Society of Robots - Robot Forum
Electronics => Electronics => Topic started by: stopgo on April 23, 2008, 04:19:37 PM
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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.
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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.
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which battery are you using , because 85 amps stall current is incredibly high
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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.
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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
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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!
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Motor specs:
Stall Current : 85A
Battery specs:
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.