3-30-06 Re: Fast Street Car with Twin DC Motors

Hello to All,

Roger Stockton wrote:

I think John’s concern is with adding *any* additional contacts and
wires into the motor loop due to their associated losses.


As soon as the motors start spinning, the back EMF is going to make
their effective resistance great enough to make a few milliohms
difference in wiring resistance unimportant.

Yes, but it’s the all-important launch at the strip, where there’s very little back emf at first, that’s the most important area where any high current losses count, big time. I agree, once the car is moving along and the motors are spooled up, there would be very little difference caused by F-R contactors.

Look at it this way; Matt’s car is running the same controller as
John’s, and a pair of 9″ motors, and he is getting *very* respectable
performance (perhaps even better than John’s when you factor in vehicle
weight) even though he does have a reversing contactor in the motor

Yes, but if he were to rewire the motor(s) circuit more directly and leave all the cables and contacts for forward-reversing out of the loop, he’d probably pick up .2 – .4 seconds! That’s not important for everyday driving for sure, but for 1/4 mile drag racing and chasing after a record, it’s HUGE.

My concern would be that a reversing contactor relies on current flowing
though the NC and NO contacts depending on the direction selected, and
usually NC contacts are rated for less current than the NO set.

Again, correct. Otmar’s 914 has experienced this problem already. Here’s a car that pulls BIG currents all the time, as it’s his rolling Zilla test bed. The NC contacts that rely on spring pressure burn more quickly than do the NO contacts that are closed under more pressure from the magnetics of the coil.

I would
think that wiring the reversing contactor so that it must be energised
for forward “gear” so that the NO contacts carry the heavy race current
would be a good thing to do.

I thought about that as well, but it only adds even more 12V current to be draining all the time, and the default mode is having the car in reverse if the 12V power is interrupted to the forward-reversing set.

For vehicles trying to set world records or for those that already own the record and are chipping away at precious fractions of seconds, every little thing counts. I’ve found that eliminating the high current losses in the motor loop circuit has improved the car’s 60 ft. and 1/8 mile times. Having just one high extra high current contactor for the front motor (that would open the field circuit so that a smaller F-R set could reverse it) with the old dual motor setup, caused an unbalanced wear condition of the brushes and coms between the two motors. Where I got rid of that contactor, the problem went away, and the 1/8 mile times improved.

For vehicles that are mainly for street with setting world records a secondary concern, the reversing contactor sets work fine. If I were to suddenly retire White Zombie from track racing and turn it into a ‘street only’ vehicle, I would have no problem installing F-R contactor sets. For now, where we are a mere .151 seconds away from hitting the 11′s, I’ll stick to having just fat 4/0 cables and thick copper bus bars inter-connecting the twin motor sections of the Siamese 8.

See Ya……John Wayland

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