Reverse the wiring of a permanent-magnet, dual-winding reversible DC motor - Page 1
Forum section: electronics / projects

author: dial-a-monkey
author_rank: Contributor
timestamp: August 27, 2025, 07:28:30 am
content: I have a permanent-magnet, dual-winding reversible DC motor - a 'Quick' brand windlass motor.It has three terminals — a common ground and two “positive” leads for Clockwise and CCW. Up & DownI discovered the relay that drives the windlass generates huge current transients from contact bounce of up to 1170 A pk-pkI’d like to replace this mechanical relay with a solid-state solution and do my switching on the low side using MOSFETs.Does anyone know if I can just reverse the wiring — feed battery +(positive) into the “common” terminal permanently, and then switch ground to either winding with a low-side solid-state relay?I checked and the case of the windlass is not grounded - it looks to be isolated ground. Not sure why the relay is high side switching - perhaps it's just by convention?I know nothing about DC motors and if the reversal might have long term consequences.Many thanks for any advice, ideas or just opinions

author: Zero999
author_rank: Super Contributor
timestamp: August 27, 2025, 08:33:36 am
content: It's not a permanent magnet motor. It has field coils with two windings in opposite directions. One winding goes to the clockwise terminal and the other to the anticlockwise terminal.Changing the polarity of the supply voltage shouldn't change the direction of the motor, because it changes the both the field and armature polarity which cancels one another out.Why do you want to replace the contactor with MOSFETs? It's bad idea. MOSFETs have a nasty tendency to fail short circuit and if power is simultaneously applied to both supply terminals bad things will happen: battery or motor overheating. Hopefully a fuse will blow, but there isn't one on the schematic.

author: dial-a-monkey
author_rank: Contributor
timestamp: August 27, 2025, 09:16:41 am
content: Hilarious! The official document actually omits the circuit breaker—LOL—I didn't notice, good spottingin practice, a hydraulic-magnetic circuit breaker is always installed.The circuit breaker is sized close to the trip current under normal operation - I guess what I mean to say is if there is anything unusual like chain snagged on a bommie, or the operator winching the yacht forward in a strong wind the breaker trips - it's not severe / unusual or prolonged abuse that trips it..I’m now looking at a MOSFET switch because the relay generates huge transients due to contact bounce. These occur while my LiFePO₄ bank is “in circuit,” and since the cells have very low internal resistance, TVS diodes and similar protections aren’t very effective. The negative portion of the transients could be back-EMF or just ringing—an ultra-fast Schottky diode might help here.Yes, that is a problem—I had contemplated, then forgotten! 🙂 (MOSFETs fail shorted ) ..There is a circuit breaker, but the design would still need some additional protective measures—perhaps a momentary button on the remote to energize the entire circuit - I could potentially use a relay/contactor here as there would be zero current - when energized + some time delay - buttons for up and down

author: Zero999
author_rank: Super Contributor
timestamp: August 27, 2025, 09:37:36 am
content: What makes you think contact bounce is causing the transients? Starting a large motor will result in transients. There isn't much which can be done about it, other than a soft start.What problems are the transients causing?Start with a Schottky diode across each winding to get rid of the back-EMF.

author: dial-a-monkey
author_rank: Contributor
timestamp: August 27, 2025, 09:51:50 am
content: Good point—I assumed the transients are caused by contact bounce. Based on their timing and shape, they are very aggressive, with high dI/dt over a short duration, occurring when the relay engages.I’m now using ChatGPT to research DC motor startup transients.The transients I measured are huge—on the order of 1170 A peak-to-peak. Obviously the energy is low—I roughly estimated around 4 J—but I’m still uncomfortable with such peaks on a 12 V supply that also powers other navigation equipment.

author: Zero999
author_rank: Super Contributor
timestamp: August 27, 2025, 10:52:29 am
content: Quote from: dial-a-monkey on August 27, 2025, 09:51:50 amBeware: AI can hallucinate.Large current surges are caused by the fact that there's no back-EMF generated by the armature, leaving purely the series resistance to limit the current. The ringing is due to resonance with the parasitic capacitance with the inductance of the circuit. It is to be expected and part of the normal operation of a motor. A started motor in a car does a similar thing.QuoteIs it causing any problems?Does the voltage drop too low when the motor is started?

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 27, 2025, 11:01:00 am
content: Quote from: dial-a-monkey on August 27, 2025, 09:51:50 amGood luck with that.