EDF question

[arslanwali]

hello all!
I had a 5 blade 70mm edf,60amp ESC,2200 mAh 3 cell lipo. I accidentally broke it's fan(impeller). couldn't get my hands on a 5 blade impeller so got one with 8 blades. now the problem is that I don't know how much amps it drew on 5 blade, but it was certainly below 60 amp. now I'm afraid of using the 8 blade fan with 60 amp, as I'm guessing it might toast the ESC and I don't want that. I don't own a watt meter and have run out budget so getting a new one is out of the question. hobby stores here dont have any either! any idea what can I do here.
the link to the motor is http://www.tiansheng-model.com/product.asp?anclassid=5
it is TS2830/3300kv.

one more thing, I was wondering of using a 55 amp or 60 amp fuse to save the ESC while testing the 8blade fan. the idea is to attach the 55amp fuse in between the negative wire of lipo which goes to the ESC.im Guessing that if the motor sucks more than 55 amps the fuse will blow, break the circuit and save the ESC. this is my theory, I'd like you guys to tell me if it'll work or not. eagerly waiting for your feedback.

 

[Hai-Lee]

I cannot answer the fan part of the question but I will make comment on the use of a fuse as protection.

A fuse is not used to protect the load, (in your case the motor and ESC), but rather to protect the power source and in extreme cases the delivery system, (the wires), from melting and fire caused by an excessive current flow through them.

A 30 Amp fuse will fuse at 30 amps but it will take a long time as most fuses rely on their resistance to cause the temperature to rise in the fuse material to a point where the material melts and breaks the circuit, With a current draw of 200% of its rating it still may take a couple of milliseconds to blow.

Modern electronics are extremely high speed devices and they are extremely sensitive to over voltage and over current situations. A sharp and very short high voltage or high current pulse will pass through the fuse and directly in to the ESC and blow it to pieces long before the fuse would be near blowing.

There are circuits designed to be electronic fuses or even current limiters but they tend ot be expensive or fiddly to construct.

One other aspect of standard fuses is that over time they can suffer internally from a form of metal fatigue, (from being used in circuits which have often switched power sources), and will eventually blow even at far lower currents than their rating.

A better device to use instead of a fuse for protecting a load whilst current and power requirements are tested is a bank of PTC resistors, (Light bulbs are the cheapest), which you can switch in or out as you need to allow the current capacity ot be increased to the maximum you wish to test.

By measuring the Voltage and Current when testing using the bank of bulbs you can determine if it will work without ever reaching the Maximum power or current of the ESC and the Motor.

It works for me!

 

[arslanwali]

Thank you hai-lee for clarifying a lot of things here! if you could guide me further, what kind of light bulbs ( as in size? regular house hold bulbs?) would be good to check the system. a bulb which will blow when 50amp or higher hit it, what size would it be?

 

[Konrad]

Are you aware of online predictive programs like this one?
https://www.ecalc.ch/fancalc.php
These are often accurate to within 10% of actual values. This is adequate for me as I like to spec my equipment for 20% head room.

Not knowing what rotor or the electrical characteristics of the motor you list I made a rough calculation based on my Hybrid fan. I'm running an FMS 8 bladed rotor with a 2500KV HET motor, on 5 lipo batteries. This combo draws 64 amps for about 1184 watts (18.5V x 64).

Your 3300Kv motor and on 3 cells should draw just a bit under half that power, for about 591watts . So 591W / 11.1V = 53 amps. Making your 60 amp ESC the smallest I'd try to use!

 

[arslanwali]

Konrad you're right about it's amp draw. it is definitely under 60 apms at full throttle. well i had everything ready and was too eager to try it. Did some test runs of the edf first, luckily there was no magic smoke from the esc!!! so decided to fly the plane. i wasn't able to get a lift off on the first two take off attempts, maybe it was extremely nose heavy. before the 3rd attempt i slid the battery towards the cg, increased the throws of elevons and gave it a go. it seemed as if it wasn't going to fly this time either, but then all of a sudden there was a gust of wind( i was taking off against the wind) and as soon as it happened there was lift off. it climbed perfectly, without stalling even at 75% throttle. at level flight i had to hold the elevator up, leaving it would make the plane's nose go down. i'm guessing it was still nose heavy as i had to trim it a little despite of giving some upward deflection prior to the flight. would try to move the batter further back towards the cg mark. hope it'll take off in a jippy then.

 

[Hai-Lee]

Thank you hai-lee for clarifying a lot of things here! if you could guide me further, what kind of light bulbs ( as in size? regular house hold bulbs?) would be good to check the system. a bulb which will blow when 50amp or higher hit it, what size would it be?

The light bulbs will not blow as they are even slower to react than the fuses. Rather being PTC (Positive Temperature Coefficient) their resistance increases as their temperature does. Their usage is as a limiter of maximum current and I normally use automotive types. A 12v 100W bulb will shine at normal brightness at a little over 8 amps.

As the bulbs resist massive current flows a bank of them can limit the current if there is a failure and you can "Guesstimate" the current being used by their illumination. For my usage I start off with just a single bulb and switch in more as I require more current. I run the bulbs at a red/orange glow. This setting reduces the voltage at the load by a few volts and provides a current limiting function.

I might lose the load still but nothing burns or explodes if the load does fail. A fuse is a fixed resistance and can pass huge current s before it fails a bulb is a variable resistance and its current is limited by its own design parameters.

For an alternate method for testing try using a current limited power supply rather than an unlimited Lipo battery.

Later!