) I have recently discovered that the DC drive motors I purchased a while back have a whopping stall current of 37 amps. Yikes!!!!
So I am asking for solutions as to how to limit that current. I really want to use these motors, initially, if I can. My first thought is, of course, an inline fuse. But I'd like something that is resetable if at all possible. The power supply I am using, for now, is a small 12 VDC lead-acid cell.
There may be no other way around it, but somehow the thought of just putting a resistor in line seems crude. Ideally, I would like something that only kicks in when the current being drawn exceeds a certain value, say 7 amps, and is fairly invisible to the circuit, otherwise. I just don't want to damage the H-bridges or anything else. Some visible over-current indicator such as an LED would be nice, too.
Depending on the battery of course, but it may not be able to dump 37 amps. The battery is the first limiter of current. If you really want to limit current to the motor, use 10 AAA heavy duty zinc batteries!
You could use a resistor, but Maxim and others make current limiting chips with foldback that do exactly what you need to do. Don't think they make one out of the box able to handle 37 amps, but you can use the design as a starting point in your solution.
You always need that anyway if you are using a battery large enough to melt your wires.
There are various options. Circuit breakers for example. There are also devices that reset themselves when the current is removed (resettable fuses).
That works, but it sounds like your limit (7A) is out of line with the circuit you are trying to limit (37A). I think it would be better to first just switch to a lower voltage battery so you don't need to consume as much power in the resistor to keep the current in line (unless you happen to be doing this on Mars and need a robot heater anyway :)).
It sounds like you are just not using a large enough H-bridge for the motor you are trying to drive at the voltage you are trying to drive it.
Here's a nice write up on the design of stepper motor current limiters that might give you some ideas:
formatting link
You basically need to put a small resistor (small ohm value, high wattage) in series with the motor supply and then measure the voltage across it. The voltage across it will be proportional to the current through it. When it gets too high, you have to either turn the motor off, or reduce the voltage supplied to it.
If you are controlling the H-bridge with a micro controller which has a free A/D input, you could use that to measure the voltage directly and make it reduce the duty cycle of the PWM based on the current. A device that can only take 5 amps continuous can take a lot more for very short periods because it has time to cool off between jolts. But, you might burn out a few H-bridges while debugging your software if you experiment with over-driving it.
If you don't have a free A/D, then you have to build some type of comparator circuit - the web page above gives some suggestions.
Thanks Gordon! Man, I must be in the forest 'cause I can't see dem trees.
I completely forgot about the whole source thing! Jeesh. Got too caught up in "the problem", I guess.
Anyway, the batteries I am currently using are 12V 5Ah AGM, "EverOn" brand. So I think that means no more than 5 amps possible, right ? (Unless I add more batteries, etc. )
The 5 AH rating has nothing to do with the max current possible. That shows how much current the battery can deliver over time before it will discharge (1 amp at 12 volts for 5 hours, or 500 ma at 12 V for 10 hours,
10 amps for 30 minutes, etc).
Doing some quick searching, I couldn't find specs for your battery, but it's likely your battery will have no problem delivering at least 50 amps based on similar sized sealed lead acid batteries I found specs on. If you put a direct short on it, it might deliver a couple hundred amps (until the wire explodes). You might have to switch to AAA alkaline batteries to get the current down to the levels you are looking for.
What motor are you using? Did you say? Do you have any specs on it?
As Curt sez elsewhere, no. Your battery is capable of instantaneously delivering more current. I'd definitely add a suitable fuse for catastrophic failure (i.e. a short circuit), and look into a switching circuit that can fold back the current if it exceeds a certain present level.
Thanks Curt and Gordon! Wow! Education never ends in this hobby ...
I am 99.99% sure this is the motor:
formatting link
I don't want to go down to AAA batteries for the run time, so I guess that means I need to invest more time into the power supply design, etc. For the electronics, I am using a "Carputer" type power supply ( picoPSU-120 ) that runs a mini-ITX type motherboard; but not for the motors. I *planned* on using one 12VDC lead-acid or gel-cell or something else for the motherboard and one for the motors. I'm not stuck on using these particular kind of batteries, but I do want some decent run time. So far, when just hooked up directly to the motors and left to run in a circle, one of these batteries has lasted 25 minutes and what seems like plenty more to go ! This is on a fairly heavy aluminum chasis. I wouldn't be too much more surprised to get 45 minutes continuous running off one of these powering both motors.
My preference would be to come up with a decent catch-all circuit such that I can protect the motor electronics decently enough to be able to swap out different types of batteries, such as nicad or li-ion, etc.
Yeah, you see the graph on that page at the bottom for the 12V motor that the current goes up steadily to 37 amps before it stalls as you put more load on it. Though the current draw when the motor is running at maximum efficiency is 6 amps, the motor will naturally draw all the way up to 37 amps for different loads.
If you click on the part number on that page, it pops up a cool interactive shockwave graph that tells you exactly how much current the motor draws and how fast it spins and how much power it consumes at different load levels (you can even adjust for different temperatures and operating voltages).
At max efficiency, it's only drawing 6 amps, spinning at 15K rpm, delivering a torque of 311 g-cm, and producing 48 W of power.
At max power, it's drawing 18.8 amps, spinning at 8K rpm, delivering a torque of 1165 g-cm, and producing 105 W of power.
With more more load (a bit before stall), it's producing 2194 g-cm of torque, running at 1K rpm, and drawing 34.7 amps of current.
If you want to use this motor in a bot and push it near it's limits, you need an H-bridge that deal with around 40 amps of current. If you spin the motor in one direction, and slam it into reverse to make it go the other way, it will stall, and then start spinning the other direction. At stall, before it starts to spin the other way, it will be drawing around 37 amps for an instant. If you don't have a big load on it, that instant may be so short as to be insignificant, but that's what will happen. If you put a large mass on it - like a bot running full speed in one direction when you slam it into reverse but have good enough grip the wheels don't spin, the "instant" it will take to stall and reverse directions will be a more substantial "instant".
Now, if the load is always small compared to what the motor can do (for example the wheels just slip long before the motor hits the max torque and max current), and the area under the current curve as it starts or reverses direction is small enough, then you might get by with simply using a 5 amp fuse. Fuses don't blow exactly at their current rating - they have to heat up and melt. So you can always run a lot more current through them if you keep the duration very short (how short depends on the fuse design).
It's just a matter of how hard you push the motor. The battery you have I think will have no problem driving it to it's limits. So if you put a heavy load on the motors, it will suck near 40 amps. If your h-bridge can't take that type of load, you can simply put a fuse at the limit it can take, and never drive the bot harder than that. But it might blow every time you start up or change directions if you do it too quickly.
Yeah, the easy solution is to use an H-bridge that can handle the current. But they cost a lot more than lower power H-bridge. ($200 for the open source one that can handle 160A). And you might have to watch burning out your motor. Smaller motors can be held stalled with no problem, larger ones burn out. I don't know what happens with that one. The spec sheet doesn't say anything about current or power limits so it might be able to be held stalled continuously.
You can use a fancy current limiting circuit - which will just limit the voltage when the draw gets to high - but it will limit it by burning up the extra power as heat - wasting battery energy as heat. Or you can limit it by keeping the PWM duty cycle low enough so the motor just never draws it's full potential. That is better because it doesn't burn up your battery energy as heat, but can be more complex. But, if you are writing your own software to drive the H-bridge, you can create the limits in software and tune it to whatever you need to keep your fuse from blowing.
Yeah, but you can also reduce to 6V for the motor and use a battery with twice the AH rating to get a similar run time. The battery size will be about the same either way. But, the higher voltage allows you to deliver more power to the motor for a given amp limit however. So the higher voltage will give you more speed and acceleration for a given amp limit.
To maximise run time, you want to try and run the motor near it's maximum efficiency as much as possible. That point is shown on the graphs for your motor (assuming the web page you gave is your motor). You can pick the gearing (or wheel size) for your application to trade off RPM and torque to allow you to operate the motor near it's max efficiency point and produce longer run times.
Use Digi-Key part number RHE750-ND, which costs 88 cents. This is a Tyco PolySwitch self-resetting fuse. When 7.5 amps are passing through it, it has a resistance of 0.01 ohm, which is its "untripped" state. Above 13A, the device heats up and the resistance goes up, limiting the current. When the load goes away, the device returns to normal.
We used many of these things on our DARPA Grand Challenge vehicle. They're cheap, small, and they work. Many current ratings are available.
PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.