Electronics > Projects, Designs, and Technical Stuff
Why have a DC motor in a mains-powered lawn mower?
calzap:
I have a B&D human-propelled electric lawn mower. It has a single horizontal spinning blade connected to the shaft of a DC motor. The 120 VAC power from the switch goes to a full wave rectifier and then to the motor, which has brushes that have to be replaced periodically. The original rectifier was 25 A, 400 V. After the first one toasted, it was replaced with another 25A, 400 V copy. After that toasted, a 35 A, 1000 V version was used. After a couple of those blew, I now have a 50 A, 1000 V version … so far, so good. The switch is a special design (SPST-NC-mom + SPST-NO-mom, sequential) that shorts the motor leads when it disconnects the AC power. I presume this is to prevent someone from being shocked from back-EMF if they should unplug it while the blade is still spinning and touch the plug pins.
Why couldn’t a brushless AC motor be used?
Mike in California
bob91343:
It may be due to safety considerations. Perhaps a dc motor stops in a shorter time, or has a better stall characteristic.
I have used many Black and Decker lawn care products and have found them to be minimalistic and very cheaply made. Repair is difficult if at all possible. Nearly every one I have owned has failed.
I solved the problem by hiring a gardener. (I am also in California.)
Kjelt:
Could be mass production to lower the cost of manufacturing, the most expensive part being the motor.
If they manufacture for the whole world and with all the different mains voltages they would need as many different motors.
SunnyDay:
--- Quote from: bob91343 on September 07, 2020, 08:38:26 pm ---It may be due to safety considerations. Perhaps a dc motor stops in a shorter time, or has a better stall characteristic.
--- End quote ---
I think that's the reason. Releasing the operating lever shorts the motor terminals, and uses the back emf to brake the rotation of the blade within about a second. An AC motor wouldn't be able to use that method to stop the spinning blade when the lever is released, so it would require a separate mechanical braking mechanism to meet blade stopping safety requirements.
mikerj:
--- Quote from: calzap on September 07, 2020, 08:06:32 pm ---I have a B&D human-propelled electric lawn mower. It has a single horizontal spinning blade connected to the shaft of a DC motor. The 120 VAC power from the switch goes to a full wave rectifier and then to the motor, which has brushes that have to be replaced periodically. The original rectifier was 25 A, 400 V. After the first one toasted, it was replaced with another 25A, 400 V copy. After that toasted, a 35 A, 1000 V version was used. After a couple of those blew, I now have a 50 A, 1000 V version … so far, so good. The switch is a special design (SPST-NC-mom + SPST-NO-mom, sequential) that shorts the motor leads when it disconnects the AC power. I presume this is to prevent someone from being shocked from back-EMF if they should unplug it while the blade is still spinning and touch the plug pins.
Why couldn’t a brushless AC motor be used?
Mike in California
--- End quote ---
The motor is shorted to create a brake for the rotating blade, stopping it much more quickly than if it were allowed to freewheel. This is one benefit of a permanent magnet DC motor that other motors do not provide. The torque/RPM characteristics of DC motors is also likely to be favourable for a lawnmower, as they are loaded and the RPM drops the provide more torque, maximum torque being acheived at 0 RPM, idealt for rapidly spinning up the blade and cutting through long grass. However being a Black and Decker, cheapness was likely the number 1 priority.
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