Re: (ET) 48v electrac

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Christopher Zach Wrote
>subscriptions aeolusdevelopment com wrote:
>> I thought the mower motors were PM?  There won't be any field 
>> weakening. 
>> The speed increase from the extra voltage might be an issue, especially
>> when it come to stopping the blades as might the extra surge current 
>> from
>> starting at 48V.  

>PM motors have a fixed field strength that is computed based on the 
>expected armature voltage and current. If you increase the arm voltage 
>without changing the field you have the equiv of field weakening. The 
>motor spins a lot faster, less torque, more current thru windings. Bad.

The motor spins faster yes (speed increases by ~1/3), but the torque for
any given current is the same.  In fact the startup torque will be higher
since the higher voltage will enable a higher current through the armature
windings.  It's possible but doubtfull that the current spike could produce
a high enough field to damage the magnets.

It's not anything like field weakening.

You can get more current through the windings due to increased resistance
of the load at speed but it's not a given that it will be anywhere near
enough to be an issue.  How the load reacts will be a big determining
factor in the increased torque requirements and thus needed current.  The
torque required to move the mower blades going at the higher speed is
likely to be significant, maybe > 2X, which will directly translate into
current and thus into heat.  It may not be due to field weakening but that
doesn't mean it won't be an issue.  You would want to measure the current
and/or the motor heating to make sure you weren't overloading the motors.

>> FWIW there exist 36V and 48V variants of the OX using the same
accessories.
>> Internal support wiring differs though.
>> 
>> I'd be cautious about assuming that 36V motors and accessories will be
>> useable or safe at 48V but that's less likely to be an issue than 
>> relays,
>> resistors etc...
>
>I'd say give it a go, but it would be sad to either torch the motors or 
>cause a blade to shatter.

Blade balancing is going to be more critical for sure.  And you are going
to have nearly twice the energy stored in the blades so if they hit
something...

I'm not sure I'd be tempted unless the 48V battery set was a lot cheaper,
especially since you'll actually lose a little capacity.  Probably not a
significant amount but you might be able to measure it.

Robert


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