BLDC motor driver failing when powered up

Question

We are building a bar telepresence robot using two BLDC motors (scavenged from a hoverboard), two 500 W motor drivers like this one, and an Arduino that processes input from a RC receiver to deliver the speed and direction inputs to the motor controllers.

The motor driver side is powered by a 36 V Li-po battery (from the original hoverboard), the Arduino is powered by a USB battery pack. The ground of the Arduino is connected to the 5 V ground of both the motor drivers.

Wide view:

Zoomed:

Left-hand driver:

Right-hand driver:

The left-hand motor and driver have worked perfectly since the start. The problem is that, so far, three right-hand drivers have failed and I have no idea why.

The damaged motor drivers:

1. Failed when not under load. They died when the 36 V battery was first connected. One was working perfectly for a while, but failed when the battery was connected again later.
2. Smoked a little, but I'm not sure from which component (and I can't see any obvious damage).
3. Two of the drivers failed in a way that shorted the 36 V power input (I'm glad they didn't cause a fire).
4. The third driver failed in a way where the inputs on the 5 V side had a greatly reduced impedance.

There are some differences between the working driver and the failing one:

1. The working driver is much older (it was bought about 3 years ago when we first looked at this pre-pandemic).
2. The failing drivers have come from different sources on Amazon, but they all look very similar.
3. The failing driver has longer leads running to the battery, and the wire used is thinner than the working driver.

Does anyone have any clues as to what might be the problem?

Finally: I am a software engineer and have very limited electronic knowledge, so please assume I am an idiot!

Answer 1

This answer

• is useful in that it addresses a common problem with components from unknown sources - and in this case the LM317 specially. Other users with similar problems may find this useful.

• addresses a highly likely problem. The real problem cannot be identified with certainty, but this is a worthwhile possibility to address and is a sensible firts step in the trouble shooting process.

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As jens has noted, the large 3 terminal device at middle right of the board is a LM317 voltage regulator. Products from a reputable manufacturer are typically rated at 40V input to output voltage differential. This usually means that they will survive about 40V + Vout
(ignore: as the regulator effectively "floats" with it's output at Vout so does not "see" the total Vin across it.)

SOME LM317's from less reputable sources have less than 40V ratings. How much less varies. With a 10S LiIon battery nominal Vout is 36V but fully charged Vout is 42V. If the regulator supplies 12V out (as it appears to) then minimum required rated voltage is 42 - 12 - 1.25* ~= 29V. [*The 1.25V is due to technical aspects of the regulator design].

A regulator from unknown suppliers MAY not meet this requirement.
This is a known real world problem with LM317 regulators.

Regrettably, once a board has failed, if this is the problem the board will probably have been damaged in other areas as well.

Solution: IF this is the problem, replacing the LM317 with one from a reputable source before powering the board will solve it.

Whether the LM317 is adequately rated can be checked by removing it from the board and operating it in a test circuit. This is relatively easy for an experienced user but not very worth trying if inexperienced - replacing the LM317 untested is easier.

In the UK there are a number of major suppliers who are liable to sock the LM317. Maximum certainty is achieved by using an LM317...HV part 0 rated at 60V, but a usual LM317 rated at 40V should suffice.

I do not know if Mouser UK do in fact deliver from a UK warehouse but they have many LM317 variants here

RS are not cheap and you need to but 10 - but they are usually very prompt and usually carry stock - see [here](https://uk.rs-online.com/web/p/voltage-regulators/7140792

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ALSO:

Check that the onboard connections on the new boards are identical to the old board. Stranger things have happened.

Note well the comment on the driver page that you referenced:

They are saying that the hall switch connections may vary and could cause misoperation or driver destruction.

• Note: Due to different motor manufacturers to make the hall of hall line order does not necessarily follow the line on the board to match the sequence, need to be adjusted, please parents who pay attention! If the line sequence wrong, does not boot properly, current, or start other phenomena not open, forced to use the drive plate will burn!!!

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Answer 2

So I think (but I don't have proof), that the different LM317s are the problem. We definitely have two different LM317M variants, the working board is different to all the failing boards. I did also notice that the LM317s were very hot when the board was working.

A friend who specialises in high-power DC motor drives says:

The driver o/p transistors are coming on before the control electronics is powered up correctly. The power transistors will be a push pull arrangement. If both top & bottom come on together, you have a dead short across the battery. Then smoke or fire.

This fits the symptoms:

1. LM317M fails with the higher voltage which means no power to the JY01 motor driver
2. No control electronics means the power transistors are shorted
3. Power transistors rapidly overheat and fail as well
4. Dead board

We have switched to using a 12V power supply (we don't really need 36V power anyway), so far the drivers are fine at this voltage.