Re: [PSUBS-MAILIST] Re: Big motor for PSUBS...ATTN: K. MARTINDALE...

[Subscuba@aol.com]

Hello Mike and Pat,

Hull speed is a complicated issue for which I know only a little. I can tell 
you more about the motor ratings.

The motor I'm using has a thrust rating of 70 pounds (Minn Kota) and an 
armature inductance of 100 uH. I investigated the motor characteristics. 
Eventually calling the people who actually made the motor. I think they were 
Lake Motors. Anyway they stated the motors are permanent magnet motors and 
have an efficiency of about 90% (Horsepower/Electrical Power Required). They 
also said the motor I'm using was about 1.2 Horsepower. Using an efficiency 
of 90% gives an electrical input of 994.7 watts [(1.2hp*746Watts/hp)/0.9] . 
>From 24 volts the current draw would be about 41 amperes.

If I scale these ratings by my parameters to a thrust of 107 pounds, I get an 
electrical requirement of 1520.5 watts (994.7x107/70). From 36 volts the 
current would be 42.2 amperes.

The thrust is measured under still water conditions with an electrical strain 
gage which may not reflect the thrust under flowing water conditions but it's 
better than the old horsepower ratings Motorguide and Minn Kota used to play 
with.

These numbers are interesting from an academic viewpoint. The best way is to 
look at the vendor's specifications.

What I also found out was that the controller was much more expensive than 
the motor which is why I designed and built my own controller. The controller 
is required to limit inrush current which can demagnetize the stator magnets. 
Direct application of 24 or 36 volts can wipe the motor out.

For what it's worth I also pasted in the equation and graph of my motor's 
horsepower versus speed in knots, the vertical axis is speed. The equation 
shown gives speed as a function of horsepower to the cube root. In other 
words if you want to increase speed by twice the horsepower has to go up 8 
times. In my case a 10 horsepower motor ought to double my speed, maybe. I 
can give more details about the equation if you care. It came from my 
research into the human powered subs. I don't know how conservative it is 
since I have no easy way to measure my Sub's speed.


Have fun and good luck,

Ken Martindale


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