Hi Peter and All,
----- Original Message Follows -----
From: "Peter VanDerWal" <
[hidden email]>
To: "Electric Vehicle Discussion List" <
[hidden email]>
Subject: Re: [EVDL] Electric motor with no tranny for
conversion
Date: Thu, 15 May 2008 14:20:44 -0600 (MDT)
>> >> My wife and I are trying to figure out the necessity
>> >> for having a tranny to do an ICE to electric
>> >> conversion on a 65 Mustang. Is the tranny required
>> >> for the conversion or can we have an adapter to go
>directly to the drive shaft (or rear end)? Would the ratio
>> >> gearing be too low? What would be the potential
>> >> problems by not having the tranny? It would save us
>>some weight and $1000 for the adaptor plate.
>> Peter VanDerWal wrote:
>>> It is technically possible to connect the motor directly
>>> to the drive shaft, but it is MUCH more expensive and
>>generally less efficient.
>> Peter is correct, but includes way to many "very"s and
>>other adverbs.
>> Essentially all vehicles built as EVs from scratch have
>> no transmissions (electric cars, trains, buses, golf
>carts, fork lifts, etc.)
>
>We aren't talking about Golf Carts, or even purpose build
>EVs. We are talking about converting a MUSTANG and
>connecting the electric motor directly to the differential.
>In THIS situation, the adverbs apply.
You can do it fairly easy without a problem if you
want.
>
>> Yes, you do
>> need a bigger motor and controller when there's no
>> transmission. But the extra size and weight, higher cost,
>> and lower efficiency in the motor and controller is more
>> than offset by eliminating the size, weight, cost, and
>efficiency of the transmission.
>
>In this situation, I dissagree.
Surprise, surprise!! ;^D
>At 30 mph, the motor is going to be turning at about 1,000
>rpm or less (depends on the exact diff ratio)
Several problems with this.
First Lee said you'd need a high diff ratio and
4.11 or 4.56-1 ratios were common with up to 7-1 available.
I'd bet his Mustang has a 4.11 gear or can easily, cheaply
be replaced by swapping out the whole axle, very easy to do
on a rear wheel drive live axle car. Get better brakes too
so could be worth it just for them!
So at 30 mph you are turning much higher rpm's than
1k rpm.
Many motors are or can be made to run eff at
1000rpm and some forklift motors are 1100rpm stock rated
power, many under 2200rpm. And if buying new, you can order
most motors at such low and eff rpm, just ask for it in your
spec's.
One can use 2 motors giving the equivalent of a 2
speed transmission, doubling the torque for the same less
expensive controller. And even less expensive contactor
controller can be used for excellent power.
Now add the 5% eff from not needing a trans and you
are ahead of the game.
10-1 gears are only used in AC motors as you know
in road EV's to make up for their lack of torque. As series
motors make much more torque, they easily do in in less rpm.
I see no reason to need to change basic
assumptions. As Lee said, almost ALL production EV's use
direct drive, Why?
But other than these, Peter is right ;^D
Jerry Dycus
>Assuming a 9" ADC or equivelent, and given the lower
>voltage needed at 30 mph, this low RPM will only be about
>65% efficient (not counting the extra power required by the
>cooling fan)
>
>Bypassing the transmission gains about 5% efficiency, but
>you loose 10-15% in the motor.
>At lower speeds it gets even worse.
>
>>
>> However, most EV conversions already have a transmission,
>> so it's "free". The mechanical design is such that it's
>> harder to remove the transmission than it is to keep it.
>> Given that you're keeping it, you can save some money by
>>using a smaller motor and controller.
>>> Without the torque multiplication provided by the
>>> transmission, you need to have a motor and controller
>>> capable producing the huge amounts of torque need for
>>hills, acceleration, etc.
>> The extra torque needed without a transmission is about
>> 2:1. You can get this by going up a size or so in the
>> motor or controller. For example, an 8" motor and 500 amp
>> controller with a transmission, versus a 9" motor and
>1000 amp controller without one.
>
>Assuming you never need to climb hills. If you do have any
>hills you'll need closer to 3:1 (i.e the typical difference
>between 1st/2nd and 4th/5th)
>
>> Note that you will need to replace your differential
>> gears to go transmissionless. A typical ICE car
>> differential ratio is around 3:1. The differential for a
>transmissionless EV is more like 5:1.
>
>Or 10:1 as found in many purpose built EVs
>
>-snip-
>
>>> Also, while electric motors are very efficient over a
>>> wide band of RPMs, efficiency tends to fall off quickly
>>at very low and very high RPMs.
>> Yes; except that your driving time at very low and very
>> high speeds is generally quite low.
>
>Unless you drive in traffic.
>
>>> around town driving will be less efficient without the
>>tranny.
>> In fact, I leave my EV in 2nd gear for almost all
>> around-town driving, because it's *more* efficient than
>shifting.
>
>Try leaving it in 4th, since that is effectively we are
>talking about. If he goes transmissionless he won't HAVE a
>second to shift down into.
>
>It's easy to win an arguement by changing the basic
>assumptions.
>
>
>
>
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