Re: (ET) Elec-trak Digest, Vol 22, Issue 73

["David Roden" <etpost drmm net>]

On 14 Oct 2024 at 19:46, Jay Donnaway via Elec-trak wrote:

>  It'd be worth some deck motor experiments to see if they run any
> cooler and if amerage drops at all, as it'll take more energy to spin
> them 1/3 faster, but the amps may still fall due to higher voltage. 

Warning: I'm not an EE.  In fact I'm an EE dropout, and this is my limited
understanding of the situation.  I invite the EEs here to correct anything
I say that's wrong.

Generally, in a DC motor, torque is proportional to current and speed is
proportional to voltage.

If you leave everything else including the load the same, when you raise
the terminal voltage, the current will increase, not decrease. The motor
will run faster and consume, and produce, more power.

However the current increase won't be defined by Ohm's law (I = E/R)
because the motor isn't a purely resistive device.  The motor's back EMF
sees to that.

>From the speed and increased power, the bearings and brushes will wear
more. From the higher current, the motor's internal temperature will rise.
There is some risk of insulation breakdown and/or field/armature failure
from both the heat and the higher voltage.

Back when converting an ICEV to EV was the only practical way to own an
electric vehicle, EV hobbyists often ran 72 volt series-wound forklift
motors at 96 volts or even 120 volts.  The motors were sturdy enough that
they usually held up to this treatment.  The builders also often used
forced cooling.

So I think, but wouldn't guarantee, that a wound-field ET drive motor would
hold up to 48 volts - maybe even more.

I'd be much more hesitant to try it with either one of the PM (permanent
magnet) drive motors or the mower motors.  Overheat those magnets and they
become less permanent, losing some of their magnetism.

An anecdote that may help:

Mark Hanson is a smart and experienced EE and EV builder. He used to be on
this list and may still be.  A good 25 years ago or so, he built his own ET
controller and dropped 8 volt batteries into the ET for 48 volts.

He did NOT apply 48 volts to the mower motors.  He used a second controller
to drop the system voltage to 36 volts for them.

As I said, Mark's a smart guy.  If he didn't drive the mower motors to 48
volts, I don't think I would either.


David Roden - 25 Years with ETs

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