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TECHNICAL ARTICLES

Electric Flight Motor Controllers
by Jim Ewing
Great Hobbies Inc

Electric Flight has come a long way in the past few years with much of the credit going to the development of sophisticated electronic motor controllers. Here are some of the features you should look for when choosing an ESC for your aircraft and what they do . . .

Electric Flight System

Pictured above is a typical setup for an electric motor system. Servos and receiver battery have been left out for clarity. Please also note that we have not shown either a protective fuse or an arming switch, both of which are recommended and quite often used by modelers.

Take special note of the electronic speed control and how small it is. The unit shown is the Castle Creations Pegasus capable of handling up to 35 amps of continuous current. It is actully physically smaller than the Sermos connectors used in the circuit.

Not only are today's motor controllers very small, they are also usually microprocessor driven and have quite a few functions that make life much easier for the electric modeler. Here we will discuss some of these features and their usefulness.

Voltage Rating
This is the voltage part of the operating range that an electronic motor controller is designed to operate over. It is generally specified as the number of NiCd cells to be used in the model. This figure can be converted to actual voltage by multiplying the number of cells by 1.2. There is 1.2 volts in each NiCd cell.

Current Rating
The current rating is the amount of current that the unit is capable of handling. There are usually two numbers involved here, one being the continuous current rating and the other being the peak current. As the term implies, the continuous current is the amount of current that the speed control can supply on an ongoing basis without damage. Peak current is the amount of current that the unit can supply in short bursts before the unit gets overheated and possibly damaged.

Current Limiter
The current limiter prevents damage to the controller should the propeller be stopped while in operation. The more load that is placed on an electric motor, the more current it draws. If the motor shaft is stopped, the motor appears to the power source as being very close to a short circuit and therefore wants to draw a great deal of current. A current limiter will reduce this maximum that it can supply so as to prevent damage to the controller, wiring and the motor.

Prop Brake
The prop brake is a feature found on some controllers to keep the prop from free-wheeling when power is off. Essentially it applies a small amount of reverse current in power off position to oppose the free-wheeling of the prop. It is not enough current to make the propeller turn backwards.

Battery Eliminator Circuit (BEC)
A Battery Eliminator Circuit permits using the main battery that powers the motor to also power the receiver and servos, thus eliminating the need for a second battery pack to supply these units. This gives a weight saving to the model, which can be crucial in electric powered aircraft. Plugging the motor controller into the receiver is all that is necessary to activate this function. Be sure the BEC for the motor controller you have selected is sufficient to power the number of servos you intend to use.

Power Cut-Off
This function works in conjunction with the BEC circuit. Obviously, if you are using your main power battery to also power your radio system, you don't want to loose control of the model once the motor has drained the battery. The Power Cut-off function will automatically cut the power off to the motor when the battery voltage has dropped to a pre-set level, ensuring that there is enough power remaining in the battery to supply the radio for the remainder of the flight.

Temperature Overload Protection
This function is also used protect the motor controller from heat damage. If the operating temperature of the unit reaches a certain point, the circuitry will automatically shut it down. Heat is the biggest enemy of electronic components.

Motor-On Safety Start
Many motor controllers have a safety feature that prevents the motor from being accidently activated when the radio is turned on. The motor is usually off when the throttle stick is in the lower portion of its range. The exact position of where the motor turns off can usually be adjusted through a potentiometer on the motor controller. The safety start ensures that the throttle stick has to be put into this lower range before the motor will start in the first place. This ensures that if the radio is turned on with the throttle stick at above the minimum position, the motor will not come on unexpectedly.

Tips:

  • Allow continuous air flow around the speed control. Do not pack it in foam. All electronic devices generate heat and their operation depends on them remaining cool. This is particularily true for devices such as ESCs that pass alot of current. The more current, the more heat.
  • Use good quality connectors and large diameter, low gauge wire with many strands. The operation of your electric flight system is inversely dependant on the total resistance in the circuit and small wire, poor solder joints, and poor grade connectors that have small surface area, dirty contact points, or that wear easily will not give you good service or good performance.