Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length.

[Jon]

Ive just started playing with brushless motors with a view to using them in my next boat.
Ive bought a couple of 1400KV 28mm diameter 28mm long outrunners to play with and be a possible substitute for a 380 motor.
Intending to run them on 6.6V with a working RPM of 5-7K rpm (according to my helicoptor friend)

http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=350434413347&ssPageName=STRK:MEWNX:IT

I intend to run on 2cell LiFe as Ive got many pouches (10Ah 3.3V) recovered from electric bikes.

So just playing with the motor on the bench, I cant seem to get any power into it at low RPM, it is very easy to stop the motor, and once stopped it wont start again unless I cycle the throttle. Now I have no idea of the speed control I'm using other than it was given to me by a friend and it's from a helicoptor (only markings are BS30A-1V2), it has clear heatshrink with a purple plate heatsink) See photo.

So I suppose the real question is should I be expecting the motor to run well at low revs (1/3-1/4 throttle) and draw current to try to keep it going. I know I am running on the low end of the voltage range of the motor, and the speed control is only a 30A (assumed) but I'm not seeing any more than about 0.8A before it stalls, and I can do that with my finger tips.

Motor works well at full throttle and I can get a current drain of 15 or so amps without it stalling.

Sorry for the long post, am I trying to do something this type of motor wasnt designed for?
Any suggestions?

[nick_75au]

A lower Kv motor and an ESC suited to boats or cars is what you need,

Dont quote me on this but I believe Heli esc's are designed for Governer mode, which means the esc has little throttle range but can maintain a steady RPM under varying load. This is obviously at the top of the RPM range.

All brushless ESCs will require a reset to neutral after a stalled motor, this is due to the esc requiring voltage (back EMF) to commutate the motor, once the motor stalls, theres no voltage to measure, so it cant commutate the motor.

Also take note that to spin a prop at very low RPM requires very little power, so its unlikely to stall unless there is a further problem.

Nick

[Jon]

Hi Nick, Thanks for the reply.
I'd come to the conclusion that the ESC I have wasn't helping so I think I'll get a car/boat ESC with reverse and a normal throttle before I continue playing.

I found that low KV motors for running on low voltages 6.6V that were relatively low power (say ~100W) are hard to come by. So my intention was to see if running a 1400 KV at lower throttle may be a 'cheaper' solution than buying a lower KV motor, with a power that was unnecessary high.

I was told out-runners had better torque at low RPM than in-runners.

The boat I'm trying to decide on a motor train for is an Egrete pusher tug, the plans ask for RS380 motors which will do over 10K RPM at 6V under load. IMHO this is way too fast for the boat, but thought the 1400KV may be an acceptable direct replacement with still half the RPM of the original RS380, with the option of capping the REVs with my controller to say ~4K (75% throttle).
Jon

[dodgy geezer]

As nick_75au has said, brushless motors have a particular design feature which makes it difficult for them to operate at low speeds.

A brushless motor works by getting the brushless ESC to energise the coils in sequence to pull the magnets in the rotor round. This has to be done at a precise point in the revolution.

Expensive specialist brushless motors designed for low speeds (crawler motors) have a special sensor to mark where this point is, and a special ESC which takes a signal from that sensor. They can go very slowly if necessary.

But the bulk of cheap brushless motors use the simple trick of detecting the back EMF which is generated in the driving coils as a magnet goes by to act as this sensor. This approach works well, so long as the magnet is going past the drive coil fast enough to generate a good signal. So the starting sequence for a standard brushless is usually:

1 - Give a burst of full-power signal to start the rotor spinning fast
2 - Detect the back EMF caused by this spin
3 - Adjust the signal rate to bring the motor speed back down to match the throttle position.

The net result is that brushless motors start with a high-speed burst, then drop to the controlled speed. Fine if you're holding a plane in your hand ready for launch. But not so good if you have a boat or a car.....   
 

[Jon]

Thanks dodgy geezer,
Of course, just realized they are sensor-less, I understand now.
Oh well, I might build the ones Ive got into  something to see what they are like, but go brushed instead in the Egrete.
Ive got a pair of monoperms kicking around, looks like they may have a home now.
Just need to find a brushed ESC that will do LiFe cutoff.

Thanks for the replies.

[essex2visuvesi]

Thanks dodgy geezer,
Of course, just realized they are sensor-less, I understand now.
Oh well, I might build the ones Ive got into  something to see what they are like, but go brushed instead in the Egrete.
Ive got a pair of monoperms kicking around, looks like they may have a home now.
Just need to find a brushed ESC that will do LiFe cutoff.

Thanks for the replies.

Why not use one of these
http://www.hobbyking.com/hobbyking/store/uh_viewitem.asp?idproduct=7367

or something like it

[Jon]

I'll have a look round HobbyKing tonight, Ideally I want something to alarm or cut down the capability of the motor.
I'm an electronics engineer so I was thinking of designing a circuit to switch out the main Life batteries at 3.0V, and switch in a suitably rated rx battery pack to get me home on 4.8V While flashing an alarm beacon on deck. I design high current DC switches and chargers for a living so the current/control wont be an issue.
Doing this may sound a little excessive, but then I can use whatever ESC I want, and have total control about the cut off voltage.

[essex2visuvesi]

If you look around hobbyking and the similar sites you will find low voltage buzzers aplenty and some with lights as well

[Subculture]

LIFE batteries aren't as sensitive to low voltage conditions as lipo, so you can do away with the warning buzzers and flashing lights etc.

You can convert any brushless motor to sensored operation by fitting hall sensors. As you're an electronics engineer, that should prove less of an issue than it would to some. Allegro systems also do some neat chips that deal with driving a sensored motor if you fancied having a crack at designing your own sensored esc, you can mate these chips up with a microcontroller for a complete system.

Hobbyking do one or two sensored controllers, but they tend to be high current ESC's with corresponding prices.

[barriew]

I think the Monoperms are a better bet for Egrete. I have a pusher similar to Egrete that use two ex windscreen washer motors at 12v - plenty of power there. Save the brushless for something FAST

Barrie

[Jon]

LIFE batteries aren't as sensitive to low voltage conditions as lipo, so you can do away with the warning buzzers and flashing lights etc.

You can convert any brushless motor to sensored operation by fitting hall sensors. As you're an electronics engineer, that should prove less of an issue than it would to some. Allegro systems also do some neat chips that deal with driving a sensored motor if you fancied having a crack at designing your own sensored esc, you can mate these chips up with a micro-controller for a complete system.

Hobbyking do one or two sensored controllers, but they tend to be high current ESC's with corresponding prices.

True Life batteries can take a lot more abuse, I treat mine well now, but some of them I charged at 4.2V for a while as I didn't know/Understand they were LiFe, and they are still perfectly fine. I have a small selection of 10Ah, 50Ah, and 100Ah cells; need to come up with something big for the latter two me thinks.

Adding the sensor is one thing, unfortunately software is not my thing, so anything too complicated and it'd never get finished. keep my eyes out for a cheap sensored ESC on eBay I think (as if).

[Jon]

I think the Monoperms are a better bet for Egrete. I have a pusher similar to Egrete that use two ex windscreen washer motors at 12v - plenty of power there. Save the brushless for something FAST

Barrie

Yes, I was just a little concerned about the pulling power of the monos' with 45mm tug props on the ends. The reason I have them is they came out of my fathers harbour tug when he upgraded to decaperms to get the pull up. Saying that he may not have got the gearing right as I don't think he experimented very much. The monos' have the reversing switch so IIRC that makes them the 8 watt version.

[Shipmate60]

If they are the ones with the reversing switch they are monoperm supers.
At their rated voltage they run at 5000 rpm.
45mm props are on the upper end of the power range for these motors so might be worth looking at something like low drain 540's.
These have a larger diameter armature which will generate more torque and power.

Bob

[CF-FZG]

Dont quote me on this but I believe Heli esc's are designed for Governer mode, which means the esc has little throttle range but can maintain a steady RPM under varying load. This is obviously at the top of the RPM range.

At any throttle setting a governor will try to maintain that setting during varying load conditions - but like you say, only of real use when you can change the pitch of the blades. 

Governor mode is 'switchable', but shouldn't stop the motor from running at low rpm.  It sounds like an esc or motor problem, as if you tell the esc to turn the motor at a certain rpm, then when you try to hold the shaft the esc/motor combination should draw more power to try to maintain that rpm

As said by others, better to get a boat esc as it'll also have reverse


Mark

[nick_75au]

Ok this I know works and can spin so slowly you can count the revolutions even at 11.1 volt, It does give a slight kick at start up but it is very gentle compared to the previous reversing ESC had

http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=6247

and this ESC

http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=11742

Definitly has the torque  to spin a 45 mm or even bigger prop

Cheers
Nick

[Jon]

At any throttle setting a governor will try to maintain that setting during varying load conditions - but like you say, only of real use when you can change the pitch of the blades. 

Governor mode is 'switchable', but shouldn't stop the motor from running at low rpm.  It sounds like an esc or motor problem, as if you tell the esc to turn the motor at a certain rpm, then when you try to hold the shaft the esc/motor combination should draw more power to try to maintain that rpm

As said by others, better to get a boat esc as it'll also have reverse

Yes I couldn't work out why the 'power' switch on the Heli ESC didn't switch of the BEC, Ive come to the conclusion is is something to do with governor mode as it seemed to change the throttle response.
I think I will get a boat/car ESC to play with, just to lean about brushless more. But I agree with you that I would have thought the motor should start to draw more current to keep it's self going, but looks like sensorless cant achieve this at LOW RPM.

Nick, The HK 30A ESC was the one I was looking at to try, Thanks for the post, good to know you had good experiences. I think I will test on my current motors first as they are lower voltage, don't really want to go to 4 cell as with my pouches it puts the weight up. Saying that I havent worked out the weight I need in the egrete yet, 1Kg of battery may be just what I need.

[nick_75au]

To clarify something, the motor (within limits) wont care what voltage its fed, also the amp/wattage stated in the specs in my experience is never reached unless
A. your an FE racer,
B. the prop is very large
C. the voltage is high in which your creeping back into FE territory

The RPM on the motor I linked to spins at 4950 RPM at your 6.6 volts which is about right for your Egret
I have a thread here somewhere, testing a 700 Kv version of this motor and

http://www.youtube.com/watch?v=tbXt4Clk8QA

on 11.1, at the beginning of the video theres a start, and near the very end theres a little bit of slow running, on a very slippery hull and this is on 11.1 volts( and I cant keep my finger off the go button    )

Nick

[Jon]

To clarify something, the motor (within limits) wont care what voltage its fed, also the amp/wattage stated in the specs in my experience is never reached unless
A. your an FE racer,
B. the prop is very large
C. the voltage is high in which your creeping back into FE territory

The RPM on the motor I linked to spins at 4950 RPM at your 6.6 volts which is about right for your Egret
I have a thread here somewhere, testing a 700 Kv version of this motor and

http://www.youtube.com/watch?v=tbXt4Clk8QA

on 11.1, at the beginning of the video theres a start, and near the very end theres a little bit of slow running, on a very slippery hull and this is on 11.1 volts( and I cant keep my finger off the go button    )

Thanks that's great footage, Ive got a cabin cruiser I'm restoring atm (about 3foot) so I may give that a go with one of my brushless motors to see then. Looks like it's still up in the air whether its a go'er or not.
I can always rewind my motors to 700KV that's simple enough, if it turns out I need to.
Jon