alec said:
But isn't that Z the impedance at 1kHz?
It's the frequency I picked. We don't know the commutation frequency. I could figure that out with a good motor and a scope. Could probably find the RPM, too just looking at the ripple current and knowing the geometry of the armature. Both I don't have. Using a frequency close to the commutation frequency would be better. I didn't look at the frequency dependence of the measurements. I only have a few frequencies available. Maybe 1 Khz was too high.
An inductor datasheet (later) says measurements were made at 1 Hz.
alec said:
I can see that enabled the inductance to be calculated, hence useful to assess inrush current rise, but not sure how it helps to determine steady-state current (which obviously is important for prolonged running).
There is some guessing going on here. Z really determines the steady state current, but we are not energizing this thing with a sine wave, so I do think there are more parameters to worry about.
A shorted driver and a power source capable of delivering high currents will take out the winding.
alec said:
How does the back-emf from the motor when spinning get calculated/estimated?
I know there are ways of measuring it and that for a DC motor V = Vm - IR. The IR drop there is the drop from the DC resistance of the windings. We don't want the Back EMF getting to the power supply. It takes out power supplies. Don't ask me how I know. So diode have to prevent it from entering the supply and make the back EMF jump over the regulator. This is missing in the OEM controller.
When the FET lets go of the coil there will be a spike. Can this take out the commutating FET? Probably. I'm not sure we can solve the who came first, the chicken or the egg UNLESS we know one side is bulletproof.
The best we can do is limit the current to 3A, prevent Back EMF from getting to your driver and placing transient protection on the motor output and reevaluating operating conditions is the only thing we can do from the driver end.
Say the commutation FET failed from a spike: with a fuse there would be no stator damage. Temperature right now could affect the integrity of the commutation FET or the winding temperature. If that FET could shut down from overtemperature, you could isolate the FET failing from a spike vs overtemperature and the winding failing from overheating.
alec said:
The more I look into these 2-phase BLDC motors/controllers the more puzzled I get. For example, I can see that rotor lock is likely if the rotor magnet poles happen to be aligned with the field winding poles at start-up, but I don't see how the lock can be overcome without human intervention (i.e. give the motor a shove).
No matter how hard you try, nothing is identical. There will be some difference and therefore some preferential way to rotate. Now as friction increases from wear or something gets caught into the impeller we might end up with a "rotor lock" condition. Orientation (additude) will be in our favor too. Rotor lock comes from a failure of the commutating FETS too.
Attempting to "restart" energize, wait, energize might free it.
And with a symmetrical winding/magnet arrangement how is the correct direction of rotation ensured at start-up? Is a trial and error approach used?
Addresssed above. Symmetrical, but not identical. Angle the pump slightly and I think all bets are off. e.g. it's less likely that rotor lock would occur on start-up assuming a good motor.
With 3-phase presumably rotor lock isn't a problem.
Bearing failure and driver failure. But there are probably no issues starting in the right direction.
I did read that with one of the hall effect switches, there is an undetermined state that occurs 10 uS after power is applied. This driver could possibly use that kind of switch. I'm thinking ahead here. I don't yet have data, nor have I presented any to back this up. Could a 10 uS glitch cause the motor to run backwards on startup? I don't know? I think it's too short.
I'm still not convinced as to what the issue or failure mechanism is because of possible issues with the commutation of the motor, stressing it (running it at 100% (24/7/365) and not running with a 3A current limit. I do believe that limiting the current to 3A would likely prevent winding damage, but not necessarily failure of the motor. I lost a post that I was going to post earlier. It's now divergent at this time divergent from the current focus.
Here is an inductor datasheet:
https://www.electro-tech-online.com/custompdfs/2012/06/AIRD03.pdf Look at the 3.3 uH data. Note the DC resistance is really small and that inductance changes with temperature. so, it might be possible to model the OEM motor with the 3.3 uH inductor, a 2 ohm resistor (correct wattage) and a two 15 uf caps in series
https://www.digikey.com/product-detail/en/EKZE630ELL150ME11D/565-1718-ND/756234 15||15 = 7.5 uf across the whole mess. Unfortunately, the wire gauge isn't right. I guess I'm not sure what the value is, but I'm just throwing it out as data.
We have to have to have a 3A current limit to mimic the OEM controller and I think the measurements indicate why it's needed. A lower voltage reduces temperature and spike generation amplitude and increases reliability.
A nice graph of commutating FET temp, winding temp, ambient temp (water) as a function of supply voltage with a 3A current limit would be really useful.
Again, I'm suspicious of the commutation circuitry. I don't have the case, armature or working motor which can add some more data.
1) If the rotor locks and the power supply is not restricted to 2.7-3A, there will be winding damage. 100% confidence.
2) Why did the rotor lock? We really can't say, but it could be thermal (commutation driver or winding) or electrical or mechanical. 100% confidence it's one of these.
3) Measurements confirm operating current and steady state current allowed in the windings.
95% confidence.
4) Measurements suggest high turn-on winding current. Current vs. Time on a good motor might reveal something. 100% confidence
5) Armature geometry combined with scope info could reveal RPM. 100% confidence
6) No temperature data and it might be difficult to obtain.
7) OEM controller reliability issues positively identified and easily fixed. It only really affects the MTBF and cost.
I do think it makes sense to separate the driver breadboard from the "controller". It may also make sense to separate the jacks from the driver. Another plug. For now, those 2A (likely) rated jacks are fine. When there is a fault, they won't be.
What is your take on using a protected low side switch for your driver. They don't appear to be that expensive, but the major issue is non SMT availability. I might have a 5A non SMT driver around. I'm not sure of the p/n.
What's your take on the suggested approach for a new commuting system? Do you undersatnd how the parts would go together? Basically the Fan driver would have all of the recommended parts except the transient protection on the internal FETS would be moved. A 5 V regulator would be added to supply the inverter. Pullups would be use on the open drains (total of 4). Now you have a logic signal to drive the protected switch. Parts count isn't excessive. Hooked up right, reverse polarity protection could be afforded too.
It's missing a few bells and whistles, but it's simple and much more reliable.