Motor debugging

[stopgo]

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.

[ALZ]

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.

[airman00]

which battery are you using , because 85 amps stall  current is incredibly high

[stopgo]

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.

[airman00]

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

[stopgo]

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!

[Admin]

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.