T-Pain Is On A Boat: Re: [Electric Boats] 7.5.kw 3 phase electric motors

Sunday, August 17, 2014

Re: [Electric Boats] 7.5.kw 3 phase electric motors

In Laymens terms. A VFD is a Variable Frequency Drive. Because they have soft
start tech built in to them, the starting current required to start the motor
is lower than what a motor typically requires to start.

It is not practical to have two motors running from the same drive because you
need to steer the boat. Without a VFD in the system and only one generator you
would have a steering problem as well.

Typically 3phs motors are larger and heavier than Permanent magnet motors (pm
motors) of the same HP.

HP requirements to move a boat in the water vary depending on hull drag . HP
is more a time/distance thing where torque is not time dependent. Multi hull
designs require enough torque to overcome a headwind as this is the largest
factor you will face with a multi hull. I have seen the nose torn off a multi
hull by the anchor rode in a blow.

To keep the desired WMG (Way Made Good) factor of a multi hull you need to
keep the weight down as low as you can. Because this it a true statement you
should consider:

* Lowest weight genset (this excludes diesel)
* Lithium type batteries
* PM motors (rare earth mags are best)
* RPM reductions using harmonic drive gear reduction. And Higher RPM lower
HP PM Motors.

You can of course reduce weight using high dollar materials in the hull design
but would likely spend more money that way.

I hope that this was easer to understand for a beginner.

Kevin Pemberton

On Saturday, August 16, 2014 09:52:41 AM you wrote:
> the new electric outboards (like 2) are possibly sufficient to drive the
> Cat.
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> -----Original Message-----
> From: Hannu Venermo gcode.fi@gmail.com [electricboats]
> <electricboats@yahoogroups.com> To: electricboats
> <electricboats@yahoogroups.com>
> Sent: Sat, Aug 16, 2014 9:44 am
> Subject: Re: [Electric Boats] 7.5.kw 3 phase electric motors
>
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> There is no inrush current, with electronic drives like VFDs.
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> An 8 kW VFD drive (or servo drive, they are similar in function and
> costs) will do a soft start, so the 3x start-up current will not be needed.
> This also applies, in general, to all modern 3-phase stuff (maybe not an
> electric cooktop) with motors.
>
>
> So 11 kW gennys and 8 kW motors are a good combo.
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> Pls send a link to the 11 kW gennys for 900 $ ?
> Where can I buy some.
>
> On 12/08/2014 06:54, zenataos@yahoo.com.au [electricboats] wrote:
> > Hi all
> >
> > The fact that I am asking a question in laymans terms is a
> >
> > clue that I have little knowledge of the subject and require responses
> > in simple terms, "electrics for Dummys" if you wish. As a boatbuilder
> > I am a mechanic, carpenter, fibreglass laminater, window installer etc
> > etc. so give me some slack. I wish to drive my boat with 3 phase
> > electric motors driven from generators. I calculate that motors
> > between 7 and 8 kw would suffice, the boat is a 15 metre catamaran. I
> > can purchase 11 kw diesel generators for approx $ 900.00 each which
> > are fitted into their own soundproof box. I have been told that I need
> > 3 times the kw to start an electric motor. This appears to be a fixed
> > fact. Could I start the motors sequentially, using both generators
> > power and manage reverse and forward manually ie. without stopping the
> > motors therefore having 11kw to run each motor? I do not wish to use
> > batteries as I want unrestricted (within reason) range. Have fun, look
> > forward to your responses.
> >
> > Al. yacht "Zenataos" (Zen And The Art Of Sailing)