[ausev] VW FOX CONVERSION, GEAR RATIO QUESTION

[Chris Robison]

loopcat wrote:
>  Hey Chris,
>  I counted 45 saparators. So about 1575 volts! WOW I could only imagine that kind of power! What would be the max current?

Between adjacent brushes?  Or around the entire commutator?  If you 
actually count 45 separators between brushes (don't count the ones under 
the brushes, just the ones that are exposed), then I'm going to guess my 
rule of thumb doesn't apply and that this is a commutator that's built 
differently from what we normally see. With so many comm bars and 
separators, these are most likely thinner separators or you'd have 
insufficient copper-to-brush contact to carry enough current. This means 
each separator would individually withstand a lower voltage difference, 
but they should still add up to a reasonable total.


While the motor is powered, each brush is at the opposite voltage from 
the brushes to either side. The copper bars form a partial "bridge" 
between the brushes, with the separators preventing a short between 
them. If voltage applied to the motor gets too high, the sum of the 
separators' dielectric strength is exceeded and you have a "flashover" 
which can result in partial or total destruction of the commutator, 
brushes and brush holder.

Makes for cool post-mortem pictures  :o)

   --chris




> All the Best, Stuart..
> 
> 
> --- On Tue, 6/10/08, Chris Robison <chris at chrisrobison.org> wrote:
> 
>> From: Chris Robison <chris at chrisrobison.org>
>> Subject: Re: [ausev] VW FOX CONVERSION, GEAR RATIO QUESTION
>> To: "AustinEV News Announcements and General Discussion" <ausev at austinev.org>
>> Date: Tuesday, June 10, 2008, 9:14 PM
>> Good advice. To get an empirical estimate for your motor
>> voltage limit, 
>> count the minimum number of comm bar separators (the black
>> part between 
>> the copper bars) visible from one brush to either of its
>> immediate 
>> neighbors. It's a really rough estimate, but each one
>> of these is 
>> typically worth about 35V of separation. Add them together
>> to get the 
>> total number of volts that's safe to put across the
>> motor. Otherwise, 
>> 150V is a good number to go with.
>>
>>    --chris
>>
>>
>>
>>
>> loopcat wrote:
>>> FYI, this is the reply from Otmar at Zilla...
>>>
>>> "Hello John,
>>> The motor looks like a good one to me.
>>> I would make sure you have a good clutch in the setup
>> so it does not slip.
>>> A safe motor voltage limit is usually 150V. I've
>> heard that some GE 
>>> motors can take much more than that, but it's a
>> good starting point. 
>>> Current limit is going to limit the torque and
>> therefore protect your 
>>> clutch and transaxle. I would start off at 400 to 600
>> amps on the 
>>> motor and see how that feels.
>>>
>>> You don't mention your batteries. If they are
>> flooded lead acid then 
>>> you may want to protect them with a 350 amps battery
>> current limit to 
>>> start and a low voltage limit of about 9V per 12V
>> block. High current 
>>> AGMs won't need a reduced battery current limit
>> but still you should 
>>> limit the voltage.
>>>
>>> Once you have it running well, you can contemplate the
>> risks to the 
>>> batteries, motor and drivetrain of turning up the
>> power.
>>> -Otmar"
>>>
>>>
>>>
>>> --- On Sun, 6/8/08, Chris Robison
>> <chris at chrisrobison.org> wrote:
>>>> From: Chris Robison <chris at chrisrobison.org>
>>>> Subject: Re: [ausev] VW FOX CONVERSION, GEAR RATIO
>> QUESTION
>>>> To: "AustinEV News Announcements and General
>> Discussion" <ausev at austinev.org>
>>>> Date: Sunday, June 8, 2008, 9:52 PM
>>>> Did you plug in these gear ratio numbers directly,
>> or did
>>>> you multiply 
>>>> them by the final drive ratio (the gearing at the
>>>> differential) first? 
>>>> Uve's calculator does not provide a separate
>> spot for
>>>> the final drive 
>>>> ratio, so you have to do the math yourself as you
>> input
>>>> each number. A 
>>>> quick google search reveals that the Fox 4-speed
>>>> transmission had a 3.89 
>>>> final drive ratio, and the 5-speed was 4.11. 
>> Multiply 3.89
>>>> by each of 
>>>> your gear ratios to get the full ratio for each
>> gear in
>>>> Uve's 
>>>> calculator. Remember that your redline with this
>> big motor
>>>> is going to 
>>>> be somewhere in the neighborhood of 5000rpm.
>>>>
>>>> Beyond that, I'll just say this -- given your
>> design
>>>> goals as you've 
>>>> stated in the past, you haven't solved your
>> primary
>>>> issue. You still 
>>>> have *way* too much motor for your car. Although
>> it may
>>>> physically fit 
>>>> within the Fox's engine bay, you've
>> actually made
>>>> this fundamental 
>>>> problem worse in that the Fox was not intended
>> even as a
>>>> diminutive 
>>>> sportscar like the NX was. In gasoline terms,
>> you're
>>>> effectively 
>>>> cramming a 350 small block into an economy car,
>> which
>>>> generally 
>>>> speaking, is something people do if they're
>> aiming to
>>>> race. What comes 
>>>> along with that intent are the modifications to
>> the
>>>> driveline -- bigger 
>>>> transmission, custom hardened gears, multi-disk
>> racing
>>>> clutch, enlarged 
>>>> axles and CV joints, wider and softer tires etc,
>> that are
>>>> necessary for 
>>>> a small car to withstand the input from a big
>> powerplant.
>>>> Without these 
>>>> modifications, your car is going to be undergoing
>> regular
>>>> repair.  With 
>>>> the stock transmission, don't be surprised if
>> you end
>>>> up needing a 
>>>> rebuild every 5 or 6 months. The stock clutch will
>> slip
>>>> under the torque 
>>>> and will definitely need upgrading and the
>> performance
>>>> aftermarket for 
>>>> Foxes may be limited (I don't know).
>>>>
>>>> Operating the motor at lower RPMs is actually
>> something to
>>>> avoid, as 
>>>> torque will be higher placing higher mechanical
>> stresses on
>>>> your 
>>>> transmission, current draw will be higher, cooling
>> less
>>>> effective, and 
>>>> generally the motor will run hotter and a bit less
>>>> efficiently.
>>>>
>>>> The way to make this motor work and not tear up
>> your car
>>>> would be to 
>>>> keep RPMs high, drive gently and limit the motor
>> current
>>>> from your 
>>>> controller, to limit the motor's output
>> torque. Doing
>>>> so will render 
>>>> unusable all the extra power the motor is capable
>> of, which
>>>> means you're 
>>>> carrying around a lot of extra mass that you
>> won't be
>>>> using. Worse yet, 
>>>> much of this will be rotating mass, which reduces
>>>> efficiency during 
>>>> acceleration.
>>>>
>>>> This motor really belongs in a full-size pickup,
>> van or
>>>> SUV, in a 
>>>> direct-drive configuration in a light truck or RWD
>>>> sportscar, or in a 
>>>> small vehicle if high performance is desired (and
>> planned
>>>> for). It will 
>>>> work in your application, but I think it is far
>> from ideal.
>>>>    --chris
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> loopcat wrote:
>>>>> Hello All,
>>>>>  After my 1st failed attemped at converting a
>> '91
>>>> Nissan NX (motor too big for car) We had to make a
>>>> decision, keep the motor or keep the car. So we
>> decided on
>>>> another donor car, a 1993 vw fox. 
>>>>> Putting the cart before the horse, last year I
>>>> purchased a rather large GE DC motor. The
>> demensions/weight
>>>> are very similar to the famed Warp-11. The motor
>> tag states
>>>> 16hp at 70v, 214amps, 1822rpm, ~210lbs. From what
>> I have
>>>> heard, The motor was designed for an
>> air-craft-tug. So it
>>>> should have a ton of torque. By most accounts the
>> motor
>>>> will function well in an EV.
>>>>> The major reason for buying the vw fox was
>> that the
>>>> engine is latitudinally mounted so my GE-11"
>> motor
>>>> will fit into the car. 
>>>>> After buying the fox, I did a bit of research
>> and
>>>> found some interesting facts on the transmission.
>> The gear
>>>> ratios are very low numerically speaking.
>> 1st-3.450,
>>>> 2nd-1.790, 3rd-1.130, 4th-0.830 R+P-4.110. So I
>> was curious
>>>> how this would perform with the motor. So I went
>> to
>>>> Uve's EV Calculator and added these
>> transmission ratios
>>>> to a vw rabbitt, zilla lv, and trojan t-105. The
>> closest
>>>> motor on the list was a Kostov. I then lowered the
>> volts
>>>> (120 t-105) and the motor rpm's (3600)  The
>> results are
>>>> interesting, the motor rpm's were low, but the
>>>> speed/distance was very repectable, 1st gear was
>> the best
>>>> with a range of 30 miles between 40-60mph. Top
>> speed
>>>> ~70mph. At first, I thought the ratio info was
>> bad. Most of
>>>> the cars on the list have a 2nd gear ratio of
>> about 8.0. But
>>>> then I saw that the Fiat Palio also had lower gear
>> ratios.
>>>> So for kicks-and grins, I lowered the volts to
>> 96v. In 2nd
>>>> gear the rpm's were below 1700 at 50-60mph. 
>>>>> And this leads me to my question. I have
>> sometimes
>>>> heard that these old GE-11" spin too slow at
>> lower
>>>> volts, "tons of torque, but no top-end
>> speed".
>>>> But if the transmission has lower gear ratios then
>> it
>>>> should produce the mph's, right? Any insights
>> would be
>>>> appeciated.
>>>>> Thanks, John Stuart in San Antonio tx..
>>>>>
>>>>>
>>>>>       
>>>>>
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