Hi Kevin, i'm pretty sure Dave is right about the
saturation volts on the bridge chip. Many of those
little 1 amp chips don't perform very well with
3v 1A motors, they are more suited for 12v to 24v
systems with lower motor currents like 500mA.

With your 6v supply you are probably getting 2.5v
on the motor and 3.5v wasted across the bridge.
Efficiency is very low with more battery power going
to heat up the chip than what turns the motor. :o)

For efficiency the best system for differential drive
robots is a DPDT relay for motor reversing, and
a low-sat transistor (or lowRds fet) to run the
motor. H-bridges are really not needed when the
thing doesn't get switched rapidly in direction.
-Roman

PS. Your reply-to address is still faulty. I had to
manually cut and paste the piclist address to reply
to you. :o(


Kevin A. Benedict wrote:
>
> Hi Dave,
> I will "measure the voltage across the motors under load" and
> see what I get.

> >
> >It looks like you are losing several volts in the bridge.  Page four of
> >the datasheet http://www-s.ti.com/sc/ds/sn754410.pdf  (High level output
> >voltage and Low level output voltage) indicates you will lose between 2
> >and 4 volts (depending upon load and temperature).  You could try
> >measuring the voltage across the motor terminals under load to verify
> >this.

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