On 2/9/2013 9:02 PM, Ds2inc wrote:
I thank you for the link am I have read it twice and as a non
scientist I don't understand how it applies to the below information
suggesting that pmdc motors should not be run in parallel. I
apologize for my ignorance.
Cor was answering a different question (on selsyn motors).
Regarding your question: There is nothing especially wrong with running
PCDC motors in parallel. You just need to understand how they will
behave. In some cases, they will work just fine. In other cases, you may
have problems.
First principle: Voltage determines the speed of a PM motor. If the
motor was perfect, its speed would not change *AT ALL* no matter what
the load.
Second principle: Current determines torque. For a perfect motor with no
load on it, torque is zero so it draws zero current. As the load
increases, the current increases. If you force the motor to turn, it
becomes a generator, and *generates* a current proportional to how hard
you "push" its shaft.
Sample characteristics of perfect 100% efficient motor. volts x amps =
watts, 746 watts = 1 horsepower, ft.lbs x rpm / 5252 = horsepower.
speed: 0v 0 rpm
14.2v 1000 rpm (speed does)
28.4v 2000 rpm (NOT change)
142v 10,000 rpm (with load!)
torque: -100a -10 ft.lb torque (negative torque and current)
-20a -2 ft.lb torque (means you are pushing it as)
-10a -1 ft.lb torque (a generator)
0a 0 torque (no load)
10a 1 ft.lb torque
20a 2 ft.lb torque
100a 10 ft.lbs torque
A less-than-perfect PM DC motor has some resistance in series (due to
the resistance of the windings and brushes, bearing friction, wind
resistance of the rotor, etc.) It behaves like an ideal motor with some
small resistance in series. This resistance causes it to slow down a
little bit under load, and speed up a little bit if you "push" it as a
generator. If you have a motor that is 90% efficient, about 10% of the
voltage you put across it is lost in this resistor.
no load full load (+100a) generating (-100a)
------- ----------------- -----------------
0v 0 rpm 0 rpm 0rpm
14.2v 1000 rpm 900 rpm (10% low) 1100 rpm (10%high)
28.4v 2000 rpm 1800 rpm 10% low) 2200 rpm (10% high)
142v 10,000 rpm 9,000 rpm (10% low) 11,000 rpm (10% high)
(torque versus current is not affected)
From this, you can see that coupling two identical PM DC motor shafts
together so they both turn at the same speed won't cause any problems.
They run at the same speed, so they have the same voltage. The current
will divide equally between them, and each handles half the load.
Trouble begins if the two motors are NOT identical, or if they are NOT
turning at the same speed. With the above 90% efficient example motors,
if one motor is turning 10% slower, then it draws *all* the current, and
produces *all* the torque. Its twin, running at the same voltage but 10%
higher rpm, draws NO current and produces NO torque. This doesn't harm
anything, but isn't what you'd expect.
Now suppose these two identical motors are forced to turn at 20%
different speeds. One is turning 10% lower than its no-load speed; it
draws full current. The other is turning 10% faster than its no-load
speed; it becomes a generator, and generates the same current that the
other motor is consuming! In other words, ALL of your power is lost by
the two motors fighting each other!
Does this help?
--
Failure is only the opportunity to begin again more intelligently.
-- Henry Ford
--
Lee A. Hart, http://www.sunrise-ev.com/LeesEVs.htm
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