khkim0102

Hi!

A few days ago, I have put a BL system into my Aquacraft Reef Racer 2.

Here is the ESC (35 A). http://www.hobbypartz.com/07e-c-seaking-35a.html

And here is the motor(4800 KV). http://www.hobbypartz.com/07e-c-sl-12t-2040-blue.html And I use 3-cell Li-Po 1,300 mAh.

When advancing the throttle to the full, the rpm goes up and the boat wants to go, but within a second, something like "Low-Voltage Cut-off" kicks in, and it slows down the boat.

And then, at the full throttle, the boat does not go fast any more. It goes in the mediup speed at the full throttle. And centainly, the battery is still fully charged.

What causes this? Why at the full throttle, does the boat go fast only for a second, and then somthing like "Low Voltage cut-off" kicks in, slowing down the boat at the full throttle?

This is very disappointing. Can anyone help me, please?

Thank you!

James

Got RPM

You have made the common mistake of choosing a motor with an excessively high Kv. Unloaded that motor would spin over 50,000 rpm, but loaded it will draw too many amps, overdraw your low-power battery and shut down the ESC. The only solution is to choose a more suitable motor with a Kv under 2000. Switching to a 2S pack may work, but you will probably see the same issue.



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khkim0102

Hi, GOT RPM!

Thank you so much for your clear explanation and advice! I really appreciate that!

As you said, even it happens with 2-cell li-po.

Q#1. Then, isn't there any solution without dumping the 4800kv motor into the garbage bin?

Q#2. If I have to replace the motor with below 2000kv, where can I use that 4800 kv motor? In a small car?

Thank you very much again for your help!

James

ryan_t888

For future mods, builds or upgrades you can visit www.radiocontrolinfo.com for a good read.

I'm sure you can look around for an application for that motor. If not, it won't be an extreme loss. That voltage cut off and you understanding something is wrong saved all 3 parts of your power system from no longer working and becoming dangerous.

Besides from selecting a motor that has too high of a kv value, you have also selected a battery that does not quite have an adequate discharge rate. Be certain you match the correct battery to your system so that it can supply no more than 80% of it's maximum discharge rating.

It would be worth reading and learning a more effective way to pick a power system.

Ryan

ironman929

Ok here's a question.. Could he put in a gear reduction say for 50% and still use the motor?

Got RPM

Yes he could, although I'd go for a 2.25:1 gear reduction. But setting up a gear box isn't simple, and the cost is probably more than what a new motor costs - particularly if he sells the old motor for 1/2 price.



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wetsleeves

did you programme your esc for the correct cell count of your batteries.

khkim0102

Yes, I did programming.

Someone said about this BL motor (4800kv)on Hobbyking.com, "Wow, this is the most ideal motor for a nice micro boat build!

I tested this with 7.4v 1800 mah lipos and a 25 amp speed control and it rocked! super fast and consistand power.

I got 50 watts at 8.2 amps and lots of thrust.

The motor got a bit warm with the 30mm prop and 6 cells but with 27mm and 3 lipoly, it should run cooler and higher RMP at the motor shaft.

Lots of applications for this motor at this price.

The runs really smooth.

I highly recommend this motor for weight saving and very good power to weight ratio output."

His statement above seems to be opposite to what Got RPM said. Why what Got RPM said does not occur to this guy? If anyone coudl explain, it will be appreciated! Thank you!

ryan_t888

30mm prop?
7.4v?
And only 8A?


If it's the same motor as above, this simply did not happen without a gearbox and/or has been explained by one who does not have a lot of experience in this area.

From many past experiences, I can say it will take 350 watts or so to turn a 30mm prop at 35 000RPM. That motor will certainly not handle this kind of output. Forcing the motor to turn this kind of load will make the motor try its hardest to approach that 350watts. With limitations such as electrical resistance in the motor and battteries it will not come all that close. There will be voltage sagging and high heat output. With no cut off, your motor, ESC, battery, or all 3 could have been permanently damaged.


Ryan

Got RPM

The explanation is simple...don't believe everything you read on the internet, particularly what you read in "testimonials" on vendor sites like the one you cited. There is no accountability for comments made there, many are made by hobbyists with almost no experience, and some are made to be purposely misleading. You are far better off relying on specialist forums like this one, although no matter where you go you will find poor information. When I wrote the FE column for RCBM I often received mail from readers challenging what I had written, claiming that "their friend said" I was completely wrong. Whatever. You have to choose your expert and not rely on only one or two. Fortunately with the Internet we all have the opportunity to read a lot of opinions on almost any subject. Then the challenge is to sort through it all to find "the truth".



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khkim0102

Thank you for your answers, boaters!

I haven't got some answers yet. Isn't there a solution with a different size of prop or a different battery?

If not, where can I use that stupid BL 4800kv battery other than the boat?

Thank you again!

ryan_t888

We would really prefer you to purchase a new motor for your existing project.

I would also recommend finding something you like and selecting the motor for that vs what you are trying to do now. It's very difficult to place a motor in to an application.

Hold on to until you find something.

Ryan

khkim0102

Hi!

Is this motor ok in the Reef Racehr 2 (with 35 A ESC)? Here is the link. http://www.hobbyking.com/hobbyking/s...idproduct=4203

Here is another one.
http://www.hobbypartz.com/96m267-2040-1800kv-30t.html

If one of these is ok, certainly I will put a water cooling coil on it. If any of you can give me an advice, it will be greatly appreciated! Thank you!

coriolan

The motor the "Reef racer" uses stock is similar to a Graupner Speed-400, the hull cannot really accomodate a gear drive and the 4800 Kv motor you first used is probably too much load for your battery which drop voltage under load, hence the LVC. I use similar power in 18" monos and hydros but with 2200 mA/20C cells. Some esc will cut-off when they become too hot. In these boats I use CC thunderbird-36 with a cold plate for the esc. A lower Kv motor will stress your battery/esc less will still giving more speed than a stock brushed motor. The "Reef racer" like the "Zig-Zag" has a limit in maximum speed before stability becomes a problem. The stock prop being submerged is too big for high Vv brushless. All my mini boats are surface drive and I use Octura X427 to X430 maximum on 2 cells for the mono and 3 with the hydro. Were do you run your boat?
http://www.rcgroups.com/forums/showp...22&postcount=1

khkim0102

Hi, Coriolan!

Thank you for your answer!

I run my boats (gas, nitro, electric) on Fraser River slough in Surrey. I don't have very high performence boats. Thye are all RTR boats with a little bit of upgrades or modifications. The place locates at the end of Alaska way. If you want to come, let me know. Where do you run your boats?

Hope to see you some time later!

James

coriolan

I run at the pond in Vanier park in Kits, but only electric.

mickvk

My Feedback: (1)

I put one of these in a zig zag racer. if you've gotten less than a few boats running well - or if you want it to run well - then I'd say it's not a good move..

But if anyone is curious about the experiment, the hull has a fixed negative prop angle so once you start developing enough thrust to do some impressive speed, it tends to stuff the nose in a nearly self destructing dive/crash. It's really stupid fast. It's not big enough to carry - or stable enough to trust - putting my GPS but I'd guess it runs up to 25MPH fun and stable at 1/2 to 3/4 throttle.