Electric lawn mower dynamic braking - Page 1
Forum section: electronics / general

author: MLXXXp
author_rank: Frequent Contributor
timestamp: May 14, 2023, 07:12:55 pm
content: I have an electric lawn mower that has a permanent magnet DC motor powered via AC mains through a bridge rectifier. It is controlled by a "dead man's" handle lever which contains a double pole, alternating single throw, break-before-make switch (when one pole is closed the other is open). When the lever is pulled, the "normally open" pole closes and switches AC to the bridge rectifier, and the other pole opens. When the lever is released, the "normally closed" pole shorts the motor connections, thus rapidly slowing the motor through dynamic braking. This is done for safety, since if only the power is removed, the motor and blade will continue to spin for around 10 seconds.I needed to replace the motor and while testing it with the cover off, I noticed that large sparks are produced between the brushes and commutator when the lever is released, due to the great amount of energy produced by shorting the motor as a generator from dynamic braking with a dead short. I can't imagine that these sparks are doing much good for the brushes and commutator.So, disregarding any safety considerations, do you think the sparking is actually doing much harm (not to mention the forces from the motor being forced to slow quickly)? How much would the life of the motor be extended if dynamic braking were disabled and the motor and blade were just allowed "free wheel" and slowly spin down from friction and air resistance?Schematic attached.

author: Gyro
author_rank: Super Contributor
timestamp: May 14, 2023, 07:40:39 pm
content: The motor braking will certainly cause additional wear on the brushes and surge current on the windings and switch contacts. How much wear compared to normal starting and running is hard to tell, and will depend on usage (ie. holding the trigger on for longer periods rather than faster stop-starts). I would expect a reasonably big spark to occur on starting too. Presumably the manufacturer will have factored in these things - if only for the warranty period!In industrial settings, things like lifts and locomotives, braking resistors would be used to dissipate the peak powers involved. You could maybe try including a reasonably low value, suitably rated wirewound resistor in the braking path - experimentation would be required, and it could get very hot in rapid start-stop operation. It would almost certainly increase the braking time beyond whatever the requirements are, but would still be far safer than allowing the mower blade to just freewheel to a stop (as they did for decades before the requirements changed).

author: MLXXXp
author_rank: Frequent Contributor
timestamp: May 14, 2023, 07:48:32 pm
content: Quote from: Gyro on May 14, 2023, 07:40:39 pmI didn't notice any sparking when the motor started. Certainly nothing like when stopping.

author: Gyro
author_rank: Super Contributor
timestamp: May 14, 2023, 08:17:43 pm
content: A dead short would certainly cause a much higher current than being limited by mains impedance, long length of cord, rectifier etc. That probably explains it, along with the current hitting the brushes at different points (?) in the commutation compared to normal running.P.S. Regarding a braking resistor value, as a finger in the air figure, maybe measure the stationary resistance value of the motor and multipy by 5 - 10 times?

author: MLXXXp
author_rank: Frequent Contributor
timestamp: May 14, 2023, 08:48:22 pm
content: Quote from: Gyro on May 14, 2023, 08:17:43 pmI was thinking the same thing.As for the legal requirements for slowing the motor vs. adding resistance to lower wear and tear; I can see that the manufacturer would be concerned about the cost of additional components. Just shorting the motor would be the cheapest solution even if it stopped faster than required (but, as you say, without causing warranty or reputation implications).However, the wires that do the shorting run from the motor, up the handle to the switch, then back down to the motor, which is a length of about 165cm. Since this is a custom cable, if the manufacturer wanted to add resistance they could have used very thin wires (thus higher resistance) for the braking circuit and saved some cost in the process, assuming no special, more costly, higher temperature insulation was required. As it is, the braking wires are the same gauge as the power wires.

author: MLXXXp
author_rank: Frequent Contributor
timestamp: May 14, 2023, 09:04:47 pm
content: Quote from: Gyro on May 14, 2023, 08:17:43 pmSince I'm the only one who uses the mower, I'm considering accepting the risk and just disconnecting one of the braking wires. I'll know to expect the motor to spin down slowly and be mindful of it.The wires are attached to the switch using standard quick-connect spade terminals, and it's easy to open the handle to get at the switch with just a few screws. It won't be hard to reconnect the wire if/when the need arises.

author: Zero999
author_rank: Super Contributor
timestamp: May 14, 2023, 09:36:09 pm
content: Quote from: Gyro on May 14, 2023, 08:17:43 pmI don't think so. I'd expect the resistance of the motor windings to dominate. The current will be a little higher than starting, but not excessively so. It should be fairly easy to test with a Hall effect sensor and oscilloscope.Quote from: MLXXXp on May 14, 2023, 09:04:47 pmI wouldn't recommend it. I doubt you'll gain that much extra life from such a modification.

author: MLXXXp
author_rank: Frequent Contributor
timestamp: May 14, 2023, 09:58:59 pm
content: Quote from: Zero999 on May 14, 2023, 09:36:09 pmOn the other hand, I don't see much risk in letting the motor slowly spin down. The main danger is that you reach under the mower while the blade is still spinning. If I know it could be spinning, I'm not going to do that. (For that matter, I'm not going to do it underanycircumstance with mains connected.) The motor makes enough noise, even when spinning under no power, that you can hear when it has stopped.Dynamic braking doesn't happen, so does nothing for you, if the blade is impeded by something while you've got the handle pulled. Then, after you hit something, so you release the switch, the damage is already done and having the motor then stop quickly likely won't help matters or prevent any more damage.

author: langwadt
author_rank: Super Contributor
timestamp: May 14, 2023, 10:09:54 pm
content: and an extra on/off switch, safety feature still functional but you can choose to turn mover off with out using itthought it probably doesn't make much difference