Motor reverses before actually starting, unreliable acceleration

apprisco

10 mW
Joined
Feb 4, 2021
Messages
31
Hi all! I'm currently extremely confused about my front motor on an e-bike. (Yes I'm running dual motors)
I'm running into an extremely weird issue where my front motor won't start unless I give it a small push.
The front motor when stopped makes a horrible groaning sound, like metal rattling, unless it is turned slightly: in which case it accelerates perfectly well.
Does this sound like a broken hall sensor? Why would it work after the initial push in that case?
Is there any easy way for me to repair said broken hall sensor? The last time I tried to open up one of these motors, it took me a literal week.
 
What controller do you have? This sounds like a broken hall sensor but that you have a sensored+sensorless controller. The initial push then lets the sensorless control find sync (it needs some rpm to get the position detection to work)

A broken hall is fixable :thumb:
 
It's a brushless DC motor, connected to a hall sin wave controller. Seems like the actual issue is just that the motor mount is loose and the wheels are SOOO unbalanced that it just vibrates like crazy. I still doubt that the front motor is okay, since I hooked it up to the rear controller and observed same behavior. The rear motor works perfectly fine.
 
apprisco said:
I'm running into an extremely weird issue where my front motor won't start unless I give it a small push.
The front motor when stopped makes a horrible groaning sound, like metal rattling, unless it is turned slightly: in which case it accelerates perfectly well.
Does this sound like a broken hall sensor? Why would it work after the initial push in that case?
Yes, that's probably a broken hall, since it also behaves the same way on the other controller (but it could be a wiring fault, or connection problem).

It works because some controllers fallback to sensorless operation when the halls aren't working, but they often dont' start well on their own vs sensored operation. Some controllers (like at least one generation of Grinfineon) have a fallback to two-sensor operation, but they may not start from a stop very well without all three. In both cases, the fallback to sensorless may also fallback to trapezoidal (noisy) phase drive, rather than sinewave (quiet), making the noise even worse at startup.


Is there any easy way for me to repair said broken hall sensor? The last time I tried to open up one of these motors, it took me a literal week.
If it's actually failed, you have to open the motor to replace it. :( (unless it is like the QSmotors QS205 I have here that has dual hall sensors; for that it will have two completely separate sets of hall wires, including power and ground, so at least five extra wires in the motor cable).

If it's a wiring fault, then if it's outside the motor you dont' have to open it to fix it.

The link by E-HP should help you test for which problem you actually have.
 
apprisco said:
the motor mount is loose
That's something you need to fix before it spins the motor in the forks, and destroys the fork, the motor cabling, (possibly including the motor halls) and the controller (by shorting the phase/hall wires and pushing battery voltage into parts not designed for that, and shorting across the FETs in the phase outputs).
 
Yeah, I definitely do lol. I just tested the voltages myself while rotating the motor( didn't open it up) and everything seemed normal, so I'm pretty certain it's just the motor mount. both motors are screaming unbalanced, so I need to fix them somehow
 
The motor mount wouldn't cause the problem of not starting until it is rotated by something else.

That is caused by:
-- a problem with either the sensors (or their connection to the controller)
-- a problem with the phase connections causing one of them to not get sufficient (or any) current
-- an incorrect phase/hall wiring combination

Any of those could *also* make the noise you describe as the controller tries to force the motor into rotation.

Additionally, with those, the noise would probably not happen if it's already rotating when started up.

But if it were a motor mount problem, the noise would probably happen anytime the motor was put under load.
 
apprisco said:
both motors are screaming unbalanced, so I need to fix them somehow

What does "screaming unbalanced" mean, exactly?
 
Vibrating as in the motor itself is "shaking"?

Can you see it doing this, or only hear it?

Can you upload an audio file of the sound, or a video of the motion, to an easily accessible site like Youtube?

Under what conditions does it happen, at what speed?

Being as precise in your descriptions as possible, with as much detail as possible, will help us help you more quickly. ;)


Also note that a rotationally "unbalanced" wheel, spun very rapidly (the rpm needed for 10-20mph or more), will shake the entire bike, if it's upside down with the wheels in the air. The larger the diameter, and the farther out the unbalancing, the worse the problem. However, unless it is an out-of-round issue, it doesn't affect the ride itself unless you're going a fair bit faster than typical bicycle speeds.
 
Yes, it's definitely unbalanced. The entire bike will shake, but I can balance it hopefully. There aren't any magnets loose, I don't need help with that, I was just concerned that my hall sensor was dead.
 
I wouldn't expect there to be magnets loose; that wouldn't cause the problem you see, it would either jam the motor or cause a clacking or grinding when tthe motor is under high load.

Since you don't state the other parameters (speed, etc) as requested it's only possible to guess what might cause the unbalance.

The most likely cause is your tires and tubes; they're often not made symmetrically all the way around, especially the tubes where the valve stem is (the rubber is often thicker around it plus the weight of the stem as well).

Slime/etc in a tube causes this as well.

Next is any reflectors on the wheel, etc.

Next is the wheel lacing, where the rim is offset on the spokes, not correctly trued.

The motors are not likely to be unbalanced enough to cause the problem described; they are too far inside the circumference of the wheel. The only issue that does cause such an unbalance like this is offset on the bearing hole in the side cover, and/or the way the cover mounts to the rotor ring.

If the motor has been disassembled at any point, it needs but reassembled with teh cover in the exact rotation position it was before, relative to the rotor itself. If it isn't, and the motor was built and balanced with it in a different position, it may be offset and cause problems.

Note that when actually riding at normal bicycle speeds, none of these things other than an out-of-true wheel are really a problem; you won't feel or hear them, except for a motor cover misaligned far enough to allow the rotor to scrape the stator, which is very rare, and obvious when hand-rotating the wheel even very slowly.

When testing motorized wheels offground at high speeds, you often see the whole bike shake from them...but as previously noted it doesn't affect the ride itself unless you're (well) above typical bicycle speeds.
 
Have you ruled out mechanical causes, like a failed or failing bearing? The "slight push and nasty metallic noises" description could easily describe any number of bearing failures I've seen.
 
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