# [EVDL] 36V battery mower (Worx WG788)



## EVDL List (Jul 27, 2007)

It is too hot at the moment to go outside, drill and tap
some necessary holes for mounting a larger IGBT in my
EV100 controller (The CM600HA has a 1/2 inch wider
footprint than the provided holes in the base plate
of the EV100, but I can make it work by moving an
isolator out of the way and adding holes. But not now.
Reason for the new IGBT is that the original ones
failed - either because they were not up to the task
or because there were some early samples handed to me
that helped for a while but eventually failed or
because the 11" GE motor is causing higher current
peaks than the controller is designed for.
I hope the 600A IGBT will have a longer life and
allow this 300-ish Amp controller to work a little
longer than it has done until now, but I digress.

I picked up a battery Electric Mower, a pretty nice
machine that sells for about $440 and has a powerful
36V motor. It should also have a 11Ah 36V lead-acid
"PowerTank" battery, but that did not come with the
used mower that I bought. In fact, when I tested it
there was no response after putting 36V on the wires
externally. Hmmm. So I opened it and sure enough, the
"IntelliCut" controller had failed - a large streak of
black soot along the heatsink, some molten wire and
a sure-dead controller. Nice hall sensor on the back
of the motor shaft though. Aha, that is how IntelliCut
works: the motor is regulated to constant RPM by the
hall sensor. There is a switch that can be rotated from
"Power" to "Silent" and this simply tells the controller
to go to a different RPMM setting. OK I get it.
Now the question is whether to connect a battery
directly to the motor or hack a new controller.
I have FETs and Schottky diode to make such a thing
and I can design a simple BMS so I can build a small
pack of Li-Ion from the stack of 18650 tool batteries
that I have sitting here...
The motor is OK, I ran it with a lab power supply.
First I though it was blown but that was operator error,
if the safety key and handle are not pulled, the motor is
shorted. Once I found that, I could test it and at no load
a voltage of 25V on the motor sent it spinning at a speed
that I can imagine the blade having plenty power to cut.
Current was about 6A but the PM motor is about 6" dia, so
I guess that it can take about 5 times more current when
loaded in wet grass, for a peak power of around 1kW.
I turned the voltage down and at 1/2V the motor was still
turning slowly at about 1A. Nice.
I have tool powerpacks of 5 series cells, which makes it
rather simple to create a 10-series pack that should have
a typical 37V and peak 42V output. 8 parallel should give
more usable Ah than the 11Ah lead-acid could ever give.
This is also confirmed by the marketing spec of the WG788
which has "up to 1 hour" run time. This sounds like the
6A for 1 hour that I measured it idling the blade...
I think the powertool cells I have are 1.3Ah so 8p would
give me just over 10Ah if they are balanced. that sounds
plenty to me. I can probably still use the original
charger for the lead-acids, which is speced for 42V so
the BMS will need to reduce the current and eventually
cut the supply when the first cell reaches bypass threshold
resp when all cells are bypassing.
I will probably order some TC54 to design a BMS that draws
a few microamps.
It will probably be a long term project as I do not have
a yard that needs mowing at the moment...

Of course connecting the battery directly to the motor will
be easier than building a controller, but I am not sure
if the PM motor will take kindly to being jolted into
action at max current. I have heard of problems with
de-magnetizing the motor, not sure if that is also here.
Then there is the hall sensor in a nice bracket, begging
to use its output to give a constant RPM. Hmmm, should
be simple to feed that into a one-shot (monostable vibrator)
to convert into a control voltage and a comparator to
create the two RPM settings depending on the Power/Silent
button setting. And there is no need for high speed
switching - my guess is that the motor will run fine
with a switching freq of a few hundred Hz. Hmmm...

Cor van de Water
Chief Scientist
Proxim Wireless Corporation http://www.proxim.com
Email: [email protected] Private: http://www.cvandewater.com
Skype: cor_van_de_water XoIP: +31877841130
Tel: +1 408 383 7626 Tel: +91 (040)23117400 x203 

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