor. Adequate and reliable if
carefully constructed, though rather heavy. Next in
complexity and a much better option is the relay type,
plugged directly into the receiver and between the flight
battery and the motor. Still rather heavy but having no
open moving parts it can be tucked conveniently out of
the way. This type of switch often provides a braking
system for the motor, particularly useful for encouraging
a folding propeller to fold. BEC (battery elimination
circuitry) is not usually available with a switch. A
switch may be all that is needed on models such as a
SPEED 400 size semi scale fighter, a SPEED 600 size
aerobatic model or a direct drive powered glider of
either size. A speed controller is a proportional device similar to the throttle control of an internal combustion engine, but it is a solid state device rather than a servo. It provides proportional control of the motor speed in relation to the throttle stick position on the transmitter. Obviously a speed controller provides for far more precise control of the model than the simple on off switch. Usually more expensive, but the benefits of a speed controller should be carefully considered. Being able to throttle back to cruise power can result in greatly extended flight times with most model types. Landings can be more precisely and accurately performed. Even with a powered glider drifting around at low throttle and low altitude on a calm summer evening can be great fun. Look for a speed controller with a brake facility and with BEC. BEC means that the speed controller can supply a regulated power supply to the receiver and therefore save the weight of a separate receiver battery, an important saving. All modern BEC systems monitor the flight battery voltage and cut the motor when the voltage drops below a certain level so that the receiver supply is maintained. The length of time for which the receiver supply is maintained will vary from controller to controller so the wise glider flyer will perform a simulated flight to determine the safe period post BEC. The unwise glider flyer should carry a plastic bag for the bits. A speed controller is connected between the flight battery and motor, two heavy wires to each, be sure not to plug it in the wrong way around. The fine three core lead connects to the receiver via the throttle socket, not the battery socket. Power can be supplied to all receivers through any socket because all of the positive and negative pins are ganged together. Be sure that the speed controller plug is compatible with your receiver. Models utilising a gearbox are a special case. The start-up shock produced by using an on/off switch places too much strain on the gears and in time (if not sooner !) will cause damage. Therefore wherever a gearbox is used the motor should be switched with a speed controller or a "Soft Start" unit. A "soft start" is a kind of a half way house system, the motor can only be switched on or off but an electronic device gradually increases the voltage to the motor over a second or two (or more) eliminating the shock on the gears. As the electronics are very similar to a speed controller the price of a soft start is virtually on a par with a similarly rated speed controller, take your choice. I recommend you use a proportional speed controller in all situations. |
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