I am trying to control a dc motor with an SN754410 h bridge. When connected directly to the battery (9V, voltage read w/ multi meter ~8.1V) it starts running immediately at a relatively high speed. However, when I connect it through the h bridge, it makes a buzzing noise and will only start rotating if I physically twist the shaft with my fingers first. If I measure the voltage delivered to the motor through the h bridge I am getting around 7.5V which should still be enough to run the motor as it rotates quickly once I start it manually.
Try reducing the PWM frequency. It could be that the motor inductance at standstill is inhibiting current flow so preventing the motor developing sufficient torque to start rotation.
Lemsixteen:
If I measure the voltage delivered to the motor through the h bridge I am getting around 7.5V which should still be enough to run the motor as it rotates quickly once I start it manually.
Did you measure that on the motor terminals when the motor was NOT turning?
Stall (startup) current could be several times higher than running current.
An ancient SN754410 (L293) could drop 2.5volt or more, and a 9volt battery is not able to deliver much current.
Leo..
jackrae:
Try reducing the PWM frequency. It could be that the motor inductance at standstill is inhibiting current flow so preventing the motor developing sufficient torque to start rotation.
The inductance won't inhibit the DC current flow, higher PWM frequencies can lead to more magnetic
losses and switching losses, but won't reduce DC current.
The problem is trying to power a motor with a small 9V battery that is incapable of sourcing enough
current for a motor, compounded by the couple of volts lost in that darlington driver chip.
Wawa:
Did you measure that on the motor terminals when the motor was NOT turning?
Stall (startup) current could be several times higher than running current.
An ancient SN754410 (L293) could drop 2.5volt or more, and a 9volt battery is not able to deliver much current.
Leo..
Yes , it seems like the problem is not enough current. I held the shaft stationary and hooked it directly to the battery, measured the current ans I was seeing about 2A. I was only around 0.4A when rotating so this makes sense based on what I was seeing. I looked up the specs on the SN754410 and it says the maximum current it could output per channel was 1A so I'm not using that anymore. I am now trying to control the motor with an IRF620 power MOSFET and getting the same problem. It is connected like: battery-->motor-->mosfet drain; arduino pin-->mosfet gate; and mosfet source-->ground. When I plug in a random tiny dc motor I had laying around it runs fine with this setup. Can the MOSFET not let through enough current either or to I need to be powering it with something other than the 9V battery? It spins fine when connected directly to the battery but I need pwm control.
The IRF620 is not a logic level mosfet, and might not work properly.
I don't know any 9volt smoke alarm battery that can provide 2Amp.
Most struggle to deliver >50mA.
The SN754410 (L293) shouldn't be used over 600mA (continuous).
But try that chip with a 9volt supply made with AA batteries.
Leo..
Wawa:
The IRF620 is not a logic level mosfet, and might not work properly.
I was able to get it to behave how I wanted (w/ pwm speed control) using that mosfet and one of these motors:
which is why I was hoping I could use it for these higher power motors
Wawa:
The SN754410 (L293) shouldn't be used over 600mA (continuous).
But try that chip with a 9volt supply made with AA batteries.
The datasheet on this said it could only do 1A per channel but I can take it to school tomorrow and use a desktop power supply to see if I can make something happen
Lets be very clear here, a small 9V battery is NOT the thing to use for powering motors.
MarkT:
Lets be very clear here, a small 9V battery is NOT the thing to use for powering motors.
what should I be using? 6xAA battery pack?
Lemsixteen:
which is why I was hoping I could use it for these higher power motors
What higher powered motors?
Can you post a link to data/specs of the motor please?
Tom... 
TomGeorge:
What higher powered motors?
Can you post a link to data/specs of the motor please?
Tom...
yes, these are the motors:
I notice they are listed on the website as 6.5A stall @ 6v but as I said above I was measuring a little over 2A when connected directly to the battery and holding the shaft in place. So when I am looking for a new motor controller/shield should I look for one that can do 2A or 6.5A? or wait until I have an appropriate battery and retake this measurement?
Lemsixteen:
yes, these are the motors:
Pololu - 99:1 Metal Gearmotor 25Dx66L mm HP 6V with 48 CPR Encoder (No End Cap)I notice they are listed on the website as 6.5A stall @ 6v but as I said above I was measuring a little over 2A when connected directly to the battery and holding the shaft in place. So when I am looking for a new motor controller/shield should I look for one that can do 2A or 6.5A? or wait until I have an appropriate battery and retake this measurement?
All batteries have INTERNAL resistance and that is what is limiting your current readings. That is also why batteries get warm when being overused.
Paul
Lemsixteen:
yes, these are the motors:
Pololu - 99:1 Metal Gearmotor 25Dx66L mm HP 6V with 48 CPR Encoder (No End Cap)I notice they are listed on the website as 6.5A stall @ 6v but as I said above I was measuring a little over 2A when connected directly to the battery and holding the shaft in place. So when I am looking for a new motor controller/shield should I look for one that can do 2A or 6.5A? or wait until I have an appropriate battery and retake this measurement?
6.5A. And for powerful motors be careful about holding the shaft, given a suitable high current supply it
might be dangerous. Torque is proportional to current for a DC motor.
You just need to measure the resistance across the motor terminals to determine the stall current, since
the winding resistance is the only thing limiting the current at stall.
A lab power supply with adjustable current limit is very handy for this sort of testing.