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Offline stopgoTopic starter

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Motor debugging
« on: April 23, 2008, 04:19:37 PM »
I have a brushed DC gearmotor (rated for up to 14.4V) such that if I hook up appropriate batteries for it, it runs for a bit and rapidly decelerates and stops.  If I watch the voltage across the battery / motor terminals with a meter while this occurs, it quickly drops from 14V (the original battery voltage) to the 1V to -1V area.  1V is not enough to drive the motor, so, given that, I'm not perplexed that the motor stops, but where is the extra energy going?

If I unplug for a while and then reconnect it repeats the same behavior.  When I unplug and watch the voltage across my batteries they seem to go back up to 14V at a roughly linear pace (maybe this is just because my digital voltmeter is averaging over time - I suspect that's the case).

Also, this is hooking the battery up to the motor directly, as opposed to interfacing the battery to the motor with a motor controller.  When I used the motor controller it seemed to work fine.  Any ideas what could be going on here?  Did I just fry my motor by flooding it with a whole lot of amps?  This seems unlikely, because the stall current is rated at 85A, and my batteries aren't even capable of putting that much out.

Update:  Actually, the motor doesn't work with the motor controller either.  It's possible I didn't let it run long enough earlier with the motor controller to see the problem occur.  The motor is also making an unusual, grinding-like noise.  It's a planetary gear-motor, and something is clearly going on inside there but the shaft is not turning.
« Last Edit: April 23, 2008, 04:36:26 PM by stopgo »

Offline ALZ

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Re: Motor debugging
« Reply #1 on: April 25, 2008, 01:28:18 AM »
Hi:

My guess is that your motor was taking more current than your battery could put out. That is why the -1 v. You are going to kill your battery if you keep that up.


I have a brushed DC gearmotor (rated for up to 14.4V) such that if I hook up appropriate batteries for it, it runs for a bit and rapidly decelerates and stops.  If I watch the voltage across the battery / motor terminals with a meter while this occurs, it quickly drops from 14V (the original battery voltage) to the 1V to -1V area.  1V is not enough to drive the motor, so, given that, I'm not perplexed that the motor stops, but where is the extra energy going?

If I unplug for a while and then reconnect it repeats the same behavior.  When I unplug and watch the voltage across my batteries they seem to go back up to 14V at a roughly linear pace (maybe this is just because my digital voltmeter is averaging over time - I suspect that's the case).

Also, this is hooking the battery up to the motor directly, as opposed to interfacing the battery to the motor with a motor controller.  When I used the motor controller it seemed to work fine.  Any ideas what could be going on here?  Did I just fry my motor by flooding it with a whole lot of amps?  This seems unlikely, because the stall current is rated at 85A, and my batteries aren't even capable of putting that much out.

Update:  Actually, the motor doesn't work with the motor controller either.  It's possible I didn't let it run long enough earlier with the motor controller to see the problem occur.  The motor is also making an unusual, grinding-like noise.  It's a planetary gear-motor, and something is clearly going on inside there but the shaft is not turning.

Offline airman00

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Re: Motor debugging
« Reply #2 on: April 25, 2008, 07:09:23 AM »
which battery are you using , because 85 amps stall  current is incredibly high
Check out the Roboduino, Arduino-compatible board!


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Offline stopgoTopic starter

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Re: Motor debugging
« Reply #3 on: April 25, 2008, 01:50:16 PM »
I'm using 12 of these guys, in series:

http://www.all-battery.com/index.asp?PageAction=VIEWPROD&ProdID=834

I think my batteries may simply have been low.  Is it possible for them to maintain a high voltage according to my voltmeter while no longer being capable of putting out much ampage?  I've recharged them and with very preliminary testing the problem is gone - I'll post back when I've done a bit more testing.

Offline airman00

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Re: Motor debugging
« Reply #4 on: April 25, 2008, 01:59:59 PM »
Quote
This seems unlikely, because the stall current is rated at 85A

Are you sure its 85 not 8.5A ? because 85 amp stall current motors powered by 14 aH batteries does not work. So when both motors are stalling the battery will last for like a minute!

The twelve 14ah  batteries do not add up to 12*14 ah  , rather only 14ah

also ,

your total battery voltage is  rated 14.4V. In reality , when fully charged, this will be more like 15-17 V . This wont kill your motor right away, but it will wear it down over time.

Please give us ALL the specs available for both your motors
Check out the Roboduino, Arduino-compatible board!


Link: http://curiousinventor.com/kits/roboduino

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Offline stopgoTopic starter

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Re: Motor debugging
« Reply #5 on: April 25, 2008, 06:38:34 PM »
I'm using these Banebots 256:1 gearmotors (http://banebots.com/pc/MP-42nnn-550/MP-42256-550)

Motor   : RS-550 (Pinion)
Operating v   : 6v - 14.4v
Nominal v   : 12v
No Load RPM   : 75
No Load A   : 1.2A
Stall Current   : 85A
Stall Torque   : 18061 oz-in   127537 mN-m
Kt   : 212.5 oz-in/A   1500 mN-m/A
Kv   : 6 rpm/v
RPM - Peak Eff   : 67
Torque - Peak Eff   : 2124.8 oz-in   15004 mN-m
Current - Peak Eff   : 10A

As you can see, there is a very wide spread of current that it eats, and I'm not planning to go anywhere near stall torque, so I think I should be OK.  I just did some more testing - I wired the motor terminals up to ground and a node partway through my freshly charged battery pack so that I was only feeding it about 11 V (to avoid damaging it with 17 V by using all the nodes) and it worked!  So the issue was clearly that my battery pack was mostly discharged... I'm just surprised that it was still reading as having a high voltage in that state. 

So I think my problem is solved.  Thanks guys!

Offline Admin

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Re: Motor debugging
« Reply #6 on: April 27, 2008, 01:31:25 PM »
Motor specs:
Quote
Stall Current   : 85A

Battery specs:
Quote
Highest Discharging rate for continuous running: 40Amp
In theory that should be enough, but I suspect that your batteries aren't fully charged and the spec is exagerated, so you're getting more like 20A . . .

If you get another 12 cells and put it in parallel, it will work. Or look into NiCad/Li batteries for higher current.

 


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