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HomeAircraftHelicopterBeginners Corner › Headspeed and current drawn on ESC + Motor
05-31-2013 03:02 PM  5 years agoPost 1
rudyy

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E. Amherst, NY

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Hope this is not a stupid question. Can someone please explain me why running at a higher headspeed will prevent current overdrawn and thus the ESC and motor from overheating? On the contrary, a lower headspeed will result in the current overdrawn, and thus heat up the ESC and motor.

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05-31-2013 05:35 PM  5 years agoPost 2
Simmer

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Massachusetts

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Ill give it a go. Think of the battery as a river.. water flowing out of a river say 12 feet across. The pressure is consistent, pressure in this case relates to the amps or current "flow"

So as I understand it, picture now the river being 8 feet wide, same amount of water "trying" to flow thru the narrowed gate.
The pressure builds up and hence in this example the battery feels that pressure, cannot release the amps as fast as before. The effectively holds back the current flow and the battery will heat up, as it cannot free up the amps wanting to spill out.

Motor I believe is similar. Best to run all out full, no wasted current, most efficient, when the current is less than the motor is designed for, its "held" back, again heat gets generated.

Now that I've typed this, I am no expert, I have heard the same from my experts, "crank up the head speed" to avoid damaging the motor or battery.

I do not know how close to accurate I am in my description above, but I think it sounds reasonable.

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05-31-2013 05:38 PM  5 years agoPost 3
Simmer

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Massachusetts

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one more thing. The Motor, Battery and Esc all work together. Must be taken into consideration when setting up a head speed. choice of battery, choice of motor, or ESC.

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05-31-2013 05:44 PM  5 years agoPost 4
dkshema

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Cedar Rapids, IA

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Wow. That's hard to follow.

Is your question written assuming all items on the heli remain constant? Or are we allowed to play with motors, pinions and other settings?

-----
Dave

* Making the World Better -- One Helicopter at a time! *

Team Heliproz

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05-31-2013 06:03 PM  5 years agoPost 5
DougPenhall

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San Jose, CA - USA

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Lower head speed requires more pitch to generate the same lift. More pitch requires more torque. More torque requires more current. More current causes more heat.

That said, higher head speed means more drag on the rotor blades. More drag + more speed requires more power. More power generates more heat. Drag and lift go up at a rate proportional to the velocity squared. The lift will be reduced by reducing the pitch, but the drag will still increase.

Because of these two opposing issues there is an optimum head speed that minimizes the required power. Increase or decrease the head speed and power (and thus also heat) will increase. In other words faster is only better up to a point. After that point faster is worse (if more heat is worse). I mean you may want a higher head speed for other reasons and just get a bigger motor, ESC, and battery to get what you want. But you also need to consider the weight and strength of the rotor blades. You can set things up so you have a head speed high enough to cause the blades to fail.

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05-31-2013 06:41 PM  5 years agoPost 6
Simmer

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Massachusetts

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yea I was kind of just guessing, but there's a good answer below mine

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05-31-2013 06:46 PM  5 years agoPost 7
rudyy

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E. Amherst, NY

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So, I would think faster up to the point the ESC can properly govern? Beyond that point, efficiency will lose and heat generated?

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05-31-2013 07:06 PM  5 years agoPost 8
Simmer

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Massachusetts

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again not the expert, but... I do not think it has to do with the governor. I do think the Motor design sets the tempo for the ESC and Battery plus head speed, to follow.

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06-01-2013 07:26 AM  5 years agoPost 9
JKos

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Redondo Beach, CA

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A lower headspeed will always require less power. Hovering an x pound heli requires x pounds of thrust. How you generate that thrust determines how much power is required.

Higher headspeeds waste power due to drag. Drag increases dramatically as headspeed is increased.
Lower head speed requires more pitch to generate the same lift.
True.
More pitch requires more torque.
At a given headspeed, yes.
More torque requires more current. More current causes more heat.
Ok.

What isn't captured in the above, though, is that power is equal to torque times angular velocity. If torque increases more slowly then angular velocity (headspeed) decreases, the total power required will decrease which means current required will decrease.

- John

RR rules!

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06-01-2013 01:55 PM  5 years agoPost 10
rudyy

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E. Amherst, NY

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What isn't captured in the above, though, is that power is equal to torque times angular velocity. If torque increases more slowly then angular velocity (headspeed) decreases, the total power required will decrease which means current required will decrease.
Let's assume Power = P, Torque = T, Angular Velocity = V required for hovering.

So P = T x V

If less headspeed, that means V decreases and T has to increase in order to give the same P. But the larger T will result in more current which means more heat generated.

If the headspeed is too low, that means more T will be required. This will cause excessive current drawn and thus overheatng.

Is that right?

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06-01-2013 04:12 PM  5 years agoPost 11
DougPenhall

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San Jose, CA - USA

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I'm not sure about this part, but I believe that for a given motor to reduce the speed you need to reduce the voltage. Since the power is the same because the torque has increased the motor will draw more current. The power disipated is volts times current (p=v*i). The current is volts divided by resistance (i=v/r). If we substitute we get p=i*i/r. So the power is proportional to the current squared. Twice the current means twice the power. However twice the current means the wires need to be twice as thick to handle the extra electrons. Since they're not twice as thick the get hotter. This in turn increases the resistance of the wires which then require more volts to overcome this resistance. A motor with no load effectively has very high resistance due to the induction of the wires and the changing current through them as the motor spins. Therefor the current stays low. As you increase the load the inductance is reduced and the current increases. As the current increases the wires heat up increasing the resistance. The power equals current squared divided by resistance is what then causes the extra heat. If we had some kind of super wires that had 0 resistance regardless of the temperature there would be no heat and the motor would be 100% efficient.

More current in the ESC also generates more heat. Basically if you have half the volts you'll need twice the current. Half the volts cause half the heat, but twice the current means 4 times the heat. So half the volts and twice the current together cause twice the heat.

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06-20-2013 03:36 PM  5 years agoPost 12
chris6414

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Sneads Ferry, NC USA

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Just lower your gearing and your Thr Curves. For scale type flying I had a 450 that could fly for 10-12 minutes with a 10T pinion on a 3700 KV Esky motor. 65% throttle at the hover. Certainly not a 3D machine but you can get them to fly longer by lower gearing.

Chris

Century Hawk Sport, OS .32, Futaba 7C, GY-601 9251
Predator Gasser SE G-23, Fut 3010 servos, JR gyro/servo

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