I have set it up so that the batteries run to a blade fuse block, with the motor controller having a 30A fuse for it (other components like lights and Raspberry Pi having 20A).
The cable to the fuse block is running 6mm Twin Core Automotive cable, and then from the fuse block, running 5mm Twin Core Automotive cable to the Roboclaw. Cable lengths range from 1-2m long.
Between the motor controller and the motors themselves, I have been testing with a 20A and 30A fuse.
I am running x2 “24V 50:1 350W Motor/single side gearbox” motors (one per channel).
My issue is that I am blowing the fuse both that is between the battery and the motor controller, and also the fuse between the motor controller and the motor. I have not been able to identify what causes which one to blow. However, with using 30A fuses between the motor controller and the motors I have been seeing less blowing here, and the majority blow between the motor controller and battery.
The issue is only happening when I am trying to run the platform over a small lip. Apart from that, running on the concrete (flat) is completely fine.
Am I running a fuse too small between the battery and the motor controller (being 30A)? Should this be bigger, and if so, what is the safest to go between?
Also, should I be changing the ones between the motor controller and motors as well?
Firstly the available current will only be as good as the smallest battery. Is your small battery able to handle the 24V requirement. Probably not due to the following.
You will probably find that the fuses are not big enough especially if they are not “slow blow”.
And your small lawn mower battery is not up to it.
The fuse problem could be fixed by fitting small circuit breakers with the correct delay curve (not instantaneous) . The 30A one could be a bit marginal. To be accurate you need to see what the maximum current is required under worst case load conditions. Circuit breaker delay should take care of the inrush current.
If the small battery capacity is a problem the only way out here is a bigger one.
Cheers Bob
Thank you for that!
I have just heard back from our motor supplier and the stall current is 36.8A.
We are currently using fast blow fuses so that is definitely something we need to change.
We are also buying a second 12V 100Ah battery to fix the smaller battery issue.
What would you suggest for a slow blow fuse value?
We are seeing with the Roboclaw that when it was trying to get over the lip, it would be reaching 60A in the motion studio reading, and blowing the 30A fast flow fuse.
We were also suggested by the controller makers not to use a fuse between the motor controller and motors.
That is going to be the start up current also as the motor is initially at a standstill and the only resistance will be the actual winding.It is not there for long (maybe a few mSec) but it is still current that has to be supplied. There are ways to tame this but sometimes can be a bit messy. Known as “soft start”. Another way is to have controlled acceleration and deceleration in the driving signals. Your driver unit might take care of this. I don’t know. I think some of the more sophisticated control the start up and sudden speed increases. Once again I just don’t know. I have done it myself a long time ago at the PWM signal generation level. From memory I think arranged to get to full speed in about 1.5Sec or so.
Good idea. As I said the supply current in a series connected scenario is only as good as the smallest device.
Yes that would do it.
If your current at any one time gets to 60A for any appreciable period then that would be minimum. Even using a “Slo Blo” fuse will rupture given enough time. The “slow” part will only take care of inrush for a very short time. You would have to take a close look at the fuse or circuit breaker specs. For instance a colleague at one time had to test some circuit breakers and the particular spec stated that the device should operate at X seconds after 100% overload was applied. That meant that for a 10A breaker he had to input 20A and measure the time it took to operate.
I know it takes about 7mSec for a 3AG fuse to fail. In which time most of the semiconductor devices have failed anyway. We used to reckon that transistors and the like were good at protecting fuses.
Cheers Bob
Add on
One good thing to remember here is that a fuse or circuit breaker DOES NOT protect the equipment connected to it like amplifiers, motors etc.
A fuse DOES protect the supply and anything on the supply to it even the wiring.
You will often see the statement an equipment like Amplifier radio motor etc is protected by this fuse. Not true. Usually the reason for a fuse to fail is there is something wrong with said equipment. So is is broken in a sense anyway. The fuse rupture is to prevent any further damage to any SUPPLY to that equipment.
