Espritmodel.com Telemetry Radio
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Old Feb 27, 2013, 11:43 AM
Current project: Electrolyte
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Knife,
This is a cool endeavor. A real question worth asking is how much more power rpm will be required simply to make up for the reduced effective fan area. Your likely spending a good amount of power simply getting back to even...
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Old Feb 27, 2013, 12:20 PM
Lee Liddle
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I guess you mean because of the dia of the motor. I don't think there is any significant loss due to the motor size. Using the 8 pole, 37mm dia motor I measured 5.75 lb of thrust and efflux speed of 300 mph at 3.5kw and 50k rpm. That maxed out the airspeed sensor. At 4.3kw it hit 55k rpm and 7.25lb. I've estimated the efflux speed at 325+.

This motor is only 5mm larger in dia, 2.5mm (aprox 3/32") all the way around. With a straight 70mm exhaust tube that doesn't squeeze the area down much, but I haven't calculated the % of FSA.

At 3.5kw l haven't seen any significant loss of thrust, so I'm not expecting any at 4.5 either. Besides it's really efflux speed I'm after, and any small reduction in area/thrust should be balanced by an increase in efflux speed, even though I won't be able to measure the increase.
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Old Feb 27, 2013, 01:38 PM
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Have you dissembled the burnt ESC's. There should be three identical chips next to each other on the controller board (these are the FET drive chips) Look next to them for any burnt components. Possible a FET shorted (BEMF) and a flyback diode got destroyed.
Look on the FET board, did more than one bank of FETs blow?
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Old Feb 27, 2013, 01:51 PM
Lee Liddle
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On this one only one row of fets was damaged. On others it was on multiple rows. I'll look at the board tonight. BTW what's the BEMF? Back feed electrical something or other?
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Old Feb 27, 2013, 02:29 PM
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Back EMF is the voltage that goes back into the FET from the motor. You need to know this value and the voltage going into the motor to find the true working voltage.
If only one bank has burnt why not just replace the components? You can set this up as testing ESC and change components as needed. Are the package sizes To-220 or SO-8?
If you use this formula;
PWM = KV x V x Poles/20 PWM= (1600 kv x 43.2 v x 6 poles)/20 = 20.7k. Can your esc be set at 20k? I wonder if 16k is just not pumping enough energy into your motor? Or if the ESC is not calculating correctly. Oscillator caused slow ADC conversion to calculate BEMF feedback?


See here for info on waveforms http://forums.offshoreelectrics.com/...less-Motor-ESC.
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Old Feb 27, 2013, 06:36 PM
Lee Liddle
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I tried to replace the fets that burned with some I salvaged off another board. Every thing looked pretty good, but when I plugged in the power, it was like a direct short, so that didn`t work.

I did salvage the brain of the esc which seems to still be working, and I have some other boards, I might be able to use all of the parts to rebuild one, but I`ll save that for a rainy day project.

To-220, or SO-8 ??? Sorry, no clue. If I post some pics would that help?

I appreciate the help 901, sorry I`m not more informed on the subject.
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Old Feb 27, 2013, 06:50 PM
Lee Liddle
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Last of the Guni Pig ESCs

OK, with the helpful input from everyone, I`ve gotten brave and decided to sacrifice one more ESC to the cause and hope for good results.

My lipos were all in need of charging, but I`ve run some preliminary tests on 10s....

I started on 8KHz and 0 degrees timing. Very smooth early, but started getting rough above 1/2 power.

6 degrees was better

12 degrees, there was a little roughness at almost full power, but got past that and smooth at full power

Went to 18 degrees (factory setting) Still at 8KHz PWM ....... This worked and I checked peak rpm just above 48k which was no man`s land before. It was hitting 95A, but 10s voltage was low, probably around 35V so I was at about 3.3kw and 48k rpm, those numbers line up with earlier tests on this rotor. So, it`s looking promising.

I`m recharging lipos right now and will push it farther when I get everything charged.

Based on what 901 is saying, I think I need to try 16KHz PWM and lower timing, that may even be better.

Right now, I`m just watching the power and the rpm carefully. If I can get it to run consistently above 50k with this esc, I`ll retest thrust and efflux speed at that level, but that won`t happen tonight..
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Old Feb 27, 2013, 08:46 PM
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Sounds like the FETs shorted other components on the board. Since you do not have a o-scope probably something like an Eagle tree RPM sensor will work. This sensor hooks up to two wires on your motor. I think they are frequency to voltage converters. So all you will have to do is calculate the poles you have to figure RPM. This will give you a good indication on the real frequency the motor is running at.
It could be the FETs are switching at 70% and causing them to heat up and blow. But you need to test the real Kv of the motor.
Can you read peak amps of the motor. Typical volt meters read RMS not peaks.
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Old Feb 27, 2013, 09:30 PM
Lee Liddle
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They are like the type on the far right.

I`m reading rpm with an optical tach, very accurate.

I`ve measured the running kv at 1650 unloaded. Loaded it`s running at 1400kv.
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Old Feb 27, 2013, 09:40 PM
Lee Liddle
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Well, these are good esc for the money, but they are limited at the high rpm levels with high pole count motors.

I decided to switch to 16KHz and try lower timing. On 6 degrees, I noticed a little surging so I changed it to 12 degrees, expecting some improvement. When I powered up slightly on 12 degrees the esc popped about 5 fetts at extremely low power??????

Oh, well............ I`ll continue to use these esc on larger motors that will be running at under 48k. But I`m not going to blow any more of them on this project. I may try a Hobbywing style 120hv because it does have a 24KHz PWM option. But I may just wait on a package from CC.

This IS going to be an incredibly powerful fan, it just isn`t going to be the bargain that I was shooting for.
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Old Feb 27, 2013, 10:10 PM
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Going up in degrees will cause more current. Looks like the FETs cannot take this at low RPMs because they are switching at 16khz and at amps that are higher than what they are rated for. What RPM do you think they blew? What are the p/n on the Fets?

This is a really bummer that you are killing ESC
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Old Feb 27, 2013, 11:41 PM
Lee Liddle
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Quote:
Originally Posted by p901P901 View Post
Going up in degrees will cause more current. Looks like the FETs cannot take this at low RPMs because they are switching at 16khz and at amps that are higher than what they are rated for. What RPM do you think they blew? What are the p/n on the Fets?

This is a really bummer that you are killing ESC
I really appreciate your help, but I'm just going to chalk this up to asking too much from an inexpensive esc. I know a Castle 120hv will do the job, so I'll wait on mine to come in.

Another option for using this esc will be to go a little larger with the fan (75 to 80mm). A larger fan will make use of the motor's power without exceeding the rpm limitations of the esc. I think I can still hit 300mph efflux speed and have more thrust to boot.
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Old Feb 28, 2013, 12:02 AM
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Would have liked to see what was going on in this ESC.
You would have seen peak amps go up with a scope. A 4 channel scope would have seen imbalance between phases from bad firmware or components.
I really applaud your efforts.

Those old ESC with the DPacks are an easy replacement and are about $1 ea. But repairing the ESC will depend if other components were damaged.
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Old Feb 28, 2013, 12:37 AM
310mph Kolibri T25 Swist
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Quote:
Originally Posted by murdnunoc View Post
We have NO WAY of truly measuring efficiency.
Usually we compare the motor's loaded kv to its unloaded kv.
Truly is a strong word... but wouldn't you get close by checking the torque at a certain rpm. power = torque*rpm, then you know the exact output power and you know from your ampmeter what you are shoving in

(Thats how you dyno a car)
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Old Feb 28, 2013, 07:16 AM
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Lee, sounds like you need a big circuit breaker so you stop killing ESCs! Not sure where you get them, but I've seen up to 400amp ones. You're not doing 4kw on 3s are you?
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