BMC 50A motor cutting out while riding

[Mudder]

First time poster:

So I'm using a BMC 50 amp motor controller with a BMC V3 motor and it often cuts out while I'm riding. I don't understand why because the batteries are fully charged, I'm using 6 gauge wire, and nothing is getting hot. I'm not abusing it and I don't believe any wires are loose (I've put this system on two different bikes using different harnesses and have seen this issue on both.

Sometimes this happens when I first plug the bike in after it's cooled off for days; I plug it in, turn the throttle, and nothing happens. A quick unplug/replug solves the problem. Sometimes I'll be riding along no problem and it'll just instantly turn off (battery voltage is fully charged: 56 volts, jumper on controller is set open to 48 volts). If I unplug the power, wait 5 seconds and plug it back in, it works just fine again. I've even had it happen on the stand with a fresh charge after sitting for days. I placed a resistor bridge in place of the throttle to see if it was the problem and it still didn't move so I imagine the controller is getting stuck in some weird state. The only option I'm using is the soft start, so all other control cables are unplugged.

I've made sure the encoder and phase wires are plugged in. To fix it, all I have to do is unplug/replug it.

Check out my progess if you're interested: electrosanity.blogspot.com

 

[amberwolf]

Best guess is if your battery has a BMS it's cutting out due to overcurrent. Could be a design flaw or a defect, or simply insufficient battery for the bike.

You can fix the problem of not turning on after plugging battery into controller by adding a precharge resistor across the connectors (search thru some of the recent threads and there are at least two or three discussing this).

The other problem either requires you lower the current limit in your controller, or fix the BMS (if it's a defect or flaw) or change to (or add) a bigger battery (if it's simply insufficient).

 

[Mudder]

Best guess is if your battery has a BMS it's cutting out due to overcurrent. Could be a design flaw or a defect, or simply insufficient battery for the bike.

You can fix the problem of not turning on after plugging battery into controller by adding a precharge resistor across the connectors (search thru some of the recent threads and there are at least two or three discussing this).

The other problem either requires you lower the current limit in your controller, or fix the BMS (if it's a defect or flaw) or change to (or add) a bigger battery (if it's simply insufficient).

Thanks for the suggestions. Unfortunately, I don't have a BMS inline with the power delivery, only to monitor voltage. In other words, the BMS can't cut the current out.

I'm using 192 A123 cells configured 12 in parallel, 16 in series, so I don't think current delivery is the option .

I'll look into the precharge resistor.

When the unit turns off it doesn't matter how much power I'm pulling. It happens sometimes when I'm pulling 10 Amps or less.

Any other thoughts?

 

[Mudder]

Tonight I rode the bike around the block for about an hour trying to figure out what was causing the controller to crap out. Every mile or so it would die; after unplugging and replugging the controller it would work again until finally it died as soon as I accelerated. Now as soon as I pull the throttle at all, the controller pulls 1500 watts and then shuts off.

I took the wheel off of the bike and noticed it had resistancec when turning. I opened the wheel and it looks fine inside. I'm just stumped on this one, but I'm thinking the motor is the problem.

 

[amberwolf]

Resistance while disconnected from the controller? Possibly something is misaligned in there, and rubbing--does it feel more resistive at one point than another, in rotation?

 

[Mark_A_W]

It's got to be the controller.


There's nothing really in the motor to "go wrong". From an electrical point of view, just some coils and hall sensors. They kinda work or they don't. It's possible the halls are moving or something when it's hot, but unlikely.


What controller are you using?

 

[fechter]

Unfortunately, it sounds like the controller blew.

If you turn the wheel (backwards) and feel a lot of resistance, it indicates a short. If you disconnect the 3 phase wires from the controller and the resistance goes away, then the problem is in the controller. If not, then the short is in the motor.

Did you ever check to see if the controller was getting hot when it was cutting out?

With the controller disconnected, you can measure resistance from each of the 3 phase wires to each of the battery input wires (6 combinations). None of these measurements should look like a dead short (0 ohms). If any do, then one or more output FET is shorted. If there are shorts DO NOT try powering it up or even more things could blow.

If you suspect the motor, with the phase wires disconnected, turn backwards while shorting and un-shorting each combination of 2 phase wires (3 combinations). If there is one combination that does not increase the resistance, then those two are shorted somewhere. It is normal to have some resistance due to the gears and friction, cogging, etc. but it should not be like the brakes are on.

 

[mr.electric]

If you have another brushless motor controller lying around try connecting it to the motor. That could tell you definitively if the controller is bad.
I run a bmc 50 amp controller and v3 motor. I use a cycle analyst to reduce the controller current limit to 40 amps and still have enough power to stay with the flow of traffic. Once you get your motor going again I would recommend reducing your current limit down from 50 amps.

 

[Mudder]

Hey all, thanks for the help. I isolated the issue down to the motor. Specifically, the green and blue wires are shorted together inside the hub because the nylon braid is too short and the wires rubbed against a sharp bend in the axle right as it comes out inside the motor by the aluminum sun.

To diagnose this, I put a DMM set to AC voltage between each of the phases and measured no generated voltage when the motor was spinned backwards. This lead me to the fact that the two wires were shorted, so I opened the motor up and on close inspection (after removing the glue) I found that the blue and green wires had rubbed against the case enough to wear through the wire.

I pulled the wires out and plan on rewiring the unit. I'm thinking that since I already have to replace the wires I might as well perform one of the upgraded flat wiring schemes I've seen on this board. I'm also considering drilling out the other side of the axle with a drill press so that I can route wires both ways. Thoughts?

 

[Mark_A_W]

You'd need to drill down through the axle, then cross drill an exit.


The axle is hardened. It's possible, but it's not going to be fun.



Nice troubleshooting by the way. I'm impressed.

 

[sandymac]

I have had a similar problem with a BMC 48501008 controller and a 1000w BMC v4c motor.
It seems that if the voltage gets below 41 volts it cuts out.
When I start the batteries deliver 50.4v (2x6 cells).
I notice that there is a 36v setting on the controller using a jumper.
If I use this will I do damage?
I have been wondering if this would set a lower cut-out voltage.

 

[jateureka]

Controller 41V LVC is typical for a 48V nominal Lipo (13s) pack, but you are using only 12s.
At 36V (10s) the controller LVC is likely to be 31.5V.

 

[fechter]

I notice that there is a 36v setting on the controller using a jumper.
If I use this will I do damage?
I have been wondering if this would set a lower cut-out voltage.

I don't think it will damage the controller. I think all it does is lower the cutoff voltage.

You could damage the battery if you discharge below a safe level. If the battery has a BMS or monitoring circuit, that should prevent cell damage from overdischarge and you really don't need the one in the controller. Dropping from 51v to 41v seems like quite a bit of sag. My 16s A123 pack starts out around 52v and by the time it reaches 48v, it's completely drained.