Trying to repair tool that I broke - BLDC motor confusion

[No_Shorty]

Hi,

So I have a cheap 'airless paint sprayer' that I bought. It wasn't really working with the batteries I was using but I figured this might be a power draw issue. The tool claims to be 500w, and run from a 21v battery. My understanding was that this is usually a regular 5s cell tool battery but using the fully charged cell voltage as the battery voltage (as opposed to the nominal voltage, which we use in the UK). However it was being very pathetic and not pumping paint. I figured maybe the Chinese batteries were more comfortable with a 25+ amp drain than typical 'western' brands, sacrificing their service life for raw power.

So... wanting to test this theory I wired up a DCDC converter capable of 20A (just to see if it worked better) to the tool battery terminals. A quick test showed this was working, the tool fired up and sounded like it had more grunt. I turned off the buck (it has an LCD control panel, so I pressed 'off') but when putting some water in the tool the buck got shifted and contacted a screwdriver and sent off some sparks. I unplugged everything, set stuff up more securely and tested... no response from the tool although the buck looked like it was working. I happened to have another identical buck so tried that, but still nothing from the tool.

So I fried something.

I took it apart to have a look. It looks like a speed controller wired from the battery and then the motor wired up from that and some mechanics for the rest of the tool.

My electronics knowledge is limited, but I checked the motor terminals with a multimeter and they all showed a short. I don't know BLDC motors at all so had no idea if this was normal, but I detached the wires to the controller and the short remained, all three of the motor terminals show a short to one another.

So I googled it and the answer that I thought I found was that this was not normal and indicated the windings were shorted to one another. I figured rewinding wouldn't be too tough and began to take it all apart.

However once I took the basic PCB off of the motor the coils were not shorted together... I checked the PCB and sure enough the coils are all shorted together by the circuit board... so I guess my Googling was incorrect?

Either that or I have something else totally wrong in what I have worked out.

Anyway - here is the motor:


And here is what I assume is a BLDC speed controller (potted):

[MarkT]

My electronics knowledge is limited, but I checked the motor terminals with a multimeter and they all showed a short. I don't know BLDC motors at all so had no idea if this was normal, but I detached the wires to the controller and the short remained, all three of the motor terminals show a short to one another.

They are not shorted, clearly they are wired in a delta (triangle) configuration, normal for 3-phase motors.  The winding resistance is low, this is normal for motors, some BLDCs have winding resistance lower than 0.1 ohm even....  You might call that a short, but its just relative - at 25A one ohm is 0.6kW wasted....

To measure small resistances well takes a four-terminal measurement (not found on cheap multimeters).

[Konkedout]

The BLDC electronics does switching between the motor windings and the DC input.   In other words the motor is "electronically commutated." You probably know that BLDC means "brushless DC."  The electronic commutation is done with semiconductor switches, probably MOSFETs which are internal to an IC.  It sounds like you have probably blown out those electronic switches.

In an old fashioned brush type DC motor you have a commutator with electro-mechanical brushes.  Those spark a bit and eventually wear out.

 But....I have a VSR electric drill which I have been using for almost 50 years.  The Variable Speed Reversable aspect is one thing that probably uses a thyristor and requires AC input (like a conventional dimmer switch).  But I am reasonably certain that the motor itself is a "universal AC/DC" type which has a brush type commutator inside.    That old puppy looks all beat up but still works great.

[No_Shorty]

Thanks for the replies, it confirms that I had a misunderstanding on now the BLDC motor should respond to a multimeter - which is obvious really looking at the PCB!

I actually ordered a cheap ESC yesterday to test out the motor and see if it spins up... should it be as simple/easy as this or am I likely to blow something else?

The ESC I bought was the 35A version of this: https://www.ebay.co.uk/itm/265684234340

[jpanhalt]

If the problem is with the electronic commutator board in the motor, I suspect the ESC will not work.  But at least you will have it for the future to control speed.

If you can remove the board, you might be able to fix it and have both.  Post a picture of both sides of that board.

[No_Shorty]

Oh I get it... I literally just soldered that board back to the motor... I didn't realise it was anything more than a passive PCB connecting up the coils. I'll pop it back off and grab a photo of the other side.

[No_Shorty]

Actually, I just had a wee look at it through the sides and top and I'm fairly sure it is passive - doesn't look like it has any components on that board.