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Author Topic: Can a Brushless Outrunner be too Powerful?  (Read 5156 times)

Tug Fanatic

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Can a Brushless Outrunner be too Powerful?
« on: March 29, 2013, 12:43:21 PM »

I am looking at using a brushless outrunner in a towing tug where big props, low revs and fine very low rev throttle control rule.
 
To get the low revs I need a low kv - 350kv is OK but lower is better - motor. I cannot find such a motor unless I go for a much bigger size than I really need. The higher the kv the lower the voltage that I would need. At 350kv I could use 12v. I know that this will give the motor an easy life but what will I loose? Obvious examples might be:
 
1) very low speed throttle control
2) motor efficiency
 
What do you think?
I have seen this motor as an example:
 
  http://www.hobbyking.com/hobbyking/store/__36825__Turnigy_Aerodrive_SK3_4250_350kv_Brushless_Outrunner_Motor_UK_Warehouse_.html
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Mad_Mike

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Re: Can a Brushless Outrunner be too Powerful?
« Reply #1 on: March 29, 2013, 03:43:03 PM »

Im not sure if i read your post right but if so then your examples of losses are completely the reverse of what to expect. a 350kv motor on 12v would have quite a varied throttle range and on a small prop be extremely efficient. What kind of size prop are we talking about here?
sensorless brushless motors have a low rev limit, so theres only a certian rpm they can go down to before they stop giving back emf feedback, at this point they run lumpy or even stall. This rpm depends on the motor.
 
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Tug Fanatic

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Re: Can a Brushless Outrunner be too Powerful?
« Reply #2 on: March 29, 2013, 04:07:56 PM »

Prop size 65mm or perhaps slightly larger.
The back emf problem is what I meant. With a lightly loaded motor I was wondering how low it might be. I am used, in model boats, to car heater motors that will run extremely slowly - less than 1 rev second & draw 3-4amps.
 
PS I know that 12v x 350kv = 4200rpm which is far too fast for a 65mm prop but to get a lower kv involves an even bigger motor. I could always try 6v!
 
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Mad_Mike

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Re: Can a Brushless Outrunner be too Powerful?
« Reply #3 on: March 29, 2013, 04:18:00 PM »

if thats too fast then use belt reduction dont drop the voltage otherwise youl lose the rev rang. The belt reduction will will keep the rpms up so the motor has no back emf issues, plus it will lessen the load of the motor and increase efficiency.
You could look into sensored types brushless motors and esc's but the only type of sensored motor i know of are car inrunners types and they have usually have quite a high kv. It is possible to fit sensors to outrunners but youd need to look further into this:
https://www.youtube.com/watch?v=XVX5T7JkBDA&NR=1&feature=fvwp
If you are getting car heater motors to work why the change? If it aint broke....
 
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Tug Fanatic

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Re: Can a Brushless Outrunner be too Powerful?
« Reply #4 on: March 29, 2013, 04:47:37 PM »

If you are getting car heater motors to work why the change? If it aint broke....

The world has changed. I used to go to the scrap yard with a battery, a tachometer & a wattmeter & have a mooch around & come out with a motor of my choice that was in good condition & which I knew was a match for 1 - that was only a couple of years ago.
 
Now they won't allow the public into the yard and want 25 for one that they have removed especially for me. I can get a new one on ebay for less but I have no idea of the sizes or specs.
 
I note that the cheap 755 & 900 motors have disappeared.
 
Just chewing the fat for alternatives but it appears that ebay pot luck will have to do although I am also looking at the scooter motors which seem to be a reasonable size, 24v 2800rpm and 80-200watt. They are also widely used in the ride on 5" gauge trains.  24v lead acid is however unfortunate and 1400rpm on 12v doesn't sound enough. Wrong forum however.
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Subculture

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Re: Can a Brushless Outrunner be too Powerful?
« Reply #5 on: March 29, 2013, 06:52:02 PM »

Ronne do a small motor rated at 300KV. Also Hi-model do a 225kv motor of reasonable proportions that is used in the Multiplex Funcopter.

For lower than that, you will either have to go for a large motor, or use some reduction, or wind your own.

For very low RPM's I find sensorless motors aren't terribly good, sensored is much better. I'm afraid there isn't much of a market for motors designed for scale boat use.

nick_75au

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Re: Can a Brushless Outrunner be too Powerful?
« Reply #6 on: March 30, 2013, 10:13:50 PM »

There is no problem running them on lower voltage, the torque is lower but it is still sufficient to drive the prop. Looks like the multi rotor models are using motors now that are a good match


http://www.himodel.com/electric/SUNNYSKY_V3508_380KV_Outrunner_Brushless_Motor_for_Multi-rotor_Aircraft.html


Cheers
Nick
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Mad_Mike

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Re: Can a Brushless Outrunner be too Powerful?
« Reply #7 on: April 01, 2013, 11:42:38 AM »

The problem here though nick is this guy wants the brushless motor to have a smooth operation right threw entire rev range. From roughly 60 rpm right up to 3000rpm. I dont know weather the motor you have chosen can run down to 60 rpm but as far as im aware to get a brushless motor to run this slow it would need to be sensored, which to my knowledge noone does a sensored esc for boats. I think this is one of those rare situations where the pros of a brushed motor outweigh those of a brushless.
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Tug Fanatic

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Re: Can a Brushless Outrunner be too Powerful?
« Reply #8 on: April 01, 2013, 12:01:04 PM »

I have decided to stick with brushed.
I fly with brushless but have no experience of the very low kv motors.
Thanks for your help.
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nick_75au

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Re: Can a Brushless Outrunner be too Powerful?
« Reply #9 on: April 02, 2013, 07:47:22 AM »

Hi Tug Fanatic, No problem,


Mike
 I have found even with 1000kv motors on 6 volts the RPM is low enough to count revs by eye, your boat wont move with this low RPM unless its a big prop, you do get a slight kick when the motor starts  so the ESC can sense the RPM, some ESCs are better than others at low RPM as well.


Nick
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Mad_Mike

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Re: Can a Brushless Outrunner be too Powerful?
« Reply #10 on: April 02, 2013, 07:38:06 PM »

Ill take your word for it then as you know scale and brushless better than i do. Tug fanatic is talking about a prop 65mm in diameter, Ive never had a boat with a prop that big so i dont know what rpms it would need before it starts to push.
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essex2visuvesi

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Re: Can a Brushless Outrunner be too Powerful?
« Reply #11 on: April 02, 2013, 11:36:17 PM »

if you need around 24v then you could consider 6 cell lipo packs.
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Re: Can a Brushless Outrunner be too Powerful?
« Reply #12 on: April 05, 2013, 01:57:43 AM »

Ive had pretty good luck so far bench testing brushless..pushing air.
Loaded of course is going to be different....There wasn't that much kick ..most of the low rpm range can be solved by messing around with the ESC settings... however to pull a 90mm prop ..I will need belt reduction on the model .. power, isn't so much the problem as inertia I figure  spinning such a large wheel.  I mean ..you can only do so much with a certain mass before things get ...errr..strange....over coming it and getting rid of it...
http://www.hobbyking.com/hobbyking/store/__36825__Turnigy_Aerodrive_SK3_4250_350kv_Brushless_Outrunner_Motor_UK_Warehouse_.html
You do realise this motor can produce 1.5 horse power right.
 
 
 
 
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Tug Fanatic

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Re: Can a Brushless Outrunner be too Powerful?
« Reply #13 on: April 05, 2013, 12:22:14 PM »

I know that it can produce 1.5bhp but that doesn't mean that it will unless I load it to the point where it need to.
I would welcome suggestions about a smaller cheaper motor that has a similar kv rating and which will turn a 65-75mm 4 blade kort nozzle prop without complaint. At least this won't get hot! The examples that were suggested above seem to be more expensive than this bigger motor. 
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Subculture

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Re: Can a Brushless Outrunner be too Powerful?
« Reply #14 on: April 05, 2013, 12:25:18 PM »

This one-

http://www.hobbyking.com/hobbyking/store/__26957__Turnigy_Multistar_4822_390Kv_22Pole_Multi_Rotor_Outrunner.html

Slightly larger KV, but not much. Cheaper, will swing that prop easily, and a more suitable power rating, plus loads of poles, which will give very good slow speed running.

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Re: Can a Brushless Outrunner be too Powerful?
« Reply #15 on: April 05, 2013, 07:01:50 PM »

there 9 on back order {:-{
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Re: Can a Brushless Outrunner be too Powerful?
« Reply #16 on: April 05, 2013, 07:08:07 PM »

Might have to wait a couple of weeks then. Never mind.

Mad_Mike

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Re: Can a Brushless Outrunner be too Powerful?
« Reply #17 on: April 05, 2013, 07:25:25 PM »

I know that it can produce 1.5bhp but that doesn't mean that it will unless I load it to the point where it need to.
I would welcome suggestions about a smaller cheaper motor that has a similar kv rating and which will turn a 65-75mm 4 blade kort nozzle prop without complaint. At least this won't get hot! The examples that were suggested above seem to be more expensive than this bigger motor.
I dont think you you will find a smaller motor than this with a similar kv. Why does the motor need to be smaller anyway? its still smaller than a scooter motor or maybe even a fan motor, not sure about the latter i cant recall ever handling one. Is it just a cost thing?
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Tug Fanatic

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Re: Can a Brushless Outrunner be too Powerful?
« Reply #18 on: April 05, 2013, 07:29:14 PM »

This one-

http://www.hobbyking.com/hobbyking/store/__26957__Turnigy_Multistar_4822_390Kv_22Pole_Multi_Rotor_Outrunner.html

Slightly larger KV, but not much. Cheaper, will swing that prop easily, and a more suitable power rating, plus loads of poles, which will give very good slow speed running.

Missed that one. Thank you. It just doesn't look big enough but the numbers suggest that it is.
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Tug Fanatic

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Re: Can a Brushless Outrunner be too Powerful?
« Reply #19 on: April 05, 2013, 07:44:14 PM »

I dont think you you will find a smaller motor than this with a similar kv. Why does the motor need to be smaller anyway? its still smaller than a scooter motor or maybe even a fan motor, not sure about the latter i cant recall ever handling one. Is it just a cost thing?

Cooling fan & heater fan motors are truly great motors for big propellers. They pull little current & can generate large amounts of thrust. They are made to run for thousands of hours and many are open motors so you can easily see the condition. I have no hesitation in recommending them for tugs & the like.
 
I am not worried about the size of the motor - it is just that I was asked if I was aware of 1.5bhp. Smaller didn't seem to be an option even though I had no need of such a powerful motor. The 26975 motor is interesting as its size opens up a number of possibilities but I am not convinced - probably my ignorance - that such a small motor will equal a fan motor.
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nick_75au

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Re: Can a Brushless Outrunner be too Powerful?
« Reply #20 on: April 06, 2013, 01:59:47 AM »

Ive used one of these in a test and in my my Axe bow


http://www.hobbyking.com/hobbyking/store/__6246__hexTronik_DT700_Brushless_Outrunner_700kv.html?strSearch=dt700


results
55mm 4 blade prop, on 6volts[/size]2200 rpm10 amp peak about 8 amps averageabout 50 watts average,


on 12 volts any of the low KV Multi Rotors motors will have enough power me thinks

Nick
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Tug Fanatic

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Re: Can a Brushless Outrunner be too Powerful?
« Reply #21 on: April 06, 2013, 11:40:31 AM »

This one-

http://www.hobbyking.com/hobbyking/store/__26957__Turnigy_Multistar_4822_390Kv_22Pole_Multi_Rotor_Outrunner.html

Slightly larger KV, but not much. Cheaper, will swing that prop easily, and a more suitable power rating, plus loads of poles, which will give very good slow speed running.

$31 is the international price & it is not available from the UK warehouse. Does anybody know what this will cost by the time I get it in the UK. A guess would be $31 + carriage + import duty + VAT + handlers fees which might add up to a lot more than the 20 price would suggest.
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essex2visuvesi

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Re: Can a Brushless Outrunner be too Powerful?
« Reply #22 on: April 06, 2013, 11:55:22 AM »

Postage on that from HK international warehouse would be $4.99 using the cheapest method


Customs duty is pot luck when it comes to Hobby King..... I have had $300 orders from them that didn't attract customs duty and then $40 ones that have


Best suggestion is be prepared to pay duty and be happy if you don't!
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Tug Fanatic

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Re: Can a Brushless Outrunner be too Powerful?
« Reply #23 on: April 06, 2013, 12:26:30 PM »

Postage on that from HK international warehouse would be $4.99 using the cheapest method


Customs duty is pot luck when it comes to Hobby King..... I have had $300 orders from them that didn't attract customs duty and then $40 ones that have


Best suggestion is be prepared to pay duty and be happy if you don't!

If you do get caught the 20% Vat is obvious but what extra charges do you incur. I am trying to figure a worst case price.
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Re: Can a Brushless Outrunner be too Powerful?
« Reply #24 on: April 06, 2013, 12:52:19 PM »

Never paid any charges on stuff from HK. Pay attention to the checkout process and you can see how you can avoid this sort of thing.
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