Brushless Motors

[Paul TParticipant @pault84577]

Dear All

I need some advice on brushless motors, as many of you know I tend to build large petrol powered models but I am tempted to power my latest with a pair of brushless motors.

The problem is that I know nothing about these motors so I am completely in the dark about the best combination of motors / battery / speed controllers.

The model is a 5ft speedboat with a empty hull weight of approx 10kilos (if fitting suitable petrol engines they would be a pair of 26cc units)

Paul

 

Edited By Paul T (fat controller) on 26/07/2012 08:12:36

[Paul TParticipant @pault84577]

[Dave MilbournParticipant @davemilbourn48782]

Paul
I have no experience of these but I know a man that does. Have a word with Alan Shillito at ASTEC; he specialises in fast electrics and has a very nice range of brushless motors and speed controllers, including some real monsters. **LINK**
Dave M

[Paul TParticipant @pault84577]

Thanks Dave

A wide selection and not as complicated as I thought it would be.

Paul

[shipwrightParticipant @shipwright]

Paul,

First I should say that I have not used i/c motors nor have I used brushless.

Looking at the specs of the motors given in Dave's link above I note that many of the motors are rated at 1 kWatt (1 HP = 746 watts). Consequently the currents drawn are very high – many tens of amps.

You will need some heavy guage wire for your connections in order to avoid wasted voltage drops. Also you will need to locate power components (ESC, Motors, Fuse, Battery) as close together as feasible. Wire connections need to be heavy duty connectors (gold plated). Switches are a problem at this level of current and often not used (remember that at 100 amps even 20 milliohms resistance will give rise to a 2 volt drop). As I have had no experience of this level of current in a model boat I don't know whether radio frequency interference (RFI) is a big issue.

The pursuit of speed brings with it some limitations – time on the water will be relatively short limited by the battery capacity (ampere hours Ah).

Ian

[Paul TParticipant @pault84577]

Ian

As a confirmed petrol head my engines of choice are always IC but as I don't want to limit where this particular model is sailed I am forced into using electric.

I have considered using traditional 12v electric motors but was curious about these new fangled brushless units.

After reading your very helpful posting the heavy gauge wiring does not present a problem however the huge current draw is a serious concern.

What would you recommend as the best choice of engines for my model.

Paul

[ashley needhamParticipant @ashleyneedham69188]

I dont think that RFI is a problem with brushless, however much the current is…however, I notice that a lot of brushless ESC come with a the reciever lead wrapped around a ferrite core?

I would think that you will need a watercooled ESC as well, given the likely high current involved.

Ashley

[shipwrightParticipant @shipwright]

Paul,

Your point re choice of motor. Given the size and displacement of your boat and the "need for speed" (because it is a speedboat not eg a tug) you will require a lot of propulsive power. If you do not want to use i/c engines then using brushless motors is the only option that I am aware of (traditional dc commutator motors cannot match the efficiency of brushless motors and for a given output power brushless motors are smaller and lighter than the equivalent dc commutator motor – brushless technology combined with high capacity LiPo battery technology has enabled the model aircraft community to use electrical propulsion rather than i/c).

Nevertheless, although very efficient brushless technology is not 100% efficient – 90% might be a realistic figure. So if you instal a 1000 watt motor you can expect to dissipate about 100 watts of heat within the motor/speed controller combination – that is a lot of heat and you will need water cooling for motor and speed controller (in model aircraft they divert airflow over the motor/controller which is a feasible cooling method because you have air at high velocity).

I regret that I am unable to contribute more because I lack experience of brushless motor and LiPo battery technology.

One last parting shot – do you know what the output BHP of a 26cc i/c engine is ?

Ian

[Paul TParticipant @pault84577]

Hello Ian

A 26cc will return between 3 to 5 bhp but with a modified fuel mix you could get as much as 8bhp

Paul

[shipwrightParticipant @shipwright]

Paul,

OK – you will not be able to achieve the performance you got with those 26cc motors by replacing them with electric motors. 1 BHP = 0.746 kW so you need, say, 4 or 5 kW motors and an unimaginable capacity of a LiPo battery !!

Regards.

Ian

[Colin BishopModerator @colinbishop34627]

It's an interesting comparison. The Electrics won't develop the same power as the I/C but the power versus speed equation is not linear so they may well come quite close speedwise given the right setup. But I agree, could be a bit expensive on the battery side though.

Colin

[Paul TParticipant @pault84577]

All of the above just confirms my own calculations.

Lack of power is the problem with larger models especially speed boats when you move away from petrol engines.

Even though the desktop study indicated a power to weight discrepancy I had hoped that someone on the forum would have practical knowledge to make the brushless / LiPo combination work in a large speed boat.

Back to the drawing board.

Paul

[shipwrightParticipant @shipwright]

I thought I would have one last look at this subject. I found this :

**LINK**

Apparently 4000 Watt continuous operation is feasible ! …. though I bet you would need forced water/pumped water cooling with a good flow rate. It is designed for model helicopter useage with a Kv of 520 rpm/volt (so for example if you had 5 LiPo cells = 5 x 3.7 = 18.5 volts the no load rpm would be a bit less than 10000) – inevitably the current would be high = about 200 amps in my example for an approx 4000 Watt output (it would be about 10% less due to losses) !!

A few points to note – a. I haven't looked for a compatible ESC b. I haven't looked at the feasibility of suitable LiPo battery and in particular safety issues (prevention of possibility of short circuit of battery would be essential because of the large stored energy and ultra low internal resistance of that type of battery) c. aircraft don't need to reverse the motor rotation so you would either live with just forwards or you would require a mechanism for going astern.

If you wanted to go further with this my recommendation would be to get in contact with Scorpion and discuss your requirements.

Regards.

Ian

[shipwrightParticipant @shipwright]

Sorry – correction to my previous post – I have just noticed that max permitted continuous current is 90 Amps. So you would need more than 5 LiPo cells to achieve 4000 watts output without exceeding 90 Amp rating. Also you would need a step down gearbox to reduce the rpm (I am sure that the helicopter models would need a gearbox as I cannot imagine a helicopter rotor running at 20000 rpm !!!) – what rpm do you run the 26 cc i/c motors at ?

Ian

[Paul TParticipant @pault84577]

Hello Ian

Thank you for researching the motors.

A 26cc unit will typically run between 1500 and 8000rpm.

Paul

[Francisco MolinerParticipant @franciscomoliner82444]

no, no, no, you need a big outrunner motor, the outrunner motors are well but they`ve got a lot of power, but you cant refrigerate it, the inrunners are expensive than the outrunners, there are a good model brushless motor for your type of boat, the Aeolian motors

[shipwrightParticipant @shipwright]

Paul,

I had a look at the motors suggested by Francisco – they are aimed at cars, helis and planes. I don't know who stocks/sells these products in uk. Nevertheless, I did find 1 motor and 1 esc on their excellent website that might be of interest :

Motor (outrunner) C6354-KV250 which can be supplied at up to 10 LiPo cells (= 37 volts) with a max continuous current rating of 90 amps and at 37 volts the no load rpm would be about 9000. Rated at 2450 watts max.

Suitable ESC is Aeolian-100A (no reverse).

See **LINK**

Also found 10 cell LiPo battery (advertised on a us website for $160 :

Turnigy nano-tech 5000 mAh 10S 45 ~ 90 C LiPo Pack

Another issue that I can't answer is whether it is permissible/feasible to operate LiPO battery packs in parallel (you would need many battery packs to give any reasonable duration on the water). It is also now evident that the brushless motor for high powers is an expensive approach !!

Ian

[Francisco MolinerParticipant @franciscomoliner82444]

i ve got the bigger motor of all and it can pull boats from 30 kgs, and in ebay costs 40 euros

[Francisco MolinerParticipant @franciscomoliner82444]

the bigger has got a descomunal power!!

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