blowing caps 180esc

[graill]

Quote:
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

John

Andy have an email? Some of the things he explains in the quote supplied by John didnt makes sense regarding FET operation states, maybe it was the wording?

[sundog]

I follow these rules

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

Excellent rules to follow to keep caps from overheating

[BakedMopar]

If this generator theory is true then why our land based motors not gernading the caps. The motors are constantly being driven through the wheels after gettin off the throttle until it comes to a dead stop.

[m4a1usr]

Andy have an email? Some of the things he explains in the quote supplied by John didnt makes sense regarding FET operation states, maybe it was the wording?

Andy Kunz is a member here on OSE so you could contact him if you believe something is not quite worded right but his basic theory is sound. That is the way FET's behave. Here is another way to look at the explanation on partial throttle.
http://www.peakeff.com/beta/PostDetail.aspx?PostID=9

John

[ray schrauwen]

If this generator theory is true then why our land based motors not gernading the caps. The motors are constantly being driven through the wheels after gettin off the throttle until it comes to a dead stop.

The gear ratios I think help reduce the problem and the weight of a car is alot easier to slow i think than most boats.

[BakedMopar]

The gear ratios I think help reduce the problem and the weight of a car is alot easier to slow i think than most boats.

I still don't buy it. Even with gear ratios your rolling for a lot longer han a boat in water. Also the gear ratios would spin the motor a lot faster than a 1:1 direct drive boat. Were not comparing "on" power consumption. The prop isn't spinning much longer after getting off the throttle. Even airplanes have props turning without power.

[ray schrauwen]

Well, I said I "think" those were some reasons. You don't have to buy it. I understand Andy's explanation and I agree with it.

[BakedMopar]

Don't take it the wrong way. I'm just trying to get a grasp on this " generator" concept. Did I miss Andys article?

[m4a1usr]

Don't take it the wrong way. I'm just trying to get a grasp on this " generator" concept. Did I miss Andys article?

The posted "Andy" article is only a reference to partial throttle operation as it relates to FET efficiency. Has nothing to do with some form of "windmill"/ Generator effect on an ESC component.

And I dont buy the logic of the generator effect working either. There are several reasons why it cannot work.

1. The Caps are on the battery side. (thats whats being discussed as a form of greater potential being generated and then stored or certainly effected) The esc will never see more voltage then what is stored in the batteries. Thats a given. Why? There is no return path "thru" the ESC to add more charge. You cant think of the ESC as a gate or a path way. Its not that simple. You start with a given amount of voltage from the batteries. That exact same amount of voltage is not available on the output phases to the motor. The ESC has losses. So you will always have a greater potential on the battery side. Thats Ohms law.

2. If you are off the throttle and "coasting" props will not necessarily free spin. Maybe with 2 pole motors with little motor iron/rotor magnetic attraction but I doubt any of the 4/6/8 pole motors that do have strong motor iron/rotor attraction will want to free spin and even if they do, because of the magnetic resistance from internal component attraction, the RPM’s will ALWAYS be lower then the RPM’s it took to get to that velocity before the “coasting” would begin. In other words,…..if the motor could feed back voltage thru the ESC it would be a lower, not higher, value. So again. You could not “charge” the caps to a level higher then what the batteries would. So the theory doesn’t work. Its an interesting concept but the operation and constraints of the components, plus the physics surrounding energy generation don’t make sense. I don’t need myth busters for this one.

John

[JackBlack26]

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...

Please explain how I missed the point? In the post you quoted I was explaining that a motor and a generator are the same in design. The difference is which end they are powered from. I never said, or agreed, that was the reason for caps blowing!WTH does that have to do with you and the motors you own? I understand your point that other than some cheap 2 pole motors most quality 4 pole motors, and higher, wont spin the prop just from forward momentum due to magnet strength. Stating that "I missed the point" when I never disagreed with you is a bit off!

Come to think of it, I don't even see why this topic is an issue in Rc boats unless these ESCs don't have reverse circuitry protection. I raced 1/8 scale electric buggies and truggies for over a year running Neu and Castle motors and never had an issue blowing caps when off throttle and the cars were still in motion, coasting with the motor still spinning.

[graill]

Andy Kunz is a member here on OSE so you could contact him if you believe something is not quite worded right but his basic theory is sound. That is the way FET's behave. Here is another way to look at the explanation on partial throttle.
http://www.peakeff.com/beta/PostDetail.aspx?PostID=9

John

Thanks John i will pm him tomorrow. Talked with a couple friends of mine at raytheon and they cleared it up, Andy omitted some stuff to simplify it and was why i asked. They also told me to ask about a dirty little secret folks that build with the mosfets know about but do not advertise, i will ask Andy about it.

[bigwaveohs]

Please explain how I missed the point? In the post you quoted I was explaining that a motor and a generator are the same in design. The difference is which end they are powered from. I never said, or agreed, that was the reason for caps blowing!WTH does that have to do with you and the motors you own? I understand your point that other than some cheap 2 pole motors most quality 4 pole motors, and higher, wont spin the prop just from forward momentum due to magnet strength. Stating that "I missed the point" when I never disagreed with you is a bit off!

Come to think of it, I don't even see why this topic is an issue in Rc boats unless these ESCs don't have reverse circuitry protection. I raced 1/8 scale electric buggies and truggies for over a year running Neu and Castle motors and never had an issue blowing caps when off throttle and the cars were still in motion, coasting with the motor still spinning.

A theory put forward in this thread is that when the throttle is quickly chopped to zero, the water would spin the prop in reverse creating a generator effect which caused the resulting (over?) voltage to blow the capacitors. Insaniac simply stated that he didn't agree that this was the cause because most motors have magnets that are strong enough to prevent the motor shaft/prop from spinning simply due to the water flow as the boat slows down.

BTW, I no longer provide "advice" on this forum...guess why...

[ScarabChris]

I don't know....I guess I'm old school and these ESC's are new technology. I was following the common sense that if the motor is turning less RPM's then its using less power thus putting less load on the ESC. So if I run my boat at low speed the batteries should last longer and the ESC stay cooler.

But from what I am reading here is seems that if I run the boat at full throttle the batteries will last longer the the ESC's will stay cooler then if I just tool around at low or half throttle.

Again thats just my old school theory.

I went ahead and ordered some cap banks for my 54" Fountain as they were getting super hot....at full throttle.

[Insaniac]

Correct, for boats running at full throttle is easier on the ESC...

[ScarabChris]

So am I going to destroy my ESC's running at slower speeds? Because I like to run it next to my real boat but I rarely run the real boat at 60 MPH. I like to run circles around the real boat as we are cruising in the Intracoastal or bay. Its mostly idle speed but some areas are 30 MPH. Even still, way slower than the maximum speed of the RC boat which is 60 MPH.

[785boats]

I'll see if I can shed some more light on this 'generator' problem.
In posts # 29 & 30, sundog & Disar got it spot on.
When you release the throttle at 30,000revs & say 60mph, the motors own inertia & the water flowing over the prop keep the revs up higher & for longer. Yes the motor is slowing down from the moment you release the trigger but it takes a lot longer than it does for a dry run on the bench. But the highest voltage is generated in the first instant that you chop the throttle.
It must be remembered that this really becomes a problem with ESC's that have a BRAKE mode, as many of the car & aircraft ESC's that a lot of us use, do. If the 'BRAKE' is programmed to the ON mode, a lot of ESC's apply an amount of brake automatically, without you actually pushing the trigger into the brake zone on the Tx.
This is where the problem arises. Especially if the battery voltage is already at the max (or close to) limits of the ESC.
If you didn't read the PDF I posted earlier in this thread here it is again. It's interesting reading.
Cheers.
Paul.

[ScarabChris]

I don't buy the generator problem, especially for these Leopard 5692's. You need a good amount of force to turn them by hand so I doubt the water flow will spin the props.

[sundog]

I don't buy the generator problem, especially for these Leopard 5692's. You need a good amount of force to turn them by hand so I doubt the water flow will spin the props.

You could easily prove or disprove the 'will a high cogging motor (difficult to turn by hand) spin in fast water?' theory by running a real boat 40 or so mph (I consider that as fast when I'm on most boats) and set the transom of your rc boat down in that water. Do you still think it won't spin? If not, then I can see I'm not going to be any help here! .

[BakedMopar]

I don't know....I guess I'm old school and these ESC's are new technology. I was following the common sense that if the motor is turning less RPM's then its using less power thus putting less load on the ESC. So if I run my boat at low speed the batteries should last longer and the ESC stay cooler.

But from what I am reading here is seems that if I run the boat at full throttle the batteries will last longer the the ESC's will stay cooler then if I just tool around at low or half throttle.

Again thats just my old school theory.

I went ahead and ordered some cap banks for my 54" Fountain as they were getting super hot....at full throttle.

I have done actual testing on runs with partial throttle to see it's ineffeciency. If you have a data logger do some real world testing and you'll see the outcome. I did two runs with boat setup running at 100% for a baseline and two runs with the speed controller limited to 75%. The partial throttle runs was less effecient by 5% average and temps around the same of 5% increase. Now before any body want to disagree this was done only for my testing and results may vary from boat to boat. I don't have the actual data file as my compy crashed a few months back but this is what I have writen down.

[ScarabChris]

You could easily prove or disprove the 'will a high cogging motor (difficult to turn by hand) spin in fast water?' theory by running a real boat 40 or so mph (I consider that as fast when I'm on most boats) and set the transom of your rc boat down in that water. Do you still think it won't spin? If not, then I can see I'm not going to be any help here! .

No way the props will turn if I lower them in the water at 40 MPH. With the motor in your hand and no coupler on the shaft you almost can't even turn it by hand.

[Rumdog]

I agree with Chris here. The water isn't even traveling in proper direction to initiate a spinning motion of the props.

[sundog]

I agree with Chris here. The water isn't even traveling in proper direction to initiate a spinning motion of the props.

Rumdog, could you elaborate?

[785boats]

Guys.
We're not talking about spinning a motor up from zero revs using the prop like a turbine. We're talking about keeping a motor spinning a bit longer when you release the throttle at 30,000 revs.
Do your motors stop dead when you release the throttle at full revs? No they don't. The inertia of the armature PLUS the turbine effect of the prop travelling through the water at 40 or 50 mph keep it spinning for a few of seconds acting as a generator. As electricity travels at 186,000 miles per second, it takes nano seconds for it to travel down 4" of wire to the ESC. That is when the damage is done. Especially if the brake mode is ON.
If you read the PDF file I posted in my previous thread you will see in fig 3. that with a 6 cell battery & an 80A load, under braking an extra 17.5 volts was generated. If a 6s ESC was being used (that is, rated to 30V) it just had 42 Volts rammed into it with a generated load of 80A, leaving the boat owner wondering why the ESC mysteriously failed.
Again I stress that this is mainly a problem when the Brake Mode is switched on & the ESC has automatic braking. But it does happen to a lesser extent with any ESC without the braking mode.
That's why it's always best to use an ESC with a higher voltage rating than you are actually using.
That's as clear as I can make it i'm afraid.
Paul.

[Rumdog]

I dont believe that. I'm sorry. When I let off throttle, the water is not going to keep the prop spinning for a longer period than when on the bench. Maybe even less spinning.Forward momentum of water on the backside of the prop wont encourage rotation.

[785boats]

Rumdog.
It's OK if you can't believe it so forget about the prop/turbine/generator thing & believe that the motor is still spinning when you release the trigger. True? At that instant it is a generator.

The water isn't even traveling in proper direction to initiate a spinning motion of the props.

But i think you should have a closer look at your props if you believe that statement.
The water flow may be acting against the back side of the prop but it will still spin it in the same direction that the motor was spinning it.
Read post #30 again.
But this is getting a bit off topic. The initial post was about why ESC's fail & caps blow. I just offered this as one of the reasons. There are lots of reasons & others have offered them too.
All the best.
Paul.

[Rumdog]

I will refer you back to post #39 then. Which I believe all to be true.

[domwilson]

The main reason caps blow is too much ripple current on the battery side.

[785boats]

Read the PDF file.
The generated voltage gets ADDED to the battery voltage in the ESC.

I install VSD,s in industrial plants. rated at up to 500 Amps & 415 Volts Ac
The manufacturers install big chokes, capacitors & electronics to deal with the back EMF generated as the pump or fan or blower winds down under dynamic braking.
This voltage is then redirected back into the electrical system.
Our ESC's are very basic VSD,s Trust me, the generated voltage is a real thing.
I'll let it go now.
Paul

[domwilson]

I just couldn't get past "Because the direction of the current flow is now opposite, also voltages caught on
resistances in the circuit have opposite polarity and they are thus added to the voltage of the battery." part. Been doing electronics for years and I'm not understanding that part.

[m4a1usr]

Read the PDF file.
The generated voltage gets ADDED to the battery voltage in the ESC.

I did read the PDF file and its very confusing. Their attempt in broken english to describe the method leaves the content still suspect by my understanding of electronics. I see nothing that definatively proves the voltage can be above the generator effect of the motor or the stored value in the batteries. And to assume the volatge is additive? That doesnt make sense. During the braking effect current use is going to be the highest. Not the lowest. That means a voltage depression, so lower, not higher voltage values. Thats Ohms law. Current drain goes up and the inverse occurs to voltage. Not a surgence. Thats the first glaring issue.

The biggest issue with a prop turning and putting back voltage in the ESC with no braking is the FET's are turned off! Let go of the trigger and there is no longer any driver voltage to provide the so called path of conduction. They only open when voltage is applied. Let go of the trigger and guess what? No voltage, no path. Its pretty simple.

And lastly the most glaring error using the logic of additive voltage thru an ESC is the fact that there are many, and I mean many of us FE boaters that have data loggers were we monitor pack voltage. I have never heard of anyone reporting back that they saw a huge spike, certainly above their battery voltage. I have been using a Eagle Tree now for 3 years and not once. Not a single time have my plots displayed a voltage above my original starting pack voltage.

Like I said. I dont need myth busters for this one. Until I see a reproduction of the "effect" from one of us who shows how this can happen its dead in my opinion.

John