Re: what is the max bus voltage, 300V?

[Brian <bjcouche@mtu.net>]

Thanks for all of the good information. Let me clarify my situation. I 
realize that the resistance losses in the motor will be the same. The losses 
in the inverter however will not. I am looking at taking a commercially 
available industrial drive and "reworking" it to make it smaller and lighter. 
I can choose either a 230 or a 460V unit. The pack voltage could be 650V 
nominal but not much higher as I don't want to hit the overvoltage limit of 
~740V during hard regen. 230V drives are significantly larger than 460V 
drives. Drive's are basically current rated devices, the more current you 
need, the bigger they get and the greater the watts losses. Take for example 
a 50HP (continuous)drive, at full load a 230V drive dissipates 2370W while a 
460V 50HP drive dissipates 1303W. I believe 1KW would be worth saving, but 
more realistically no small truck will need 50HP continuous to operate so the 
losses won't be that drastic. In general though, a drive at twice the voltage 
will have half the losses.
   As for working with that high DC voltage, I don't touch anything over 48V 
so I'd be using insulated tools and MV gloves while working with live battery 
conections.
  I knew there was a small battery efficiency gain by halphing current but 7% 
sounds like a substantial gain to me, what %eff does LRR tires give all those 
people who use them?
   As for a DC-DC, Vicor has an in depth description of just how to connect 
two DC-DC converters in series at their input. I'd just follow their 
expertise.
  To sum it all up, the real drive for the higher bus voltage is the drive 
due to size, weight, and drive efficiency.

ev@listproc.sjsu.edu wrote:
>
>Brian wrote:
>> Has Anyone ever seen or heard of someone using a 625V pack?
>
>GM's Electrovair (1966 Corvair conversion) used a 532v pack. Large EVs
>like electric trucks, buses, trains, mining equipment, and ships also
>routinely use higher voltages.
>
>> higher voltage beckons to me because I see the following advantages:
>> lower current thus lower resistive losses in motor, drive, wire, etc.
>> lower discharge rate for possible longer range?
>> smaller individual battery AH rating, but twice as many of them.
>
>None of these work our as advantages. For example, doubling motor
>voltage halves the current. Each winding thus has twice as many turns of
>half the wire size. Winding resistance is thus FOUR times higher. When
>you figure out its resistive losses (I^2 x R), it turns out to be
>exactly the same.
>
>Same for the batteries. No matter how you wire them, you get exactly the
>same current per battery, and the overall resistive losses in the pack
>and its wiring is also the same.
>
>Sometimes there are benefits in the controller for particular voltages,
>because certain semiconductor technologies are better suited for certain
>voltage ranges. But the difference is minor, since controller losses are
>not a large fraction of total losses.
>-- 
>Lee A. Hart                Ring the bells that still can ring
>814 8th Ave. N.            Forget your perfect offering
>Sartell, MN 56377 USA      There is a crack in everything
>leeahart_at_earthlink.net  That's how the light gets in - Leonard Cohen
>


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