DC motor (from a treadmill) inrush current trips breaker - Page 1
Forum section: electronics / projects

author: scott216
author_rank: Regular Contributor
timestamp: October 12, 2024, 11:20:30 am
content: I have a DC motor I took out of a treadmill. It's connected to a SCR voltage controller (like this one on Amazon) and a bridge rectifier. The voltage controller uses a potentiometer to control the speed. I've got a 16 amp circuit breaker on the A/C input to the voltage controller. If the motor is off and the pot is set to about 75% speed or higher, then when I turn the motor on there is an inrush of current that trips the breaker. I can run the motor at any speed if I start it off a little slower. I was thinking you could either put some sort of surge suppression on there or somehow do a soft start. Please let me know if you have any ideas.BTW - I have this motor on my woodworking lathe. I wanted to be able to change the speed with a knob and not mess around with the v-belt and pulleys.

author: Ice-Tea
author_rank: Super Contributor
timestamp: October 12, 2024, 11:27:26 am
content: Look into NTCs:https://www.ametherm.com/blog/inrush-current/dc-motor-inrush-current

author: themadhippy
author_rank: Super Contributor
timestamp: October 12, 2024, 11:31:31 am
content: what type of breaker if its a b tyype maybe changing it for a slightly slower c type might help

author: scott216
author_rank: Regular Contributor
timestamp: October 12, 2024, 11:41:00 am
content: Quote from: themadhippy on October 12, 2024, 11:31:31 amThis is the breaker from Amazon:https://www.amazon.com/gp/product/B09WF5QKLTI guess it's a C because it says C16. It's from China, so they may not use the same B/C designation.

author: scott216
author_rank: Regular Contributor
timestamp: October 12, 2024, 11:45:14 am
content: Quote from: Ice-Tea on October 12, 2024, 11:27:26 amWould a NTC go on the AC side or DC side?

author: Ice-Tea
author_rank: Super Contributor
timestamp: October 12, 2024, 11:50:03 am
content: I'd say in your case it probably doesn't matter as you don't have a cap after your bridge (I think?) but I'd still put it in front of your bridge in case you ever add smoothing...

author: scott216
author_rank: Regular Contributor
timestamp: October 12, 2024, 11:56:31 am
content: Quote from: Ice-Tea on October 12, 2024, 11:50:03 amI do not have a capacitor after the bridge rectifier. I do have a big DC choke after the bridge rectifier.

author: Gyro
author_rank: Super Contributor
timestamp: October 12, 2024, 01:05:49 pm
content: Surely, from the point of view of mechanical shock, not compromising the grip of the headstock on the lump of wood you're turning, extending the life of the motor, etc. You want to implement a proper soft start. Depending on the mass of the work-piece, you could end up burning out the motor during acceleration. I don't think breaker characteristics or NTC thermistors are going to be flexible enough to cope with the potential load and spinning mass scenarios.Go for a ramp-up, even if that means manually turning the pot. My DC motor driven metal lathe enforces starting with the speed control at zero, I would think it's even more important for a wood lathe with it's relatively lower grip on the softer work-piece, possibility of serious out-of-balance situations etc.

author: andrewtaylor
author_rank: Contributor
timestamp: October 12, 2024, 03:19:21 pm
content: Quote from: scott216 on October 12, 2024, 11:45:14 amWill work on both sides, because it`s series connection .-)

author: schmitt trigger
author_rank: Super Contributor
timestamp: October 12, 2024, 03:30:04 pm
content: When the speed is zero, there is no back-EMF to oppose a sudden application of voltage. Inrush current is limited ONLY by the rotor winding's resistance, which is very low.You should always soft-start a DC motor from standstill. NTCs is a way to accomplish that, but I would rather start the motor with a low conduction angle and then slowly bring it up to the correct speed.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 13, 2024, 02:07:30 pm
content: Quote from: schmitt trigger on October 12, 2024, 03:30:04 pmIndeed. Inrush current can easily be 40 times higher then the nominal motor current. I would not be surprised if it would be over 200A.An NTC is not a good solution here. A lathe motor is likely to have a lot of start cycles, and the NTC does not have enough time to cool down during the time it is stopped. It also always introduces losses during operation.Big audio amplifiers (with old fashioned mains transformer and big electrolytics) also form a near short when turning on. A common way to prevent overcurrent during turn-on is to put a power resistor in series with the mains voltage (something like 30 Ohm 20Watt) and then short circuit the resistor with a relay a few seconds after startup. Wire wound resistors (the green ones) are very robust, and the "cemented" type in squarish white ceramic housing too. These resitors can easily handle power peaks that are 40x higher then their nominal power. ( Surge rating of resistors is a niche part of electronics. Melf resistors are apparently the best in the SMT department. Vishay has written extensive application notes about surge power handling of various resistor types)But with the SCR in your circuit, you already have a reliable means to limit the input surge current. Just add a little circuit that always ramps up the conduction angle "slowly" when the motor is turned on. This also mitigates high torque pulses on your timing belts and gears in the lathe. Just the chuck is a heavy piece of metal spinning at high speed, but your work piece can be one or two magnitudes heavier still.

author: scott216
author_rank: Regular Contributor
timestamp: October 13, 2024, 03:36:31 pm
content: Quote from: Doctorandus_P on October 13, 2024, 02:07:30 pmI like the idea of a circuit on the SCR to ramp up the speed, but I don't know how to design one. There's a potentiometer that controls the speed. At about 120K is the slow speed, 0 ohms is full speed. I put a voltmeter on it to try and get an idea of the voltage, but it jumps around a lot.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 13, 2024, 04:24:21 pm
content: Post the schematic.The classic way is to have:1. Zero cross detector.2. Saw tooth generator.3. Potentiometer with a DC voltage.4. Comparator that controls the triac.With this setup, it's easy to manipulate the DC voltage that comes out of the potentiometer, with a start switch and RC smoothing, maybe add a simple opamp or emitter follower buffer.All these sub circuits can be very simple. The saw tooth generator can be just a single current source that charge a capacitor, and gets short circuited on any zero crossing. There must be hundreds of usable schematics floating around the 'net.Designing a circuit like this can also be a lot of fun. It is only the mains voltage related part that makes it iffy for beginners. You can get around that by using a 24V transformer, and design and test the whole thing with that (and a smaller DC motor fit for a lower voltage), and then with a few small modifications you can make it fit your local mains voltage.I once designed and built a microcontroller controlled TRIAC light dimmer with mostly 6Vac transformer and bicycle light bulbs. Only after the final PCB was made and 90% of the software was finished I put mains voltage on the thing.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 13, 2024, 04:29:35 pm
content: Just realized: The simplest way for you is to use a potentiometer with a built in switch. Just turn it fully counter clockwise to turn the motor off. If you add a big dial (or even a movable mechanical stop), then it is also easy to ramp up to a certain speed.For a mechanical stop, you can use a disk with a nail in it that fits over the poteniometer shaft. Then you can rotate the knob until some extrusion on the knob hits the nail. This is easy to operate "blindly" while you have your eyes on your lathe.