RPM too high shotening run time

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  • Updated 2 years ago
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I purchased the mower last year and all was well.  This year during the third time mowing there was an incident where my tire slipped off the grass edge into the garden causing the deck height to drop and bogged the mower down for a second.  Once recovered from the bog-down, the RPM was instantly higher.  I thought initially that it sensed the need for more power and entered a turbo mode.  However, the RPM never returned to normal even after future uses and charge cycles.  This is a problem since it drastically shortened run time which before was 40+ minutes dropping down to 22 minutes at best.

See the video for a side by side comparison of the old and new mowers.

I contacted customer support and was told they had never heard of this condition.  They sent me a new mower and charger (sans a new battery).  I was able to compare the two mowers using the same battery and the new mower very clearly runs at a slower RPM and it's cutting and run time performance is as it should be.

I could not find anyone else claiming to have this experience but perhaps it is happening to others and they are not really noticing the RPM difference.  I do see where folks are having trouble with run time being shortened though so maybe the higher RPM issue could be the cause in some cases.  It certainly was in mine.

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Posted 5 years ago

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(a)Typical Engineer

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Couple thoughts on the matter:

1) The mower running with the handles in the collapsed position, seems like an unrelated issue, e.g. there could be another mower with the high RPM issue, and it may or may not exhibit the safety symptom of running with the handle collapsed.  Both the load sensing circuit and the safety cut-offs go to the motor controller (well of course they do, the safety circuit is a sub-system to the controller).

2) The load sensing could be based on two methods: a) hall effect sensor, or b) current sensing.  I am pretty sure it's a hall effect sensor setup because there appears to be a magnet on the top end of the armature shaft, which sits in close proximity to other electronic gizmos above the brush assembly.

For Michael's case, but guess is that the motor controller is the issue, not the speed sensor, since the controller is the common piece that could cause both issues.

I have also noticed that when the motor senses the blade slowing down due to increased load (e.g. when I hit a patch of thick grass), the mower will attempt to compensate by increasing the RPM (ramps up quite quickly), and then when it sense the load is no longer there, it coasts back down to normal RPM (ramps down slowly).  This has only happened a few times (maybe 1 in 5 times mowing).

In the picture below: a) the motor controller is right below the rear wheel, and it sits on top of the motor, b) the white block is a ceramic enclosed resistor, c) the red/blue/black connect to the battery, d) the hanging black plastic is the RH pivot, which contains the safety switch to ensure the handles are in the raised position.

See this post on why I had to tear this apart. =)

20" Mower "field" repair of the MOTOR