Protective elements for powering dc motors

Thread Starter

Capitankl

Joined Mar 8, 2021
3
Hi everyone!

I am building a robot that carries a drill to make holes in the ground. I have not been able to find similar size/power references regarding how to power the motors. I would like to know how to protect the battery and motor drivers, my specific questions are:
- Where to place fuses.
- If a voltage control is needed or the batteries/motor driver can absorb the voltages produced by

I am running 7 motors all of them 24V all of them brushed DC motors except for the last one
4 for the wheels, 36 amp max
1 for drill, 19.2 amp max
1 that moves the drill up and down, 2.1 A
1 that moves the drill to the sides, 1.5 A -stepper motor

I will use 3 or 4 motor drivers which have overcurrent protection, and over and under-voltage protection.

The battery is a 24V 100Ah Li-ion.

So I have a Battery, Motor Drivers, and Motors. Where the fuses should go?
The actual size of the fuses I will decide when I will be able to test how much current the motors are drawing in the operation or in max load situations.
Is there another element I miss? I heard shunt regulators are used but maybe the motor drivers already can handle them. Is it ok to just connect all motor drivers to the battery directly or maybe restrict max current with a fuse? I am pretty lost if they can share overload for example.. most projects I have found are with only a single motor driver.

Thanks a lot in advance,
Carlos


Electric Configuration.png
 

LowQCab

Joined Nov 6, 2012
1,110
Fuses are fine for Fire protection, but the Motors should be individually protected by
"Electronic-Fuses" and PWM-Current Limiting Circuitry.

If the Electronic-Fuses or other Current-Limiting measures should fail, or,
something causes a direct-short across any heavy Battery Wiring,
The Fuses will prevent a complete melt-down-FIRE.

The Motors need Temperature-Protection,
not so much Peak-Current-Protection.
The Motors can withstand "Short-Term" "Locked-Rotor" Current-Draw for very short times,
and this could actually be required under certain circumstances.

As an example .........
Let's say that the Machine is drilling a hole, and runs into a rock, which locks-up the Drill-Motor,
this will cause the Current to spike-up very high.
This could easily pop a Fuse,
but what needs to happen, is that when the Current Spikes-up over a set-level,
for more than, let's say, 0.5 seconds, the Motor should immediately reverse direction for
~2-seconds, then revert back to forward.
This can repeat until the Motor reaches a specified Temperature, at which point,
all Power to the Motor will be stopped until the Motor cools-down.

What I am getting at is that,
the Motors should not necessarily be protected by an absolute, "One-Time" Fuse,
but should be protected instead by a Temperature-Sensor / PWM-Current-Limiting scheme.

Fuses will simply cause the mission to FAIL miserably, possibly leaving the device stuck in place.

Here are some very nice Power-Distribution/Fuse-Blocks ..........
https://www.delcity.net/store/MEGA-Fuse-Holders/p_795652.h_795653
https://www.waytekwire.com/products/1612/Fuse-Blocks/&Current-Rating=300A
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. 300A 5 Position LMG Mega Fuse Holder 46030 WayTek $38.00 .png
 

Thread Starter

Capitankl

Joined Mar 8, 2021
3
Solution for now:
1. Having a main fuse to avoid any dangerous short circuit and fire. The fuse will be sized according to the maximum consumption process (In my case when the robot is moving uphill, only using the 4 wheel motors).
As this is a prototype version, we will have time to supervise its operation so we can change the fuse if it fails.
I will solve this soon by getting an automatic fuse. Ideally, this automatic fuse pops only if there is a short circuit and not in a jam or in a problem because of the operation. Thanks @LowQCab for the guidelines.

2. Fuses after splitter:
- The wheels could draw 36x4=144A, the wheel drivers 30x2x2=120A, and the battery can output max 100A. So maybe a fuse is not necessary but a mechanism to recognize and respond quickly if a wheel gets stuck. Can the battery deliver more than the specified current as "max output current"? So here I am assuming that the answer for that is no.
- Microstep driver will be fused accordingly let's say 5 Amp(?)
- Motor driver 2x20 will be also fused. I will share the values after measuring in the operation. Thanks @MaxHeadRoom
Fuse blocks seem a bit expensive for our prototype I will just separate those components.

3. Jam/Overcurrent sensors: I will add hall sensors and amp meters to check if motors are stuck or overdrawing current. We have also end-of-run sensors (but they wouldn't know instantly if something went wrong). So this will be followed by routines to retry or abort operations in case of finding big rocks either in the path of the robot or when it digs.

Thank you all for your help!!
 

LowQCab

Joined Nov 6, 2012
1,110
That Li-Ion Battery can put out, ( guessing ),
around ~6 to ~10 Times its Amp-Hour Rating without even blinking an eye.
Yes, that's ~1000-Amps with a Direct-Short.
Be careful.
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Thread Starter

Capitankl

Joined Mar 8, 2021
3
That Li-Ion Battery can put out, ( guessing ),
around ~6 to ~10 Times its Amp-Hour Rating without even blinking an eye.
Yes, that's ~1000-Amps with a Direct-Short.
Be careful.
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Yes, Li-Ion. I got those 100A at the info stated in the datasheet. Still, I dont know if the battery can output more than that and I could have those 1000A lightnings. I didn't find in the datasheet something like "maximum discharge rate".

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Last edited:

LowQCab

Joined Nov 6, 2012
1,110
"Peak-Discharge-Current" is not an absolute maximum rating,
this is the Maximum-Peak-Current that the Battery is designed to deal with on a routine basis,
with little, or no, deleterious effects to the Life-Expectancy of Battery.
This rating is normally based on the amount of Internal-HEAT generated
at that stated "Peak" Discharge-Rate.
Notice that it gives a Time-Limit of ~2-Seconds,
that's because HEAT is being generated faster than it can be dissipated.

You can bet that the Peak "Short-Circuit" Current is over ~1000 Amps.
This kind of Current can turn a 2-Gauge Battery-Cable into
an impromptu Light-Bulb-Filament in a matter of Seconds.
This is not a joke.
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