There are always exceptions---look up John Wayland's White Zombie electric Datsun. For years he found the best way to get performance down the drag strip was 2 series wound DC motors coaxially driving the differential. Electrically, his controller had several combinations to work with in optimally driving this. Ultimately, he worked with a team to fully integrate the dual motor windings and commutators into a single housing and shaft. The car now does 10 second 1/4 mile times.
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From: "cnc sales (hanermo)" <gcode.fi@gmail.com>
To: <electricboats@yahoogroups.com>
Subject: [Electric Boats] RE: RE: Re: using dual DC motors with one controller
Date: Thu, Oct 3, 2013 11:26 pm
These types of questions come up every now and then in cnc where someone
wants to use one controller to control to motors.
Or two motors to drive one table, etc.
The answer is both yes, .. and no.
In a gross sense, two rigidly coupled motors will move a load, and if
the acceleration is slow, and the load is small and the rpm is slow it
works, apparently, ok.
(Rpm is few rotationes per sec at most).
In another sense it does NOT work very well.
What happens is that no two motors are absolutely equal.
Thus there are small differences in the acceleration abilities of each
motor.
This is because it takes a finite amount of time to raise and lower a
current (power) on every piece of wire (coil, winding). Impedance.
The faster the rise (voltage) and the harder the push (load, ie
current) the larger the difference is.
So, two rigidly coupled motor will drive a load, but they will not work
well and will fight each other.
This is independent of the fact of using two controllers or one.
There are controllers that can drive two motors at once.
They can even drive two rigidly coupled motors quite well, but do this
using an optical encoder to control the position of each axis in real
time (very fast, microseconds). Called DC servo motors.
Granitedevices.fi makes one.
Electronically, they are two separate controllers in one package.
Otherwise, they are just dc motor controllers, in essence.
Even using timing belts and two controllers will work badly, unless the
controllers are servos and controls the rpm and angular position of the
shaft.
Servos are not very expensive.
They can be cheaper than many overpriced older dc motor competitors.
I use AC bruhless servos, but DC servos will work just as well (the
electronics inside are different).
An AC brushless servo package of 2.5 kW costs about 850€.
Motor, servo drive, encoder, cables (3000 rpm, 5000 count encoder, 220V).
The price is an example, only for reference, shipped here in the EU.
In a sense DC brushed, brushless, stepper motors, VFD 3-phase motors and
servo motors are very similar.
Some inside differences that are not relevant for users, mostly.
Brushless is much better than brushed.
(Noise, sparks, ozone, wear).
--
-hanermo
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