Motors, voltage input and amperage

[Old School]

I am looking at a 3674 motor rated for 27 volts and 2400 watts (around 2200kv). Now I was planning to only run 4s batteries. Will a 2400w motor always want to run at this wattage or is it voltage dependent? At 4s input and 2400 watts the current draw theoretically will be around 150 amps. At 6s input the amperage is around 90 amps.

Thus using a lower voltage than the motor is rated at will the amperage draw necessarily be a lot higher than at a higher input voltage or will the motor output simply reflect the voltage input? I realise that a lower voltage input for the same kv value will result in lower rpm at the prop and thus reduced speed.

[Fluid]

You can run at whatever wattage/amperage you want based on the load (propeller). The maker's rating are based on the maximum that he thinks the motor will tolerate. Different makers have different ratings based on the quality of materials/quality even on the "same" motor specs. The voltage rating is usually based on the max rpm the maker deems safe, you can certainly run a lower voltage successfully.


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[Old School]

Many thanks

[CraigP]

I might add, that you also have to look at the motor's max amp value. There is a spec for voltage AND current. The manufacturer intends that the motor not be used above either of those two values. So to get the best efficiency and achieve the highest power available from the motor, it should be ran fairly close the voltage spec (-about 25%) with the amperage controlled by the load placed on the motor, via the prop selection and/or the hull setup. Note that the motor will NOT just pull it's max wattage value and hold there. It will continue to pull power as long as the prop and hull continue to place demands on it.

I bet that motor has about a 80-90 Max Amp rate, so if you run 4s, and the motor is rated for let's say 90A, then the most power (wattage, the electrical force that pushes the boat through the water) would be 14.8*90=1332 watts. Now you can push more current into the motor, but it will start to run hotter which you can help to overcome with a good water cooling system. But 140% of rating is about the end of the safe line... so that gives you 1865W. It is much better to get the volt rating closer to your expected battery voltage. Racers will push this line until it breaks, but to be fair to motor manufacturers, they gave you a spec, and it is your decision if you are going to respect that...

[Old School]

If I step down to a 3660 motor then I have the following considerations using Leopard Hobby specs -

3660/2.5D 112A 15 volt 1700w 3800kv....... 3660/3D 94A 18V 1700W 3180kv.........3660/3.5D 78A 22V 1700W 2700kv

Basically then if I run lower voltages then I need to expect higher amperages. The motor amperages above are obviously the wattage divided by the voltage, so possibly I may not be drawing so much power (hopefully).

[CraigP]

Exactly! If you had a motor running let's say 30v and it was using 90 amps to push a given hull, and the motor kv yielded a prop rpm of 35,000, then keeping the same prop, changing the motor so it's kv yielded 35,000 at 15v, the amps would about double, to 180. But in reality, the amp draw will be slightly higher, because wires, batts and connectors lose more power at higher amps. If your a sport boater and like to keep your stuff from blowing up, then using a higher voltage, with a properly matched motor kv for a given prop, will use less current. This will increase your runtime by lowering losses and if both examples had 8000 mah batteries, the runtime would double on high voltage. It's basic electrical theory... Note: it does NOT reduce the number of batteries. A 4s system may have 2, 4000mah batteries in parallel. An 8s would have to have 2, 8000mah in series, so there's more battery in the latter.

[CraigP]

Note that racers are often confined to lower voltage packs due to class rules, that's why there are really high amp numbers being tossed around all the time on the site.

[Old School]

I thought that this was a hobby, but it seems that it really is rocket science!!!!!!!!!!

[rol243]

I thought that this was a hobby, but it seems that it really is rocket science!!!!!!!!!!

Don,t worry Old School your not the only one trying to learn the concepts of electric power, i am in the same boat going from glow power to electric chemistry.

[Old School]

I am trying to get my head around so many amps travelling through the boat's electronics, plus the amount of heat generated.

[CraigP]

The amount of power in these little boats is really beyond imagination! I've been an EE for 35 years, and I'm blown away with the amount of HP these tiny motors can put out. But acceleration and fun is also indescribable. I love this sport!

[Hydrocarbon82]

We're also looking at burst currents of 1sec or less. These motors are practically controlled 3-phase shorts.

[Old School]

I had no idea how complicated the decisions can be for these boats. When I decided to 'restomod' an older hull I thought it was simply just purchase a motor, a stinger and an esc and start running. Then we need to choose a suitable propellor, then tune/trim the hull. Decisions, decisions, decisions!!!!!!!

[rol243]

Old School, all you need to do is select the suitable size motor for your hull then buy a matching esc and a suitable lipo battery pack and a prop. i am sure one of the guys here that know there electrics would point you in the right direction for this motor etc etc selection, your not racing so your only wanting some sport fun. even if you pick a rtr boat thats about the same size as yours just find pout what motor / esc battery and prop is used and then your off and going with no electric nightmares.

[Old School]

I looked at a similar size hull at Hobbyking and the motor they supply standard is a 3660 with approx. 2100 - 2200kv. Thus I am interested in a Leopard Hobby 3660 2120kv 4 pole or a Leopard Hobby 3660/X2-4D 2380kv 4 pole. Either motor would run with a 120 amp Seaking ESC on 4S 45C batteries (4000 - 5000mah). I am interested in a propellor aprox. 40mm in diamter (2-blade) or a 3-blade around 37mm.

[rol243]

a 36 size motor from 3650 to 3674 and up to 2200 kv,s plus an esc from 150 to 180 amps [ safer option size ] and go for a 4s pack of around 5000 mah / 60c . the prop your wanting i would say a 440 or equilevent size 2 blade. HK Australia will have all this.

[Old School]

Many thanks

[785boats]

Old School.
What type & size hull are we talking about here? Mono? Cat? Hydroplane? Rigger?
It would help people point you in the right direction for an efficient & workable setup, with regards to Motor size & Kv, ESC, batteries, props etc.

[Old School]

Many thanks for your response. The hull is a 28 inch shallow vee mono.

[CraigP]

Sounds close to a DF29 Mono...

[Old School]

Sounds close to a DF29 Mono...

The hull is similar but was made over twenty years ago in thick ABS.

[rol243]

have you converted the drive line to surface yet Old School ??

[Old School]

I am awaiting carbon fibre to reinforce the transom and hull interior before I can fit any hardware. Then I can proceed with the stinger and rudder mounting.

[rol243]

that should give some strength.