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Consequences of exceeding servo rated values

Old 05-01-2002, 06:57 AM
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Default Consequences of exceeding servo rated values

In another thread ("Mike, please bolster my confidence") you replied to assure me that my HS-5925 servo had been properly repaired.

You wrote:
"Aerosplat- Sorry to hear about the problem. Yes, the technician did determine the amplifier board was bad and replaced it. Yes, the problem was eliminated but you do want to take a look at potentially why it happened. With a single 5925 on the rudder of that particular airplane, it may draw too much current. If the amp draw exceeds 3 amps-- the FET's maximum rating, the amplifier board can fail. The 5945 may be a better choice since it has a lower gear ratio and will draw less current under heavy load that can be produced by a big rudder in a 3D ship.
Mike "

Your comments about servo size, current draw, and consequential servo failure give me great cause for concern. This is going to be a little lengthy, but please bear with me. I think this situation needs to be fully understood and accurately stated for the safety and peace of mind of all RC modelers.

Your statements infer that overloading a servo (exceeding rated capacity) is likely to cause the electronic components of that servo to self-destruct. That's scary, because servos are overloaded every day. Many servos (none of mine) are setup so that full travel causes control surface binding and are effectively stalled (overloaded) at that point. Also, many servos are simply under sized for the load presented to them.

The servo in question here (HS-5925) is rated for 103 oz torque and 309 oz holding power at 4.8 volts. My understanding is this:
Rated torque = maximum force which servo arm can overcome and continue to move in the requested direction. Any greater force will cause the servo motor to stall and the arm to stop moving.
Holding power - Maximum force against which the servo can maintain its requested position. Any greater force will cause the servo gears and motor shaft to move in reverse of the requested position / direction. (Push it backwards).

Now, the rated torque of a servo is a direct function of the voltage applied, wattage rating of the motor, and gear ratio of motor shaft to output shaft. For a given voltage, as load is increased the required wattage output is increased right up to the maximum. At this point the motor stalls and gears stop turning. During this process current draw increases (Wattage = Voltage x Current). At motor stall, current reaches its maximum. The motor is doing all it can do, and additional load will not result in additional wattage or current. For a given voltage (4.8 volts) the maximum current draw can be (SHOULD BE) determined by the designing engineer. Electrical components for the circuitry should be selected to handle this maximum load condition. In fact, all quality electronic circuits I have dealt with have at least a 20% safety margin engineered into the design, meaning the electronics should be able to handle a 20% overload without damage to the components. (I am an Electronics Technician by trade – 35 years) I understand that if you hold a servo in a stalled condition for an extended period of time (say 20 seconds or more) the motor will heat up and possibly fail. But the Electronic components should not suffer if properly engineered for overload. Also, I run my servos at 4.8 volts. This servo is rated for a higher output at 6 volts. Higher output equates to higher wattage, thus more current. If the components are rated to handle increased current at 6 volts, then 4.8 volts should NEVER overload the components.

I would be very surprised, disappointed, and apprehensive about flying if Hitec, or any other manufacturer engineered their servos to be at risk of Electronic component failure as a result of brief, occasional overload. With the possibility of damage or injury resulting from an out of control RC Airplane, I would hope (EXPECT) the manufactures to have a substantial safety margin Engineered into their electronic designs.

Will my Edge 540 and the way I fly it overload the HS-5925 ? I don’t think so, but that is another whole story and discussion………..

Comments, Mike or anyone ???
Old 05-01-2002, 05:17 PM
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Default Consequences of exceeding servo rated values

Aerosplat- You're getting a little beyond my knowledge of how it all works. I do know there is software to help prevent an overload so you should not have to worry, but occationally any electronic component can fail.


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