48v or 72v or 96v ?

[Ohms_Cousin]

First off, newb here and just starting out on the learning path. So please excuse the dumb questions I may ask.

I am wanting to put an electric motor setup in my sail boat.

I have selected Agni Motors Pancake motors (axial flux) - these motors have been developed in conjunction with Cedric Lynch, the inventor of the so called Axial Flux Pancake motor.

I will be running 2 x 8kW motors - I am buying the plans to make the mount for the motors and the reduction gearing etc.

I need to select a voltage to run at. I was initially going to use 48vnom as my inverters are 48vnom and it makes sense to try to keep voltages on the boat to a minimum. Then there is the safety aspect - 48vnomn is regarded as "safe" although I wouldnt want to get a bite from a 48vnom system !

After much reading I came across some info that said that the heat output from a 96v motor is 1/4 the heat output of a 48v motor setup. Of course double the volts and halve the amps - that's always a good thing !

So what are the pros/cons of 96v Vs 48v? (72v also an option - although if going above 48v why not just to to 96v instead of 72v?)

What I know so far - 48v considered safe/speed of the motor is slower at 48v so reduction gearing is more manageable/lot more switch gear available for 48v compared to 96v.

In general is it wise or unwise to consider 96v over 48v? I am concerned about heat build up down in the engine bay of the boat (diesel removed for the electric) and if 96v really does run so much cooler it would seem to be the way to go.

I am wary of 96v "hazards" - however how big a problem is this really? The batteries will be buried under the floor with limited access so there really is no way that anyone could accidently come in touch with a 96v battery system. The only time an issue could arise is when working on the system for maintenance / repairs etc - then it becomes a case of being VERY VERY careful and using the right safety gear while working on it and always having another person present to hit a kill switch or similar to turn off the current if one does get hooked up.

One thing I do like about 96v is that it would be a 32S1P battery configuration - thats a lot of energy in the battery I will build. I have EVE LF304 LFP cells for the battery build. I intend to build 2 x 32S1P batteries - each on thier own BMS. (or 4 x 16S1P batteries at 48v if I go that way) - all of these batteries will be under the floor out of the way.

I am sure that many have been down this road before and have experience of the issues I am considering.

Looking forward to be educated on this.

 

[Chalo]

If the Agni motor is brushed, as most Lynch motors have been, then the brushes are not indifferent to the voltage used. Higher voltage will mean more frequent brush replacement and more debris inside the commutators.

Higher voltage raises RPM proportionally, and props also aren't indifferent to RPM increases. If you have to use a speed controller to reduce the speed to the RPM you'd be running on a lower voltage, then you might as well use the lower voltage.

 

[Ohms_Cousin]

Thanks Chalo.

Yes it is the 119R motor - brushed. 2 motors to be installed as a Pair. My main concern is heat. It is definitely easier to run the whole boat at 48v, but the technical parameters of the motors come first - if the heat is a big reduction at the higher voltage then that would seem to be the way to go - but your points have been taken onboard.

48v Vs 72v - how much of a difference are we talking when it comes to brush replacement/debris in the commutator ?

The higher speed of the motors at the higher voltage can be dealt with in gearing, so that is not a major concern (it is a concern but not at the top of the list for concerns)

 

[harrisonpatm]

How expensive are brushes, and how difficult are they to replace? Chalo is right, but it is almost always better to volt up, gear down. Better efficiency, cabling doesn't need to be as thick... You mentioned 8kw per motor. 8kw/48v nominal = 166 amp draw, that's gonna need something like 2/0AWG, quick google search says $6.56 per foot. At 96v nominal the same 8kw would be 83 amps, now you only need 4 or 6AWG, quick googling says $2.36 per foot. In addition, higher voltage means fewer losses through the cabling; I upgraded my motorcycle battery from 72v to 102v, kept everything else the same (motor, controller, total motorcycle weight, speed), and saw a 10% increase in wh/kilometer. I'd have to chalk that up to increased efficiency from higher voltage.

So you can weigh the cost of brushes and the difficulty of replacing them against that. Like you said, you'll need to gear it down either way. But these are just a few variables, I'm sure you have a thousand to consider. You're right that 48v is practically touch-safe, and 96v isn't.

 

[Ohms_Cousin]

I would also think that the higher voltage would add to the efficiency of the setup and that is pretty important in a boat where recharge is difficult. Losses through the cable can be overcome by using thicker cable - but yes the cost and difficulty of working with 4 ought cable is a thing to consider.

One of the advantages of the Lynch motors is the ease of changing out brushes and the low cost of these brushes.

However I did not know that the higher voltage has an effect on the brushes/commutator - so learnt something here.

 

[gromike]

I believe there is a USCG 48 dcv limit for watercraft, with any higher voltage subject to additional regulations. Just something stuck in my mind.

 

[amberwolf]

As noted there are two problems using brushed motors at higher voltage; RPM and brushes.

The RPM isn't just the output RPM (which you could gear down in various ways) but also the commutator. If you spin a comm faster than it's designed for, there's a risk of "grenading" it, where the segments come off the rotor and fly at that speed outward, destroying the comm, brushes, and possibly windings if they get entangled in them. If they are going fast enough and mass enough, they can penetrate the motor casing and damage things outside the motor--anything in the flight path is at risk. Somewhere on the DIY Electric Car forums there is a thread about a Saturn conversion where this happened (on a downhill I think)....

Brushes arc to the comm while it's rotating, passing some of their current that way, creating plasma between the brush and comm. This is normal, but the higher the voltage (and current) the worse this problem is, and the greater the heat from it. That heat is at least as much of a problem as the higher brush (and comm) wear--if it's hot enough, it can weld the brushes to the comm once the motor stops. If it's really bad it could distort the comm segments enough to pry them off the rotor, and you get the first problem above.


The brush arc is also a gigantic source of broadband RF noise, and it can destroy a controller that's not properly designed. 4QD.co.uk has some info on this issue (or did a decade ago when I was learning about this type of controller design) including some examples of the destruction that can occur. So increasing the brush arc can also increase the risk of controller damage.

 

[offGridDownUnder]

I have selected Agni Motors Pancake motors (axial flux)

Does the manufacturer make any statements about the voltage to use? There might be a reason for the choice.

 

[Ohms_Cousin]

Does the manufacturer make any statements about the voltage to use? There might be a reason for the choice.

I think they like 72v - they dont seem to be keen on 96v, and they can supply 48v if that is my request. Before I selected Agni, I was working with LEMCO - they told me the upper limit of the motor design was 120v - anything over that and as Amberwolf describes, the motor will self destruct. (96vnom is 110v with a fully charged LFP 96v pack - so close to the limit).

My rationale on this is if I have to go to a higher voltage it may as well be 96vnom - but there is no doubt it is far far easier and simpler to leave everything including these motors at 48vnom - but the heat output while the motors are working concerns me - I just wont know till I do it how much heat I will have to deal with and if it it excessive then its too late as the rebuild is over.

Having said all that, its looking like I will have to settle for 48v or 72v. At least 72vnom is 50% higher then 48vnom - so there is a reduction in amps - so better then nothing.

How much safer would 72vnom be compared to 96vnom?

 

[Ohms_Cousin]

I believe there is a USCG 48 dcv limit for watercraft, with any higher voltage subject to additional regulations. Just something stuck in my mind.

I am in darkest Africa - so no real rules to follow other then common sense.