blowing caps 180esc

[scoota]

more tests that show a brussless motor can be turned into a GENERATOR

http://www.youtube.com/watch?v=1thlLKZ5_c8&feature=youtube_gdata

[keithbradley]

i have asked the same question down at the club my self , i will try and explain the best i can , the half throttle will get the fets working over time which will cause the ESC to run hot but not blow the caps .

i was told if you back off from full throttle to a stop , the the caps will have a full charge in them still & as the boat slows down in the water ,the prop starts to turn the motor , causing it to turn into a generator & putting more charge into the caps causing them to blow.

so now insted of coming off full throttle to a stop i run my boat to a gradual stop for a half a lap before bring it back to shore . Since i have been doing this i have not blowen any caps ...

I have to imagine this is happening more commonly with the Asian motors? I cant imagine my prop spinning my Neu's but I could definitely see them spinning the "free-er feeling motors".

[m4a1usr]

I have to imagine this is happening more commonly with the Asian motors? I cant imagine my prop spinning my Neu's but I could definitely see them spinning the "free-er feeling motors".

You are most likely correct. There is absolutely no way the water resistance on a SV27 6 pole motor off the prop is going to rotate the drive line. Its almost impossible to do on the bench using ones big strong mits. Water friction with the prop dia we use in said hull does not have enough force to make that happen.

Now a 2 pole 540 can I can see it happening. At least the possibility. Maybe even some 4 pole motors. Magnet force is weaker, rotor mass is not significant, drivelines are about as free as we desire them to operate at.

No matter how this thread develops you cannot charge the caps far above their rating or capability. Its a matter of design. Your not going to get 60 volts stored on a 30 volt cap. Not for long. It leaks. It self discharges. Its design. So the question really is how far above the rated voltage can a capacitor safely maintain voltage and hold it? And for how long? The problem with ultra low ESR caps is they have been designed with excellent voltage surge characteristics. A weakness so to speak in our ESC designs.

This might develop into a very informative thread providing the right folks speak up.

John

[Steven Vaccaro]

You are most likely correct. There is absolutely no way the water resistance on a SV27 6 pole motor off the prop is going to rotate the drive line. Its almost impossible to do on the bench using ones big strong mits.

John

Thats one of the reasons the sv27r motor is a bit different than the sv27. The sv27 motor acts as a brake, causing handling issues when slowing down.

[Insaniac]

i guess my generator theory has a bit of fact behind it , thanks to Pauls info

All i can say before i changed the way i brought my my boats to a stop , i was blowing caps on my ESCs all the time , and now it does not happen anymore !!!

The people that i ask these sort of questions to, know there stuff & would not give me an answer unless there was some sort of testing to back there answer!

Or it could be that by gradually backing off the throttle you are slowly discharging the capacitors rather than abruptly chopping the throttle (ESC rapidly reduces voltage across caps to zero causing rapid discharge and failure).
It just seems hard to believe that there is enough torque created by the water flow to spin the motor against the resistance of the magnets-at least in my motors the force required to turn the shaft is significant).
JMHO

[JackBlack26]

more tests that show a brussless motor can be turned into a GENERATOR

http://www.youtube.com/watch?v=1thlL...=youtube_gdata

The only difference between a generator and a motor is the supplied power to create ether. Spin the motor shaft and you get a generator producing power at the motor leads. Provide power to the leads and you get a motor. Internally both are the same, stator(windings)/rotor.

There is always great info on this site about caps. I love reading it.

Thanks to all that have contributed

[Insaniac]

The only difference between a generator and a motor is the supplied power to create ether. Spin the motor shaft and you get a generator producing power at the motor leads. Provide power to the leads and you get a motor. Internally both are the same, stator(windings)/rotor.

There is always great info on this site about caps. I love reading it.

Thanks to all that have contributed

You missed the point...again...there is not enough prop "torque" when you slow down to rotate the motor/drive shaft against the resistance of the magnets in most brushless motors. The only exception that I own is the Ammo 36-50-2300 which spins freely...

[ray schrauwen]

Just a question about the caps on the esc and what causes them to blow?
Just put in a new Hobbywing 180amp after the second run a cap let go I think
the problem was caused by half throttle running as the boat is to unstable at WOT
its in a 1/10 hydro 7XL 4S 5000mah pack spinning a x437.The eagle tree showed a spike 150 amps


http://www.youtube.com/watch?v=OoypEAo1Kbg

Who makes the 7xl ? it is a 2 pole motor which is not so easy to run on a Turnigy 180. Standard timing is a bit high for a 2 pole motor BUT, sometimes caps are crap.

Thats all, sometimes there are bad ones from the factory. In fact there are allot of Capacitor failures due to some poor manufacturing & poor quality raw materials.

Do research, The Cap King has great information on what is good and not so good. He also has a great slection of low ESR caps.

[ScarabChris]

Is it bad to run these ESCs at low or half throttle for any period of time? Because sometimes I just like to put the boat in the canal behind my house and tool around.

[ray schrauwen]

It partly depends on how hot your setup is but, in general, the ESC is working much harder at 1/2 throttle.

[ScarabChris]

It partly depends on how hot your setup is but, in general, the ESC is working much harder at 1/2 throttle.

When running it hard the motors and batteries stay very cool but the ESCs do get pretty hot, and the caps are even hotter. I was thinking of adding extra caps to help aid in this.

Can you give me a brief explanation on why the ESC works harder when run at low speeds?

[dasboata]

Whereabouts are you?

I just added an Etti cap board to my 180 Turnigy after the caps had discoloured a bit. Seem to be finenow.

can I see some pics of that and where did you purchase thanks Chris

[m4a1usr]

Can you give me a brief explanation on why the ESC works harder when run at low speeds?

If you mean at partial throttle then here you go!

John

To understand why low speed throttle is bad, you need to know a little about how ESCs work.

In general, with a constant load a motor's RPM changes as a result of the average voltage it sees. Since a battery is a (relatively) fixed voltage, we need some way to vary voltage seen by the motor. A microcontroller is a digital device - it can generate only "on" and "off" conditions. How does one take a fixed input (battery) and an on/off control and make an analog (variable) output?

The answer is by going on and off very quickly, varying the relative amount of on and off pulses. The longer you are "on" and the shorter "off", the more average voltage the motor sees. This is called "Pulse Width Modulation" or PWM. The on-time is called "duty cycle" and is repeated at a certain rate (times per second) called "frequency." The duty cycle is controlled by the transmitter, the frequency is controlled by the speed control.

In a brushed control, this goes directly into the two motor leads, with the coils and mass of the motor acting to average out the pulses into a more-or-less linear value.

In a brushless control, the same thing is going on, with an added twist. Not only are the pulses varying the voltage, but the voltage must be moved between the 3 different wires in order to simulate the plates of the commutator (comm) on the armature.

Now, why is low speed bad? The on/off switching is done with a device called a FET. A FET is a low-resistance switch when fully turned on. When it's not fully turned on, it has a resistance which varies depending on exactly how turned-on it is. This is called the "linear mode." When in linear mode, a FET is like a big resistor - you know, those things inside your slot car handle that get HOT, or like the ceramic thing in your old mechanical speed control. The problem is, FETs don't like to be in linear mode - as they warm up, they get more and more resistive. But the motor at low RPM is less and less resistive, meaning it will draw more current.

What you have is a situation that can quickly degenerate into what we call "Thermal Runaway" - the heating of something causes it to heat itself even more.

So why don't ESC designers not run FETs in linear mode? Well, we try real hard not to, but it's a matter of physics - you can't completely eliminate it. If we make the linear mode time very short, it adds bigger capacitors to the board to provide the necessary energy, and people complain about size. They also complain about glitching their radios, as the speed at which the FETs are switched then generates radio frequencies. If we spend more time in linear mode, we can eliminate the glitching in the radio - but then we run into potential heat problems. Engineering always is a balance of competing problems and solutions.

What happens at low throttle is that the FET goes into the linear mode for a larger percentage of the on-time. That means the FET spends more time heating than it should - and hot FETs have more resistance, meaning next time they are in linear mode, they will heat up even hotter. At the same time, the motor is at lower RPM and therefore pulling more amps (a stalled motor will draw the maximum current).

You can get a lower RPM (ie, a lower average voltage) at maximum duty cycle only by lowering the input voltage. The speed control will run better with a lower input voltage even though it may be passing more amps simply because there will be less time for the FETs to be in linear mode. It is not an exaggeration to say that the FET may have 10X or more resistance when in linear mode. Heat generated is directly related to resistance.


Andy Kunz

[sundog]

there is not enough prop "torque" when you slow down to rotate the motor/drive shaft against the resistance of the magnets in most brushless motors. The only exception that I own is the Ammo 36-50-2300 which spins freely...

But when at speed and initially slowing down there is plenty enough torque from a prop in water to turn even a high cogging motor. Never underestimate the power of water

[DISAR]

Agree, even in big merchant ships-tankers etc... if travelling at 15 knots at full power and the engine suddenly stops the propeller and engine still rotate for about 10-15 mins more due to the inertia and water resistance(I have witnessed that). So for our boats, if you go from full speed (full throttle) to neutral the motor and prop still rotates, the time depends on speed-motor-boat size. That's when the motor acts as generator supplying extra volts to the esc, and a very short time 1-2secs is enough to exceed the esc's volts limit, that's why it is best practise to run the esc 1-2s below the max. I follow these rules

1-2s below the esc max volts
full throttle as much as possible
smooth stopping as musch as possible