Lathe DC motor / brushless controller tripping breaker seemingly at random  - Page 1
Forum section: electronics / projects

author: r-mm
author_rank: Contributor
timestamp: January 02, 2021, 01:26:30 pm
content: Hello allI have looked for help on this from my machinist forum but believe it may be a more detailed electrical question so I'm looking for additional insight here.My lathe is tripping breakers onstartup. With the motoryet to turn. I tried it on two circuits, both are new wiring, new non gfi breakers (15a, 20a), outlets located just feet from the panel. I have wired lots of circuits for far larger loads (50a welder, 240v oven, knee mill) and never once had a false trip, ever.Lathe is late model PM-1030 with 1hp/1000w brushless DC motor and controller. Running/no load my clamp meter tells me I pull less than 1amp.As trouble shooting I have removed the drive belt (zero load on motor), removed the contactor and safety switched from the circuit (line voltage going directly to control board). I have a spare control board and swapped that in with same result:~1/20 times I start the lathe it instantly trips the breaker. Important to note that the control board will not let the motor run unless the speed pot is turned to min. However the trips happen no matter what position the pot is in when the machine is turned on. The pot can be in min in which case the motor will spin at ~50rpm or it can be at max in which case the motor does not move whatsoever.My next step was going to be to simply unplug the motor from the control board but it occurred to me I have no idea how the board works or if this could damage it.For the record - the mfgr is being helpful but doesn't want to throw parts at is and asking me to troubleshoot hence my efforts. Thanks for any advice.https://www.precisionmatthews.com/wp-content/uploads/2020/10/PM-1030V-v5-2020-10.pdf(wiring diagram is not exactly right in that PDF but it gives an idea)

author: ace1903
author_rank: Regular Contributor
timestamp: January 02, 2021, 06:01:11 pm
content: From the manual, I can see that controller is for a brushed motor. Uses SCR with phase angle control.If you have lathe like that probably it is a good idea to check brushes and carbon build-up on the collector(copper part under the brushes).You can check if part of the rotor is burnt. Disconnect motor from the controller and measure resistance between the two wires.Rotate shaft by hand slowly. If suddenly resistance drops significantly then part of the rotor is burnt. If the motor stops in that position, the next time it is powered up it will trip the breaker.If you have a lathe with brushless motor post pictures of the control board. I expect to see large capacitors and some circuit to limit inrush current.Maybe some resistor is burnt and with lower resistance or melted contacts on startup relay.

author: Paul Moir
author_rank: Frequent Contributor
timestamp: January 02, 2021, 06:08:48 pm
content: I wouldn't suspect anything downstream of the control board; the SCRs would have surely blown if you had a short downstream. Maybe get away with one breaker trip but certainly no more. Even excessive carbon arc tracking bad enough to open a breaker will short the SCRs, and then they drive will just blow fuses.I would go after the chuck guard switch, gear guard switch, etc looking for a short to ground. Lots of opportunity for swarf to get in somewhere nasty there.

author: r-mm
author_rank: Contributor
timestamp: January 02, 2021, 06:16:35 pm
content: Here is the control board.During my test that connected line voltage directly to the control board the chuck guard, E-stop, on/off button and gear cover safeties were all bypassed. That leaves the pot, tach, board and motor.

author: r-mm
author_rank: Contributor
timestamp: January 02, 2021, 06:17:56 pm
content: The board looks very close to this -https://www.aliexpress.com/i/1005001279538818.html?spm=2114.12057483.0.0.42804074F0RaE0

author: ace1903
author_rank: Regular Contributor
timestamp: January 02, 2021, 07:19:18 pm
content: OK, this is controller for brushless motor. There is NTC in space between two capacitors. It limits inrush current at startup when capacitors are empty.Relay contacts are in parallel with it and shortly after start-up short it to prevent overheating. Turn off lathe for several minutes so capacitor can drain completely.Turn on the power and listen for relay click. after 1~5 sec relay should click. If not, contacts are welded.That relay is really small for 1kw at 110V. You can also check the resistance of the NTC, it should be 20 Ohm but these fail catastrophically most often.Please before taking any measurement wait 10min for complete capacitor drainage.

author: r-mm
author_rank: Contributor
timestamp: January 02, 2021, 07:58:08 pm
content: Thanks for your suggestions. In all conditions I tested the relay seems to click just about instantly on startup.

author: WattsThat
author_rank: Frequent Contributor
timestamp: January 02, 2021, 08:23:01 pm
content: The manual shows a 15 amp fuse in series with the incoming power. What type of fuse is that? Please post a photo or the type printed on the fuse. It make little sense for a residential 15 amp breaker to trip while a 15 amp fuse in the same circuit survives. That is possibly pointing to a problem with the wiring or termination of the supply before it ever gets to the lathe.PS: what does “~1/20 times I start the lathe it instantly trips the breaker.” mean? That approximately 1 out of 20 times the breaker trips? Does the lathe run correctly when the breaker does not trip?

author: r-mm
author_rank: Contributor
timestamp: January 02, 2021, 09:23:17 pm
content: When the breaker does not trip the lathe runs perfectly and draws less than an amp with no load. I haven't done a measurement with load.Yes 1 out of 20 starts is a very rough approximation of how often I'll get a trip.The fuse on the rear panel is a glass type, the embossing on it is tiny but I believe it reads 10amp / 250v. I agree it does not make sense. All I can say is I have never had one single breaker issue in the entire time I've run equipment in my garage or for that matter home (same panel). I did try the lathe on two different circuits, the first of which is the one I ran my Seig Mini Lathe (also brushless DC) for years without a problem.

author: WattsThat
author_rank: Frequent Contributor
timestamp: January 03, 2021, 02:48:40 am
content: Well, that’s certainly not a time delay fuse. So, it makes zero sense that the breaker would trip before that fuse would clear as residential breakers are inherently time delay to allow for motor starting in appliances. With a 20 amp breaker on the circuit, it’s even more puzzling.Time to stress test the branch circuit. Got a big shop vacuum? Plug it in the outlet being used for the lathe and see if you can get it to trip by cycling the vacuum (or other high starting current load) on and off, allowing it to fully stop before switching on again.BTW, if the outlet is connected using the back stab-in connections, it is time to rewire using the screws. Also beware the sub-dollar diy home center outlets, they’re junk with poor contact resistance. Buy the commercial grade stuff, it’s worth it.

author: r-mm
author_rank: Contributor
timestamp: January 03, 2021, 02:57:10 am
content: Good idea - but only points a finger at the lathe. I used a "professional" (home depot nomenclature) 20amp receptacle and wired the side terminals with 14ga. Voltage drop not an issue as its practically mounted to the side of the panel. Ran my 4.5hp shop vac and saw up to 15amp peak at startup using the clamp meter. No trips after a few dozen startups...

author: WattsThat
author_rank: Frequent Contributor
timestamp: January 03, 2021, 04:36:37 am
content: Have you inspected all mains wiring inside the switch and anywhere it may lead? Pull tested everything?I’ve seen some dodgy wiring inside stuff imported from that big place on the other side of the planet. An arcing connection somewhere could explain the behavior. Whatever it is, it’s not obvious but then you know that better than anyone

author: tautech
author_rank: Super Contributor
timestamp: January 03, 2021, 07:58:41 am
content: C and D curve breakers handle motor start inrush currents far better than conventional breakers.

author: fourtytwo42
author_rank: Super Contributor
timestamp: January 03, 2021, 10:14:29 am
content: Quote from: r-mm on January 02, 2021, 01:26:30 pmThis is the second time in a few weeks I have seen a thyristor controller with no or insufficient EMC suppression blowing breakers. I did not see anywhere in that manual any FCC compliance data, maybe I missed it ? Did this thing originate in China ? They seem to specialise in non-compliant products.

author: r-mm
author_rank: Contributor
timestamp: January 03, 2021, 01:45:20 pm
content: Thanks guys. From doing automotive wiring I am familiar with inductive load breakers, I used one as a main breaker on an older car and it worked perfectly - did not trip when the starter pulled over a hundred amps. For residential panelboards - Mine is a GM Load center - I am not aware of there being options for breakers. I simply use the amperage / form factor of GE's Q-line I need for a given position. This is the breaker the lathe is on now:https://www.homedepot.com/p/GE-Q-Line-20-Amp-1-in-Single-Pole-Circuit-Breaker-THQL1120/100356508I have some confidence in residential and automotive wiring (for older cars) but not electronics so the comments about thyristors and EMC suppression have gone above me.

author: PaulAm
author_rank: Frequent Contributor
timestamp: January 03, 2021, 07:18:48 pm
content: This isn't your problem but you really need 12ga on a 20A branch circuit. If you're going to stick with 14ga, use a 15A breaker.This would be a good place for a clamp on current probe and scope to see what is actually happening. There must be a very large current spike to trip that breaker; there's nothing special about it.If you don't have a scope current probe, you might have a peak hold on your clamp on ammeter. Some of those are so-so but you might catch the spike if you're lucky. For it to trip immediately, it's going to be more than 25-30A.Another thing to try is to check the trip point of the breaker by putting enough load on it to trip it. It is pretty cheap, but I'm thinking it's unlikely the breaker is the problem.

author: r-mm
author_rank: Contributor
timestamp: January 03, 2021, 08:06:27 pm
content: You are right. I planned this circuit for 15amp but changed gears to figure out this trip. I'll go back to a 15amp breaker when I solve this.I actually just purchased my first scope, an AESwave uScope. I don't have the current clamp yet but suppose this is a nudge to get one. This is the one AESwave sells which is geared to automotive work but should help with just about any home load I'll encounter.https://www.aeswave.com/Current-Probe-Low-range-with-Banana-Plugs-p9501.html

author: r-mm
author_rank: Contributor
timestamp: January 04, 2021, 12:50:57 am
content: I truly can't see how the problem is anything besides the board. I ran line voltage directly to the board from two different circuits. Repeated for the spare board. Same thing. Trips about one in twenty.I will speak to the mfgr on Monday but if the only suggestion is more of the same... curious to hear ideas on:-add-on current limiting device-alternative 'name brand' replacement controller for brushless DC 1hp motor

author: ace1903
author_rank: Regular Contributor
timestamp: January 04, 2021, 08:59:20 am
content: The board design is problematic. The relay should click after one or two seconds after powerup.But if they do like that they will need to use bigger, more expensive NTC.Like as it is, they are selling cheap lathe and then they sell several controllers at 100$ each.Good business model especially when the controller board has components worth 30$ but they sell at 100$.If you want to add an external inrush current limiting circuit then that is the basic idea: quality relay + more powerful NTC plus some capacitor for 1sec delay.