Originally Posted by
HarryC
You seem to be saying that a servo is a constant wattage device, so that if the voltage falls the current will rise. In that case as voltage falls to near zero, the current will rise to near infinite. On the other hand according to you, if the voltage rises the current will fall!
That is not how things behave. A servo can not maintain a constant wattage, the wattage is whatever voltage is present and the current that can be driven by that voltage. The voltage drives the current, so as the voltage falls the current driven by it will also fall, and the total wattage at the servo will fall dramatically. A voltage sag means a current sag.
I understand what you are saying. However, when the power is not being provided to the servo that it needs to match the Servo's Pot setting to the signal's resolution, then the servo attempts to match the two. This is what causes the demand for power to increase even though the voltage decreases for whatever reason. Thus when there is a demand for a certain amount of power and the voltage is lower than normal (i.e. sagging for whatever reason) current increases. P=IR
A flap servo has a constant demand for power (load) when the flap is deployed. To keep that flap down, at the same speed, takes a constant amount of power. If you need 30 watts to keep the flap down at any given speed and then reduce the voltage, the current needs to increase to give you the correct power level needed to match the servo's Pot to the signal resolution. Thus when you reduce voltage, current demand goes up. P=IR.
For the other servos, they are momentary loads. But the same concepts apply, only momentarily. However, as digital servos move, the loads can be cumulative.The more servos moving simultaneously the higher the demand on the battery through the Rxer buss. At times, (e.g. when they all are moving) the load is higher than the power system can supply thus the voltage sags through the resistance caused by the Rxer buss, servo leads and connector, and the limits of the battery, etc.. Thus with reduced voltage it takes more current to accomplish the same work. Increase the current too much for too long and the unit's electronics will heat up and fail.
Shutting off the voltage is not a "sag." and yes when you shut off the voltage, there is no force to move the current. But on servos you are not shutting off the voltage. The voltage is always there. When the servo sees a variance between the resolution setting on the signal line it is receiving and the resistance across the pot in the servo, it uses current to move the servo's motor to match the pot to the resolution setting. When this takes place, the more voltage that is present, (withing the limits of the servos design) the less current used for the same amount of movement. A High Voltage movement is also faster as there is more force behind the current. High voltage servos are designed to take advantage of this variable.
I am not talking about shutting off the voltage and then the current cuts off. That never happens, until you shut off the power switch disconnect the battery or the servo's power circuit fails open, etc. When the system is on, the voltage is always present at the servo. Some folks even bypass the Rxer with the servo's Ground and VDC line Only the signal line comes from the Rer. That is how Smartfly eliminates the limiting effects of the Rxer buss.. Point being, the voltage is always present on the servo. If you don't want the servo's available voltage to be affected by Rxer resistance, you bypass the Rxer. Also, when you bypass the Rxer, you can increase the voltage to the servos so that the resistance of long leads does not impact the voltage at the servo. And, there are no voltage sags/fluctuations on the Rxer as you can provide a dedicated regulated power circuit to the Rxer that is totally isolated from the servo's power circuits.
Back to the point: If you need, say, 30 watts of power to hold a flap in place and decrease the working voltage, the current will go up. P= IR. No way around it. If the current is increased above the servo specs, then eventually it may heat up and will fail. The flap servo's load and the load's effects on the power circuits make the flap servo more susceptible to failure as it's load is constant, once deployed. So, all things being equal, I would expect to see a voltage or undersized servo problem first show up on the flap.