Need Power!

[lastRites]

I have six motors on my battle bot, each of which draw 7A when stalled and a wiper motor of 8A. I tried to power my bot using two pc smpss(450w) but it does not seem to be enough. Moreover the motors seem to run at a lower speed. So I am thinking of purchasing a motorcycle battery(14Ah). Would that be enough and is there any cheaper alternative?
Thanks

[lastRites]

I am participating in a competition today. I am running each side of the bot(3 motors) with a separate smps. Still if one motor gets stalled, the other 2 barely move. Moreover the stall torque is a fraction of what the motors really deliver.

[rbtying]

There's no way the stall torque is a fraction of what the motors can deliver - stall torque is, by definition, the maximum torque the motor can provide (or to be accurate, the torque to be applied to the motor shaft to keep it from turning). 

Either way, I don't see why you're regulating power into the motors - motors run just fine off of unregulated power, and you'll need speed control anyways (probably via PWM). 

My question is, of course, what powers your SMPSs?  If you're running that from a limited-current wall wart, that is probably your bottleneck.

[Soeren]

Hi,

I am participating in a competition today. I am running each side of the bot(3 motors) with a separate smps. Still if one motor gets stalled, the other 2 barely move. Moreover the stall torque is a fraction of what the motors really deliver.

Bet you a dime to a dollar that you triggered a protection circuit in the switcher. A stalled motor can look suspiciously close to a short circuit.

What powers the switchers?


You didn't write the average consumption of your motors, but a 14Ah battery can deliver 28A for something much less than half an hour (I'll guess ~15 minutes) as the apparent capacity falls with an increasing current draw.
It cannot deliver 14A for an hour though, as automotive batteries are specified based on a 20 hour discharge at C20 (1/20 of the nominal capacity), so it can give 700mA for 20 hours. Any higher current equals a lower apparent capacity.
If the competition is over in eg. 5..10 minutes you don't need to worry that much, but you'd need a fresh battery for each melee as it takes a long time to charge a lead-acid battery properly.

[lastRites]

There's no way the stall torque is a fraction of what the motors can deliver - stall torque is, by definition, the maximum torque the motor can provide (or to be accurate, the torque to be applied to the motor shaft to keep it from turning).

I meant that when I run three motors in parallel, it takes much less torque to keep a motor from turning than when I run a single motor from the source. I am using PC SMPSs.

[garrettg84]

http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=8583
11.1v 
5Ah
That battery by itself would handle your load as described, and has a bit of room for variation if ALL motors were stalled simultaneously. I use them in my RC gear.

[Conscripted]

Are your motors wired in series or in parallel?

[lastRites]

3 motors in parallel on each side(skid steer). BTW can I reconfigure those SMPSs(3 450W 24pin PC SMPS) in any way such that I get a 12V 60A source??

[rbtying]

You're running this thing off of the mains?  SMPSs have to be powered by something, after all.

And since you haven't told us what exactly the output of your current power supply is, we can't help you too much with tell you what it can and can't do.  Being that they are 450W sources, and 12V * 60A = 720W, I don't see how that would work. 

[Soeren]

Hi,

Time to back up and take a deep breath now.

I have six motors on my battle bot, each of which draw 7A when stalled

What do they each draw when run freely (i.e. with no load)?
And what do they each draw, on average, when loaded (i.e. running the bot)?
What are their nominal voltage?

and a wiper motor of 8A.

Is this when stalled?
Better tell us the currents of this motor as well - unloaded, at average load and at stall.
I assume it's a 12V motor?

I tried to power my bot using two pc smpss(450w) but it does not seem to be enough.

The 450W is the total of all outputs. Does the PSU's say how much current it can deliver at 12V (assuming that's what you use)?

Moreover the motors seem to run at a lower speed. So I am thinking of purchasing a motorcycle battery(14Ah). Would that be enough and is there any cheaper alternative?

First things first, so let's have the specs for the motors and we take it from there.

[lastRites]

200 rpm dc geared motors: 12V
Average:3A
Stall:7-8A
No load:.7A

Wiper motor: 12V
Stalled: 8A
No load: 1~2A

They(PSUs) can each supply about 10A at 12V.

[garrettg84]

http://www.powersupplydepot.com/Power_Supplies/Single_Output_Supplies/12V,_50A_SUPPLY_PARALLEL_OPERATION_18402_PS.asp

They have a 12v 50amp power supply on there, of course, you could always just use a car battery =)

[Soeren]

Hi,

200 rpm dc geared motors: 12V
Average:3A
Stall:7-8A
No load:.7A

Wiper motor: 12V
Stalled: 8A
No load: 1~2A

They(PSUs) can each supply about 10A at 12V.

Now we can get a proper look at it.
The average is thus 9A each side, but with only ~10% margin (on the PC-PSU) to negotiate obstacles, so, as I mentioned before, your PSU's probably have some fold back limiting or similar circuitry - if you measure the voltage over the motor terminals, you'll probably see it reduces to something very low, the second you overload even one of the motors.
For testing the motors, just run only one or two at a time when on wall power. Remove the connector to one of the terminals of the motor(s) not running and it won't be that much of a load to drag.

A battery won't have that sort of protection, but it's advisable to arrange some, just with a higher trigger point.
A circuit to pause eg. 10..50ms when changing direction of rotation will help reducing stall currents as well.

You'd probably need up to around 30A worst case when maneuvering around (assuming you make proper stall protection).

A 14Ah lead/acid can handle that, but you'll have to find the exact runtime to expect by test driving it in a similar fashion as you would in a fight.

[lastRites]

A battery won't have that sort of protection, but it's advisable to arrange some, just with a higher trigger point.
A circuit to pause eg. 10..50ms when changing direction of rotation will help reducing stall currents as well.

Thank you!!:)
How can I make those circuits??

[Soeren]

That depends on your motor drivers. Do you have schematics for them?

[lastRites]

Not using motor drivers:P I control the motors manually.

[Soeren]

Not using motor drivers:P I control the motors manually.

How?
Sitting on the robot during a battle?

And how do you use mains fed switch mode supplies in a competition?

Sounds a bit fishy to me.

[TrickyNekro]

I bet that he's using an ATX that gets power from a 12V source, like the ones for portable PC's and such...

Well, well... First of all, with that kind of consumption, you are gonna need some serious mosfets... there is no other way...
By manually, he must mean that he controls the power to the ATXs through relays with a RC remote...


But... That's not for us to sit down and guess... that's something you SHOULD tell us...

So... how's your electronics??? With that kinda power you need..... well... if you don't know many things... Go look for a mosfet based motor controller...

But seriously now... if you seriously want help... Don't just troll us around...

[Soeren]

Hi,

I bet that he's using an ATX that gets power from a 12V source, like the ones for portable PC's and such...

Powering 12V loads from a 12V battery driven switcher? That doesn't sound likely.

[lastRites]

Sorry. I have been a little busy.. Power source is off board. The bot is controlled by two DPDT(reversing) switches, which get power from three smpss which i took out from junked PCs. I just purchased the motors and built the whole thing out of junk. The bot(more of a car than a robot:P) is wired as I really do not possess the knowledge or the means to go RC. Here is link to a event which can give you some idea about the competition here:
http://www.edg.co.in/edge11/index.php?option=com_content&view=article&id=51&Itemid=55
These are the spares I have access to:
http://robokits.co.in/shop/
Thank you for your help:)

[Soeren]

Hi,

Sorry. I have been a little busy.. Power source is off board. The bot is controlled by two DPDT(reversing) switches, which get power from three smpss which i took out from junked PCs.

If the outputs from the switchers is very close (like within 0.1V) when loaded, you could parallel their outputs and feed all motors (through your switches) from the same supply.

Another solution could be a smaller battery on the vehicle, acting as a huge capacitor and buffer, so that you still use the SMPSUs but feed them power to the battery and the motors get their power off the combined battery plus cable.
Problem with this is that a 12V lead acid needs more than 12V to charge and I don't think you would know how to change the output voltage on the switchers.
If you just charge the battery well before each start, it would still get you over the points where you need more current than your switchers can handle.

If you get two automotive relays (mounted near the battery) and control them with your switches, you can feed the battery from your SMPSUs with a single power cable and you take the inductive load away from your switches, which should otherwise have a diode or an RC-circuit to keep them from burning out , get contact pitting or weld together. Automotive relays are made for such and can be found in 30A or 40A versions for relatively cheap. You'll need a couple of low current wires for your switches, running along with the power cable.

If you wanna keep the cable setup that you have now, a single battery and two large diodes (20..50A) could make it buffer both sides, without them interfering. It could be placed next to the switchers, but will be better mounted at the vehicle, as you probably have quite some loss in 10m of cable.

What's the gauge or cross section of the wires in the cable?