Mac/BMC type motor drive cutting out.

Justin.Clements

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Apr 1, 2015
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Hi, I'm having a problem with my front wheel mac motor (geared hub motor similar to BMC). Firstly I believe the clutch is stuck, it sounds like the motor is turning when I'm freewheeling with the throttle off. I replaced the gears on it fairly recently and used a black coloured multi purpose lithium based grease, I've read somewhere that you shouldn't use a moly grease but I don't think this is. Secondly the drive is sometimes cutting out, power seems to reduce and I get a rough sound from the motor (possibly spinning??) then it cuts out after a few seconds. If I keep trying it eventually runs as normal. This usually happens once every 15 miles, any ideas? I'm thinking it's probably related to the clutch but not sure what's happening. I've pulled the motor out and it all looks ok, the gears are all intact but if I hold the outside edge of the clutch I can't turn it around the spindle in either direction. In truth I'm not sure if I ever could, I don't remember it turning like that but looking at a video about BMC clutches online it looks like I should be able to and it would make sense if I could. The other possible is that there were some water droplets inside the motor as well.
I've found a similar post on endless sphere but it was never concluded so I don't know what the OP discovered.
Any ideas?
 

Nealh

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Aug 7, 2014
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Likely you may have damaged the clutch nylon gears when locating /interlocking with the sun gear if not done correctly. Also did you make sure that there is a small 1mm gap between clutch and armature, a too large a gap may mean the clutch rubs against the hub inner face of the casing.
 

vfr400

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Jun 12, 2011
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If you turn the motor wheel in each direction by hand, you should be able to feel whether the clutch disengages.
 

Justin.Clements

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Apr 1, 2015
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Likely you may have damaged the clutch nylon gears when locating /interlocking with the sun gear if not done correctly. Also did you make sure that there is a small 1mm gap between clutch and armature, a too large a gap may mean the clutch rubs against the hub inner face of the casing.
Hi Nealh, thanks for replying. The old nylon clutch gears were replaced with composite a while back, I have destroyed 2 sets of nylon gears over the past 6 years but the composite gears look ok, no teeth stripped. I don't remember there being a problem interlocking the gears but as I've decided to order a new clutch and new gears it would be good to know about the correct and incorrect way of doing this.
 

Justin.Clements

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Apr 1, 2015
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If you turn the motor wheel in each direction by hand, you should be able to feel whether the clutch disengages.
Thanks vfr400, it feels the same in both directions, the motor feels engaged and turning so yes I think the bearing is stuck. I've ordered a new clutch so hopefully that will fix the problem.
 

vfr400

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Jun 12, 2011
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When your motor runs rough and loses power, nine times out of 10 the cause is electrical. It happens when the controller goes out of sync with the motor. Every time it's happened to one of my bikes, it only did it when using high power. It feels exactly the same as slipping or jamming gears. There are lots of potential causes. Weak battery, bad motor connection, damaged motor wire, software letting in the controller, controller incompatibility, bad MOSFETs in the controller, bad capacitor/s in the controller, etc. You have to look at how and when the symptoms started to get to the cause.
 
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Nealh

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Nealh

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Aug 7, 2014
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As vfr has said it depends on the noise type, maybe a short vid of said noise with motor operating though don't use full throttle power as you may incur more damage.
Grinding/rubbing or rougher bearings you should be able to feel in motor rotation with out power, if the noise only manifests with power then the likely cause is electrical as stated and usually resonating electrically from the motor via the controller or phase connection.
 

Justin.Clements

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Apr 1, 2015
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I'm sure you're both right about the problem being electrical, but I'll replace the clutch anyway as it's definitely not disengaging. When I freewheel downhill the motor makes the same whine as when I'm riding normally and the resistance is the same whichever way I turn the wheel.
Finding the electrical problem is going to be harder, as I've been focusing on the clutch I've not actually connected the tester to it yet. Hope it's not a hall sensor, last time I replaced them I put so much epoxy in there it's not going to be easy to get them out! Pretty sure I've got some more sensors and even a new circuit board somewhere. I'll try and video it next time with my phone, as long as it's not when I'm rushing to get to work. It happened today on both legs of my journey. It's odd that if I keep trying to get it to run by turning the throttle on and off that after maybe the 20th time it runs without cutting out and will then go on for another 7 or 8 miles to get me home or to work without a problem.
Thanks, I'll post any developments.
 

vfr400

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Sounds like a hall sensor problem, but could still be the MOSFETS. They're easy to test, so do that first. With the controller disconnected, measure the resistance between each of its battery wires and each phase wire to get six readings. Each group of three should be the same as each other and in the range 7k to 14K.

MOSFETs can half work, but hall sensors either work or they don't. As your problem is intermittent, it can be a bad connection to a hall sensor or damaged cable, but not the hall sensor itself.
 
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Justin.Clements

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Apr 1, 2015
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MOSFETs can half work, but hall sensors either work or they don't. As your problem is intermittent, it can be a bad connection to a hall sensor or damaged cable, but not the hall sensor itself.
That's very useful info, thanks. I didn't know mosfets could partially work. I suspect a damaged hall sensor cable or loose connection is more likely but I might just try using a spare controller first as I have one, to see if it's a mosfet issue.