I have a 130 size dc motor that runs at 6 volts(http://www.pololu.com/product/1117). And I'm using this motor shield(http://osepp.com/products/shield-arduino-compatible/motor-servo-shield/). The motor runs perfectly fine through 4 AA batteries. When I run it through the motor shield with a battery power pack plugged in, it doesn't run at all. I measured the current through a multimeter and it's there. Does this mean the board is limiting the current? This doesn't make sense since the FA-130 motor(http://www.pololu.com/product/77) which draws twice as much current at least ran, though not as fast as it ran on battery. What could be the problem? For some reason, with 4 AA batteries I only see 4 volts on the multimeter. The motor works with those 4 batteries without the motor even though the multimeter says the batteries only provide 4 volts with the motor shield. Is the board somehow limiting the current? And how could the FA-130 motor run while the more efficient motor doesn't?
Does this mean the board is limiting the current?
No it is limiting the voltage. The chip the shield uses is not really suited to being used at such low voltages.
It's not suited to providing 6 volts? But it says on the website it's suited for 4.5 - 25V DC. I figured out the problem, anyway. It was a connection problem. But the motor still runs much slower than it was running on just the 4 AA batteries.
But it says on the website it's suited for 4.5 - 25V DC.
The web site lies. Just look at the data sheet for the h- bridge chip it uses.
A chip like this, uses 4 transistors (inside the chip, and depending on the exact application), and each transistor "eats up" about 0.7 volts. 0.7 * 4 == 2.8 volts. So those 4 volts you can see using your multimeter, are about right.
Since your motor shield will drop about 3 volts, you just need a power supply with 3 extra volts.
The outputs of that chip are called darlington pairs, which is an arrangement of two transistors with lots of current gain (useful for driving heavy loads), but which necessarily uses up one Vbe diode drop (0.7..1.1V) plus one Vsat (0.2..0.5V), leading to about 1 to 1.5V loss to the output load - but in an Hbridge you have both high-side and low-side darlington pairs, so a total of 2 to 3V is lost, taking the 6V down to 3.5V or thereabouts.
Also this means the chip gets hot as is dissipating 2.5 x I for each motor, which at stall would be about 1.5W.
For higher voltage motors this voltage loss is small and acceptable, for 6V motors its large and unsatisfactory - however the L293 is cheap.
For larger currents and for low voltages MOSFET H-bridges are much superior, and also they switch a lot faster too. MOSFETs have an on-resistance, rather than a fixed voltage loss, so you can reduce the power wastage by selecting one with less resistance.
If you want your 6V motor to work with an L293, use a supply of about 8 to 9V.