Thanks a lot for that detailed explanation Max, I'll order the rest of the parts I'll need to perform the test.
The concern is with the motors stalling during a ball throw under load, this can occasionally happen if the throwing wheels can't grip the ball and it becomes jammed between the wheels. Commercial tennis ball machines have a inline fuse at the battery and what I think is a 20A 'push to reset' circuit breaker on the control panel.
Is there a simple way to protect the motor drivers and motors in the event of a stall?

Michael

 

One often used way is with a op amp monitoring voltage (current) across a very low resistance that is in the ground connection of the motor driver devices, if current is exceeded it throttles back the drive.
Max.

 

Ok, I'll have to look into that. Would a fast acting fuse work as a simple method of protection?
Thanks

Michael

 

A fast acting fuse would be a bit of a nuisance issue IMO.
Max.

 

Could a slow blow fuse be any better? I did also look into circuit breakers as an option too.

Michael

 

Would 20w be an appropriate wattage for the 10 Ohm resistor? Considering P=VxI, so in this case 12x1.2=14.4w

Michael

 

Yes, it will usually only be engaged for a short period anyway.
Max.

 

Thanks Max

 

Something like this, and a vice, makes the job real easy:

 

Thanks Joey, unfortunately I've just got a basic DMM.

Max, I had to go with your original suggestion and order a 10W 10 Ohm resistor as I couldn't get hold of 20w one.
When I perform the test do I need to take the readings quickly so the motor doesn't get damaged while it's stalled? or is the current so low that there's no real risk

Thanks

Michael

 

With a 10Ω resistor I would not expect any problem with that motor, but normally it does not take that long.
Just turn the motor shaft until the highest reading seen across the resistor.
Max.

 

Ok thanks, so just a matter of incrementally turning and clamping the motor shaft until I find the highest reading

Michael

 

Yes, then while in this position, confirm the supply voltage.
Max.

 

Ok, and that's just reading the voltage at the 12v battery terminals

 

Yes.
Max.

 

Thanks Max. So I have performed the test with a 12v SLA battery and the 10 Ohm resistor. The motor shaft actually wouldn't turn under no load in certain positions on the commutator, but would spin slowly in other positions. Is that still OK for the test?

The highest voltage measured across the resistor with the shaft locked was 12.26
With power still going to the circuit and the motor shaft still locked, I measured the 12v battery voltage in this position as 12.59. With the battery disconnected it read 12.71. I did notice the battery voltage reading would slowly drop and not remain on an exact measurement while the circuit was live.

Does all this sound ok?

Michael

 

Pretty much jives with the ohm reading of 1.2Ω.
Should be safe to go with that figure.
Max.

 

Thanks Max, could you explain how I work out the stall current of the motor with these readings?

Michael

 

Voltage across 10Ω resistor = 12.26 = 12.26/10 = 1.226amps.
For a Supply voltage 12.59v and a current of 1.226 amps = total resistance of 12.59/1.226 = 10.269ΩΩ.
Subtract 10Ω res which leaves the armature at .269Ω.
The stall current with a uncontrolled drive supplying 12v = 12/.269 = 44AMPs!
Which if allowed to remain would either collapse the supply or burn out the motor.!!
Max.

 

Thanks a lot for all your help and patience Max, it's been invaluable.
Blimey, 44 Amps! That is more than I was hoping for. Does that mean the 43A BTS7960 motor driver would be out of the question? It says it has current limitation and over temperature protection, if that makes a difference. As you can probably tell, I'm looking for an 'off the shelf' driver I could use.

Michael