Re: Multiple Curtis controllers

[Lee Hart <leeahart@earthlink.net>]

bernie2002ev@att.net wrote:
> I have a number of Curtis 1205 controllers and because I
> am accustomed to using what is at hand to build
> everything...I wonder what problems you forsee in a
> staged series of 1205 controllers to achieve 96 volts.

The most certain way to do it is trade several 1205's for a 1231 :-)

Another way is to use multiple motors, each with a controller. For
example, three 36v controllers, motors, and battery packs.

It is possible, but difficult to connect controllers in series for
higher voltages, or parallel for higher currents. It is very difficult
to get them to accurately and reliably share the voltage and load under
all conditions.

If I really felt inclined to do this (and had a lot of controllers I was
willing to sacrifice to the gods of experimentation :-) I would try
something like this: View with fixed width font:
   ___________________________________
  |          |                       _|_
__|__ +      |                      /   \ armature
 ___  48v     / S1b                 \___/
  |   -      |             _______    |   series
  |____/_____|____________|       |   |_  motor
  |   S2     |            |B+     |    _|
  |        __|__ +        |       |    _| field
   / S1a    ___  48v      |     M-|_____|
  |          |   -        |       |
  |__________|____________| B-    | Controller
                          |_______|

With S2 open, and S1a and S1b closed, you have two 48v batteries in
parallel driving a 48v controller and series motor in a completely
normal fashion. The controller adjusts effective motor voltage from
0-48v.

With S1a and S1b open, and S2 closed, the two 48v batteries are wired in
series for 96v. But the controller still only sees 48v between its B+
and B- terminals. The controller still adjusts the effective voltage at
M- from 0-48v. But since the top connection to the motor is 48v higher,
the motor sees 48-96v.

Thus you have two ranges; 0-48v, and 48-96v. Note that the controller's
current limit won't work in the 96v range. That's because even if the
controller is fully off, the motor is still connected to the upper 48v
battery thru the controller's freewheel diode (between M- and B+). Thus,
you must not use the 96v range unless motor current is below the
controller's current limit.

In operation, you accellerate at 48v until the controller comes out of
current limit, release the throttle, switch to 96v, then re-apply the
throttle. If you're in the 96v range and start climbing a hill, you must
switch back to 48v range before the motor current rises above the
controller's current limit.

Another quirk of this circuit is that the batteries are unequally
discharged in the 96v position; the upper battery always carries motor
current, while the lower one carries motor current x controller % duty
cycle. But the batteries will re-balance if you spend time in the 48v
position.

> Besides that one smell, how do you tell when you
> have too much voltage in a large DC motor?

Very roughly speaking... series DC motors don't have an explicit maximum
voltage. Things just get slowly worse as you go higher. As you raise
voltage, they run faster and draw more current (making more horsepower
and heat in the process). Brush arcing gets worse, though you can adjust
brush timing to reduce this somewhat. Eventually, arcing is so bad that
brush life is too short to be practical. If you get really crazy with
the voltage, the commutator can flash over or fireball (an arc forms all
the way around the commutator). Motor life is a matter of seconds when
this happens.
--
Lee A. Hart                Ring the bells that still can ring
814 8th Ave. N.            Forget your perfect offering
Sartell, MN 56377 USA      There is a crack in everything
leeahart_at_earthlink.net  That's how the light gets in - Leonard Cohen