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Old Oct 10, 2001, 07:21 AM
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Electronites! Put on your thinking CAPs

A pal of mine has a Jeti JES-70-3P Opto brushless ESC. After running for about a minute at half throttle on his test bench, the ESC blew its capacitor (condenser). He was running 16xCP1700 on Mega 22-20-2 motor (~42A full throttle).

He was using a clamp meter and his leads from battery to ESC totaled about 7", so there are no apparent problems with his setup.

The Jeti 70-3P Opto has one 470/25V cap.

Should this cap size be sufficient?

Would a 470/63V cap do a better job? Or should he use two of the 470/25V?

This question was left dangling in another thread, so I'd really appreciate some help from an electron-enabled member - I don't know enough to advise my buddy on this issue.
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Old Oct 10, 2001, 08:23 AM
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I can see several problems here:

1: 25V is perhaps a little short of what a freshly charged 16-cell batterypack will put out. But i don't think it is that big a problem. But please don't put in a new 25V cap. Find a 35V type (or more).

2: I don't know what type of cap that are used on the Jeti controllers, but it may not have been able to handle the high switching currents. Replace it with a high current / low ESR type.

3: A Mega 22-20-2 on 16 cells really doesn't sound like a combination i would try, even in my dreams. Isn't the 22-20-2 designed for 7 cells use?
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Old Oct 10, 2001, 08:27 AM
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If you can measure the voltage waveform on the cap with an oscilloscope it may give you a clue as to why it failed. It seems common for these large electrolytics to get hot in our brushless controllers. The 16 cell pack could have exceeded the voltage rating of the cap when freshly charged. At 1.6 volts per cell that comes to 25.6V. This of course will drop rapidly as soon as any significant current is extracted from the battery. Part throttle operation at high currents will produce current pulsations in the battery circuit. It is the purpose of the cap to act as a buffer to reduce the effect this would have on the ESC circuitry. If the combined wire length of the battery and ESC is long, or if the wire is undersized, the pulses can be larger. Inductive effects in the wires can then magnify the voltage, so it is again possible that the 25V rating was exceeded. Also, these electrolytic caps tend to heat up when subjected to high power / high frequency duty. The heating reduces the performance of the cap and it should be de-rated as a result. I'd suggest using a higher voltage capacitor at those higher cell counts, perhaps 35V. Also be sure that the wire is at least 14 or even 12 guage to support the high current.

All that said, I recommend the controller be returned to the manufacturer for repair before any mods are made if it is still under warranty.
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Old Oct 10, 2001, 10:30 AM
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Quote:
Originally posted by Mikael Pedersen
I can see several problems here:

1: 25V is perhaps a little short of what a freshly charged 16-cell batterypack will put out. But i don't think it is that big a problem. But please don't put in a new 25V cap. Find a 35V type (or more).

2: I don't know what type of cap that are used on the Jeti controllers, but it may not have been able to handle the high switching currents. Replace it with a high current / low ESR type.

3: A Mega 22-20-2 on 16 cells really doesn't sound like a combination i would try, even in my dreams. Isn't the 22-20-2 designed for 7 cells use?
1) So should I use two 25V caps in parallel, or one 67V? I don't have a souce for 35V Caps and have plenty of 25V and 63V in hand.

2) Says 470/25V, looks just like the cap you'd find on any other ESC - don't know more than that...

3) Actually, the Mega 22-20-2 on 16 cells, 5:1 gear and 13x8 prop is just about ideal - extremely efficient. Also runs fine on 27 cells at 7:1. Seven cells is just the max recommended for DIRECT drive, as the Kv is 2875, but it's much better on more cells geared.
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Old Oct 10, 2001, 11:17 AM
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Quote:
Originally posted by NipponDave


snip

I don't have a souce for 35V Caps and have plenty of 25V and 63V in hand.
They really should be low ESR types as a previous poster
mentioned. Low ESR caps look identical to normal capacitors so
the only way to tell is by checking the manufacturers specs.
Chances are that normal caps will get hot and won't last long.

Colin
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Old Oct 10, 2001, 11:19 AM
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Quote:
Originally posted by NipponDave


1) So should I use two 25V caps in parallel, or one 67V
No.

Two 25V caps in parallel have a working voltage of 25V not 50V.

You might get away with two caps in series but then the capacitance is halved.

Colin
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Old Oct 10, 2001, 01:46 PM
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Originally posted by NipponDave


1) So should I use two 25V caps in parallel, or one 67V? I don't have a souce for 35V Caps and have plenty of 25V and 63V in hand.


Definately go with the 67V. But be prepared that if it isn't a low ESR type cap, it just may die at some time.

2) Says 470/25V, looks just like the cap you'd find on any other ESC - don't know more than that...

I have just received my Jeti controller (for my first brushless ) and it doesn't seem like it is a low ESR type.

3) Actually, the Mega 22-20-2 on 16 cells, 5:1 gear and 13x8 prop is just about ideal

Oh, geared. That explains it
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Old Oct 11, 2001, 06:20 AM
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Quote:
Originally posted by NipponDave


1) So should I use two 25V caps in parallel, or one 67V? I don't have a souce for 35V Caps and have plenty of 25V and 63V in hand.
IMHO, 63V would be fine.
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Old Oct 11, 2001, 07:00 PM
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Why dont you get in touch with Jety and ask them, they did after all design the contoller, or probably know who did. You never know, they might be able to give you some good information about their product. They might be able to tell you why the blocking capacitor failed in your aplication. You never know, they might even be able to tell you how to get it fixed so that it will work as it should, they may even tell you what not to do so that it does not blow again. Give it a try, it might just be the rite thing to do.
Robert.
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Old Oct 11, 2001, 11:34 PM
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TVSD

You could try putting a TVSD across the motor... A TVSD is a transient voltage suppression diode - much like a high current zener. Digikey sells them. Pick a voltage above your battery voltage.
The capacitor should be low ESR, but at the same time be twice the expected working voltage at a minimum. Even given this, the capacitor could fail as this one, if the voltage across it was inadvertantly reversed - even momentarily. A higher ESR cap will not fail sooner than a low ESR cap. In fact I'd expect the opposite. With a high ESR cap your ESC could fail. The lower the ESR the more current the cap will be passing. Shock due to a crash could be a factor too. Tantalum and other caps will emit high voltage spikes with shock events - maybe enough to cause failure(?)
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Old Oct 12, 2001, 02:34 AM
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Re: TVSD

Quote:
Originally posted by bklein
You could try putting a TVSD across the motor...
I wouldn't do that with a brushless motor.
If it has to be done, it will take 6 diodes in order to make it work properly. And I don't see the need for them. Brushless motors doesn't make sparks and big transient voltages in the first place.
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Old Oct 12, 2001, 07:27 AM
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After a bit of thinking, I realised that 3 diodes are enough if they are coupled as a star, with the center joint left "floating".
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Old Oct 12, 2001, 08:44 PM
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Oops I didn't catch the fact that it was a brushless motor. Regardless, a 470uf cap will not be placed across it in some way - it will be on VCC for the ESC. Definitely there can be overvoltages here with a brushless motor. Say you're running full bore and crash. The back EMF has to go somewhere and it will be the ESC and its bypass cap. This cap could be damaged it wouldn't be apparent until some future point in time when it decides to short. I work for a disk drive company and we use a TVSD on each drive to protect against overvoltage from the motor or crappy power supply... Truthfully though, I'd just put another cap in like the one you had unless the manufacturer advises otherwise. Be advised that the cap could be very critical in the design. If the ESR is not right the motor may not start because the BEMF on the cap isn't large enough to determine motor speed. The circuit determines this by the ripple frequency across the cap.
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Old Oct 13, 2001, 06:05 AM
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Thanks for all the good responses. I have replaced the blown cap with a 470/25V, just to see if the rest of the ESC survived. It seems fine, but we've only tested it at 7 cells. We'll repeat the 16-cell, part-throttle torture test again soon and see what happens. If it blows again, we might conclude that 25V caps aren't sufficient. If it runs OK, then we'll suspect that the original cap was weak.

If the 25V does blow again, we'll install the 63V cap and see how the ESC likes it. Further reports to follow...

Thanks again for all the illumination.

David
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Old Oct 13, 2001, 09:27 PM
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When I design a DC circuit, I always use capacitors that are rated at about twice the voltage I'm using. If it's a 5V power supply, I use a 10V cap.

If I was building a 24VDC supply, I sure wouldn't use 25V caps.
I've seen too many of them pop.. To be safe, use at least a 25% safety margin.
Rich<>
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