DC Motor Tripping Circuit Breaker

[Dumpsterholic]

Hi all.

My latest Dumpster score, a Bowflex treadmill, yielded me a bunch of goodies. Since I had plenty of time and all the right tools, I believe I managed to salvage the entire circuit, which encompasses a DC motor, an AC-powered linear actuator, a motor controller and all the various switches, safety features and blinky lights. Leaving all this stuff hooked up, I wasn't able to get the unit working, probably because of the speed sensor and another idiot-protection circuit which shuts off the motor when the user's hands aren't on the metal hand grips. The controller board is house-labeled, but is probably an MC-2100, about which there is tons of hookup info available online. When I have some free time I may try to get that working.

So I tried connecting the motor to my test jig, which is nothing more than a variac and a 50A bridge rectifier. With smaller motors, I have been able to use this jig to test for functionality, but the new motor is giving me some trouble. Per the label, this beast is rated at 90VDC @20A and supposedly can put out 3HP. When I hooked it up and gradually turned up the voltage, the motor never turned or made any funny noises or sparks. Instead, it tripped the GFCI, turning off the power in my garage. I know this is true, because I reset the switch inside the breaker panel and tried testing the motor again and got the same result.

So my question is this: Big motor circuits often have either an inductor or a capacitor, and sometimes both. My understanding is that either of these passive devices allows an electrical charge to build up slowly, eventually delivering the massive surge of current the motor needs to start turning from a full stop. Motors I've salvaged in the past often had an 0.1uF capacitor rated at 600V or so; this seems to be a standard value. If I get such a capacitor, how should I go about hooking it up? Do I need an inductor to protect the rectifier from back EMF when the motor powers down, and if so, how do I go about choosing its value? If I had a value in mind of so many henries (or microhenries), I could probably figure out how many turns of some particular gauge of wire to wind on a bobbin to make my own "choke", or whatever it's called.... Or am I on the wrong track entirely?

For those of you who are concerned about safety, not to worry: My test jig plugs into a fused power strip on a long extension cord, and I generally wear rubber-soled shoes and hide behind a heavy desk when I first turn it on. And once it's energized, I'm very careful not to touch anything metallic that might deliver a jolt. Thanks for reading.

[Bratster]

Are you sure it is tripping the GFCI breaker due to a ground fault or an overcurrent fault?

Before more in-depth troubleshooting can go on and adding parts etc you would need to identify what exactly is the breaker trip caused by.

Ground fault or overcurrent.






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[jh15]

some gfci's and a lot of arc-fault breakers nuisance trip with transients.

try plugging into the garage door opener plug or elsewhere in house. Seems you now safety enough, so I won't drag on.

[BrianHG]

The motor should be able to hand turn easily to any speed with the power lines open, there should be very little or no denten as these motors were designed to run smoothly.  Short the 2 power lines and turning it should be really difficult.

If these tests fail, check the brushes.  You should be able to remove them as they are serviceable on these large DC motors.  Is there a burned or damaged commutator.
Otherwise, if the brushes and commutator looks fine, the coils has internally shorted.

[Dumpsterholic]

Thanks, Bratster. Guess I was so sure that the GFCI was at fault--I hate those things--that I just assumed I knew what the problem was, namely a big current spike. I believe most house circuits in the U.S. are only rated for 15 or 20 amps, and I figured that the motor was pulling too much current. Well, plugging the same rig into a different outlet will provide some clue; if the motor doesn't work then I'll have to rethink things. But first I need to verify that there is no ground fault. Hmmm.... more coffee is needed.

[Dumpsterholic]

BrianHG: Not sure I get what you're saying.

First of all, the treadmill was thrown away, so I don't know that any part of it still works. But let's just say I feel lucky.

Sounds like what you're suggesting is to hand-spin the motor with no power applied, then to short the disconnected + and - leads of the motor and try again to turn it by hand to see if I feel a mechanical drag? Or should I hook up a DMM to the leads, set it to detect AC, and then spin the motor, which if in working condition should act as an AC power source? Then, depending on whether or not I see a voltage generated by hand-spinning,  maybe start to suspect that the motor is burned up and probe the inside to look at the brushes?

I will try these tests and post my results. Part of my goal in posing these questions is to generate a dialogue which anybody can read and hopefully profit from without getting themselves hurt. If there is one treadmill in the Dumpster today, there is likely to be another one somewhere else on another day, and I know that lots of people are salvaging these big motors without necessarily realizing just how hazardous they can be. To me, something that draws 20 Amperes at 90 Volts is about as hazardous as a loaded gun, and I don't intend to be looking down the barrel when I pull the trigger, if you catch my drift...

Here is a photo of the inside of the motor with the cover removed:

[rstofer]

That small capacitor across the motor leads doesn't have anything to do with startup.  It is there to help suppress the EMI of the brushes.

AC motors use larger capacitors to get a phase shift for one of the two windings and this shifted phase winding pulls the armature ahead of the main winding.  But that's for AC motors.

Some DC motors with certain types of control will always trip a GFCI.  I have this exact situation on a trimming tool.  No other motor in my shop does this but, for that particular motor, it is a well know effect.  It's a PITA.

I can't imagine this situation on a treadmill.  It is far to common for it to have electrical problems.

[BrianHG]

My test, the no drag when hand spun with no connection of the motor's +&-, then, big drag if you try to short the motor's +/- together is normal expected function for a treadmill's main belt DC motor.  I've done a few projects with them.  Even though the motor is rated at 90v, you should be able to spin it up with 12v dc perfectly fine.  If you throw 90vdc into a still treadmill motor as such, it's initial powerup current from stand-still will be over 50 amps, even maybe 100 amps.  If you feed the motor AC at any voltage, forget it, it will be like a dead short.  You cannot use a simple light dimmer and DC rectifier to spin-up this motor slowly, they are not designed to regulate voltage that way, however, a real variac and a good 50 amp diode bridge rectifier could do the trick, slowly ramping up the voltage from 0v otherwise you will blow your diodes into a short, but, I would personally use a regulated DC lab supply at 12-20vdc, 5amp current limit to test the motor first.  Make sure everything is floating since the chasis of the motor may be wired to it's GND wire creating another type of ground fault short if you are working on a metal workbench which may have a connection to GND due to outlets and just physical contact.

Yes, if you put a DC voltmeter on the motor's +/- and hand spin it, it will generate DC voltage.

I'd say start with the 12v test after my first test.  Then if there is still a problem, we need to know what that black gunk on the motor winding coils in you photo is.  Is it burned coils, of, just an odd black paint/sealant which I haven't seen before.

[BrianHG]

This doesn't look too good, but, it might be some sort of potting compound.
If it is in fact charred melted insulation, that motor is garbage.

[Fludo]

Depending on your motor winding configuration, the motor could draw over 100A when you first turn it on.  The inrush current may wreaking havoc on your protection systems.  They need to be brought up slowly by increasing the voltage from zero, or by adding a start resistor in series with one lead.  This can be implemented with a regular relay, short the resistor after the motor reaches speed.

[Dumpsterholic]

Many thanks to all who responded!

I am happy to report that all is well with my motor. Below is a photo for reference. For the sake of completeness,  I wish to add a couple of details about my motor test jig. In addition to a variac which plugs into a fused power strip driving a 50A bridge rectifier, so that I can gradually ramp up the DC voltage to the motor, I mounted the rectifier on a heat sink and securely bolted the motor to a wooden table.

Per jh15's comment: The simple fix you suggested of plugging the AC cord into the garage-door opener outlet did the trick. This leads me to suspect that the GFCI was indeed the culprit.

Per BrianHG's thoughtful commentary: I did try shorting the motor leads and trying to turn the motor by hand, then opened the leads and spun it again. With the leads shorted, there did seem to be more drag. But because of the massive flywheel, this test wasn't as convincing as I might have liked. Luckily, I also took your suggestion of hooking up a DMM to the motor leads and checking for a DC voltage while spinning the motor shaft. The voltage was small, but definitely there.

To anybody new to electronics, please observe the safety precautions outlined above! High voltages can literally kill you, and you could just buy a motor like  this for a hundred bucks or so if you were in the market for a DC-powered lathe, drill press, belt/disc sander, etc.

[jh15]

At a Portland Maine makerfaire a couple years ago someone was making wind generators out of tossed treamill motors and I think, scrap metal or garbage can lids or something.

He had specs, and sharing more than trying to sell something.

When married, I absorbed a dreadmill and just tripped over it in the garage for years preferring long distance cycling, etc.  Recently I've had a high wattage age hit to the meatbag and don't think I'll be biking for awhile.

I came close to gutting it for wind experiments, but lubed it up and legs getting back in shape.

The Maker had a display of his junkbox wind generator and they really put out!

So grab free ones at end of yard sales for experimentation.

With this thread, I may experiment setting a couple up along with grid tied Chinese solar panels to charge my Tesla, although we never see a spike in our electric bill. Down in the noise versus our electric kitchen and plasma tv I guess.