RE: (ET) motors

["Andrew Hirzel" <adhintl chartermi net>]

Nic, Neil

A good question, run two 12V motors in series at 24V supply.  Sounds easy
but....
Assume both motors have equal resistances (R) and Back-EMF (BEMF). We'll
ignore any inductance, since for your DC machines this is only at startup.
So measure R on your motor, and calculate V = I*R.  Yikes, way too much
current!  But it is also the BEMF that provides an opposite voltage to your
supply voltage, in proportion to the motor speed.  It is the R and BEMF
together that limit the amount of current to your motor, and keeps the 
wires
from burning insulation, and making black smoke, Ok you follow. If you are
an engineering type email me for the equation, there's only one really for
this machine.

Normally, as your motor slows down (like with some load), then the BEMF 
goes
down, meaning more current can come in, and the motor speeds up again, 
until
the BEMF (and R) make the current/voltage equation balance.

Concerning R, when current comes in, the wires get hot, and that reduces 
the
current coming in.  The motor designer never relies on this to keep the
current at normal levels, but generally plans on wires at 125 degrees C,
which means R is about 1.4 times R than at room temp, ok enough of that.

Now back to your idea.  If both machines have identical R and BEMF, and 
stay
that way for all temperatures and loads, no problem, your idea is good.
This will mean that the 24V supply will exactly split between the motors 
and
you have 12V on each.  But if at any time the R and/or BEMF are different,
what happens is that the 24V divides in proportion to the loads.  Example,
assume the total R + BEMF for 1st motor is 2/3 of that for the 2nd motor.
Now the split will not be 50-50, it will be 40-60.  So the 1st motor will
have a 60% of 24 = 14.4V and the 2nd will have only 40% of the 24V = 9.6V.
This will probably be in the range of what the motor designer can tolerate.
(I say PROBABLY, no guarantee).

If one motor stalls, then the current goes very high for both machines, and
the voltage split will be much worse, more like 1:23, and the other motor
will spin like crazy at 23V until both motors burn up together (same 
current
remember), but the second one might fail the bearings or brushes, or the
load it is attached to, depending how long it spins very fast, since you 
are
running a 12V motor at 23V.

A problem is that most motors have a fan internal to keep them cool.  Does
your motor have internal fan, or rely on the fact that it originally drove 
a
fan that washed air over it?  Example, say the motor with high R+BEMF will
get less voltage and go slower and its internal fan will spin slower,
meaning it could get hot faster, making R worse, and the voltage goes 
lower,
and the cycle keeps up until this motor stalls, and the other motor is
spinning like crazy and you get the idea.

Now to the decision.  Does this scare me?  No way, I like your idea, belt
losses are very high, and are a known failure point.  You are mowing grass,
exact motor speed is unimportant.  But it would be nice if you had an
engineering lab and could mount thermometers on the motors as you mow.  How
about try the idea, at no load?  Check motor temp with your hands, from
startup until you can't take any more.  If one gets hotter a lot faster, 
the
imbalance is high, otherwise give it a try in the application.  I can also
guess this is a particularly fun project if the motors are cheap (free)...

By the way, I am paid to design electric motors, so what I've lectured on 
is
straight, although glossed over the engineering.  I can give you simple
proper equations and methods to test your motors, before you start, to see
how balanced they are, all you need is a multi-meter and electric drill.

Good-luck

Andy


-----Original Message-----
From: Nic Hirzel [mailto:Nic Hirzel kellogg com]
Sent: Friday, June 14, 2002 11:39 AM
To: Neil Dennis
Cc: Elec-trak; Andrew Hirzel
Subject: Re: (ET) motors




Neil, this is a really interesting question.  I also like to know if 
someone
knows the answer to this.  Also, there a couple of items on semi-trucks
(blower
fan, sleeper accessories) that are already on 24V.  Food for thought.

nic




Neil Dennis <wombat RealNS com> on 05/28/2002 04:01:29 PM



 To:      Elec-trak <elec-trak cosmos phy tufts edu>

 cc:      (bcc: Nic Hirzel/BattleCreek/USA/Kellogg)



 Subject: (ET) motors






Just wondering, what might be HP of the radiator fan mators that are
used on trucks ?

Put a pair of those in series (only 30% overvolted) a mower motors.

The simplicity of seperate mower motors over a single motor and belts is
a big advantage IMHO.

wombat