Car alternator as R/C engine dyno?

Sounds good ta me !!

snipped-for-privacy@hotmail.com says...


You sure can, though you'd have to factor in the low efficiency of
the alternator, should be possible to calibrate it with a known
electric motor and power supply. If you were in Perth, I'd give you
a hand and use a HP6813A as power source ;)

No reason why an alternator wouldn't work if the revs and power
outputs match... and provided you don't mind the error due to
efficiency losses.

Since the power involved presumably isn't very great it's probably
also relatively easy to cobble up a dinkum Prony Brake.  Only needs a
flat pulley, a lever actuated brake band and a spring balance.

The efficiency of the alternator would introduce some error. It would be OK
for comparing engines at a fixed RPM value, since more current from the
alternator would always require more torque from the engine, but it would
not be so useful for any kind of quantitative measurements, nor for
questions line "can I get more power at 4000rpm or 4500rpm", since there is
no guarantee that the efficiency of the alternator will be the same at
different RPM.

You can make a surprisingly effective adjustable brake by passing some DC
through one of the windings of an old induction motor (single phase or
three phase, they all work and you don't need many volts).  You could mount
the induction motor casing in some large ball race bearings coaxial with
the motor shaft, and make an arm of known radius attached to the motor
casing, with a spring balance or load cell hooked to the end of it.  This
would allow you to measure the torque accurately.  The RPM could be counted
using an optical sensor and a microcontroller or frequency counter.  If you
use a load cell instead of spring balance, then you could hook the lot up
to a PC and plot all sorts of curves to find the optimal gear ratios etc.

Chris

as an engine dyno?

ISTM that the model engine maay run too fast for a direct connection.
check that out first.

I guess the rotor could be replaced with a solid lump of aluminium and a
regulated current applied to the output windings, might need new bearings too.

Would a geared down auto cabin fan (permanent magnet) motor  connected to a
variable DC load using a couple of '3055's or whatever be more useful
perhaps.

Depending on the gearing you use the alternator might have too much inertia.

gearing down reduces innertia.
I never heard of a  motor that ran worse with a large flywheel.

personally I'd go for a perforated disc in a tank and adjkust the water
level in the tank to get the load I wanted, output would be measured by
measuring the torqe on the engine mount and the RPM.

Bye.
   Jasen

I once read an article that said the plastic/metal construction of the R/C
cars is so efficient that you can just measure the cars acceleration and
wheel rotation with a P.E beam across spokes etc. and get extremely accurate
value anyway. Would be a lot of work trying to get rpm and power with an
alternator.