Motor amp draw

[Old School]

Manufacturers list a motor's maximum amp draw and voltage for a given kv value. The lower the kv value the lower the amp draw at the expense of a higher voltage used. This is useful in determining a suitable capacity ESC. However, what I cannot understand is not everyone runs at the max voltage listed by the manufacturer, thus if max power is wanted then obviously the amp draw would increase. If however, a lower voltage and suitably smaller prop were chosen, would the user exceed the max current draw listed on the motor specifications?

When would one be drawing the full wattage (power) of the motor - using too high a kv to maximise rpm or using too large a diameter prop? How does one select a suitable esc when a manufacturer lists a lower numerical amperage draw against a max voltage that most would never consider? Does a lower amp draw/higher voltage low kv motor differ in construction (wire diameter,number of windings) to a higher kv motor or is it simply that a lower voltage implies a higher amp draw for a given wattage output?

If one chooses to use 6s packs then the specification amp draw places one with the choice of a minimum 180 amp esc and most likely higher, say a 200, 220 or 240 amp unit. A manufacturer will use the amp draw as simply a product of wattage and voltage, however how does one know what his final amp draw is going to be (other than by failure of the esc) when the motor specifications are not related to suitable prop sizes? Final selection seems to be based on trial and error.

For example, say a 5692 motor from Leopard Hobby. The manufacturer states a power output of 5500 watts. At 50 volt, the amp draw is 110 for 730kv. At 35v the amp draw is 160a and 1090 kv. At 29 volts the amp draw is 180 for a kv of 1340. All these examples will give us a theoretical rpm around 35,000 - 38000 rpm, which even allowing for slippage is still relatively high for "normal" use (not racing). What determines the kv value of a motor?

Of cause one could choose a lower kv motor and then prop up, but other than failure how can one safely achieve a given prop size for a given motor amp/voltage limitations? A prerequisite for this hobby seems to be that one has a degree in electrical engineering.

[NativePaul]

Motor basics. Electric motors are made up of 3 or more electromagnets pulling and/or pushing against 1 or more fixed magnet.

An electomagnet's magnetism is increased by the amound of turns of copper and the amount of current runnig through those turns.

If you have lots of turns of wire, that wire has to be thin to fit inside the motor, so it can't handle many amps, but it has strong magnetism, and lots of torque. This is a Low KV motor

If you have few turns of wire you can make that wire much fatter and still fit inside the motor, so while it has much less magnetism at the same current, you can push much more current through those fat wires and get some torque back. This is a high KV motor.

Generally the Max voltage rating of the motor is the short duration RPM rating of the bearings (often around 50,000) divided by the KV, or sometimes the RPM over which the manufaturer thinks the rotor will come apart. Either way running at RPMs up near where the manufacturor thinks things will fail is risky, and wears things out faster.

If you want any decent length of runtime while spinning a prop at 50k rpm, that prop would have to be tiny, and tiny props dont have a lot of thrust so acceleration suffers, and they are more prone to cavitation so aceleration suffers again. Prop sizing becomes problematic at small sizes too, going up one size from an x427 to an x430 is a 3mm increase as is going up one size from a x442 to an x445, but the 45 is only 15% more blade area, and going from 27 to 30 is 24% more blade area so it is much easier to accidentally overload your motor.

There is no common maximum current rating system to adhere to, but there is a current at which a wire will fuse quickly and as it happens quickly no amount of cooling will save it, so whilst some manufacturers underate their motors and some are rather optimistic, but generally amp ratings are reasonably realistic.

Manufacturer max power ratings however should be taken with a very large pinch of salt, again there is no common rating system to adhere to, and while one manufacturer may rate their motor at 1000W (for 10 minutes with no cooling), the next manufacturer may rate their similar motor at 3000W (for 10 seconds with excelent cooling).

Those 5692s are big heavy motors often put in big heavy boats, that need big props to push, and even though the RPM limits are lower than average at under 40k (due to the big heavy rotor), you would likely be better off spinning them slower than that, for example using the 730KV on 10s for 30,660rpm unloaded.

[Old School]

Paul,
many thanks. The ideal situation seems to be the sum of all its parts. One needs the right motor with the right battery voltage with the right prop to avoid disaster. It looks like experience is golden.

[CraigP]

Your comment that you need to be an EE is funny! Yes, it seems like that... The entire electrical system has to be in balance. Experience is the biggest helper, by far. But the knowledge can be learned faster using some tools. Data loggers are very useful to "see" what is really happening. They are very powerful in determining if your guesses or estimates are on track. At some point, you learn the basic playing field, and the need to data log everything starts to fall away. I like to make my data loggers mobile, meaning I use connectors to move them from boat to boat.

One of the best tools is an IR Meter with a laser sight. Heat is the big tattle tale on where the resistance is in the circuit. I recommend to folks to keep a notebook on temp recordings (I know, PIA, but golden info, like a caddy's notebook). Compare these readings to the data logger output to learn what parts can take what current.

Look, it's scary at first! I realize that... But I love that anticipation of a new setup, will it run? Will it burn up? That's part of whole experience! It's like skiing a big hill, scary at the top, exhilarating scooting down hill, and a big "Yahoo!" at the bottom of the hill! Each time you ski it, you learn more...

You asked great questions about manufacturer's specs! Really good questions... But as Paul says, many of these "specs" aren't worth the paper they are written on. You learn what size/manufacturer can take what. There are some many good facts posted here, one of the great things about the forum! Try to keep in mind, there are two factions of fundementally different camps in rc boats, racers and sport boaters. Both are great, but the goals and means to accomplish those goals can be skewed quite a bit. Racers have to push the lines, hard. They are our scouts, doing things that haven't been done. If your racing, follow suite. If you're sport boating then know you can pull it back a bit and still have a great day boating. Everyone has something to bring to the table!

[Old School]

Craig,
many thanks. One can teach an old dog new tricks. After a lifetime in commercial civil aviation I was always glad to handover a frustrating incoming defect to the avionics engineers. They worked magic with their meters and their zeroes and ones. Always left me in awe. Perhaps my reluctance to embrace the electrical field is my ignorance as to what is actually going on.

[Speed3]

Some manufacture give max rating for short burst. Example 15 seconds max. To be on the safe side, take the recommendation given by the experience racers. 2200kv is good for 4s. A 3674 2200kv leopard should be good with a x442, x642, grimracer 4255 on 4s. The 1900kv in the same motor will take 5s with the same props. The 1700 6s with the same props. A bigger motor like the 4082 will use the same cell count for a given similar kV but will take a 45mm prop. You might be able to push the listed motors a little harder and get a way. But I think your equipment will be ok using the above guidelines.

[Old School]

Many thanks. After a little googling around I have found the following to be popular with a similar sized hull - 4092, 1300 - 1390kv, 6s, 180 amp esc and a prop 45 - 47mm.

Edit: I found the following info helpful.

http://www.radiocontrolinfo.com/brushless-motor-poles/

[NativePaul]

The best method for someone new to Fe is to copy thesetups of others, not single setups but if you browse the forum you will find a bunch of setups that have been used by several people, pick one used on a boat of similar size to yours, buy the typical prop and 2 more, 1 and 2 sizes smaller. Start with the smallest prop and run for a minute checking for temperatures, if temps are good add another 30 secs to the runtime, do some trimming along the way, when you get to the point where you have only about 20% left in your packs at thread of the run you have a choice, either stick with the prop you have and trim to perfection, or if you want more speed and less runtime than you have, then go to the next size up prop, and start again repeating the process.

You don't have to be an EE, and it won't take too long before you are learning the usefully relationships and picking your own power systems.

Mass is a fairly good indicator of power and power required, a 125g motor should be able to handle 500w for a few minute with water cooling, a 250g motor 1kW, a 500g 2kW, etc.

My boats all have about 500 w per Kg of all up weight. And while not being as fast a sprint race setup gives me good performance for 5+ minutes.

My motor mass is normally about the same as my hull mass, my ESC is about half that, and my battery is about double that.

[Old School]

Paul,
many thanks

[Hydrocarbon82]

The whole "more torque" idea only works with stall torque. What they really mean by "more winds, more torque" is more torque PER AMP.

Example: 17.5t vs 3.5t motors (fairly extreme since they're 2080kv vs 6600kv)

http://www.rctech.net/forum/attachments/electric-road/441068d1239686995-dyno-homemade-using-novak-sentry-data-logger-continued-experimental-thread-lrp-x11-35-nickel-rotor-3-13-09005.jpg

http://www.rctech.net/forum/attachments/electric-road/441307d1239749211-dyno-homemade-using-novak-sentry-data-logger-continued-experimental-thread-novak-ss-17-5001.jpg

The 17.5t makes almost twice the stall torque, but look at 8000rpm on both. That's peak power for the 17.5t & half it's max RPM, but it's making 10% less torque than the 3.5t. However, the 17.5t is also drawing half the amps. At 5% max rpm (2700 & 875 rpm), the 3.5t makes 75 watts of power & the 17.5t only 25 watts. Geared down to the same max wheel speed, the 3.5t will have more power everywhere. If you instead gave the 17.5t 3.5x more voltage to match RPM's, the 17.5t would make more power everywhere (if it survived).

It comes down to this: you want the most RPM you can efficiently run (25-30k usually), the most volts you can use, and a motor with the KV & voltage rating that makes it happen. You will notice high KV motors don't have high voltage ratings & vice versa.

[Old School]

Many thanks