[PSUBS-MAILIST] Motor Suggestion?

Tue May 15 21:21:59 EDT 2018

Hi James & Alan,
My trials with the Maytech 6374 brushless motor did not go well, although
the VESC is amazing and I cannot recommend it highly enough.  Bit of a
learning curve, but I can now change all the parameters and monitor
everything (temperatures/voltage/current/RPM) via bluetooth and Android
smartphone.

Maytech recommended the 6374 motors based on the specs I gave them, and
promised they would have certain resistance, etc.  Well, they arrived and
didn't, so were only running at 50% efficiency with the propellor loads
(roughly 27V, max 20A, ~500W, 800RPM) I put on them, and quickly triggered
overheating protection as a result.  So if anyone wants a 6374 60Kv or 70Kv
(I have one of each) let me know - I'm hoping to try the 8085 which is the
next size up, but may have to go elsewhere if Maytech don't want to take
the 6374s back or refund me on them.

It's critically important to match (at least ballpark) the motor and prop -
and both are highly variable (just think about the infinitely variable
number of coils and prop pitch).  Otherwise a huge chunk of your electrical
input power is turning to heat and you may not even realise (I suspect this
may have been happening to Alan with some of his issues).  Just because a
motor is rated to however many volts, amps or watts, doesn't mean it will
perform well outside of the sweet spot.  Efficiency is difficult to
determine because it's hard to measure power out - but a good approximation
can be found from the motor curves.  I have a spreadsheet/calculator if
anyone is interested - email me direct at
stephen.fordyce at tfmengineering.com.au .

The prop has a performance curve and so does the motor and matching them
scientifically is a bit of an art form.  But apparently a good rule of
thumb is to start with your prop RPM and torque under desired load.  To
find nominal unloaded motor speed, multiply loaded RPM by 1.25.  Use this
with your battery voltage to determine the Kv of the motor.  Then use the
required prop torque to determine motor current and size.

I'm finding that often the RC motors are not well designed for continuous
operation and the associated cooling, so they need to be derated
significantly, or better cooled.

Cheers,
Steve

On Wed, May 16, 2018 at 9:01 AM, Brian Cox via Personal_Submersibles <
personal_submersibles at psubs.org> wrote:

> Yes I am a proponent of golf cart motors as well.   If you can find a
> place that repairs them you can find some good deals.  Best plan is to just
> make multiple back up motors and switch them out if you have any problem.
>
> Brian
>
>
>
> --- personal_submersibles at psubs.org wrote:
>
> From: hank pronk via Personal_Submersibles <personal_submersibles at psubs.
> org>
> To: Alan via Personal_Submersibles <personal_submersibles at psubs.org>
> Subject: Re: [PSUBS-MAILIST] Motor Suggestion?
> Date: Tue, 15 May 2018 22:15:05 +0000 (UTC)
>
> James, for an inboard motor, you can't go wrong with golf cart motors.
> They are available in 48 volt and up to 8 hp I think.  These motors are
> extremely durable and cost effective.  Gamma has a 2 hp traction motor from
> a tenant sweeper, same as golf cart.  At full load it barely gets warm.
> Hank
>
> On Tuesday, May 15, 2018, 3:28:36 PM MDT, Alan via Personal_Submersibles <
> personal_submersibles at psubs.org> wrote:
>
>
> Hi James,
> just had a look at the motor & it's a bit of a beast.
> Looks like an in-runner. It says elsewhere it is cooled with a fan but has
> the option in the check box for air or water cooling.
>    If you have a look at the Dragon submersible, they have gone with some
> large diameter but short motors that resemble the high torque direct drive
> bicycle hub motors that are popular. On the Dragon submersible it looks
> like they have developed thier own propeller so that the thrust isn't
> hitting
> the body of the thruster.
> The cooling system on your motor isn't going to work if you put it in a
> water/
> pressure proof enclosure. With the in-runner the coils are on the outside
> & can dissipate their heat through the can, so there would be the option
> of fitting the motor tightly in to a housing & letting the heat go through
> the
> housing into the water; but the fins on the motor won't let that happen in
> this case. Maybe grind them off. It has a temperature sensor so there is
> the
> option of monitoring the temperature & running it at a suitable power.
> Some
> escs can drop the power automatically if the temperature gets too high.
> The motor is very expensive. There is this Maytech motor designed for
> electric
> surfboards that is rated at 600W
> http://www.michobby.com/product/electric-surfboard-
> skateboard-e-bike-brushless-motor-8085-160kv-motor/
> It is an 8085 160 kv out-runner & priced at $188- US. I am wanting to test
> one of these but will test my smaller motors first to get a better idea of
> what
> kv winding I should have. Maytech will wind to your specifications; I
> think you
> have some knowledge of the RC world so you will understand this.
> This motor being an out-runner has the coils on the inside & so I am
> relying
> on oil for cooling.
> You will need to replace the propeller shaft with a longer stainless shaft.
> Your motor is designed to drive a pulley & you would have to check
> what axial load the bearings take. In my thruster design I have my motors
> suspended between two bearings in the thruster housing that take the
> forward & reverse axial load.
> Alien Power in England have some large brushless motors & motor
> controllers.
> I think he's a small operator & he will respond to questions. Got an idea
> he's
> in Birmingham.
> I am focussed on my thruster testing at the moment, but could not put a
> time on when I will test the 8085 motor. You can keep an eye on my progress
> if you want to hold off your thruster build.
> There is an electric surfboard site that are using large DC motors, that
> may be
> of help, but these boards are doing 40kph, so are more like planning boats
> rather than submarines that fall in to the work boat category.
> Good luck.
> Alan
>
>
>
>
> Sent from my iPad
>
> On 16/05/2018, at 3:51 AM, James Frankland via Personal_Submersibles <
> personal_submersibles at psubs.org> wrote:
>
> hi all
>
> I am thinking about what motor I should use for my main thruster on
> Skadoc.  Its got to be big.
>
> All these ideas are just thoughts at the moment, as I am working on
> battery pods now.
>
> Expecting delivery of pipe from UK today.
>
> So I have time to think how motor configuration is going to work.
>
> What do you guys think about this motor choice here?  As a possibility.
>
> https://www.miromax.lt/en/m-6/c-39/c-45/product-500-bldc_
> motor_hpm-5000b_-_nominal_power_5-86kw
>
> I have room for 12 x12v batteries, so all voltages are a possibility.
>
> I am also thinking of running an inverter and working at 120v AC.
> investigating this.
>
> Regards
> James
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