On Fri, 6 Jan 2006 10:51:57 +0000, Clive wrote:

In message . 170,
Adrian writes
No engine, exhaust, cooling system, fuel tank, 4wd transmission... Just
batteries and lightweight hub motors.
This is rubbish, it'll be lighter to have a smaller motor running faster
and geared down. A hub motor would have very low starting torque
extremely high currents and need to be force ventilated at town speeds.

No, it wouldn't.

At town speeds the power is very low.

You do need a lot of poles on a bhub motr to get useable efficiencies at
low RPM, and I accept that I haven;t done the calculations totally - but
there are some protypes using that.

Its certainly possible to get down to less an 100.RPM or so on a big
diametr multipole motor. From memory in top gear my old midgets on 15"
wheels had about 4:1 final ratios on 6K RPM, so peak wheel RPM was about
1500..so a hub motor is not totally out of court.

As far as heating goes..well full power is the worst cae mainly..you
wouldn't slap full current through the motor to start it - too destructive.
If we take a 50bhp motor - about 37KW - and go for a shoddy 90% efficiency,
thats 3.7KW at full chat loss wise.

With a finned casing out on the air,it should not be hard to hold that down
to the 100C or so necessary to protect te magnets from demagnetising.

BUT as with all things car, its unlikley you would hold full power for any
length of time. Thernal sensors in teh motor would in any case hold temp
rises down to what was deemed acceptable.

If permanent magnets are not used (or cobalt magnets) up to 200C is
probably permissible.

If it was necessary to gear, I'd expect to use epicyclic boxes in the hubs.

Unlike a car, where about 3/4 of the input power has to be got rid of,
electric motors generate MUCH less heat. This may cause cities to become a
lot colder, but so what?